WorldWideScience

Sample records for superlattice strength enhancement

  1. Anisotropic behavior of quantum transport in graphene superlattices

    DEFF Research Database (Denmark)

    Pedersen, Jesper Goor; Cummings, Aron W.; Roche, Stephan

    2014-01-01

    We report on the possibility to generate highly anisotropic quantum conductivity in disordered graphene-based superlattices. Our quantum simulations, based on an efficient real-space implementation of the Kubo-Greenwood formula, show that in disordered graphene superlattices the strength of multi......We report on the possibility to generate highly anisotropic quantum conductivity in disordered graphene-based superlattices. Our quantum simulations, based on an efficient real-space implementation of the Kubo-Greenwood formula, show that in disordered graphene superlattices the strength...

  2. Dielectric enhancement of BaTiO3/SrTiO3 superlattices with embedded Ni nanocrystals

    International Nuclear Information System (INIS)

    Xiong Zhengwei; Sun Weiguo; Wang Xuemin; Jiang Fan; Wu Weidong

    2012-01-01

    Highlights: ► The BaTiO 3 /SrTiO 3 superlattices with embedded Ni NCs were successfully fabricated by L-MBE. ► The influence with the various concentrations of Ni nanocrystals embedded in BaTiO 3 /SrTiO 3 superlattices was also discussed. ► The BaTiO 3 /SrTiO 3 superlattices with lower concentration of embedded Ni NCs had higher permittivity and dielectric loss compared with the pure BaTiO 3 /SrTiO 3 superlattices. ► The dielectric enhancement of BaTiO 3 /SrTiO 3 superlattices with embedded Ni NCs was proposed to explained by Drude quasi-free-electron theory. - Abstract: The self-organized Ni nanocrystals (NCs) were embedded in BaTiO 3 /SrTiO 3 superlattices using laser molecular beam epitaxy (L-MBE). The stress of the composite films was increased with the increasing concentration of embedded Ni NCs, as investigation in stress calculation. The influence with the various concentrations of Ni NCs embedded in BaTiO 3 /SrTiO 3 superlattices was also discussed. The internal stress of the films was too strong to epitaxial growth of BaTiO 3 /SrTiO 3 superlattices. Compared with the pure BaTiO 3 /SrTiO 3 superlattices, the BaTiO 3 /SrTiO 3 superlattices with lower concentration of embedded Ni NCs had higher permittivity and dielectric loss. Furthermore, the dielectric enhancement of BaTiO 3 /SrTiO 3 superlattices with embedded Ni NCs was proposed to explained by Drude quasi-free-electron theory.

  3. Superlattices in thermoelectric applications

    International Nuclear Information System (INIS)

    Sofo, J.O.; Mahan, G.D.; Tennessee Univ., Knoxville, TN

    1994-08-01

    The electrical conductivity, thermopower and the electronic contribution to the thermal conductivity of a superlattice, are calculated with the electric field and the thermal gradient applied parallel to the interfaces. Tunneling between quantum wells is included. The broadening of the lowest subband when the period of the superlattice is decreased produces a reduction of the thermoelectric figure of merit. However, we found that a moderate increase of the figure of merit may be expected for intermediate values of the period, due to the enhancement of the density of states produced by the superlattice structure

  4. Rare earth superlattices

    International Nuclear Information System (INIS)

    McMorrow, D.F.

    1997-01-01

    A review is given of recent experiments on the magnetism of rare earth superlattices. Early experiments in this field were concerned mainly with systems formed by combining a magnetic and a non-magnetic element in a superlattice structure. From results gathered on a variety of systems it has been established that the propagation of magnetic order through the non-magnetic spacer can be understood mostly on the basis of an RKKY-like model, where the strength and range of the coupling depends on the details of the conduction electron susceptibility of the spacer. Recent experiments on more complex systems indicate that this model does not provide a complete description. Examples include superlattices where the constituents can either be both magnetic, adopt different crystal structures (Fermi surfaces), or where one of the constituents has a non-magnetic singlet ground state. The results from such systems are presented and discussed in the context of the currently accepted model. (au)

  5. Application of long-range ordering in the synthesis of a nanoscale Ni2 (Cr,Mo) superlattice with high strength and high ductility

    International Nuclear Information System (INIS)

    Tawancy, H.M.; Aboelfotoh, M.O.

    2009-01-01

    We demonstrate that bulk nanoscale materials with high strength and high ductility can be synthesized by using long-range ordering in certain alloy systems. In the case of a Ni-18.6 atomic % Mo-15.1 atomic % Cr, a bulk nanoscale superlattice of Ni 2 (Cr,Mo) isomorphous with Pt 2 Mo has been synthesized by thermal aging at 700 deg. C. The superlattice is shown to have high strength and high ductility as well as high thermal stability. Although the yield strength is nearly doubled in the ordered state exceeding 800 MPa, the material is found to maintain about 70% of its initial tensile ductility corresponding to 42% engineering strain. This behavior has been related to the crystallography of the ordering transformation. Although most of the slip systems of the parent face-centered cubic lattice are suppressed by ordering, most of the twinning systems remain energetically favorable. Therefore, deformation in the ordered state is found to predominantly occur by twinning rather than by slip giving rise to the observed combination of high strength and high ductility

  6. Tunable superlattice in graphene to control the number of Dirac points.

    Science.gov (United States)

    Dubey, Sudipta; Singh, Vibhor; Bhat, Ajay K; Parikh, Pritesh; Grover, Sameer; Sensarma, Rajdeep; Tripathi, Vikram; Sengupta, K; Deshmukh, Mandar M

    2013-09-11

    Superlattice in graphene generates extra Dirac points in the band structure and their number depends on the superlattice potential strength. Here, we have created a lateral superlattice in a graphene device with a tunable barrier height using a combination of two gates. In this Letter, we demonstrate the use of lateral superlattice to modify the band structure of graphene leading to the emergence of new Dirac cones. This controlled modification of the band structure persists up to 100 K.

  7. Application of long-range ordering in the synthesis of a nanoscale Ni{sub 2} (Cr,Mo) superlattice with high strength and high ductility

    Energy Technology Data Exchange (ETDEWEB)

    Tawancy, H.M. [Center for Engineering Research, Research Institute, King Fahd University of Petroleum and Minerals, P.O. Box 1639, Dhahran 31261 (Saudi Arabia)], E-mail: tawancy@kfupm.edu.sa; Aboelfotoh, M.O. [Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7909 (United States)

    2009-01-25

    We demonstrate that bulk nanoscale materials with high strength and high ductility can be synthesized by using long-range ordering in certain alloy systems. In the case of a Ni-18.6 atomic % Mo-15.1 atomic % Cr, a bulk nanoscale superlattice of Ni{sub 2}(Cr,Mo) isomorphous with Pt{sub 2}Mo has been synthesized by thermal aging at 700 deg. C. The superlattice is shown to have high strength and high ductility as well as high thermal stability. Although the yield strength is nearly doubled in the ordered state exceeding 800 MPa, the material is found to maintain about 70% of its initial tensile ductility corresponding to 42% engineering strain. This behavior has been related to the crystallography of the ordering transformation. Although most of the slip systems of the parent face-centered cubic lattice are suppressed by ordering, most of the twinning systems remain energetically favorable. Therefore, deformation in the ordered state is found to predominantly occur by twinning rather than by slip giving rise to the observed combination of high strength and high ductility.

  8. Enhancement of dielectric and ferroelectric properties of PbZrO3/PbTiO3 artificial superlattices

    International Nuclear Information System (INIS)

    Choi, Taekjib; Lee, Jaichan

    2005-01-01

    PbZrO 3 (PZO)/PbTiO 3 (PTO) artificial superlattices have been grown on La 0.5 Sr 0.5 CoO 3 (LSCO) (100)/MgO (100) substrate by pulsed laser deposition with various stacking periods from 1 to 100 unit cells. The PZO/PTO artificial lattice exhibited a diffraction pattern characteristic of a superlattice structure, i.e., a main diffraction peak with satellite peaks. The electrical properties of the superlattices were investigated as a function of the stacking period. The dielectric constant and remnant polarization improved on decreasing the stacking periodicity. The dielectric constant of the superlattice reached 800 at a stacking period of 1unit cell/1unit cell (PZO 1 /PTO 1 ), which is larger than that of the single PZT solid-solution film. Moreover, the remnant polarization reached a maximum, 2Pr = 38.7 μC/cm 2 , at a 2-unit-cell stacking period. Progressive enhancement of dielectric constant and remnant polarization in artificial PZO/PTO superlattice was accompanied by expansion of the (100)-plane spacing on decreasing the stacking periodicity. These results suggest that the lattice strain developed in the PZO/PTO superlattice may have influence on dielectric constant and ferroelectric behavior.

  9. Noise-enhanced chaos in a weakly coupled GaAs/(Al,Ga)As superlattice

    Science.gov (United States)

    Yin, Zhizhen; Song, Helun; Zhang, Yaohui; Ruiz-García, Miguel; Carretero, Manuel; Bonilla, Luis L.; Biermann, Klaus; Grahn, Holger T.

    2017-01-01

    Noise-enhanced chaos in a doped, weakly coupled GaAs /Al0.45Ga0.55As superlattice has been observed at room temperature in experiments as well as in the results of the simulation of nonlinear transport based on a discrete tunneling model. When external noise is added, both the measured and simulated current-versus-time traces contain irregularly spaced spikes for particular applied voltages, which separate a regime of periodic current oscillations from a region of no current oscillations at all. In the voltage region without current oscillations, the electric-field profile consist of a low-field domain near the emitter contact separated by a domain wall consisting of a charge accumulation layer from a high-field regime closer to the collector contact. With increasing noise amplitude, spontaneous chaotic current oscillations appear over a wider bias voltage range. For these bias voltages, the domain boundary between the two electric-field domains becomes unstable and very small current or voltage fluctuations can trigger the domain boundary to move toward the collector and induce chaotic current spikes. The experimentally observed features are qualitatively very well reproduced by the simulations. Increased noise can consequently enhance chaotic current oscillations in semiconductor superlattices.

  10. Characterization of the Nb-B superlattice system

    Energy Technology Data Exchange (ETDEWEB)

    Franco, D.G.; Sarmiento-Chavez, A.; Schenone, N.; Llacsahuanga Allcca, A.E.; Gómez Berisso, M.; Fasano, Y.; Guimpel, J., E-mail: jguimpel@cab.cnea.gov.ar

    2016-12-15

    Highlights: • In this manuscript we study the crystalline and superconducting properties of this system, as a possible material to be used in solid state neutron detector sensors. • The results show that this superlattice system can be grown even for very thin layers, in spite of the Nb-B binary system showing many possible compounds, which could enhance interdifussion at the interfaces. • Also, the superconducting properties are not degraded, and they are even enhanced with respect to those of single Nb films of the same thickness. • In conclusion, we find that this system is a good potential candidate for the design and construction of solid state neutron Transition Edge Sensors. - Abstract: We study the growth, stacking and superconducting properties of Nb and B thin films and superlattices. The interest in these resides in their possible use in transition edge neutron sensors. The samples were grown by magnetron sputtering over Si (1  0  0) substrates. The X-ray diffraction patterns for all Nb containing samples show a Nb (1  1  0) preferential orientation. From the low-angle X-ray reflectivity we obtain information on the superlattice structure. The superconducting transition temperatures of the superlattices, obtained from the temperature dependence of the magnetization, are higher than those of single Nb films of similar thickness. The temperature dependence of the perpendicular and parallel upper critical fields indicate that the superlattices behave as an array of decoupled superconducting Nb layers.

  11. Characterization of the Nb-B superlattice system

    International Nuclear Information System (INIS)

    Franco, D.G.; Sarmiento-Chavez, A.; Schenone, N.; Llacsahuanga Allcca, A.E.; Gómez Berisso, M.; Fasano, Y.; Guimpel, J.

    2016-01-01

    Highlights: • In this manuscript we study the crystalline and superconducting properties of this system, as a possible material to be used in solid state neutron detector sensors. • The results show that this superlattice system can be grown even for very thin layers, in spite of the Nb-B binary system showing many possible compounds, which could enhance interdifussion at the interfaces. • Also, the superconducting properties are not degraded, and they are even enhanced with respect to those of single Nb films of the same thickness. • In conclusion, we find that this system is a good potential candidate for the design and construction of solid state neutron Transition Edge Sensors. - Abstract: We study the growth, stacking and superconducting properties of Nb and B thin films and superlattices. The interest in these resides in their possible use in transition edge neutron sensors. The samples were grown by magnetron sputtering over Si (1  0  0) substrates. The X-ray diffraction patterns for all Nb containing samples show a Nb (1  1  0) preferential orientation. From the low-angle X-ray reflectivity we obtain information on the superlattice structure. The superconducting transition temperatures of the superlattices, obtained from the temperature dependence of the magnetization, are higher than those of single Nb films of similar thickness. The temperature dependence of the perpendicular and parallel upper critical fields indicate that the superlattices behave as an array of decoupled superconducting Nb layers.

  12. Enhanced thermoelectric figure-of-merit in thermally robust, nanostructured superlattices based on SrTiO3

    KAUST Repository

    Abutaha, Anas I.

    2015-03-24

    Thermoelectric (TE) metal oxides overcome crucial disadvantages of traditional heavy-metal-alloy-based TE materials, such as toxicity, scarcity, and instability at high temperatures. Here, we report the TE properties of metal oxide superlattices, composed from alternating layers of 5% Pr3+-doped SrTiO3-δ (SPTO) and 20% Nb5+-doped SrTiO3-δ (STNO) fabricated using pulsed laser deposition (PLD). Excellent stability is established for these superlattices by maintaining the crystal structure and reproducing the TE properties after long-time (20 h) annealing at high temperature (∼1000 K). The introduction of oxygen vacancies as well as extrinsic dopants (Pr3+ and Nb5+), with different masses and ionic radii, at different lattice sites in SPTO and STNO layers, respectively, results in a substantial reduction of thermal conductivity via scattering a wider range of phonon spectrum without limiting the electrical transport and thermopower, leading to an enhancement in the figure-of-merit (ZT). The superlattice composed of 20 SPTO/STNO pairs, 8 unit cells of each layer, exhibits a ZT value of 0.46 at 1000 K, which is the highest among SrTiO3-based thermoelectrics. © 2015 American Chemical Society.

  13. Enhanced thermoelectric figure-of-merit in thermally robust, nanostructured superlattices based on SrTiO3

    KAUST Repository

    Abutaha, Anas I.; Sarath Kumar, S. R.; Li, Kun; Dehkordi, Arash Mehdizadeh; Tritt, Terry M.; Alshareef, Husam N.

    2015-01-01

    Thermoelectric (TE) metal oxides overcome crucial disadvantages of traditional heavy-metal-alloy-based TE materials, such as toxicity, scarcity, and instability at high temperatures. Here, we report the TE properties of metal oxide superlattices, composed from alternating layers of 5% Pr3+-doped SrTiO3-δ (SPTO) and 20% Nb5+-doped SrTiO3-δ (STNO) fabricated using pulsed laser deposition (PLD). Excellent stability is established for these superlattices by maintaining the crystal structure and reproducing the TE properties after long-time (20 h) annealing at high temperature (∼1000 K). The introduction of oxygen vacancies as well as extrinsic dopants (Pr3+ and Nb5+), with different masses and ionic radii, at different lattice sites in SPTO and STNO layers, respectively, results in a substantial reduction of thermal conductivity via scattering a wider range of phonon spectrum without limiting the electrical transport and thermopower, leading to an enhancement in the figure-of-merit (ZT). The superlattice composed of 20 SPTO/STNO pairs, 8 unit cells of each layer, exhibits a ZT value of 0.46 at 1000 K, which is the highest among SrTiO3-based thermoelectrics. © 2015 American Chemical Society.

  14. Transport in a magnetic field modulated graphene superlattice.

    Science.gov (United States)

    Li, Yu-Xian

    2010-01-13

    Using the transfer matrix method, we study the transport properties through a magnetic field modulated graphene superlattice. It is found that the electrostatic barrier, the magnetic vector potential, and the number of wells in a superlattice modify the transmission remarkably. The angular dependent transmission is blocked by the magnetic vector potential because of the appearance of the evanescent states at certain incident angles, and the region of Klein tunneling shifts to the left. The angularly averaged conductivities exhibit oscillatory behavior. The magnitude and period of oscillation depend sensitively on the height of the electrostatic barrier, the number of wells, and the strength of the modulated magnetic field.

  15. Computer simulation of the anomalous elastic behavior of thin films and superlattices

    International Nuclear Information System (INIS)

    Wolf, D.

    1992-10-01

    Atomistic simulations are reviewed that elucidate the causes of the anomalous elastic behavior of thin films and superlattices (the so-called supermodulus effect). The investigation of free-standing thin films and of superlattices of grain boundaries shows that the supermodulus effect is not an electronic but a structural interface effect intricately connected with the local atomic disorder at the interfaces. The consequent predictions that (1) coherent strained-layer superlattices should show the smallest elastic anomalies and (2) the introduction of incoherency at the interfaces should enhance all anomalies are validated by simulations of dissimilar-material superlattices. 38 refs, 10 figs

  16. Reactive molecular beam epitaxial growth and in situ photoemission spectroscopy study of iridate superlattices

    Directory of Open Access Journals (Sweden)

    C. C. Fan

    2017-08-01

    Full Text Available High-quality (001-oriented perovskite [(SrIrO3m/(SrTiO3] superlattices (m=1/2, 1, 2, 3 and ∞ films have been grown on SrTiO3(001 epitaxially using reactive molecular beam epitaxy. Compared to previously reported superlattices synthesized by pulsed laser deposition, our superlattices exhibit superior crystalline, interface and surface structure, which have been confirmed by high-resolution X-ray diffraction, scanning transmission electron microscopy and atomic force microscopy, respectively. The transport measurements confirm a novel insulator-metal transition with the change of dimensionality in these superlattices, and our first systematic in situ photoemission spectroscopy study indicates that the increasing strength of effective correlations induced by reducing dimensionality would be the dominating origin of this transition.

  17. Electronic properties of superlattices on quantum rings.

    Science.gov (United States)

    da Costa, D R; Chaves, A; Ferreira, W P; Farias, G A; Ferreira, R

    2017-04-26

    We present a theoretical study of the one-electron states of a semiconductor-made quantum ring (QR) containing a series of piecewise-constant wells and barriers distributed along the ring circumference. The single quantum well and the superlattice cases are considered in detail. We also investigate how such confining potentials affect the Aharonov-Bohm like oscillations of the energy spectrum and current in the presence of a magnetic field. The model is simple enough so as to allow obtaining various analytical or quasi-analytical results. We show that the well-in-a-ring structure presents enhanced localization features, as well as specific geometrical resonances in its above-barrier spectrum. We stress that the superlattice-in-a-ring structure allows giving a physical meaning to the often used but usually artificial Born-von-Karman periodic conditions, and discuss in detail the formation of energy minibands and minigaps for the circumferential motion, as well as several properties of the superlattice eigenstates in the presence of the magnetic field. We obtain that the Aharonov-Bohm oscillations of below-barrier miniband states are reinforced, owing to the important tunnel coupling between neighbour wells of the superlattice, which permits the electron to move in the ring. Additionally, we analysis a superlattice-like structure made of a regular distribution of ionized impurities placed around the QR, a system that may implement the superlattice in a ring idea. Finally, we consider several random disorder models, in order to study roughness disorder and to tackle the robustness of some results against deviations from the ideally nanostructured ring system.

  18. Enhanced hardness in epitaxial TiAlScN alloy thin films and rocksalt TiN/(Al,Sc)N superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Saha, Bivas [School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907 (United States); Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907 (United States); Lawrence, Samantha K.; Bahr, David F. [School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907 (United States); Schroeder, Jeremy L.; Birch, Jens [Thin Film Physics Division, Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping (Sweden); Sands, Timothy D. [School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907 (United States); Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907 (United States); School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907 (United States)

    2014-10-13

    High hardness TiAlN alloys for wear-resistant coatings exhibit limited lifetimes at elevated temperatures due to a cubic-AlN to hexagonal-AlN phase transformation that leads to decreasing hardness. We enhance the hardness (up to 46 GPa) and maximum operating temperature (up to 1050 °C) of TiAlN-based coatings by alloying with scandium nitride to form both an epitaxial TiAlScN alloy film and epitaxial rocksalt TiN/(Al,Sc)N superlattices on MgO substrates. The superlattice hardness increases with decreasing period thickness, which is understood by the Orowan bowing mechanism of the confined layer slip model. These results make them worthy of additional research for industrial coating applications.

  19. Magneto-transport studies of InAs/GaSb short period superlattices

    International Nuclear Information System (INIS)

    Broadley, Victoria Jane

    2002-01-01

    This thesis studies the transport properties of short period semiconducting InAs/GaSb superlattices in the presence of strong electric and magnetic fields applied parallel to the growth axis. Electrical transport parallel to the growth axis occurs through the superlattice miniband, which have widths varying from three to 30meV. Resonant scattering between confined Landau levels and Stark levels is observed at low temperatures (4.2K). In addition LO-phonon assisted scattering between Landau levels is observed in both type-I GaAs/AIAs and type-ll inAs/GaSb superlattices, which are enhanced in the type-ll system due to the strong interband coupling. K·p band structure calculations show that the interband coupling causes the superlattice miniband energy dispersion to be strongly dependent on the in-plane wavevector and the applied magnetic field. For large applied electric fields, where the miniband is split into discrete Stark levels, strong stark-cyclotron resonance (SCR) features are observed, which occur when the Landau level separation equals to the stark level separation. These resonances are enhanced when compared to SCR in type-I superlattices due to the suppression of miniband conduction in higher lying Landau levels. At low electric fields electrical transport through the superlattice miniband yields characteristic miniband transport features, which are modelled using the Esaki-Tsu miniband transport model. Strong electron - LO-phonon scattering is also observed in InAs/GaSb superlattices, where we report the first observation of miniband transport assisted via the emission of LO-phonons between stark levels in adjacent wells. Below 50K thermally activated behaviour is reported and at high magnetic fields (in the quantum limit) complete localisation of carriers is observed. In this regime LO-phonon delocalised transport in also observed. (author)

  20. Perovskite Superlattices as Tunable Microwave Devices

    Science.gov (United States)

    Christen, H. M.; Harshavardhan, K. S.

    2003-01-01

    Experiments have shown that superlattices that comprise alternating epitaxial layers of dissimilar paraelectric perovskites can exhibit large changes in permittivity with the application of electric fields. The superlattices are potentially useful as electrically tunable dielectric components of such microwave devices as filters and phase shifters. The present superlattice approach differs fundamentally from the prior use of homogeneous, isotropic mixtures of base materials and dopants. A superlattice can comprise layers of two or more perovskites in any suitable sequence (e.g., ABAB..., ABCDABCD..., ABACABACA...). Even though a single layer of one of the perovskites by itself is not tunable, the compositions and sequence of the layers can be chosen so that (1) the superlattice exhibits low microwave loss and (2) the interfacial interaction between at least two of the perovskites in the superlattice renders either the entire superlattice or else at least one of the perovskites tunable.

  1. Monolayer atomic crystal molecular superlattices

    Science.gov (United States)

    Wang, Chen; He, Qiyuan; Halim, Udayabagya; Liu, Yuanyue; Zhu, Enbo; Lin, Zhaoyang; Xiao, Hai; Duan, Xidong; Feng, Ziying; Cheng, Rui; Weiss, Nathan O.; Ye, Guojun; Huang, Yun-Chiao; Wu, Hao; Cheng, Hung-Chieh; Shakir, Imran; Liao, Lei; Chen, Xianhui; Goddard, William A., III; Huang, Yu; Duan, Xiangfeng

    2018-03-01

    Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach to a new class of stable superlattices in which monolayer atomic crystals alternate with molecular layers. Using black phosphorus as a model system, we show that intercalation with cetyl-trimethylammonium bromide produces monolayer phosphorene molecular superlattices in which the interlayer distance is more than double that in black phosphorus, effectively isolating the phosphorene monolayers. Electrical transport studies of transistors fabricated from the monolayer phosphorene molecular superlattice show an on/off current ratio exceeding 107, along with excellent mobility and superior stability. We further show that several different two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties. These studies define a versatile material platform for fundamental studies and potential technological applications.

  2. Critical properties of a ferroelectric superlattice described by a transverse spin-1/2 Ising model

    International Nuclear Information System (INIS)

    Tabyaoui, A; Saber, M; Baerner, K; Ainane, A

    2007-01-01

    The phase transition properties of a ferroelectric superlattice with two alternating layers A and B described by a transverse spin-1/2 Ising model have been investigated using the effective field theory within a probability distribution technique that accounts for the self spin correlation functions. The Curie temperature T c , polarization and susceptibility have been obtained. The effects of the transverse field and the ferroelectric and antiferroelectric interfacial coupling strength between two ferroelectric materials are discussed. They relate to the physical properties of antiferroelectric/ferroelectric superlattices

  3. Magnetic Rare-Earth Superlattices

    DEFF Research Database (Denmark)

    Majkrzak, C.F.; Gibbs, D.; Böni, P.

    1988-01-01

    The magnetic structures of several single‐crystal, magnetic rare‐earth superlattice systems grown by molecular‐beam epitaxy are reviewed. In particular, the results of recent neutron diffraction investigations of long‐range magnetic order in Gd‐Y, Dy‐Y, Gd‐Dy, and Ho‐Y periodic superlattices...... are presented. In the Gd‐Y system, an antiphase domain structure develops for certain Y layer spacings, whereas modified helical moment configurations are found to occur in the other systems, some of which are commensurate with the chemical superlattice wavelength. References are made to theoretical interaction...

  4. Structural, electronic properties and enhancement of electrical polarization in Er2NiMnO6/La2NiMnO6 superlattice by first-principles calculations

    Directory of Open Access Journals (Sweden)

    Haipeng Lu

    2016-03-01

    Full Text Available Employing first-principles calculations, structural, electronic properties of new multiferroic material Er2NiMnO6/La2NiMnO6 perovskite superlattice are investigated. This structure is computed as monoclinic phase with obvious distortion. The average in-plane anti-phase rotation angle, average out-of-plane in-phase rotation angle and other microscopic features are reported in this paper. Ni and Mn are found in this superlattice that stay high spin states. These microscopic properties play important roles in multiferroic properties. Based on these microscopic features, the relationship between the direction of spontaneous polarization and the order of substitution in neighboring A-O layers is explained. Finally, we try to enhance the electrical polarization magnitude by 32% by altering the previous superlattice as LaEr2NiMnO7 structure. Our results show that both repulsion force of A site rare earth ions and the arrangement of B site ions can exert influences on spontaneous polarization.

  5. Magnetic modes in superlattices

    International Nuclear Information System (INIS)

    Oliveira, F.A.

    1990-04-01

    A first discussion of reciprocal propagation of magnetic modes in a superlattice is presented. In the absence of an applied external magnetic field a superllatice made of alternate layers of the type antiferromagnetic-non-magnetic materials presents effects similar to those of phonons in a dielectric superlattice. (A.C.A.S.) [pt

  6. Spectral properties of waves in superlattices with 2D and 3D inhomogeneities

    International Nuclear Information System (INIS)

    Ignatchenko, V. A.; Tsikalov, D. S.

    2011-01-01

    We investigate the dynamic susceptibility and one-dimensional density of states in an initially sinusoidal superlattice containing simultaneously 2D phase inhomogeneities simulating correlated rough-nesses of superlattice interfaces and 3D amplitude inhomogeneities of the superlattice layer materials. The analytic expression for the averaged Green’s function of the sinusoidal superlattice with two phase inhomogeneities is derived in the Bourret approximation. It is shown that the effect of increasing asymmetry in the peak heights of dynamic susceptibility at the Brillouin zone boundary of the superlattice, which was discovered earlier [15] upon an increase in root-mean-square (rms) fluctuations, also takes place upon an increase in the correlation wavenumber of inhomogeneities. However, the peaks in this case also become closer, and the width and depth of the gap in the density of states decrease thereby. It is shown that the enhancement of rms fluctuations of 3D amplitude inhomogeneities in a superlattice containing 2D phase inhomogeneities suppresses the effect of dynamic susceptibility asymmetry and leads to a slight broadening of the gap in the density of states and a decrease in its depth. Targeted experiments aimed at detecting the effects studied here would facilitate the development of radio-spectroscopic and optical methods for identifying the presence of inhomogeneities of various dimensions in multilayer magnetic and optical structures.

  7. Exchange bias in Fe/Cr double superlattices

    International Nuclear Information System (INIS)

    Jiang, J. S.; Felcher, G. P.; Inomata, A.; Goyette, R.; Nelson, C.; Bader, S. D.

    1999-01-01

    Utilizing the oscillatory interlayer exchange coupling in Fe/Cr superlattices, we have constructed ''double superlattice'' structures where a ferromagnetic (F) and an antiferromagnetic (AF) Fe/Cr superlattice are coupled through a Cr spacer. The minor hysteresis loops in the magnetization are shifted from zero field, i.e., the F superlattice is exchange biased by the AF one. The double superlattices are sputter-deposited with (211) epitaxy and possess uniaxial in-plane magnetic anisotropy. The magnitude of the bias field is satisfactorily described by the classic formula for collinear spin structures. The coherent structure and insensitivity to atomic-scale roughness makes it possible to determine the spin distribution by polarized neutron reflectivity, which confirms that the spin structure is collinear. The magnetic reversal behavior of the double superlattices suggests that a realistic model of exchange bias needs to address the process of nucleating local reverse domains

  8. Exchange bias in Fe/Cr double superlattices

    International Nuclear Information System (INIS)

    Jiang, J. S.; Felcher, G. P.; Inomata, A.; Goyette, R.; Nelson, C. S.; Bader, S. D.

    2000-01-01

    Utilizing the oscillatory interlayer exchange coupling in Fe/Cr superlattices, we have constructed ''double superlattice'' structures where a ferromagnetic (F) and an antiferromagnetic (AF) Fe/Cr superlattice are coupled through a Cr spacer. The minor hysteresis loops in the magnetization are shifted from zero field, i.e., the F superlattice is exchange biased by the AF one. The double superlattices are sputter deposited with (211) epitaxy and possess uniaxial in-plane magnetic anisotropy. The magnitude of the bias field is satisfactorily described by the classic formula for collinear spin structures. The coherent structure and insensitivity to atomic-scale roughness makes it possible to determine the spin distribution by polarized neutron reflectivity, which confirms that the spin structure is collinear. The magnetic reversal behavior of the double superlattices suggests that a realistic model of exchange bias needs to address the process of nucleating local reverse domains. (c) 2000 American Vacuum Society

  9. Planar channeling in superlattices: Theory

    International Nuclear Information System (INIS)

    Ellison, J.A.; Picraux, S.T.; Allen, W.R.; Chu, W.K.

    1988-01-01

    The well-known continuum model theory for planar channeled energetic particles in perfect crystals is extended to layered crystalline structures and applied to superlattices. In a strained-layer structure, the planar channels with normals which are not perpendicular to the growth direction change their direction at each interface, and this dramatically influences the channeling behavior. The governing equation of motion for a planar channeled ion in a strained-layer superlattice with equal layer thicknesses is a one degree of freedom nonlinear oscillator which is periodically forced with a sequence of δ functions. These δ functions, which are of equal spacing and amplitude with alternating sign, represent the tilts at each of the interfaces. Thus upon matching an effective channeled particle wavelength, corresponding to a natural period of the nonlinear oscillator, to the period of the strained-layer superlattice, corresponding to the periodic forcing, strong resonance effects are expected. The condition of one effective wavelength per period corresponds to a rapid dechanneling at a well-defined depth (catastrophic dechanneling), whereas two wavelengths per period corresponds to no enhanced dechanneling after the first one or two layers (resonance channeling). A phase plane analysis is used to characterize the channeled particle motion. Detailed calculations using the Moliere continuum potential are compared with our previously described modified harmonic model, and new results are presented for the phase plane evolution, as well as the dechanneling as a function of depth, incident angle, energy, and layer thickness. General scaling laws are developed and nearly universal curves are obtained for the dechanneling versus depth under catastrophic dechanneling

  10. Thermal conductivity and heat transfer in superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Chen, G; Neagu, M; Borca-Tasciuc, T

    1997-07-01

    Understanding the thermal conductivity and heat transfer processes in superlattice structures is critical for the development of thermoelectric materials and devices based on quantum structures. This work reports progress on the modeling of thermal conductivity of superlattice structures. Results from the models established based on the Boltzmann transport equation could explain existing experimental results on the thermal conductivity of semiconductor superlattices in both in plane and cross-plane directions. These results suggest the possibility of engineering the interfaces to further reduce thermal conductivity of superlattice structures.

  11. Magnetism in lanthanide superlattices

    DEFF Research Database (Denmark)

    Goff, J.P.; Sarthour, R.S.; McMorrow, D.F.

    2000-01-01

    Neutron diffraction studies of heavy rare-earth superlattices have revealed the stabilization of novel magnetic phases chat are not present in bulk materials. The most striking result is the propagation of the magnetic ordering through nonmagnetic spacer materials. Here we describe some recent X......-ray magnetic resonant scattering studies of light rare-earth superlattices, which illuminate the mechanism of interlayer coupling, and provide access to different areas of Physics. such as the interplay between superconductivity and magnetism. Magnetic X-ray diffraction is found to be particularly well suited...... to the study of the modulated magnetic structures in superlattices, and provides unique information on the conduction-electron spin-density wave responsible for the propagation of magnetic order. (C) 2000 Elsevier Science B.V. All rights reserved....

  12. MBE growth and characterisation of light rare-earth superlattices

    DEFF Research Database (Denmark)

    Ward, R.C.C.; Wells, M.R.; Bryn-Jacobsen, C.

    1996-01-01

    The molecular beam epitaxy growth techniques which have already successfully produced a range of heavy rare-earth superlattices have now been extended to produce superlattices of two light rare-earth elements, Nd/Pr, as well as superlattices and alloy films of a heavy/light system, Ho/Pr. High......-resolution X-ray diffraction analysis shows the Nd/Pr superlattices to be of high structural quality, while the Ho/Pr superlattices are significantly less so. In the Ho/Pr superlattices, Pr is found to retain its bulk dhcp crystal structure even in thin layers (down to 6 atomic planes thick) sandwiched between...... thick layers of hcp Ho. In addition, neutron diffraction studies of the He/Pr superlattices have shown that the helical Ho magnetic order is not coherent through the dhcp Pr layers, in contrast to previous hcp/hcp superlattices Ho/Y, Ho/Lu and Ho/Er. The series of Ho:Pr alloy films has shown structural...

  13. Strength gradient enhances fatigue resistance of steels

    Science.gov (United States)

    Ma, Zhiwei; Liu, Jiabin; Wang, Gang; Wang, Hongtao; Wei, Yujie; Gao, Huajian

    2016-02-01

    Steels are heavily used in infrastructure and the transportation industry, and enhancing their fatigue resistance is a major challenge in materials engineering. In this study, by introducing a gradient microstructure into 304 austenitic steel, which is one of the most widely used types of stainless steel, we show that a strength gradient substantially enhances the fatigue life of the material. Pre-notched samples with negative strength gradients in front of the notch’s tip endure many more fatigue cycles than do samples with positive strength gradients during the crack initiation stage, and samples with either type of gradient perform better than do gradient-free samples with the same average yield strength. However, as a crack grows, samples with positive strength gradients exhibit better resistance to fatigue crack propagation than do samples with negative gradients or no gradient. This study demonstrates a simple and promising strategy for using gradient structures to enhance the fatigue resistance of materials and complements related studies of strength and ductility.

  14. Performance enhancement in p-channel charge-trapping flash memory devices with Si/Ge super-lattice channel and band-to-band tunneling induced hot-electron injection

    International Nuclear Information System (INIS)

    Liu, Li-Jung; Chang-Liao, Kuei-Shu; Jian, Yi-Chuen; Wang, Tien-Ko; Tsai, Ming-Jinn

    2013-01-01

    P-channel charge-trapping flash memory devices with Si, SiGe, and Si/Ge super-lattice channel are investigated in this work. A Si/Ge super-lattice structure with extremely low roughness and good crystal structure is obtained by precisely controlling the epitaxy thickness of Ge layer. Both programming and erasing (P/E) speeds are significantly improved by employing this Si/Ge super-lattice channel. Moreover, satisfactory retention and excellent endurance characteristics up to 10 6 P/E cycles with 3.8 V memory window show that the degradation on reliability properties is negligible when super-lattice channel is introduced. - Highlights: ► A super-lattice structure is proposed to introduce more Ge content into channel. ► Super-lattice structure possesses low roughness and good crystal structure. ► P-channel flash devices with Si, SiGe, and super-lattice channel are investigated. ► Programming/erasing speeds are significantly improved. ► Reliability properties can be kept for device with super-lattice channel

  15. Quantitative x-ray structure determination of superlattices and interfaces

    International Nuclear Information System (INIS)

    Schuller, I.K.; Fullerton, E.E.

    1990-01-01

    This paper presents a general procedure for quantitative structural refinement of superlattice structures. To analyze a wide range of superlattices, the authors have derived a general kinematical diffraction formula that includes random, continuous and discrete fluctuations from the average structure. By implementing a non-linear fitting algorithm to fit the entire x-ray diffraction profile, refined parameters that describe the average superlattice structure, and deviations from this average are obtained. The structural refinement procedure is applied to a crystalline/crystalline Mo/Ni superlattices and crystalline/amorphous Pb/Ge superlattices. Roughness introduced artificially during growth in Mo/Ni superlattices is shown to be accurately reproduced by the refinement

  16. Electronic structure of silicon superlattices

    International Nuclear Information System (INIS)

    Krishnamurthy, S.; Moriarty, J.A.

    1984-01-01

    Utilizing a new complex-band-structure technique, the electronic structure of model Si-Si/sub 1-x/Ge/sub x/ and MOS superlattices has been obtained over a wide range of layer thickness d (11 less than or equal to d less than or equal to 110 A). For d greater than or equal to 44 A, it is found that these systems exhibit a direct fundamental band gap. Further calculations of band-edge effective masses and impurity scattering rates suggest the possibility of a band-structure-driven enhancement in electron mobility over bulk silicon

  17. Thermal Conductivity of Graphene-hBN Superlattice Ribbons.

    Science.gov (United States)

    Felix, Isaac M; Pereira, Luiz Felipe C

    2018-02-09

    Superlattices are ideal model systems for the realization and understanding of coherent (wave-like) and incoherent (particle-like) phonon thermal transport. Single layer heterostructures of graphene and hexagonal boron nitride have been produced recently with sharp edges and controlled domain sizes. In this study we employ nonequilibrium molecular dynamics simulations to investigate the thermal conductivity of superlattice nanoribbons with equal-sized domains of graphene and hexagonal boron nitride. We analyze the dependence of the conductivity with the domain sizes, and with the total length of the ribbons. We determine that the thermal conductivity reaches a minimum value of 89 W m -1 K -1 for ribbons with a superlattice period of 3.43 nm. The effective phonon mean free path is also determined and shows a minimum value of 32 nm for the same superlattice period. Our results also reveal that a crossover from coherent to incoherent phonon transport is present at room temperature for BNC nanoribbons, as the superlattice period becomes comparable to the phonon coherence length. Analyzing phonon populations relative to the smallest superlattice period, we attribute the minimum thermal conductivity to a reduction in the population of flexural phonons when the superlattice period equals 3.43 nm. The ability to manipulate thermal conductivity using superlattice-based two-dimensional materials, such as graphene-hBN nanoribbons, opens up opportunities for application in future nanostructured thermoelectric devices.

  18. Enhancing Carrier Injection Using Graded Superlattice Electron Blocking Layer for UVB Light-Emitting Diodes

    KAUST Repository

    Janjua, Bilal

    2014-12-01

    We have studied enhanced carrier injection by having an electron blocking layer (EBL) based on a graded superlattice (SL) design. Here, we examine, using a selfconsistent 6 × 6 k.p method, the energy band alignment diagrams under equilibrium and forward bias conditions while also considering carrier distribution and recombination rates (Shockley-Read-Hall, Auger, and radiative recombination rates). The graded SL is based on AlxGa1-xN (larger bandgap) Al0:5Ga0:5N (smaller bandgap) SL, where x is changed from 0.8 to 0.56 in steps of 0.06. Graded SL was found to be effective in reducing electron leakage and enhancing hole injection into the active region. Due to our band engineering scheme for EBL, four orders-of-magnitude enhancement were observed in the direct recombination rate, as compared with the conventional bulk EBL consisting of Al0:8Ga0:2N. An increase in the spatial overlap of carrier wavefunction was obtained due to polarization-induced band bending in the active region. An efficient single quantum-well ultraviolet-B light-emitting diode was designed, which emits at 280 nm. This is the effective wavelength for water disinfection application, among others.

  19. Magnetic structure of holmium-yttrium superlattices

    DEFF Research Database (Denmark)

    Jehan, D.A.; McMorrow, D.F.; Cowley, R.A.

    1993-01-01

    We present the results of a study of the chemical and magnetic structures of a series of holmium-yttrium superlattices and a 5000 angstrom film of holmium, all grown by molecular-beam epitaxy. By combining the results of high-resolution x-ray diffraction with detailed modeling, we show...... that the superlattices have high crystallographic integrity: the structural coherence length parallel to the growth direction is typically almost-equal-to 2000 angstrom, while the interfaces between the two elements are well defined and extend over approximately four lattice planes. The magnetic structures were...... determined using neutron-scattering techniques. The moments on the Ho3+ ions in the superlattices form a basal-plane helix. From an analysis of the superlattice structure factors of the primary magnetic satellites, we are able to determine separately the contributions made by the holmium and yttrium...

  20. Plasmon Modes of Vertically Aligned Superlattices

    DEFF Research Database (Denmark)

    Filonenko, Konstantin; Duggen, Lars; Willatzen, Morten

    2017-01-01

    By using the Finite Element Method we visualize the modes of vertically aligned superlattice composed of gold and dielectric nanocylinders and investigate the emitter-plasmon interaction in approximation of weak coupling. We find that truncated vertically aligned superlattice can function...

  1. Dresselhaus spin-orbit coupling induced spin-polarization and resonance-split in n-well semiconductor superlattices

    International Nuclear Information System (INIS)

    Ye Chengzhi; Xue Rui; Nie, Y.-H.; Liang, J.-Q.

    2009-01-01

    Using the transfer matrix method, we investigate the electron transmission over multiple-well semiconductor superlattices with Dresselhaus spin-orbit coupling in the potential-well regions. The superlattice structure enhances the effect of spin polarization in the transmission spectrum. The minibands of multiple-well superlattices for electrons with different spin can be completely separated at the low incident energy, leading to the 100% spin polarization in a broad energy windows, which may be an effective scheme for realizing spin filtering. Moreover, for the transmission over n-quantum-well, it is observed that the resonance peaks in the minibands split into n-folds or (n-1)-folds depending on the well-width and barrier-thickness, which is different from the case of tunneling through n-barrier structure

  2. Surface electron structure of short-period semiconductor superlattice

    International Nuclear Information System (INIS)

    Bartos, I.; Czech Academy Science, Prague,; Strasser, T.; Schattke, W.

    2004-01-01

    Full text: Semiconductor superlattices represent man-made crystals with unique physical properties. By means of the directed layer-by-layer molecular epitaxy growth their electric properties can be tailored (band structure engineering). Longer translational periodicity in the growth direction is responsible for opening of new electron energy gaps (minigaps) with surface states and resonances localized at superlattice surfaces. Similarly as for the electron structure of the bulk, a procedure enabling to modify the surface electron structure of superlattices is desirable. Short-period superlattice (GaAs) 2 (AlAs) 2 with unreconstructed (100) surface is investigated in detail. Theoretical description in terms of full eigenfunctions of individual components has to be used. The changes of electron surface state energies governed by the termination of a periodic crystalline potential, predicted on simple models, are confirmed for this system. Large surface state shifts are found in the lowest minigap of the superlattice when this is terminated in four different topmost layer configurations. The changes should be observable in angle resolved photoelectron spectroscopy as demonstrated in calculations based on the one step model of photoemission. Surface state in the center of the two dimensional Brillouin zone moves from the bottom of the minigap (for the superlattice terminated by two bilayers of GaAs) to its top (for the superlattice terminated by two bilayers of AlAs) where it becomes a resonance. No surface state/resonance is found for a termination with one bilayer of AlAs. The surface state bands behave similarly in the corresponding gaps of the k-resolved section of the electron band structure. The molecular beam epitaxy, which enables to terminate the superlattice growth with atomic layer precision, provides a way of tuning the superlattice surface electron structure by purely geometrical means. The work was supported by the Grant Agency of the Academy of Sciences

  3. Current responsivity of semiconductor superlattice THz-photon detectors

    DEFF Research Database (Denmark)

    Ignatov, Anatoly A.; Jauho, Antti-Pekka

    1999-01-01

    The current responsivity of a semiconductor superlattice THz-photon detector is calculated using an equivalent circuit model which takes into account the finite matching efficiency between a detector antenna and the superlattice in the presence of parasitic losses. Calculations performed for curr......The current responsivity of a semiconductor superlattice THz-photon detector is calculated using an equivalent circuit model which takes into account the finite matching efficiency between a detector antenna and the superlattice in the presence of parasitic losses. Calculations performed...... for currently available superlattice diodes show that both the magnitudes and the roll-off frequencies of the responsivity are strongly influenced by an excitation of hybrid plasma-Bloch oscillations which are found to be eigenmodes of the system in the THz-frequency band. The expected room temperature values...... of the responsivity (2–3 A/W in the 1–3 THz-frequency band) range up to several percents of the quantum efficiency e/[h-bar] omega of an ideal superconductor tunnel junction detector. Properly designed semiconductor superlattice detectors may thus demonstrate better room temperature THz-photon responsivity than...

  4. Crack-tips enriched platinum-copper superlattice nanoflakes as highly efficient anode electrocatalysts for direct methanol fuel cells.

    Science.gov (United States)

    Zheng, Lijun; Yang, Dachi; Chang, Rong; Wang, Chengwen; Zhang, Gaixia; Sun, Shuhui

    2017-07-06

    We have developed "crack-tips" and "superlattice" enriched Pt-Cu nanoflakes (NFs), benefiting from the synergetic effects of "crack-tips" and "superlattice crystals"; the Pt-Cu NFs exhibit 4 times higher mass activity, 6 times higher specific activity and 6 times higher stability than those of the commercial Pt/C catalyst, respectively. Meanwhile, the Pt-Cu NFs show more enhanced CO tolerance than the commercial Pt/C catalyst.

  5. Manganite/Cuprate Superlattice as Artificial Reentrant Spin Glass

    KAUST Repository

    Ding, Junfeng

    2016-05-04

    Emerging physical phenomena at the unit-cell-controlled interfaces of transition-metal oxides have attracted lots of interest because of the rich physics and application opportunities. This work reports a reentrant spin glass behavior with strong magnetic memory effect discovered in oxide heterostructures composed of ultrathin manganite La0.7Sr0.3MnO3 (LSMO) and cuprate La2CuO4 (LCO) layers. These heterostructures are featured with enhanced ferromagnetism before entering the spin glass state: a Curie temperature of 246 K is observed in the superlattice with six-unit-cell LSMO layers, while the reference LSMO film with the same thickness shows much weaker magnetism. Furthermore, an insulator-metal transition emerges at the Curie temperature, and below the freezing temperature the superlattices can be considered as a glassy ferromagnetic insulator. These experimental results are closely related to the interfacial spin reconstruction revealed by the first-principles calculations, and the dependence of the reentrant spin glass behavior on the LSMO layer thickness is in line with the general phase diagram of a spin system derived from the infinite-range SK model. The results of this work underscore the manganite/cuprate superlattices as a versatile platform of creating artificial materials with tailored interfacial spin coupling and physical properties. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  6. Structural, electronic properties and enhancement of electrical polarization in Er{sub 2}NiMnO{sub 6}/La{sub 2}NiMnO{sub 6} superlattice by first-principles calculations

    Energy Technology Data Exchange (ETDEWEB)

    Lu, Haipeng; Deng, Longjiang [National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, Chengdu, Sichuan, 610054 (China); State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, Sichuan, 610054 (China); Sun, Xun, E-mail: sunxunphy@hotmail.com; Hou, Zhihua; Yang, Wen; Wang, Siyuan; Xie, Jianliang [State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, Sichuan, 610054 (China)

    2016-03-15

    Employing first-principles calculations, structural, electronic properties of new multiferroic material Er{sub 2}NiMnO{sub 6}/La{sub 2}NiMnO{sub 6} perovskite superlattice are investigated. This structure is computed as monoclinic phase with obvious distortion. The average in-plane anti-phase rotation angle, average out-of-plane in-phase rotation angle and other microscopic features are reported in this paper. Ni and Mn are found in this superlattice that stay high spin states. These microscopic properties play important roles in multiferroic properties. Based on these microscopic features, the relationship between the direction of spontaneous polarization and the order of substitution in neighboring A-O layers is explained. Finally, we try to enhance the electrical polarization magnitude by 32% by altering the previous superlattice as LaEr{sub 2}NiMnO{sub 7} structure. Our results show that both repulsion force of A site rare earth ions and the arrangement of B site ions can exert influences on spontaneous polarization.

  7. Tunneling of electrons through semiconductor superlattices

    Indian Academy of Sciences (India)

    Unknown

    Tunneling of electrons through semiconductor superlattices. C L ROY. Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721 302, India. Abstract. The purpose of the present paper is to report a study of tunneling of electrons through semicon- ductor superlattices (SSL); specially, we have ...

  8. Photostimulated attenuation of hypersound in superlattice

    International Nuclear Information System (INIS)

    Mensah, S.Y.; Allotey, F.K.; Adjepong, S.K.

    1992-10-01

    Photostimulated attenuation of hypersound in semiconductor superlattice has been investigated. It is shown that the attenuation coefficient depends on the phonon wave vector q in an oscillatory manner and that from this oscillation the band width Δ of superlattice can be found. (author). 14 refs, 1 fig

  9. Dissipative chaos in semiconductor superlattices

    Directory of Open Access Journals (Sweden)

    F. Moghadam

    2008-03-01

    Full Text Available In this paper the motion of electron in a miniband of a semiconductor superlattice (SSL under the influence of external electric and magnetic fields is investigated. The electric field is applied in a direction perpendicular to the layers of the semiconductor superlattice, and the magnetic field is applied in different direction Numerical calculations show conditions led to the possibility of chaotic behaviors.

  10. Touching points in the energy band structure of bilayer graphene superlattices

    International Nuclear Information System (INIS)

    Pham, C Huy; Nguyen, V Lien

    2014-01-01

    The energy band structure of the bilayer graphene superlattices with zero-averaged periodic δ-function potentials are studied within the four-band continuum model. Using the transfer matrix method, the study is mainly focused on examining the touching points between adjacent minibands. For the zero-energy touching points the dispersion relation derived shows a Dirac-like double-cone shape with the group velocity which is periodic in the potential strength P with the period of π and becomes anisotropic at relatively large P. From the finite-energy touching points we have identified those located at zero wave-number. It was shown that for these finite-energy touching points the dispersion is direction-dependent in the sense that it is linear or parabolic in the direction parallel or perpendicular to the superlattice direction, respectively. We have also calculated the density of states and the conductivity which demonstrates a manifestation of the touching points examined. (paper)

  11. Solvent-driven symmetry of self-assembled nanocrystal superlattices-A computational study

    KAUST Repository

    Kaushik, Ananth P.

    2012-10-29

    The preference of experimentally realistic sized 4-nm facetted nanocrystals (NCs), emulating Pb chalcogenide quantum dots, to spontaneously choose a crystal habit for NC superlattices (Face Centered Cubic (FCC) vs. Body Centered Cubic (BCC)) is investigated using molecular simulation approaches. Molecular dynamics simulations, using united atom force fields, are conducted to simulate systems comprised of cube-octahedral-shaped NCs covered by alkyl ligands, in the absence and presence of experimentally used solvents, toluene and hexane. System sizes in the 400,000-500,000-atom scale followed for nanoseconds are required for this computationally intensive study. The key questions addressed here concern the thermodynamic stability of the superlattice and its preference of symmetry, as we vary the ligand length of the chains, from 9 to 24 CH2 groups, and the choice of solvent. We find that hexane and toluene are "good" solvents for the NCs, which penetrate the ligand corona all the way to the NC surfaces. We determine the free energy difference between FCC and BCC NC superlattice symmetries to determine the system\\'s preference for either geometry, as the ratio of the length of the ligand to the diameter of the NC is varied. We explain these preferences in terms of different mechanisms in play, whose relative strength determines the overall choice of geometry. © 2012 Wiley Periodicals, Inc.

  12. Topotactic interconversion of nanoparticle superlattices.

    Science.gov (United States)

    Macfarlane, Robert J; Jones, Matthew R; Lee, Byeongdu; Auyeung, Evelyn; Mirkin, Chad A

    2013-09-13

    The directed assembly of nanoparticle building blocks is a promising method for generating sophisticated three-dimensional materials by design. In this work, we have used DNA linkers to synthesize nanoparticle superlattices that have greater complexity than simple binary systems using the process of topotactic intercalation-the insertion of a third nanoparticle component at predetermined sites within a preformed binary lattice. Five distinct crystals were synthesized with this methodology, three of which have no equivalent in atomic or molecular crystals, demonstrating a general approach for assembling highly ordered ternary nanoparticle superlattices whose structures can be predicted before their synthesis. Additionally, the intercalation process was demonstrated to be completely reversible; the inserted nanoparticles could be expelled into solution by raising the temperature, and the ternary superlattice could be recovered by cooling.

  13. Simple theoretical analysis of the photoemission from quantum confined effective mass superlattices of optoelectronic materials

    Directory of Open Access Journals (Sweden)

    Debashis De

    2011-07-01

    Full Text Available The photoemission from quantum wires and dots of effective mass superlattices of optoelectronic materials was investigated on the basis of newly formulated electron energy spectra, in the presence of external light waves, which controls the transport properties of ultra-small electronic devices under intense radiation. The effect of magnetic quantization on the photoemission from the aforementioned superlattices, together with quantum well superlattices under magnetic quantization, has also been investigated in this regard. It appears, taking HgTe/Hg1−xCdxTe and InxGa1−xAs/InP effective mass superlattices, that the photoemission from these quantized structures is enhanced with increasing photon energy in quantized steps and shows oscillatory dependences with the increasing carrier concentration. In addition, the photoemission decreases with increasing light intensity and wavelength as well as with increasing thickness exhibiting oscillatory spikes. The strong dependence of the photoemission on the light intensity reflects the direct signature of light waves on the carrier energy spectra. The content of this paper finds six different applications in the fields of low dimensional systems in general.

  14. Phase diagrams of a spin-1 Ising superlattice with alternating transverse field

    International Nuclear Information System (INIS)

    Saber, A.; Ez-Zahraouy, H.; Lo Russo, S.; Mattei, G.; Ainane, A.

    2003-01-01

    The effects of alternating transverse fields Ω a and Ω b on the critical behavior of an alternating spin-1 Ising superlattice are studied within an effective field theory with a probability distribution technique that accounts for the single-site spin correlation. Critical temperatures are calculated as a function of the thickness of the superlattice and the strength of the transverse field. Depending on the values of the transverse fields Ω a and Ω b , the critical temperature can increase or decrease with increasing the thickness of the film, such result is not obtained in the uniform transverse field case (Ω a = Ω b ). Furthermore, for each thickness L of the film, a long range ordered phase persist at low temperature for selected values of the transverse field Ω a and arbitrary values of Ω b . The effects of interlayer and intralayer exchange interactions are also examined

  15. Phase diagrams of a spin-1 Ising superlattice with alternating transverse field

    International Nuclear Information System (INIS)

    Saber, A.; Ez-Zahraouy, H.

    2000-09-01

    The effects of alternating transverse fields Ω a and Ω b on the critical behavior of an alternating spin-1 Ising superlattice are studied within an effective field theory with a probability distribution technique that accounts for the single-site spin correlations. Critical temperatures are calculated as a function of the thickness of the superlattice and the strength of the transverse field. Depending on the values of the transverse fields Ω a and Ω b , the critical temperature can increase or decrease with increasing the thickness of the film, such result is not obtained in the uniform transverse field case (Ω a = Ω b ). Furthermore, for each thickness L of the film, a long range ordered phase persists at low temperature for selected values of the transverse field Ω a and arbitrary values of Ω b . The effects of interlayer and intralayer exchange interactions are also examined. (author)

  16. Ground state energy of a polaron in a superlattice

    International Nuclear Information System (INIS)

    Mensah, S.Y.; Allotey, F.K.A.; Nkrumah, G.; Mensah, N.G.

    2000-10-01

    The ground state energy of a polaron in a superlattice was calculated using the double-time Green functions. The effective mass of the polaron along the planes perpendicular to the superlattice axis was also calculated. The dependence of the ground state energy and the effective mass along the planes perpendicular to the superlattice axis on the electron-phonon coupling constant α and on the superlattice parameters (i.e. the superlattice period d and the bandwidth Δ) were studied. It was observed that if an infinite square well potential is assumed, the ground state energy of the polaron decreases (i.e. becomes more negative) with increasing α and d, but increases with increasing Δ. For small values of α, the polaron ground state energy varies slowly with Δ, becoming approximately constant for large Δ. The effective mass along the planes perpendicular to the superlattice axis was found to be approximately equal to the mass of an electron for all typical values of α, d and Δ. (author)

  17. Investigation of switching region in superlattice phase change memories

    Science.gov (United States)

    Ohyanagi, T.; Takaura, N.

    2016-10-01

    We investigated superlattice phase change memories (PCMs) to clarify which regions were responsible for switching. We observed atomic structures in a superlattice PCM film with a stack of GeTe / Sb2Te3 layers using atomically resolved EDX maps, and we found an intermixed region with three atom species of the Ge, Sb and Te around the top GeTe layer under the top electrode. We also found that a device with a GeTe layer on an Sb2Te3 layer without superlattice structure had the same switching characteristics as a device with a superlattice PCM, that had the same top GeTe layer. We developed and fabricated a modified superlattice PCM that attained ultra low Reset / Set currents under 60 μ A .

  18. Nanophysics in graphene: neutrino physics in quantum rings and superlattices.

    Science.gov (United States)

    Fertig, H A; Brey, Luis

    2010-12-13

    Electrons in graphene at low energy obey a two-dimensional Dirac equation, closely analogous to that of neutrinos. As a result, quantum mechanical effects when the system is confined or subjected to potentials at the nanoscale may be quite different from what happens in conventional electronic systems. In this article, we review recent progress on two systems where this is indeed the case: quantum rings and graphene electrons in a superlattice potential. In the former case, we demonstrate that the spectrum reveals signatures of 'effective time-reversal symmetry breaking', in which the spectra are most naturally interpreted in terms of effective magnetic flux contained in the ring, even when no real flux is present. A one-dimensional superlattice potential is shown to induce strong band-structure changes, allowing the number of Dirac points at zero energy to be manipulated by the strength and/or period of the potential. The emergence of new Dirac points is shown to be accompanied by strong signatures in the conduction properties of the system.

  19. Epitaxy, thin films and superlattices

    International Nuclear Information System (INIS)

    Jagd Christensen, Morten

    1997-05-01

    This report is the result of structural investigations of 3d transition metal superlattices consisting of Fe/V, Cr/Mn, V/Mn and Fe/Mn, and a structural and magnetic study of a series of Ho/Pr alloys. The work includes preparation and characterization of substrates as well as growth of thin films and Fe/V superlattices by molecular beam epitaxy, including in-situ characterization by reflection high energy electron diffraction and Auger electron spectroscopy. Structural characterization has been done by x-ray diffraction and neutron diffraction. The x-ray diffraction experiments have been performed on the rotating copper anode at Risoe, and at synchrotron facilities in Hamburg and Brookhaven, and the neutron scattering was done at the Danish research reactor DR3 at Risoe. In addition to longitudinal scans, giving information about the structural parameters in the modulation direction, non-specular scans were also performed. This type of scans gives information about in-plane orientation and lattice parameters. From the analysis, structural information is obtained about lattice parameters, epitaxial strain, coherence lengths and crystallographic orientation for the superlattice systems, except Fe/Mn superlattices, which could not be modelled. For the Ho/Pr alloys, x-ray magnetic scattering was performed, and the crystal and magnetic structure was investigated. (au)

  20. Epitaxy, thin films and superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Jagd Christensen, Morten

    1997-05-01

    This report is the result of structural investigations of 3d transition metal superlattices consisting of Fe/V, Cr/Mn, V/Mn and Fe/Mn, and a structural and magnetic study of a series of Ho/Pr alloys. The work includes preparation and characterization of substrates as well as growth of thin films and Fe/V superlattices by molecular beam epitaxy, including in-situ characterization by reflection high energy electron diffraction and Auger electron spectroscopy. Structural characterization has been done by x-ray diffraction and neutron diffraction. The x-ray diffraction experiments have been performed on the rotating copper anode at Risoe, and at synchrotron facilities in Hamburg and Brookhaven, and the neutron scattering was done at the Danish research reactor DR3 at Risoe. In addition to longitudinal scans, giving information about the structural parameters in the modulation direction, non-specular scans were also performed. This type of scans gives information about in-plane orientation and lattice parameters. From the analysis, structural information is obtained about lattice parameters, epitaxial strain, coherence lengths and crystallographic orientation for the superlattice systems, except Fe/Mn superlattices, which could not be modelled. For the Ho/Pr alloys, x-ray magnetic scattering was performed, and the crystal and magnetic structure was investigated. (au) 14 tabs.; 58 ills., 96 refs.

  1. The solitary electromagnetic waves in the graphene superlattice

    International Nuclear Information System (INIS)

    Kryuchkov, Sergey V.; Kukhar', Egor I.

    2013-01-01

    d’Alembert equation written for the electromagnetic waves propagating in the graphene superlattice is analyzed. The possibility of the propagation of the solitary electromagnetic waves in the graphene superlattice is discussed. The amplitude and the width of the electromagnetic pulse are calculated. The drag current induced by such wave across the superlattice axis is investigated. The numerical estimate of the charge dragged by the solitary wave is made.

  2. Picosecond electron bunches from GaAs/GaAsP strained superlattice photocathode

    International Nuclear Information System (INIS)

    Jin, Xiuguang; Matsuba, Shunya; Honda, Yosuke; Miyajima, Tsukasa; Yamamoto, Masahiro; Utiyama, Takashi; Takeda, Yoshikazu

    2013-01-01

    GaAs/GaAsP strained superlattices are excellent candidates for use as spin-polarized electron sources. In the present study, picosecond electron bunches were successfully generated from such a superlattice photocathode. However, electron transport in the superlattice was much slower than in bulk GaAs. Transmission electron microscopy observations revealed that a small amount of variations in the uniformity of the layers was present in the superlattice. These variations lead to fluctuations in the superlattice mini-band structure and can affect electron transport. Thus, it is expected that if the periodicity of the superlattice can be improved, much faster electron bunches can be produced. - Highlights: • GaAs/GaAsP strained superlattices are excellent candidates for spin-polarized electron beam. • Pulse spin-polarized electron beam is required for investigating the magnetic domain change. • Picosecond electron bunches were achieved from GaAs/GaAsP superlattice photocathode. • TEM observation revealed a small disorder of superlattice layers. • Improvement of superlattice periodicity can achieve much faster electron bunches

  3. Microwave absorption in YBCO/PrBCO superlattices

    International Nuclear Information System (INIS)

    Carlos, W.E.; Kaplan, R.; Lowndes, D.H.; Norton, D.P.

    1992-01-01

    In this paper, non-resonant microwave absorption is employed to probe YBCO/PrBCO superlattices and compare the response to that of a YBCO film. Near the transition temperatures, the response of the superlattice samples and the YBCO film have similar amplitudes and orientation dependencies. At lower temperatures, the response of the superlattices is much stronger than that of the YBCO film and, while both responses are hysteretic at low temperatures, the widths of the hysteresis have opposite orientation dependencies, which the authors attribute to the role of the PrBCO layers

  4. Photoluminescence and electrical properties of silicon oxide and silicon nitride superlattices containing silicon nanocrystals

    International Nuclear Information System (INIS)

    Shuleiko, D V; Ilin, A S

    2016-01-01

    Photoluminescence and electrical properties of superlattices with thin (1 to 5 nm) alternating silicon-rich silicon oxide or silicon-rich silicon nitride, and silicon oxide or silicon nitride layers containing silicon nanocrystals prepared by plasma-enhanced chemical vapor deposition with subsequent annealing were investigated. The entirely silicon oxide based superlattices demonstrated photoluminescence peak shift due to quantum confinement effect. Electrical measurements showed the hysteresis effect in the vicinity of zero voltage due to structural features of the superlattices from SiOa 93 /Si 3 N 4 and SiN 0 . 8 /Si 3 N 4 layers. The entirely silicon nitride based samples demonstrated resistive switching effect, comprising an abrupt conductivity change at about 5 to 6 V with current-voltage characteristic hysteresis. The samples also demonstrated efficient photoluminescence with maximum at ∼1.4 eV, due to exiton recombination in silicon nanocrystals. (paper)

  5. Ab initio study of thermoelectric properties of doped SnO_2 superlattices

    International Nuclear Information System (INIS)

    Borges, P.D.; Silva, D.E.S.; Castro, N.S.; Ferreira, C.R.; Pinto, F.G.; Tronto, J.; Scolfaro, L.

    2015-01-01

    Transparent conductive oxides, such as tin dioxide (SnO_2), have recently shown to be promising materials for thermoelectric applications. In this work we studied the thermoelectric properties of Fe-, Sb- and Zn-uniformly doping and co-doping SnO_2, as well as of Sb and Zn planar (or delta)-doped layers in SnO_2 forming oxide superlattices (SLs). Based on the semiclassical Boltzmann transport equations (BTE) in conjunction with ab initio electronic structure calculations, the Seebeck coefficient (S) and figure of merit (ZT) are obtained for these systems, and are compared with available experimental data. The delta doping approach introduces a remarkable modification in the electronic structure of tin dioxide, when compared with the uniform doping, and colossal values for ZT are predicted for the delta-doped oxide SLs. This result is a consequence of the two-dimensional electronic confinement and the strong anisotropy introduced by the doped planes. In comparison with the uniformly doped systems, our predictions reveal a promising use of delta-doped SnO_2 SLs for enhanced S and ZT, which emerge as potential candidates for thermoelectric applications. - Graphical abstract: Band structure and Figure of merit for SnO2:Sb superlattice along Z direction, P. D. Borges, D. E. S. Silva, N. S. Castro, C. R. Ferreira, F. G. Pinto, J. Tronto and L. Scolfaro, Ab initio study of thermoelectric properties of doped SnO2 superlattices. - Highlights: • Thermoelectric properties of SnO_2-based alloys and superlattices. • High figure of merit is predicted for planar-doped SnO_2 superlattices. • Nanotechnology has an important role for the development of thermoelectric devices.

  6. Dielectric function of semiconductor superlattice

    International Nuclear Information System (INIS)

    Qin Guoyi.

    1990-08-01

    We present a calculation of the dielectric function for semiconductor GaAs/Ga 1-x Al x As superlattice taking account of the extension of the electron envelope function and the difference of both the dielectric constant and width between GaAs and Ga 1-x Al x As layers. In the appropriate limits, our results exactly reduce to the well-known results of the quasi two-dimensional electron gas obtained by Lee and Spector and of the period array of two-dimensional electron layers obtained by Das Sarma and Quinn. By means of the dielectric function of the superlattice, the dispersion relation of the collective excitation and the screening property of semiconductor superlattice are discussed and compared with the results of the quasi two-dimensional system and with the results of the periodic array of the two-dimensional electron layers. (author). 4 refs, 3 figs

  7. ZnSe/ZnSeTe Superlattice Nanotips

    Directory of Open Access Journals (Sweden)

    Young SJ

    2010-01-01

    Full Text Available Abstract The authors report the growth of ZnSe/ZnSeTe superlattice nanotips on oxidized Si(100 substrate. It was found the nanotips exhibit mixture of cubic zinc-blende and hexagonal wurtzite structures. It was also found that photoluminescence intensities observed from the ZnSe/ZnSeTe superlattice nanotips were much larger than that observed from the homogeneous ZnSeTe nanotips. Furthermore, it was found that activation energies for the ZnSe/ZnSeTe superlattice nanotips with well widths of 16, 20, and 24 nm were 76, 46, and 19 meV, respectively.

  8. Effects of WC phase contents on the microstructure, mechanical properties and tribological behaviors of WC/a-C superlattice coatings

    Energy Technology Data Exchange (ETDEWEB)

    Pu, Jibin [Key Laboratory of Marine Materials and Related Technologies, Key Laboratory of Marine Materials and Protective Technologies of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201 (China); He, Dongqing [State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000 (China); University of Chinese Academy of Sciences, Beijing 100039 (China); Wang, Liping, E-mail: lpwang@licp.cas.cn [Key Laboratory of Marine Materials and Related Technologies, Key Laboratory of Marine Materials and Protective Technologies of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201 (China)

    2015-12-01

    Graphical abstract: - Highlights: • WC/a-C superlattice coatings were synthesized with various WC phase content. • Superlattice structure diminished residual stress and densified microstructure. • Nanocomposite coating with W 5.43 at.% achieved the optimal tribological properties. • Friction triggered WO{sub 3} lead to a low friction coefficient at 200 °C. - Abstract: Nanocomposite WC/a-C coatings with variable contents of tungsten carbide (WC{sub 1−x}) and amorphous carbon (a-C) were successfully fabricated using a magnetron sputtering process. The microstructure, mechanical properties and tribological behaviors of the as-fabricated coatings were investigated and compared. The results showed that the “superlattice coating” feature of an alternating multilayer structure with a-C and WC{sub 1−x} nanocrystallites layers on the nanoscale was formed. These multilayer superlattice structures led to diminished residual stress and improved the strength of the adhesion to the substrate. The WC/a-C coating with W 5.43 at.% exhibited low friction coefficients of 0.05 at 25 °C and 0.28 at 200 °C. This significant improvement in the tribological performances of the WC/a-C coating was mainly attributed to the superior “superlattice” microstructure and the formation of a continuously compacted tribofilms, which was rich in graphitized carbon at 25 °C and dominated by the friction triggered WO{sub 3} at 200 °C. Moreover, the WC/a-C coating with W 5.43 at.% achieved optimal anti-wear properties at 25 °C due to the synergistic combination of the enhancement effects of the WC{sub 1−x} nanoparticles and the partition effect from the transfer film that restricted direct contact of the steel ball with the coating and thus prevented further intense wear. The accelerated wear of the WC/a-C coating with the increase of the WC phase content at 200 °C might be due to the combination of oxidation wear and abrasive wear that originated from the WC{sub 1−x} phase.

  9. Thermoelectric power of Bi and Bi{sub 1{minus}x}Sb{sub x} alloy thin films and superlattices grown by MBE

    Energy Technology Data Exchange (ETDEWEB)

    Cho, S; DiVenere, A; Wong, G K; Ketterson, J B; Meyer, J R; Hoffman, C A

    1997-07-01

    The authors have measured the thermoelectric power (TEP) of MBE-grown epitaxial Bi and Bi{sub 1{minus}x} alloy thin films and superlattices as a function of temperature in the range 20--300 K. They have observed that the TEP of a Bi thin film of 1 {micro}m thickness is in good agreement with the bulk single crystal value and that the TEPs for superlattices with 400 {angstrom} and 800 {angstrom} Bi well thicknesses are enhanced over the bulk values. For x = 0.072 and 0.088 in Bi{sub 1{minus}x}Sb{sub x} thin films showing semiconducting behavior, TEP enhancement was observed by a factor of two. However as Bi or Bi{sub 1{minus}x}Sb{sub x} well thickness decreases in superlattice geometry, the TEP decreases, which may be due to unintentional p-type doping.

  10. Interfacial magnetic coupling in ultrathin all-manganite La0.7Sr0.3MnO3-TbMnO3 superlattices

    KAUST Repository

    Tian, Y. F.

    2014-04-14

    We report the growth and magnetic properties of all-manganite superlattices composed of ultrathin double-exchange ferromagnetic La0.7Sr0.3MnO3 and noncollinear multiferroic TbMnO3 layers. Spontaneous magnetization and hysteresis loops are observed in such superlattices with individual La0.7Sr0.3MnO3 layers as thin as two unit cells, which are accompanied by pronounced exchange bias and enhanced coercivity. Our results indicate substantial interfacial magnetic coupling between spin sublattices in such superlattices, providing a powerful approach towards tailoring the properties of artificial magnetic heterostructures.

  11. Interfacial magnetic coupling in ultrathin all-manganite La0.7Sr0.3MnO3-TbMnO3 superlattices

    KAUST Repository

    Tian, Y. F.; Lebedev, O. I.; Roddatis, V. V.; Lin, W. N.; Ding, J. F.; Hu, S. J.; Yan, S. S.; Wu, Tao

    2014-01-01

    We report the growth and magnetic properties of all-manganite superlattices composed of ultrathin double-exchange ferromagnetic La0.7Sr0.3MnO3 and noncollinear multiferroic TbMnO3 layers. Spontaneous magnetization and hysteresis loops are observed in such superlattices with individual La0.7Sr0.3MnO3 layers as thin as two unit cells, which are accompanied by pronounced exchange bias and enhanced coercivity. Our results indicate substantial interfacial magnetic coupling between spin sublattices in such superlattices, providing a powerful approach towards tailoring the properties of artificial magnetic heterostructures.

  12. The dependence of the tunneling characteristic on the electronic energy bands and the carrier’s states of Graphene superlattice

    Science.gov (United States)

    Yang, C. H.; Shen, G. Z.; Ao, Z. M.; Xu, Y. W.

    2016-09-01

    Using the transfer matrix method, the carrier tunneling properties in graphene superlattice generated by the Thue-Morse sequence and Kolakoski sequence are investigated. The positions and strength of the transmission can be modulated by the barrier structures, the incident energy and angle, the height and width of the potential. These carriers tunneling characteristic can be understood from the energy band structures in the corresponding superlattice systems and the carrier’s states in well/barriers. The transmission peaks above the critical incident angle rely on the carrier’s resonance in the well regions. The structural diversity can modulate the electronic and transport properties, thus expanding its applications.

  13. Ab initio study of thermoelectric properties of doped SnO{sub 2} superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Borges, P.D., E-mail: pdborges@gmail.com [Instituto de Ciências Exatas e Tecnológicas, Universidade Federal de Viçosa, 38810-000 Rio Paranaíba, MG (Brazil); Silva, D.E.S.; Castro, N.S.; Ferreira, C.R.; Pinto, F.G.; Tronto, J. [Instituto de Ciências Exatas e Tecnológicas, Universidade Federal de Viçosa, 38810-000 Rio Paranaíba, MG (Brazil); Scolfaro, L. [Department of Physics, Texas State University, 78666 San Marcos, TX (United States)

    2015-11-15

    Transparent conductive oxides, such as tin dioxide (SnO{sub 2}), have recently shown to be promising materials for thermoelectric applications. In this work we studied the thermoelectric properties of Fe-, Sb- and Zn-uniformly doping and co-doping SnO{sub 2}, as well as of Sb and Zn planar (or delta)-doped layers in SnO{sub 2} forming oxide superlattices (SLs). Based on the semiclassical Boltzmann transport equations (BTE) in conjunction with ab initio electronic structure calculations, the Seebeck coefficient (S) and figure of merit (ZT) are obtained for these systems, and are compared with available experimental data. The delta doping approach introduces a remarkable modification in the electronic structure of tin dioxide, when compared with the uniform doping, and colossal values for ZT are predicted for the delta-doped oxide SLs. This result is a consequence of the two-dimensional electronic confinement and the strong anisotropy introduced by the doped planes. In comparison with the uniformly doped systems, our predictions reveal a promising use of delta-doped SnO{sub 2} SLs for enhanced S and ZT, which emerge as potential candidates for thermoelectric applications. - Graphical abstract: Band structure and Figure of merit for SnO2:Sb superlattice along Z direction, P. D. Borges, D. E. S. Silva, N. S. Castro, C. R. Ferreira, F. G. Pinto, J. Tronto and L. Scolfaro, Ab initio study of thermoelectric properties of doped SnO2 superlattices. - Highlights: • Thermoelectric properties of SnO{sub 2}-based alloys and superlattices. • High figure of merit is predicted for planar-doped SnO{sub 2} superlattices. • Nanotechnology has an important role for the development of thermoelectric devices.

  14. Superlattice design for optimal thermoelectric generator performance

    Science.gov (United States)

    Priyadarshi, Pankaj; Sharma, Abhishek; Mukherjee, Swarnadip; Muralidharan, Bhaskaran

    2018-05-01

    We consider the design of an optimal superlattice thermoelectric generator via the energy bandpass filter approach. Various configurations of superlattice structures are explored to obtain a bandpass transmission spectrum that approaches the ideal ‘boxcar’ form, which is now well known to manifest the largest efficiency at a given output power in the ballistic limit. Using the coherent non-equilibrium Green’s function formalism coupled self-consistently with the Poisson’s equation, we identify such an ideal structure and also demonstrate that it is almost immune to the deleterious effect of self-consistent charging and device variability. Analyzing various superlattice designs, we conclude that superlattice with a Gaussian distribution of the barrier thickness offers the best thermoelectric efficiency at maximum power. It is observed that the best operating regime of this device design provides a maximum power in the range of 0.32–0.46 MW/m 2 at efficiencies between 54%–43% of Carnot efficiency. We also analyze our device designs with the conventional figure of merit approach to counter support the results so obtained. We note a high zT el   =  6 value in the case of Gaussian distribution of the barrier thickness. With the existing advanced thin-film growth technology, the suggested superlattice structures can be achieved, and such optimized thermoelectric performances can be realized.

  15. Passive high-frequency devices based on superlattice ferromagnetic nanowires

    International Nuclear Information System (INIS)

    Ye, B.; Li, F.; Cimpoesu, D.; Wiley, J.B.; Jung, J.-S.; Stancu, A.; Spinu, L.

    2007-01-01

    In this paper we propose to tailor the bandwidth of a microwave filter by exploitation of shape anisotropy of nanowires. In order to achieve this control of shape anisotropy, we considered superlattice wires containing varying-sized ferromagnetic regions separated by nonferromagnetic regions. Superlattice wires of Ni and Au with a nominal diameter of 200 nm were grown using standard electrodeposition techniques. The microwave properties were probed using X-band (9.8 GHz) ferromagnetic resonance (FMR) experiments performed at room temperature. In order to investigate the effectiveness of the shape anisotropy on the superlattice nanowire based filter the FMR spectrum of superlattice structure is compared to the FMR spectra of nanowires samples with constant length

  16. Positron probing of electron momentum density in GaAs-AlAs superlattices and related materials

    International Nuclear Information System (INIS)

    Arutyunov, N.Y.; Sekkal, N.

    2008-08-01

    The band structure calculations based on the method proposed by Jaros et al. (Phys. Rev. B 31, 1205 (1985)) have been performed for the defect-free GaAs-AlAs superlattice and related AlAs and GaAs single crystals; the electron-positron momentum density distributions have been computed and analyzed. The results of calculations are in good agreement with the experimental data obtained ad hoc for GaAs and AlAs bulk materials by measuring the angular correlation of the annihilation radiation (ACAR). Small (but marked) features of the electron-positron momentum density of the valence band have been revealed both for constituent materials and GaAs-AlAs superlattice. The delocalization of positron in 'perfect' defect-'free' AlAs and GaAs single crystals to be observed experimentally is borne out by the results of pseudo-potential band calculations performed on the basis of method proposed by Sekkal et al. (Superlattices and Microstructures, 33, 63 (2003)). The prediction of the possibility of a certain confinement of positron in the interstitial area of GaAs- AlAs superlattice is confirmed by the agreement between the results of calculations and relevant experimental data obtained for GaAs and AlAs single crystals. No considerable effect of the enhancement of the annihilation rate (due to electron-positron interaction) upon the electron-positron momentum density distribution both in the superlattice and its constituent bulk materials has been found. The results of ACAR measurements and calculations performed suggest that a tangible improvement of the sensitivity of existing positron annihilation techniques is necessary for studying details of the electron-positron momentum density distributions in defect-'free' superlattices to be created on the basis of the diamond-like semiconductors possessing close values of the electron momentum densities. On the contrary, the positron-sensitive vacancy-type defects of various types in the superlattice may become a source of the

  17. Density functional theory computational study of ferroelectricity and piezoelectricity in BaTiO3/PbTiO3 (0 1 1) superlattices

    Science.gov (United States)

    Lou, Yaoding; Deng, Junkai; Zhe Liu, Jefferson

    2018-04-01

    The structure, ferroelectricity (FE), and piezoelectricity of epitaxial BaTiO3/PbTiO3 (BTO/PTO) (0 1 1) superlattices are studied using density functional theory calculations. Our results show that compressive strain arising from the SrTiO3 (0 1 1) substrate stabilizes the (BTO) m /(PTO) n (0 1 1) superlattices in orthorhombic phase with the FE polarization along [0 1 1] direction. Tuning the BTO contents significantly changes the structural, ferroelectric and piezoelectric properties. The FE polarization of superlattices significantly drops with increasing BTO contents, which can be attributed to depolarization of the PTO layers. The averaged c/a ratio of the whole superlattices exhibits anomalous non-monotonic relation with respect to BTO contents. Interestingly, our results predict the (0 1 1) superlattices can enhance the piezoelectric coefficient e 33 with a maximum value at ~67% BTO concentration. This result suggests a potential avenue to design high performance piezoelectric materials with less Pb contents. In-depth analysis reveals the B-site Ti cation as the origin for the enhanced e 33 value, which implies the potential of B-site cation engineering in perovskite heterostructure designs.

  18. Magnetic superlattices

    International Nuclear Information System (INIS)

    Kwo, J.; Hong, M.; McWhan, D.B.; Yafet, Y.; Fleming, R.M.; DiSalvo, F.J.; Waszczak, J.V.; Majkrzak, C.F.; Gibbs, D.; Goldmann, A.I.; Boni, P.; Bohr, J.; Grimm, H.; Bohr, J.; Chien, C.L.; Grimm, H.; Cable, J.W.

    1988-01-01

    Single crystal magnetic rare earth superlattices were synthesized by molecular beam epitaxy. The studies include four rare earth systems: Gd-Y, Dy-Y, Ho-Y, and Gd-Dy. The magnetic properties and the long-range spin order are reviewed in terms of the interfacial behavior, and the interlayer exchange coupling across Y medium

  19. Theoretical study of nitride short period superlattices

    Science.gov (United States)

    Gorczyca, I.; Suski, T.; Christensen, N. E.; Svane, A.

    2018-02-01

    Discussion of band gap behavior based on first principles calculations of electronic band structures for various short period nitride superlattices is presented. Binary superlattices, as InN/GaN and GaN/AlN as well as superlattices containing alloys, as InGaN/GaN, GaN/AlGaN, and GaN/InAlN are considered. Taking into account different crystallographic directions of growth (polar, semipolar and nonpolar) and different strain conditions (free-standing and pseudomorphic) all the factors influencing the band gap engineering are analyzed. Dependence on internal strain and lattice geometry is considered, but the main attention is devoted to the influence of the internal electric field and the hybridization of well and barrier wave functions. The contributions of these two important factors to band gap behavior are illustrated and estimated quantitatively. It appears that there are two interesting ranges of layer thicknesses; in one (few atomic monolayers in barriers and wells) the influence of the wave function hybridization is dominant, whereas in the other (layers thicker than roughly five to six monolayers) dependence of electric field on the band gaps is more important. The band gap behavior in superlattices is compared with the band gap dependence on composition in the corresponding ternary and quaternary alloys. It is shown that for superlattices it is possible to exceed by far the range of band gap values, which can be realized in ternary alloys. The calculated values of the band gaps are compared with the photoluminescence emission energies, when the corresponding data are available. Finally, similarities and differences between nitride and oxide polar superlattices are pointed out by comparison of wurtzite GaN/AlN and ZnO/MgO.

  20. Mixing of III-V compound semiconductor superlattices

    International Nuclear Information System (INIS)

    Mei, Ping.

    1989-01-01

    In this work, the methods as well as mechanisms of III-V compound superlattice mixing are discussed, with particular attention on the AlGaAs based superlattice system. Comparative studies of ion-induced mixing showed two distinct effects resulting from ion implantation followed by a thermal anneal; i.e. collisional mixing and impurity induced mixing. It was found that Ga and As ion induced mixing are mainly due to the collisional effect, where the extent of the mixing can be estimated theoretically, with the parameters of ion mass, incident energy and the implant dose. The impurity effect was dominant for Si, Ge, Be, Zn and Te. Quantitative studies of impurity induced mixing have been conducted on samples doped with Si or Te during the growth process. It was discovered that Si induced AlGaAs superlattice mixing yielded an activation energy of approximately 4 eV for the Al diffusion coefficient with a high power law dependence of the prefactor on the Si concentration. In the Te doped AlGaAs superlattice the Al diffusion coefficient exhibited an activation energy of ∼3.0 eV, with a prefactor approximately proportional to the Te concentration. These results are of importance in examining the current diffusion models. Zn and Si induced InP/InGaAs superlattice mixing are examined. It was found that Zn predominantly induces cation interdiffusion, while Si induces comparable cation and anion interdiffusion. In addition, widely dispersed Zn rich islands form with Zn residing in the InP layers in the form of Zn 3 P 2 . With unstrained starting material, the layer bandgap disparity increases due to mixing induced strain, while in the Si diffused sample the mixed region would be expected to exhibit bandgaps intermediate between those of the original layers. Semiconductor superlattice mixing shows technological potential for optoelectronic device fabrication

  1. Quasiperiodic AlGaAs superlattices for neuromorphic networks and nonlinear control systems

    Energy Technology Data Exchange (ETDEWEB)

    Malyshev, K. V., E-mail: malyshev@bmstu.ru [Electronics and Laser Technology Department, Bauman Moscow State Technical University, Moscow 105005 (Russian Federation)

    2015-01-28

    The application of quasiperiodic AlGaAs superlattices as a nonlinear element of the FitzHugh–Nagumo neuromorphic network is proposed and theoretically investigated on the example of Fibonacci and figurate superlattices. The sequences of symbols for the figurate superlattices were produced by decomposition of the Fibonacci superlattices' symbolic sequences. A length of each segment of the decomposition was equal to the corresponding figurate number. It is shown that a nonlinear network based upon Fibonacci and figurate superlattices provides better parallel filtration of a half-tone picture; then, a network based upon traditional diodes which have cubic voltage-current characteristics. It was found that the figurate superlattice F{sup 0}{sub 11}(1) as a nonlinear network's element provides the filtration error almost twice less than the conventional “cubic” diode. These advantages are explained by a wavelike shape of the decreasing part of the quasiperiodic superlattice's voltage-current characteristic, which leads to multistability of the network's cell. This multistability promises new interesting nonlinear dynamical phenomena. A variety of wavy forms of voltage-current characteristics opens up new interesting possibilities for quasiperiodic superlattices and especially for figurate superlattices in many areas—from nervous system modeling to nonlinear control systems development.

  2. Sm cluster superlattice on graphene/Ir(111)

    Science.gov (United States)

    Mousadakos, Dimitris; Pivetta, Marina; Brune, Harald; Rusponi, Stefano

    2017-12-01

    We report on the first example of a self-assembled rare earth cluster superlattice. As a template, we use the moiré pattern formed by graphene on Ir(111); its lattice constant of 2.52 nm defines the interparticle distance. The samarium cluster superlattice forms for substrate temperatures during deposition ranging from 80 to 110 K, and it is stable upon annealing to 140 K. By varying the samarium coverage, the mean cluster size can be increased up to 50 atoms, without affecting the long-range order. The spatial order and the width of the cluster size distribution match the best examples of metal cluster superlattices grown by atomic beam epitaxy on template surfaces.

  3. Piezoelectricity in the dielectric component of nanoscale dielectric-ferroelectric superlattices.

    Science.gov (United States)

    Jo, Ji Young; Sichel, Rebecca J; Lee, Ho Nyung; Nakhmanson, Serge M; Dufresne, Eric M; Evans, Paul G

    2010-05-21

    The origin of the functional properties of complex oxide superlattices can be resolved using time-resolved synchrotron x-ray diffraction into contributions from the component layers making up the repeating unit. The CaTiO3 layers of a CaTiO3/BaTiO3 superlattice have a piezoelectric response to an applied electric field, consistent with a large continuous polarization throughout the superlattice. The overall piezoelectric coefficient at large strains, 54  pm/V, agrees with first-principles predictions in which a tetragonal symmetry is imposed on the superlattice by the SrTiO3 substrate.

  4. Fine structure of the exciton electroabsorption in semiconductor superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Monozon, B.S., E-mail: borismonozon@mail.ru [Physics Department, Marine Technical University, 3 Lotsmanskaya Str., 190008 St.Petersburg (Russian Federation); Schmelcher, P. [Zentrum für Optische Quantentechnologien, The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg (Germany)

    2017-02-15

    Wannier-Mott excitons in a semiconductor layered superlattice (SL) are investigated analytically for the case that the period of the superlattice is much smaller than the 2D exciton Bohr radius. Additionally we assume the presence of a longitudinal external static electric field directed parallel to the SL axis. The exciton states and the optical absorption coefficient are derived in the tight-binding and adiabatic approximations. Strong and weak electric fields providing spatially localized and extended electron and hole states, respectively, are studied. The dependencies of the exciton states and the exciton absorption spectrum on the SL parameters and the electric field strength are presented in an explicit form. We focus on the fine structure of the ground quasi-2D exciton level formed by the series of closely spaced energy levels adjacent from the high frequencies. These levels are related to the adiabatically slow relative exciton longitudinal motion governed by the potential formed by the in-plane exciton state. It is shown that the external electric fields compress the fine structure energy levels, decrease the intensities of the corresponding optical peaks and increase the exciton binding energy. A possible experimental study of the fine structure of the exciton electroabsorption is discussed.

  5. Superlattice configurations in linear chain hydrocarbon binary mixtures

    Indian Academy of Sciences (India)

    Unknown

    Long-chain alkanes; binary mixtures; superlattices; discrete orientational changes. 1. Introduction ... tem and a model of superlattice configuration was proposed4, in terms of .... C18 system,4 the angle with value = 3⋅3° was seen to play an ...

  6. Wave-function reconstruction in a graded semiconductor superlattice

    DEFF Research Database (Denmark)

    Lyssenko, V. G.; Hvam, Jørn Märcher; Meinhold, D.

    2004-01-01

    We reconstruct a test wave function in a strongly coupled, graded well-width superlattice by resolving the spatial extension of the interband polarisation and deducing the wave function employing non-linear optical spectroscopy. The graded gap superlattice allows us to precisely control the dista...

  7. Sodium effect on self-organization of amphiphilic carboxylates: formation of structured micelles and superlattices.

    Science.gov (United States)

    Rosenlehner, Karin; Schade, Boris; Böttcher, Christoph; Jäger, Christof M; Clark, Timothy; Heinemann, Frank W; Hirsch, Andreas

    2010-08-16

    Not only the self-aggregation of dendritic polycarboxylates into structurally persistent micelles, but also that of the micelles themselves into superlattices is controlled by alkali-metal counterions and shows a pronounced sodium effect. Our combined experimental and computational work has revealed the formation of superlattices for the first time. The behavior of a variety of amphiphilic carboxylates and the different effects of the alkali cations Li(+), Na(+), and K(+) have been investigated by conductivity measurements, cryogenic transmission electron microscopy (cryo-TEM), and molecular-dynamics (MD) simulations. Together, these show that sodium salts of the amphiphiles give the most stable micelles, followed by lithium and potassium. Our results suggest that ion multiplets in bridging positions, rather than contact ion pairs, are responsible for the enhanced stability and the formation of hexagonally ordered superlattices with sodium counterions. Potassium ions do not form such ion multiplets and cannot therefore induce aggregation of the micelles. This sodium effect has far-reaching consequences for a large number of biological and technical systems and sheds new light on the origin of specific-ion effects.

  8. Epitaxial rare-earth superlattices and films

    International Nuclear Information System (INIS)

    Salamon, M.B.; Beach, R.S.; Flynn, C.P.; Matheny, A.; Tsui, F.; Rhyne, J.J.

    1992-01-01

    This paper reports on epitaxial growth of rare-earth superlattices which is demonstrated to have opened important new areas of research on magnetic materials. The propagation magnetic order through non-magnetic elements, including its range and anisotropy, has been studied. The importance of magnetostriction in determining the phase diagram is demonstrated by the changes induced by epitaxial clamping. The cyrstallinity of epitaxial superlattices provides the opportunity to study interfacial magnetism by conventional x-ray and neutron scattering methods

  9. Antiferromagnetic spinor condensates in a bichromatic superlattice

    Science.gov (United States)

    Tang, Tao; Zhao, Lichao; Chen, Zihe; Liu, Yingmei

    2017-04-01

    A spinor Bose-Einstein condensate in an optical supelattice has been considered as a good quantum simulator for understanding mesoscopic magnetism. We report an experimental study on an antiferromagnetic spinor condensate in a bichromatic superlattice constructed by a cubic red-detuned optical lattice and a one-dimensional blue-detuned optical lattice. Our data demonstrate a few advantages of this bichromatic superlattice over a monochromatic lattice. One distinct advantage is that the bichromatic superlattice enables realizing the first-order superfluid to Mott-insulator phase transitions within a much wider range of magnetic fields. In addition, we discuss an apparent discrepancy between our data and the mean-field theory. We thank the National Science Foundation and the Oklahoma Center for the Advancement of Science and Technology for financial support.

  10. Scaling properties of optical reflectance from quasi-periodic superlattices

    International Nuclear Information System (INIS)

    Wu Xiang; Yao Hesheng; Feng Weiguo

    1991-08-01

    The scaling properties of the optical reflectance from two types of quasi-periodic metal-insulator superlattices, one with the structure of Cantor bars and the other with the structure of Cantorian-Fibonaccian train, have been studied for the region of s-polarized soft x-rays and extreme ultraviolet. By using the hydrodynamic model of electron dynamics and transfer-matrix method, and be taking into account retardation effects, we have presented the formalism of the reflectivity for the superlattices. From our numerical results, we found that the reflection spectra of the quasi-superlattices have a rich structure of self-similarity. The interesting scaling indices, which are related to the fractal dimensions, of the spectra are also discussed for the two kinds of the quasi-superlattices. (author). 10 refs, 7 figs

  11. Superlattice to nanoelectronics

    CERN Document Server

    Tsu, Raphael

    2005-01-01

    Superlattice to Nanoelectronics provides a historical overview of the early work performed by Tsu and Esaki, to orient those who want to enter into this nanoscience. It describes the fundamental concepts and goes on to answer many questions about todays 'Nanoelectronics'. It covers the applications and types of devices which have been produced, many of which are still in use today. This historical perspective is important as a guide to what and how technology and new fundamental ideas are introduced and developed. The author communicates a basic understanding of the physics involved from first principles, whilst adding new depth, using simple mathematics and explanation of the background essentials. Topics covered include * Introductory materials * Superlattice, Bloch oscillations and transport * Tunneling in QWs to QDs * Optical properties: optical transitions, size dependent dielectric constant, capacitance and doping * Quantum devices: New approaches without doping and heterojunctions - quantum confinement...

  12. Novel electronic structures of superlattice composed of graphene and silicene

    International Nuclear Information System (INIS)

    Yu, S.; Li, X.D.; Wu, S.Q.; Wen, Y.H.; Zhou, S.; Zhu, Z.Z.

    2014-01-01

    Highlights: • Graphene/silicene superlattices exhibit metallic electronic properties. • Dirac point of graphene is folded to the Γ-point in the superlattice system. • Significant changes in the transport properties of the graphene layers are expected. • Small amount of charge transfer from the graphene to the silicene layers is found. - Abstract: Superlattice is a major force in providing man-made materials with unique properties. Here we report a study of the structural and electronic properties of a superlattice made with alternate stacking of graphene and hexagonal silicene. Three possible stacking models, i.e., the top-, bridge- and hollow-stacking, are considered. The top-stacking is found to be the most stable pattern. Although both the free-standing graphene and silicene are semi-metals, our results suggest that the graphene and silicene layers in the superlattice both exhibit metallic electronic properties due to a small amount of charge transfer from the graphene to the silicene layers. More importantly, the Dirac point of graphene is folded to the Γ-point of the superlattice, instead of the K-point in the isolated graphene. Such a change in the Dirac point of graphene could lead to significant change in the transportation property of the graphene layer. Moreover, the band structure and the charge transfer indicate that the interaction between the stacking sheets in the graphene/silicene superlattice is more than just the van der Waals interaction

  13. Band structure of superlattice with δ-like potential

    International Nuclear Information System (INIS)

    Gashimzade, N.F.; Gashimzade, F.M.; Hajiev, A.T.

    1993-08-01

    Band structure of superlattice with δ-like potential has been calculated taking into account interaction of carriers of different kinds. Superlattices of semiconductors with degenerated valence band and zero-gap semiconductors have been considered. For the latter semimetal-semiconductor transition has been obtained. (author). 8 refs, 1 fig

  14. Almost 'magnetic dead' of the Co layer in Co/Zr sub 3 superlattice

    CERN Document Server

    Kwon, Y S; Hong, S C; Lee, Y P

    1999-01-01

    The magnetic and electronic properties of Co/Zr sub 3 (0001) superlattice and unsupported Co and Zr monolayers (ML) with the hcp bulk Zr two dimensional lattice parameters have been calculated by employing the full potential linearized augmented plane wave (FLAPW) method with general gradient approximation (GGA) for exchange-correlation potential. The unsupported Zr and Co ML were calculated to be stable in paramagnetic and ferromagnetic states, respectively. The magnetic moment of the unsupported Co ML was 2.08 mu B. The Co-Zr interlayer spacing of the Co/Zr sub 3 superlattice was calculated to be reduced significantly by 9.80 % compared to that expected from Co and Zr bulk assuming preservation of the atomic volumes. On the other hand, the Zr-Zr interlayer spacing was calculated to be enhanced by 3.35 % compared to that of bulk Zr. Both of the reduced Zr-Co and the enhanced Zr-Zr interlayer spacings decreased the magnetic moment of Co and eventually led to almost 'magnetic dead' (0.33 mu B). Surprisingly, t...

  15. Formation Energies of Native Point Defects in Strained-Layer Superlattices (Postprint)

    Science.gov (United States)

    2017-06-05

    potential; bulk materials; total energy calculations; entropy; strained- layer superlattice (SLS) 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF...AFRL-RX-WP-JA-2017-0217 FORMATION ENERGIES OF NATIVE POINT DEFECTS IN STRAINED- LAYER SUPERLATTICES (POSTPRINT) Zhi-Gang Yu...2016 Interim 11 September 2013 – 5 November 2016 4. TITLE AND SUBTITLE FORMATION ENERGIES OF NATIVE POINT DEFECTS IN STRAINED- LAYER SUPERLATTICES

  16. Materials science and technology strained-layer superlattices materials science and technology

    CERN Document Server

    Pearsall, Thomas P; Willardson, R K; Pearsall, Thomas P

    1990-01-01

    The following blurb to be used for the AP Report and ATI only as both volumes will not appear together there.****Strained-layer superlattices have been developed as an important new form of semiconducting material with applications in integrated electro-optics and electronics. Edited by a pioneer in the field, Thomas Pearsall, this volume offers a comprehensive discussion of strained-layer superlattices and focuses on fabrication technology and applications of the material. This volume combines with Volume 32, Strained-Layer Superlattices: Physics, in this series to cover a broad spectrum of topics, including molecular beam epitaxy, quantum wells and superlattices, strain-effects in semiconductors, optical and electrical properties of semiconductors, and semiconductor devices.****The following previously approved blurb is to be used in all other direct mail and advertising as both volumes will be promoted together.****Strained-layer superlattices have been developed as an important new form of semiconducting ...

  17. Control of the interparticle spacing in gold nanoparticle superlattices

    Energy Technology Data Exchange (ETDEWEB)

    MARTIN,JAMES E.; WILCOXON,JESS P.; ODINEK,JUDY G.; PROVENCIO,PAULA P.

    2000-04-06

    The authors have investigated the formation of 2-D and 3-D superlattices of Au nanoclusters synthesized in nonionic inverse micelles, and capped with alkyl thiol ligands, with alkane chains ranging from C{sub 6} to C1{sub 18}. The thiols are found to play a significant role in the ripening of these nanoclusters, and in the formation of superlattices. Image processing techniques were developed to reliably extract from transmission electron micrographs (TEMs) the particle size distribution, and information about the superlattice domains and their boundaries. The latter permits one to compute the intradomain vector pair correlation function, from which one can accurately determine the lattice spacing and the coherent domain size. From these data the gap between the particles in the coherent domains can be determined as a function of the thiol chain length. It is found that as the thiol chain length increases, the nanoclusters become more polydisperse and larger, and the gaps between particles within superlattice domains increases. Annealing studies at elevated temperatures confirm nanocluster ripening. Finally, the effect of the particle gaps on physical properties is illustrated by computing the effective dielectric constant, and it is shown that the gap size now accessible in superlattices is rather large for dielectric applications.

  18. Quasi free-standing silicene in a superlattice with hexagonal boron nitride

    KAUST Repository

    Kaloni, T. P.

    2013-11-12

    We study a superlattice of silicene and hexagonal boron nitride by first principles calculations and demonstrate that the interaction between the layers of the superlattice is very small. As a consequence, quasi free-standing silicene is realized in this superlattice. In particular, the Dirac cone of silicene is preserved. Due to the wide band gap of hexagonal boron nitride, the superlattice realizes the characteristic physical phenomena of free-standing silicene. In particular, we address by model calculations the combined effect of the intrinsic spin-orbit coupling and an external electric field, which induces a transition from a semimetal to a topological insulator and further to a band insulator.

  19. Electronic structure of superlattices of graphene and hexagonal boron nitride

    KAUST Repository

    Kaloni, Thaneshwor P.

    2011-11-14

    We study the electronic structure of superlattices consisting of graphene and hexagonal boron nitride slabs, using ab initio density functional theory. We find that the system favors a short C–B bond length at the interface between the two component materials. A sizeable band gap at the Dirac point is opened for superlattices with single graphene layers but not for superlattices with graphene bilayers. The system is promising for applications in electronic devices such as field effect transistors and metal-oxide semiconductors.

  20. Electronic structure of superlattices of graphene and hexagonal boron nitride

    KAUST Repository

    Kaloni, Thaneshwor P.; Cheng, Yingchun; Schwingenschlö gl, Udo

    2011-01-01

    We study the electronic structure of superlattices consisting of graphene and hexagonal boron nitride slabs, using ab initio density functional theory. We find that the system favors a short C–B bond length at the interface between the two component materials. A sizeable band gap at the Dirac point is opened for superlattices with single graphene layers but not for superlattices with graphene bilayers. The system is promising for applications in electronic devices such as field effect transistors and metal-oxide semiconductors.

  1. Superlattice doped layers for amorphous silicon photovoltaic cells

    Science.gov (United States)

    Arya, Rajeewa R.

    1988-01-12

    Superlattice doped layers for amorphous silicon photovoltaic cells comprise a plurality of first and second lattices of amorphous silicon alternatingly formed on one another. Each of the first lattices has a first optical bandgap and each of the second lattices has a second optical bandgap different from the first optical bandgap. A method of fabricating the superlattice doped layers also is disclosed.

  2. InN/GaN Superlattices: Band Structures and Their Pressure Dependence

    DEFF Research Database (Denmark)

    Gorczyca, Iza; Suski, Tadek; Staszczak, Grzegorz

    2013-01-01

    Creation of short-period InN/GaN superlattices is one of the possible ways of conducting band gap engineering in the green-blue range of the spectrum. The present paper reports results of photoluminescence experiments, including pressure effects, on a superlattice sample consisting of unit cells...... with one monolayer of InN and 40 monolayers of GaN. The results are compared with calculations performed for different types of superlattices: InN/GaN, InGaN/GaN, and InN/InGaN/GaN with single monolayers of InN and/or InGaN. The superlattices are simulated by band structure calculations based on the local...... density approximation (LDA) with a semi-empirical correction for the ‘‘LDA gap error’’. A similarity is observed between the results of calculations for an InGaN/GaN superlattice (with one monolayer of InGaN) and the experimental results. This indicates that the fabricated InN quantum wells may contain...

  3. Stability and Dynamic of strain mediated Adatom Superlattices on Cu<111>

    OpenAIRE

    Kappus, Wolfgang

    2012-01-01

    Substrate strain mediated adatom density distributions have been calculated for Cu surfaces. Complemented by Monte Carlo calculations a hexagonal close packaged adatom superlattice in a coverage range up to 0.045 ML is derived. Conditions for the stability of the superlattice against nucleation and degradation are analyzed using simple neighborhood models. Such models are also used to investigate the dynamic of adatoms within their superlattice neighborhood. Collective modes of adatom diffusi...

  4. Electronic structure of superlattices

    International Nuclear Information System (INIS)

    Altarelli, M.

    1987-01-01

    Calculations of electronic states in semiconductor superlattices are briefly reviewed, with emphasis on the envelope-function method and on comparison with experiments. The energy levels in presence of external magnetic fields are discussed and compared to magneto-optical experiments. (author) [pt

  5. Participation of mechanical oscillations in thermodynamics of crystals with superlattice

    International Nuclear Information System (INIS)

    Jacjimovski K, S.; Mirjanicj Lj, D.; Shetrajchicj P, J.

    2012-01-01

    The superlattice, consisting of two periodically repeating films, is analyzed in proposal paper. Due to the structural deformations and small thickness, the acoustic phonons do not appear in these structures. The spontaneous appearance of phonons is possible in an ideal structure only. Therefore the thermodynamical analysis of phonon subsystems is the first step in investigations of superlattice properties. Internal energy as well as specific heat will be analyzed, too. Low-temperature behavior of these quantities will be compared to the corresponding quantities of bulk structures and of thin films. The general conclusion is that the main thermodynamic characteristics of superlattices are considerably lower than those of the bulk structure. Consequently, their superconductive characteristics are better than the superconductive characteristics of corresponding bulk structures. Generally considered, the application field of superlattices is wider than that of bulk structures and films. (Author)

  6. Manganites in Perovskite Superlattices: Structural and Electronic Properties

    KAUST Repository

    Jilili, Jiwuer

    2016-07-13

    structure of bulk CaMnO3 and LaNiO3. An onsite Coulomn interaction term U is tested for both the Mn and Ni atoms. G-type antiferromagnetism and insulating properties of CaMnO3 are reproduced with U = 3 eV and ferromagnetic ordering is favorable when CaMnO3 is strained to the substrate lattice constant. This implies that the CaMnO3 magnetism is sensitive to both strain and the U parameter. Antiparallel orientation of the Mn and Ti moments has been found experimentally in the BiMnO3/SrTiO3 superlattice. By introducing O defects at different layers, we find similar patterns when the defect is located in the BiO layer. The structural, electronic and magnetic properties are analysed. Strong hybridization between the d3z2−r2 orbitals of the Mn and Ti atoms near the O defect is found. The effect of uniaxial strain for the formation of a two-dimensional electron gas and the interfacial Ti magnetic moments of the (LaMnO3)2/(SrTiO3)2 superlattice are investigated. By tuning the strain state from compressive to tensile, we predict under which conditions the spin-polarization of the electron gas is enhanced. Since the thickness ratio of the superlattice correlates with the strain state, we also study the structural, electronic and magnetism trends of (LaMnO3)n/(SrTiO3)m superlattices with varying layer thicknesses. The main finding is that half-metallicity will vanish for n, m > 8. Reduction of the minority band gaps with increasing n and m originates mainly from an energetic downshift of the Ti dxy states. Along with these, the interrelation between the interface geometry and the electronic properties of the antiferromagnetic/ferromagnetic superlattice BiFeO3/ La0.7Sr0.3MnO3 is investigated. The magnetic and optical properties are also analysed by first principles calculations. The half-metallic character of bulk La0.7Sr0.3MnO3 is maintained in the superlattice, which implies potential applications on spintronics and memory devices.

  7. Band structure and optical properties of sinusoidal superlattices: ZnSe1-xTex

    International Nuclear Information System (INIS)

    Yang, G.; Lee, S.; Furdyna, J. K.

    2000-01-01

    This paper examines the band structure and optical selection rules in superlattices with a sinusoidal potential profile. The analysis is motivated by the recent successful fabrication of high quality ZnSe 1-x Te x superlattices in which the composition x varies sinusoidally along the growth direction. Although the band alignment in the ZnSe 1-x Te x sinusoidal superlattices is staggered (type II), they exhibit unexpectedly strong photoluminescence, thus suggesting interesting optical behavior. The band structure of such sinusoidal superlattices is formulated in terms of the nearly-free-electron (NFE) approximation, in which the superlattice potential is treated as a perturbation. The resulting band structure is unique, characterized by a single minigap separating two wide, free-electron-like subbands for both electrons and holes. Interband selection rules are derived for optical transitions involving conduction and valence-band states at the superlattice Brillouin-zone center, and at the zone edge. A number of transitions are predicted due to wave-function mixing of different subband states. It should be noted that the zone-center and zone-edge transitions are especially easy to distinguish in these superlattices because of the large width of the respective subbands. The results of the NFE approximation are shown to hold surprisingly well over a wide range of parameters, particularly when the period of the superlattice is short. (c) 2000 The American Physical Society

  8. Binding of biexcitons in GaAs/AlxGa1-xAs superlattices

    DEFF Research Database (Denmark)

    Mizeikis, Vygantas; Birkedal, Dan; Langbein, Wolfgang Werner

    1997-01-01

    Properties of the heavy-hole excitons and biexcitons in GaAs/Al0.3Ga0.7As superlattices are studied using linear and nonlinear optical techniques. In superlattices with miniband halfwidths less than the exciton binding energy, the biexciton binding energy is found to be the same as in the noninte......Properties of the heavy-hole excitons and biexcitons in GaAs/Al0.3Ga0.7As superlattices are studied using linear and nonlinear optical techniques. In superlattices with miniband halfwidths less than the exciton binding energy, the biexciton binding energy is found to be the same...

  9. Optical characterization of nanocrystals in silicon rich oxide superlattices and porous silicon

    International Nuclear Information System (INIS)

    Agocs, E.; Petrik, P.; Milita, S.; Vanzetti, L.; Gardelis, S.; Nassiopoulou, A.G.; Pucker, G.; Balboni, R.; Fried, M.

    2011-01-01

    We propose to analyze ellipsometry data by using effective medium approximation (EMA) models. Thanks to EMA, having nanocrystalline reference dielectric functions and generalized critical point (GCP) model the physical parameters of two series of samples containing silicon nanocrystals, i.e. silicon rich oxide (SRO) superlattices and porous silicon layers (PSL), have been determined. The superlattices, consisting of ten SRO/SiO 2 layer pairs, have been prepared using plasma enhanced chemical vapor deposition. The porous silicon layers have been prepared using short monopulses of anodization current in the transition regime between porous silicon formation and electropolishing, in a mixture of hydrofluoric acid and ethanol. The optical modeling of both structures is similar. The effective dielectric function of the layer is calculated by EMA using nanocrystalline components (nc-Si and GCP) in a dielectric matrix (SRO) or voids (PSL). We discuss the two major problems occurring when modeling such structures: (1) the modeling of the vertically non-uniform layer structures (including the interface properties like nanoroughness at the layer boundaries) and (2) the parameterization of the dielectric function of nanocrystals. We used several techniques to reduce the large number of fit parameters of the GCP models. The obtained results are in good agreement with those obtained by X-ray diffraction and electron microscopy. We investigated the correlation of the broadening parameter and characteristic EMA components with the nanocrystal size and the sample preparation conditions, such as the annealing temperatures of the SRO superlattices and the anodization current density of the porous silicon samples. We found that the broadening parameter is a sensitive measure of the nanocrystallinity of the samples, even in cases, where the nanocrystals are too small to be visible for X-ray scattering. Major processes like sintering, phase separation, and intermixing have been

  10. Superior Properties of Energetically Stable La2/3Sr1/3MnO3/Tetragonal BiFeO3 Multiferroic Superlattices

    KAUST Repository

    Feng, Nan; Mi, Wenbo; Wang, Xiaocha; Cheng, Yingchun; Schwingenschlö gl, Udo

    2015-01-01

    The superlattice of energetically stable La2/3Sr1/3MnO3 and tetragonal BiFeO3 is investigated by means of density functional theory. The superlattice as a whole exhibits a half-metallic character, as is desired for spintronic devices. The interfacial electronic states and exchange coupling are analyzed in details. We demonstrate that the interfacial O atoms play a key role in controlling the coupling. The higher ferroelectricity of tetragonal BiFeO3 and stronger response to the magnetic moment in La2/3Sr1/3MnO3/BiFeO3 superlattice show a strongly enhanced electric control of the magnetism as compared to the rhombohedral one. Therefore, it is particularly practical interest in the magnetoelectric controlled spintronic devices.

  11. Superior Properties of Energetically Stable La2/3Sr1/3MnO3/Tetragonal BiFeO3 Multiferroic Superlattices

    KAUST Repository

    Feng, Nan

    2015-04-30

    The superlattice of energetically stable La2/3Sr1/3MnO3 and tetragonal BiFeO3 is investigated by means of density functional theory. The superlattice as a whole exhibits a half-metallic character, as is desired for spintronic devices. The interfacial electronic states and exchange coupling are analyzed in details. We demonstrate that the interfacial O atoms play a key role in controlling the coupling. The higher ferroelectricity of tetragonal BiFeO3 and stronger response to the magnetic moment in La2/3Sr1/3MnO3/BiFeO3 superlattice show a strongly enhanced electric control of the magnetism as compared to the rhombohedral one. Therefore, it is particularly practical interest in the magnetoelectric controlled spintronic devices.

  12. Designing Optical Properties in DNA-Programmed Nanoparticle Superlattices

    Science.gov (United States)

    Ross, Michael Brendan

    A grand challenge of modern science has been the ability to predict and design the properties of new materials. This approach to the a priori design of materials presents a number of challenges including: predictable properties of the material building blocks, a programmable means for arranging such building blocks into well understood architectures, and robust models that can predict the properties of these new materials. In this dissertation, we present a series of studies that describe how optical properties in DNA-programmed nanoparticle superlattices can be predicted prior to their synthesis. The first chapter provides a history and introduction to the study of metal nanoparticle arrays. Chapter 2 surveys and compares several geometric models and electrodynamics simulations with the measured optical properties of DNA-nanoparticle superlattices. Chapter 3 describes silver nanoparticle superlattices (rather than gold) and identifies their promise as plasmonic metamaterials. In chapter 4, the concept of plasmonic metallurgy is introduced, whereby it is demonstrated that concepts from materials science and metallurgy can be applied to the optical properties of mixed metallic plasmonic materials, unveiling rich and tunable optical properties such as color and asymmetric reflectivity. Chapter 5 presents a comprehensive theoretical exploration of anisotropy (non-spherical) in nanoparticle superlattice architectures. The role of anisotropy is discussed both on the nanoscale, where several desirable metamaterial properties can be tuned from the ultraviolet to near-infrared, and on the mesoscale, where the size and shape of a superlattice is demonstrated to have a pronounced effect on the observed far-field optical properties. Chapter 6 builds upon those theoretical data presented in chapter 5, including the experimental realization of size and shape dependent properties in DNA-programmed superlattices. Specifically, nanoparticle spacing is explored as a parameter that

  13. pi-phase magnetism in ferromagnetic-superconductor superlattices

    CERN Document Server

    Khusainov, M G; Proshin, Y N

    2001-01-01

    The Larkin-Ovchinnikov-Fylde-Ferrel new 0 pi- and pi pi-states are forecasted for the ferromagnetic metal/superconductor superlattices with antiferromagnetic magnetization orientation in the neighbouring layers. The above-mentioned states are characterized under certain conditions by higher critical temperature T sub c as compared to the earlier known LOFF 00- and pi 0-states with the FM-layers ferromagnetic ordering. It is shown that the nonmonotonous behavior of the T sub c of the FM/S superlattices by the thickness of the S-layers lower than the d sub s suppi value is connected with the cascades of the 0 pi-pi pi-0 pi phase transitions. The character of the T sub c oscillations by the d sub s > d sub s suppi is related to the 00-pi 0-00 transitions. The logical elements of the new type, combining the advantages of the superconducting and magnetic information recording in one sample are proposed on the basis of the FM/S superlattices

  14. Induced magnetism at the interfaces of a Fe/V superlattice investigated by resonant magnetic x-ray scattering

    Energy Technology Data Exchange (ETDEWEB)

    Magnuson, Martin, E-mail: Martin.Magnuson@ifm.liu.se

    2017-01-15

    The induced magnetic moments in the V 3d electronic states of interface atomic layers in a Fe(6ML)/V(7ML) superlattice was investigated by x-ray resonant magnetic scattering. The first V atomic layer next to Fe was found to be strongly antiferromagnetically polarized relatively to Fe and the magnetic moments of the next few atomic layers in the interior V region decay exponentially with increasing distance from the interface, while the magnetic moments of the Fe atomic layers largely remain bulk-like. The induced V moments decay more rapidly as observed by x-ray magnetic scattering than in standard x-ray magnetic circular dichroism. The theoretical description of the induced magnetic atomic layer profile in V was found to strongly rely on the interface roughness within the superlattice period. These results provide new insight into interface magnetism by taking advantage of the enhanced depth sensitivity to the magnetic profile over a certain resonant energy bandwidth in the vicinity of the Bragg angles. - Highlights: • Magnetic moments of buried layers are probed by XRMS in a Fe/V superlattice. • The induced V magnetic moments in XRMS are more rapidly decaying than previously observed by XMCD. • The magnetic depth profile sensitivity is enhanced at an energy bandwidth in the vicinity of the Bragg angles.

  15. Fabrication of C60/amorphous carbon superlattice structures

    International Nuclear Information System (INIS)

    Kojima, Nobuaki; Ohshita, Yoshio; Yamaguchi, Masafumi

    2001-01-01

    The nitrogen doping effects in C 60 films by RF plasma source was investigated, and it was found that the nitrogen ion bombardment broke up C 60 molecules and changed them into amorphous carbon. Based on these results, formation of C 60 /amorphous carbon superlattice structure was proposed. The periodic structure of the resulted films was confirmed by XRD measurements, as the preliminary results of fabrication of the superlattice structure

  16. Dependence of Fe/Cr superlattice magnetoresistance on orientation of external magnetic field

    International Nuclear Information System (INIS)

    Ustinov, V.V.; Romashev, L.N.; Minin, V.I.; Semerikov, A.V.; Del', A.R.

    1995-01-01

    The paper presents the results of investigations into giant magnetoresistance of [Fe/Cr] 30 /MgO superlattices obtained using molecular-beam epitaxy under various orientations of magnetic field relatively to the layers of superlattice and to the direction of current flow. Theory of orientation dependence of superlattice magnetoresistance enabling to describe satisfactorily behaviour of magnetoresistance at arbitrary direction of magnetic field on the ground of results of magnetoresistance measurements in magnetic field parallel and perpendicular to plane of layers, is elaborated. It is pointed out that it is possible to obtain field dependence of superlattice magnetization on the ground of measurement results. 9 refs., 6 figs

  17. Epitaxial superlattices with titanium nitride as a plasmonic component for optical hyperbolic metamaterials

    DEFF Research Database (Denmark)

    Naik, Gururaj V.; Saha, Bivas; Liu, Jing

    2014-01-01

    , we address these issues by realizing an epitaxial superlattice as an HMM. The superlattice consists of ultrasmooth layers as thin as 5 nm and exhibits sharp interfaces which are essential for high-quality HMM devices. Our study reveals that such a TiN-based superlattice HMM provides a higher PDOS...

  18. Structural and magnetic properties of holmium-scandium alloys and superlattices

    DEFF Research Database (Denmark)

    Bryn-Jacobsen, C.; Cowley, R.A.; McMorrow, D.F.

    1997-01-01

    The properties of Ho-Sc alloys and superlattices grown by molecular-beam epitaxy have been investigated using x-ray and neutron-diffraction techniques. Structural studies reveal that the alloy samples have different a lattice parameters for the Sc-seed layer and the Ho:Sc alloy grown on top...... of the seed layer; while the superlattices have different a lattice parameters for the Sc seed, and for both the Ho and Sc in the superlattice layers. The structural characteristics are related to the large lattice mismatches (of the order 7%) between the constituent elements. The magnetic moments...

  19. Exploring graphene superlattices: Magneto-optical properties

    Science.gov (United States)

    Duque, C. A.; Hernández-Bertrán, M. A.; Morales, A. L.; de Dios-Leyva, M.

    2017-02-01

    We present a detailed study of magnetic subbands, wave functions, and transition strengths for graphene superlattices (SLs) subject to a perpendicular magnetic field. It is shown that, for a weak magnetic field, the flat subbands of a SL exhibiting extra Dirac points are grouped into subsets, each of which consists of a singlet subband and a nearly degenerate doublet subband, and one nearly degenerate triplet subband. It was found that the wave functions corresponding to a singlet or to a doublet are always located around the image in real space of the central or extra Dirac points in k-space. The latter properties were explained by assuming that the electron motion is quasi-classical. Our study revealed that, for an intermediate field, the general characteristics of the wave functions are very similar to those of the pristine graphene, while for weak field, their behavior is drastically different. The latter is characterized by rapid oscillations which were understood using the solutions provided by the formalism of Luttinger-Kohn. The study on transition strengths allows us to obtain, for SLs with extra Dirac points in a weak magnetic field and different polarizations, the conditions under which transitions between multiplets are approximately allowed. It was shown that these conditions correspond to an unusual selection rule that is broken when the magnetic field intensity increases from weak to an intermediate value.

  20. Enhancement of breakdown voltage for fully-vertical GaN-on-Si p-n diode by using strained layer superlattice as drift layer

    Science.gov (United States)

    Mase, Suguru; Hamada, Takeaki; Freedsman, Joseph J.; Egawa, Takashi

    2018-06-01

    We have demonstrated a vertical GaN-on-Si p-n diode with breakdown voltage (BV) as high as 839 V by using a low Si-doped strained layer superlattice (SLS). The p-n vertical diode fabricated by using the n‑-SLS layer as a part of the drift layer showed a remarkable enhancement in BV, when compared with the conventional n‑-GaN drift layer of similar thickness. The vertical GaN-on-Si p-n diodes with 2.3 μm-thick n‑-GaN drift layer and 3.0 μm-thick n‑-SLS layer exhibited a differential on-resistance of 4.0 Ω · cm2 and a BV of 839 V.

  1. Propagation of Nd magnetic phases in Nd/Sm(001) superlattices

    International Nuclear Information System (INIS)

    Soriano, S; Dufour, C; Dumesnil, K; Stunault, A

    2006-01-01

    The propagation of Nd long range magnetic order in the hexagonal and cubic sublattices has been investigated in double hexagonal compact Nd/Sm(001) superlattices by resonant x-ray magnetic scattering at the Nd L 2 absorption edge. For a superlattice with 3.7 nm thick Sm layers, the magnetic structure of the hexagonal sublattice propagates coherently through several bilayers, whereas the order in the cubic sublattice remains confined to single Nd blocks. For a superlattice with 1.4 nm thick Sm layers, the magnetic structures of both sublattices appear to propagate coherently through the superlattice. This is the first observation (i) of the long range coherent propagation of Nd order on the cubic sites between Nd blocks and (ii) of a different thickness dependence of the propagation of the Nd magnetic phases associated with the hexagonal and cubic sublattices. The propagation of the Nd magnetic order through Sm is interpreted in terms of generalized susceptibility of the Nd conduction electrons

  2. Ordered quantum-ring chains grown on a quantum-dot superlattice template

    International Nuclear Information System (INIS)

    Wu Jiang; Wang, Zhiming M.; Holmes, Kyland; Marega, Euclydes; Mazur, Yuriy I.; Salamo, Gregory J.

    2012-01-01

    One-dimensional ordered quantum-ring chains are fabricated on a quantum-dot superlattice template by molecular beam epitaxy. The quantum-dot superlattice template is prepared by stacking multiple quantum-dot layers and quantum-ring chains are formed by partially capping quantum dots. Partially capping InAs quantum dots with a thin layer of GaAs introduces a morphological change from quantum dots to quantum rings. The lateral ordering is introduced by engineering the strain field of a multi-layer InGaAs quantum-dot superlattice.

  3. Quasi free-standing silicene in a superlattice with hexagonal boron nitride

    KAUST Repository

    Kaloni, T. P.; Tahir, M.; Schwingenschlö gl, Udo

    2013-01-01

    We study a superlattice of silicene and hexagonal boron nitride by first principles calculations and demonstrate that the interaction between the layers of the superlattice is very small. As a consequence, quasi free-standing silicene is realized

  4. Phonon-induced optical superlattice.

    Science.gov (United States)

    de Lima, M M; Hey, R; Santos, P V; Cantarero, A

    2005-04-01

    We demonstrate the formation of a dynamic optical superlattice through the modulation of a semiconductor microcavity by stimulated acoustic phonons. The high coherent phonon population produces a folded optical dispersion relation with well-defined energy gaps and renormalized energy levels, which are accessed using reflection and diffraction experiments.

  5. Magnetic rare earth superlattices

    DEFF Research Database (Denmark)

    Majkrzak, C.F.; Kwo, J.; Hong, M.

    1991-01-01

    Advances in molecular beam epitaxy deposition techniques have recently made it possible to grow, an atomic plane at a time, single crystalline superlattices composed of alternating layers of a magnetic rare earth, such as Gd, Dy, Ho, or Er, and metallic Y, which has an identical chemical structure...

  6. Ultra-low Thermal Conductivity in Si/Ge Hierarchical Superlattice Nanowire.

    Science.gov (United States)

    Mu, Xin; Wang, Lili; Yang, Xueming; Zhang, Pu; To, Albert C; Luo, Tengfei

    2015-11-16

    Due to interfacial phonon scattering and nanoscale size effect, silicon/germanium (Si/Ge) superlattice nanowire (SNW) can have very low thermal conductivity, which is very attractive for thermoelectrics. In this paper, we demonstrate using molecular dynamics simulations that the already low thermal conductivity of Si/Ge SNW can be further reduced by introducing hierarchical structure to form Si/Ge hierarchical superlattice nanowire (H-SNW). The structural hierarchy introduces defects to disrupt the periodicity of regular SNW and scatters coherent phonons, which are the key contributors to thermal transport in regular SNW. Our simulation results show that periodically arranged defects in Si/Ge H-SNW lead to a ~38% reduction of the already low thermal conductivity of regular Si/Ge SNW. By randomizing the arrangement of defects and imposing additional surface complexities to enhance phonon scattering, further reduction in thermal conductivity can be achieved. Compared to pure Si nanowire, the thermal conductivity reduction of Si/Ge H-SNW can be as large as ~95%. It is concluded that the hierarchical structuring is an effective way of reducing thermal conductivity significantly in SNW, which can be a promising path for improving the efficiency of Si/Ge-based SNW thermoelectrics.

  7. Superlattice photonic crystal as broadband solar absorber for high temperature operation.

    Science.gov (United States)

    Rinnerbauer, Veronika; Shen, Yichen; Joannopoulos, John D; Soljačić, Marin; Schäffler, Friedrich; Celanovic, Ivan

    2014-12-15

    A high performance solar absorber using a 2D tantalum superlattice photonic crystal (PhC) is proposed and its design is optimized for high-temperature energy conversion. In contrast to the simple lattice PhC, which is limited by diffraction in the short wavelength range, the superlattice PhC achieves solar absorption over broadband spectral range due to the contribution from two superposed lattices with different cavity radii. The superlattice PhC geometry is tailored to achieve maximum thermal transfer efficiency for a low concentration system of 250 suns at 1500 K reaching 85.0% solar absorptivity. In the high concentration case of 1000 suns, the superlattice PhC absorber achieves a solar absorptivity of 96.2% and a thermal transfer efficiency of 82.9% at 1500 K, amounting to an improvement of 10% and 5%, respectively, versus the simple square lattice PhC absorber. In addition, the performance of the superlattice PhC absorber is studied in a solar thermophotovoltaic system which is optimized to minimize absorber re-emission by reducing the absorber-to-emitter area ratio and using a highly reflective silver aperture.

  8. X-ray diffraction of multilayers and superlattices

    International Nuclear Information System (INIS)

    Bartels, W.J.; Hornstra, J.; Lobeek, D.J.W.

    1986-01-01

    Recursion formulae for calculating the reflected amplitude ratio of multilayers and superlattices have been derived from the Takagi-Taupin differential equations, which describe the dynamical diffraction of X-rays in deformed crystals. Calculated rocking curves of complicated layered structures, such as non-ideal superlattices on perfect crystals, are shown to be in good agreement with observed diffraction profiles. The kinematical theory can save computing time only in the case of an ideal superlattice, for which a geometric series can be used, but the reflections must be below 10% so that multiple reflections can be neglected. For a perfect crystal of arbitrary thickness the absorption at the center of the dynamical reflection is found to be proportional to the square root of the reflectivity. Sputter-deposited periodic multilayers of tungsten and carbon can be considered as an artificial crystal, for which dynamical X-ray diffraction calculations give results very similar to those of a macroscopic optical description in terms of the complex index of refraction and Fresnel reflection coefficients. (orig.)

  9. Magnetic structures of holmium-lutetium alloys and superlattices

    DEFF Research Database (Denmark)

    Swaddling, P.P.; Cowley, R.A.; Ward, R.C.C.

    1996-01-01

    Alloys and superlattices of Ho and Lu have been grown using molecular beam epitaxy and their magnetic structures determined using neutron-scattering techniques. The 4f moments in the alloys form a helix at all compositions with the moments aligned in the basal plane perpendicular to the wave vector...... of the helix remaining coherent through the nonmagnetic Lu blocks. The neutron scattering from the superlattices is consistent with a model in which there are different phase advances of the helix turn angle through the Ho and Lu blocks, but with a localized moment on the Ho sites only. A comparison...... of Ho and Lu. At low temperatures, for superlattices with fewer than approximately twenty atomic planes of Ho, the Ho moments within a block undergo a phase transition from helical to ferromagnetic order, with the coupling between successive blocks dependent on the thickness of the Lu spacer....

  10. Second harmonic generation in generalized Thue-Morse ferroelectric superlattices

    International Nuclear Information System (INIS)

    Wang Longxiang; Yang Xiangbo; Chen Tongsheng

    2009-01-01

    In this paper the second harmonic generation (SHG) in generalized Thue-Morse (GTM(m, n)) ferroelectric superlattices is studied. Under the small-signal approximation, the SHG spectra in both real and reciprocal spaces are investigated. It is found that: (1) only when the structure parameters l, l A , and l B are all chosen to be proper, can SHG in GTM(m, n) ferroelectric superlattices be generated; (2) for Family A of generalized Thue-Morse, GTM(m, 1) ferroelectric systems, with the increase of parameter m, the intense peaks of SHG concentrate on the long wavelength 1.4-1.5μm (the fundamental beam (FB) wavelength is within 0.8-1.5μm), but for Family B of generalized Thue-Morse, GTM(1, n) ferroelectric superlattices, with the increase of parameter n, the intense peaks of SHG concentrate on the middle wavelength 1.1-1.2μm; and (3) for GTM(m, 1) ferroelectric superlattices, the bigger the m, the stronger the relative integral intensity (RII) of SHG would be, but for GTM(1, n) ferroelectric systems, the bigger the n, the weaker the RII of SHG would be.

  11. COMPORTAMENTO A CORROSIONE E TRIBOCORROSIONE DI RIVESTIMENTI CERMET E CERMET/ SUPERLATTICE

    OpenAIRE

    Monticelli, C.; Zucchi, F.

    2009-01-01

    È stato studiato il comportamento a corrosione e tribocorrosione di riporti cermet e cermet/superlattice,applicati su campioni di acciaio. I riporti cermet consistono in riporti termici HVOF a spessore,di tipo WC-12Co o Cr3C2-37WC-18Me. I doppi riporti cermet/superlattice sono ottenuti sovrapponendoai depositi cermet citati un superlattice a base di nitruri, in cui si alternano strati di CrN e di NbN. Unasoluzione al 3.5 % di NaCl costituisce l’ambiente aggressivo. Le condizioni di tribocorro...

  12. Transmission of electrons with flat passbands in finite superlattices

    International Nuclear Information System (INIS)

    Barajas-Aguilar, A H; Rodríguez-Magdaleno, K A; Martínez-Orozco, J C; Enciso-Muñoz, A; Contreras-Solorio, D A

    2013-01-01

    Using the transfer matrix method and the Ben Daniel-Duke equation for variable mass electrons propagation, we calculate the transmittance for symmetric finite superlattices where the width and the height of the potential barriers follow a linear dependence. The width and height of the barriers decreases from the center to the ends of the superlattice. The transmittance presents intervals of stopbands and quite flat passbands.

  13. GeTe sequences in superlattice phase change memories and their electrical characteristics

    Energy Technology Data Exchange (ETDEWEB)

    Ohyanagi, T., E-mail: ohyanagi@leap.or.jp; Kitamura, M.; Takaura, N. [Low-Power Electronics Association and Projects (LEAP), Onogawa 16-1, Tsukuba, Ibaraki 305-8569 (Japan); Araidai, M. [Department of Computational Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603 (Japan); Kato, S. [Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571 (Japan); Shiraishi, K. [Department of Computational Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603 (Japan); Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571 (Japan)

    2014-06-23

    We studied GeTe structures in superlattice phase change memories (superlattice PCMs) with a [GeTe/Sb{sub 2}Te{sub 3}] stacked structure by X-ray diffraction (XRD) analysis. We examined the electrical characteristics of superlattice PCMs with films deposited at different temperatures. It was found that XRD spectra differed between the films deposited at 200 °C and 240 °C; the differences corresponded to the differences in the GeTe sequences in the films. We applied first-principles calculations to calculate the total energy of three different GeTe sequences. The results showed the Ge-Te-Ge-Te sequence had the lowest total energy of the three and it was found that with this sequence the superlattice PCMs did not run.

  14. Tunable porous nanoallotropes prepared by post-assembly etching of binary nanoparticle superlattices

    Science.gov (United States)

    Udayabhaskararao, Thumu; Altantzis, Thomas; Houben, Lothar; Coronado-Puchau, Marc; Langer, Judith; Popovitz-Biro, Ronit; Liz-Marzán, Luis M.; Vuković, Lela; Král, Petr; Bals, Sara; Klajn, Rafal

    2017-10-01

    Self-assembly of inorganic nanoparticles has been used to prepare hundreds of different colloidal crystals, but almost invariably with the restriction that the particles must be densely packed. Here, we show that non-close-packed nanoparticle arrays can be fabricated through the selective removal of one of two components comprising binary nanoparticle superlattices. First, a variety of binary nanoparticle superlattices were prepared at the liquid-air interface, including several arrangements that were previously unknown. Molecular dynamics simulations revealed the particular role of the liquid in templating the formation of superlattices not achievable through self-assembly in bulk solution. Second, upon stabilization, all of these binary superlattices could be transformed into distinct “nanoallotropes”—nanoporous materials having the same chemical composition but differing in their nanoscale architectures.

  15. Superlattices of platinum and palladium nanoparticles

    Energy Technology Data Exchange (ETDEWEB)

    MARTIN,JAMES E.; WILCOXON,JESS P.; ODINEK,JUDY G.; PROVENCIO,PAULA P.

    2000-04-06

    The authors have used a nonionic inverse micelle synthesis technique to form nanoclusters of platinum and palladium. These nanoclusters can be rendered hydrophobic or hydrophilic by the appropriate choice of capping ligand. Unlike Au nanoclusters, Pt nanoclusters show great stability with thiol ligands in aqueous media. Alkane thiols, with alkane chains ranging from C{sub 6} to C{sub 18} were used as hydrophobic ligands, and with some of these they were able to form 2-D and/or 3-D superlattices of Pt nanoclusters as small as 2.7 nm in diameter. Image processing techniques were developed to reliably extract from transmission electron micrographs (TEMs) the particle size distribution, and information about the superlattice domains and their boundaries. The latter permits one to compute the intradomain vector pair correlation function of the particle centers, from which they can accurately determine the lattice spacing and the coherent domain size. From these data the gap between the particles in the coherent domains can be determined as a function of the thiol chain length. It is found that as the thiol chain length increases, the gaps between particles within superlattice domains increases, but more slowly than one might expect, possibly indicating thiol chain interdigitation.

  16. Transport in semiconductor nanowire superlattices described by coupled quantum mechanical and kinetic models.

    Science.gov (United States)

    Alvaro, M; Bonilla, L L; Carretero, M; Melnik, R V N; Prabhakar, S

    2013-08-21

    In this paper we develop a kinetic model for the analysis of semiconductor superlattices, accounting for quantum effects. The model consists of a Boltzmann-Poisson type system of equations with simplified Bhatnagar-Gross-Krook collisions, obtained from the general time-dependent Schrödinger-Poisson model using Wigner functions. This system for superlattice transport is supplemented by the quantum mechanical part of the model based on the Ben-Daniel-Duke form of the Schrödinger equation for a cylindrical superlattice of finite radius. The resulting energy spectrum is used to characterize the Fermi-Dirac distribution that appears in the Bhatnagar-Gross-Krook collision, thereby coupling the quantum mechanical and kinetic parts of the model. The kinetic model uses the dispersion relation obtained by the generalized Kronig-Penney method, and allows us to estimate radii of quantum wire superlattices that have the same miniband widths as in experiments. It also allows us to determine more accurately the time-dependent characteristics of superlattices, in particular their current density. Results, for several experimentally grown superlattices, are discussed in the context of self-sustained coherent oscillations of the current density which are important in an increasing range of current and potential applications.

  17. Terahertz emission of Bloch oscillators excited by electromagnetic field in lateral semiconductor superlattices

    International Nuclear Information System (INIS)

    Dodin, E.P.; Zharov, A.A.

    2003-01-01

    The effect of the strong high-frequency electromagnetic field on the lateral semiconductor superlattice is considered on the basis of the quasi-classical theory on the electron transport in the self-consistent wave arrangement. It is theoretically identified, that the lateral superlattice in the strong feed-up wave field may emit the terahertz radiation wave trains, which are associated with the periodical excitation of the Bloch oscillations in the superlattice. The conditions, required for the Bloch oscillators radiation observation, are determined. The spectral composition of the radiation, passing through the superlattice, and energy efficiency of multiplying the frequency, related to the Bloch oscillator excitation, are calculated [ru

  18. Possible THz gain in superlattices at a stable operation point

    DEFF Research Database (Denmark)

    Wacker, Andreas; Allen, S. J.; Scott, J. S.

    1997-01-01

    We demonstrate that semiconductor superlattices may provide gain at THz frequencies at an operation point which is stable against fluctuations at lower frequency. While an explicit experimental demonstration for the sample considered could not be achieved, the underlying principle of quantum resp...... response is quite general and may prove successful for differently designed superlattices....

  19. The cross-plane thermoelectric properties of p-Ge/Si0.5Ge0.5 superlattices

    International Nuclear Information System (INIS)

    Ferre Llin, L.; Samarelli, A.; Weaver, J. M. R.; Dobson, P. S.; Paul, D. J.; Cecchi, S.; Chrastina, D.; Isella, G.; Etzelstorfer, T.; Stangl, J.; Müller Gubler, E.

    2013-01-01

    The electrical conductivity, Seebeck coefficients, and thermal conductivities of a range of p-type Ge/Si 0.5 Ge 0.5 superlattices designed for thermoelectric generation and grown by low energy plasma enhanced chemical vapor deposition have been measured using a range of microfabricated test structures. For samples with barriers around 0.5 nm in thickness, the measured Seebeck coefficients were comparable to bulk p-SiGe at similar doping levels suggesting the holes see the material as a random bulk alloy rather than a superlattice. The Seebeck coefficients for Ge quantum wells of 2.85 ± 0.85 nm increased up to 533 ± 25 μV/K as the doping was reduced. The thermal conductivities are between 4.5 to 6.0 Wm −1 K −1 which are lower than comparably doped bulk Si 0.3 Ge 0.7 but higher than undoped Si/Ge superlattices. The highest measured figure of merit ZT was 0.080 ± 0.011 obtained for the widest quantum well studied. Analysis suggests that interface roughness is presently limiting the performance and a reduction in the strain between the quantum wells and barriers has the potential to improve the thermoelectric performance

  20. Terahertz radiation induced chaotic electron transport in semiconductor superlattices with a tilted magnetic field

    Energy Technology Data Exchange (ETDEWEB)

    Wang, C., E-mail: cwang@mail.sim.ac.cn; Wang, F.; Cao, J. C., E-mail: jccao@mail.sim.ac.cn [Key Laboratory of Terahertz Solid-State Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050 (China)

    2014-09-01

    Chaotic electron transport in semiconductor superlattice induced by terahertz electric field that is superimposed on a dc electric field along the superlattice axis are studied using the semiclassical motion equations including the effect of dissipation. A magnetic field that is tilted relative to the superlattice axis is also applied to the system. Numerical simulation shows that electrons in superlattice miniband exhibit complicate nonlinear oscillating modes with the influence of terahertz radiation. Transitions between frequency-locking and chaos via pattern forming bifurcations are observed with the varying of terahertz amplitude. It is found that the chaotic regions gradually contract as the dissipation increases. We attribute the appearance of complicate nonlinear oscillation in superlattice to the interaction between terahertz radiation and internal cooperative oscillating mode relative to Bloch oscillation and cyclotron oscillation.

  1. Terahertz radiation induced chaotic electron transport in semiconductor superlattices with a tilted magnetic field

    International Nuclear Information System (INIS)

    Wang, C.; Wang, F.; Cao, J. C.

    2014-01-01

    Chaotic electron transport in semiconductor superlattice induced by terahertz electric field that is superimposed on a dc electric field along the superlattice axis are studied using the semiclassical motion equations including the effect of dissipation. A magnetic field that is tilted relative to the superlattice axis is also applied to the system. Numerical simulation shows that electrons in superlattice miniband exhibit complicate nonlinear oscillating modes with the influence of terahertz radiation. Transitions between frequency-locking and chaos via pattern forming bifurcations are observed with the varying of terahertz amplitude. It is found that the chaotic regions gradually contract as the dissipation increases. We attribute the appearance of complicate nonlinear oscillation in superlattice to the interaction between terahertz radiation and internal cooperative oscillating mode relative to Bloch oscillation and cyclotron oscillation

  2. Terahertz radiation induced chaotic electron transport in semiconductor superlattices with a tilted magnetic field.

    Science.gov (United States)

    Wang, C; Wang, F; Cao, J C

    2014-09-01

    Chaotic electron transport in semiconductor superlattice induced by terahertz electric field that is superimposed on a dc electric field along the superlattice axis are studied using the semiclassical motion equations including the effect of dissipation. A magnetic field that is tilted relative to the superlattice axis is also applied to the system. Numerical simulation shows that electrons in superlattice miniband exhibit complicate nonlinear oscillating modes with the influence of terahertz radiation. Transitions between frequency-locking and chaos via pattern forming bifurcations are observed with the varying of terahertz amplitude. It is found that the chaotic regions gradually contract as the dissipation increases. We attribute the appearance of complicate nonlinear oscillation in superlattice to the interaction between terahertz radiation and internal cooperative oscillating mode relative to Bloch oscillation and cyclotron oscillation.

  3. Estimation of fatigue strength enhancement for carburized and shot-peened gears

    Science.gov (United States)

    Inoue, Katsumi; Kato, Masana

    1994-05-01

    An experimental formula has been proposed to estimate the bending fatigue strength of carburized gears from the hardness and the residual stress. The derivation of the formula is briefly reviewed, and the effectiveness of the formula is demonstrated in this article. The comparison with many test results for carburized and shot-peened gears verifies that the formula is effective for the approximate estimation of the fatigue strength. The formula quantitatively shows a way of enhancing fatigue strength, i.e., the increase of hardness and residual stress at the fillet. The strength is enhanced about 300 MPa by an appropriate shot peening, and it can be improved still more by the surface removal by electropolishing.

  4. ZnO: Hydroquinone superlattice structures fabricated by atomic/molecular layer deposition

    International Nuclear Information System (INIS)

    Tynell, Tommi; Karppinen, Maarit

    2014-01-01

    Here we employ atomic layer deposition in combination with molecular layer deposition to deposit crystalline thin films of ZnO interspersed with single layers of hydroquinone in an effort to create hybrid inorganic–organic superlattice structures. The ratio of the ZnO and hydroquinone deposition cycles is varied between 199:1 and 1:1, and the structure of the resultant thin films is verified with X-ray diffraction and reflectivity techniques. Clear evidence of the formation of a superlattice-type structure is observed in the X-ray reflectivity patterns and the presence of organic bonds in the films corresponding to the structure of hydroquinone is confirmed with Fourier transform infrared spectroscopy measurements. We anticipate that hybrid superlattice structures such as the ones described in this work have the potential to be of great importance for future applications where the precise control of different inorganic and organic layers in hybrid superlattice materials is required. - Highlights: • Inorganic–organic superlattices can be made by atomic/molecular layer deposition. • This is demonstrated here for ZnO and hydroquinone (HQ). • The ratio of the ZnO and HQ layers is varied between 199:1 and 14:1. • The resultant thin films are crystalline

  5. ZnO: Hydroquinone superlattice structures fabricated by atomic/molecular layer deposition

    Energy Technology Data Exchange (ETDEWEB)

    Tynell, Tommi; Karppinen, Maarit, E-mail: maarit.karppinen@aalto.fi

    2014-01-31

    Here we employ atomic layer deposition in combination with molecular layer deposition to deposit crystalline thin films of ZnO interspersed with single layers of hydroquinone in an effort to create hybrid inorganic–organic superlattice structures. The ratio of the ZnO and hydroquinone deposition cycles is varied between 199:1 and 1:1, and the structure of the resultant thin films is verified with X-ray diffraction and reflectivity techniques. Clear evidence of the formation of a superlattice-type structure is observed in the X-ray reflectivity patterns and the presence of organic bonds in the films corresponding to the structure of hydroquinone is confirmed with Fourier transform infrared spectroscopy measurements. We anticipate that hybrid superlattice structures such as the ones described in this work have the potential to be of great importance for future applications where the precise control of different inorganic and organic layers in hybrid superlattice materials is required. - Highlights: • Inorganic–organic superlattices can be made by atomic/molecular layer deposition. • This is demonstrated here for ZnO and hydroquinone (HQ). • The ratio of the ZnO and HQ layers is varied between 199:1 and 14:1. • The resultant thin films are crystalline.

  6. Observation of interface dependent spin polarized photocurrents in InAs/GaSb superlattice

    Energy Technology Data Exchange (ETDEWEB)

    Li, Yuan, E-mail: liyuan12@semi.ac.cn; Liu, Yu; Zhu, Laipan; Qin, Xudong; Wu, Qing; Huang, Wei; Chen, Yonghai, E-mail: yhchen@semi.ac.cn [Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, 100083 Beijing (China); Niu, Zhichuan; Xiang, Wei; Hao, Hongyue [The State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, 100083 Beijing (China)

    2015-05-11

    In this letter, we investigated the spin polarized photocurrents excited by mid-infrared radiation and near-infrared radiation, respectively, in InAs/GaSb type II superlattices with different kinds of interfaces. By periodically varying the polarization state of the radiation, we analyzed Rashba-type and Dresselhaus-type spin polarized photocurrents, which present different features depending on the interface types and excitation conditions. Under mid-infrared excitation, the ratio of Rashba-type and Dresselhaus-type spin polarized photocurrents of the superlattice with InSb-like interface is obviously larger than that of the superlattice with GaAs-like interface, the ratio of the superlattice with alternate interface is in the middle. Whereas under near-infrared excitation, the ratios of the three superlattices are nearly the same. Further researches reveal the synactic effects of interface dependent strain and asymmetric interface potential on the spin splitting. Besides, the polarized Raman spectroscopies of these structures were also analyzed.

  7. Observation of interface dependent spin polarized photocurrents in InAs/GaSb superlattice

    International Nuclear Information System (INIS)

    Li, Yuan; Liu, Yu; Zhu, Laipan; Qin, Xudong; Wu, Qing; Huang, Wei; Chen, Yonghai; Niu, Zhichuan; Xiang, Wei; Hao, Hongyue

    2015-01-01

    In this letter, we investigated the spin polarized photocurrents excited by mid-infrared radiation and near-infrared radiation, respectively, in InAs/GaSb type II superlattices with different kinds of interfaces. By periodically varying the polarization state of the radiation, we analyzed Rashba-type and Dresselhaus-type spin polarized photocurrents, which present different features depending on the interface types and excitation conditions. Under mid-infrared excitation, the ratio of Rashba-type and Dresselhaus-type spin polarized photocurrents of the superlattice with InSb-like interface is obviously larger than that of the superlattice with GaAs-like interface, the ratio of the superlattice with alternate interface is in the middle. Whereas under near-infrared excitation, the ratios of the three superlattices are nearly the same. Further researches reveal the synactic effects of interface dependent strain and asymmetric interface potential on the spin splitting. Besides, the polarized Raman spectroscopies of these structures were also analyzed

  8. Surface phonon polaritons in semi-infinite semiconductor superlattices

    International Nuclear Information System (INIS)

    Nkoma, J.S.

    1986-07-01

    Surface phonon polaritons in a semi-infinite semiconductor superlattice bounded by vacuum are studied. The modes associated with the polaritons are obtained and used to obtain the dispersion relation. Numerical results show that polariton bands exist between the TO and LO phonon frequencies, and are found to approach two surface mode frequencies in the limit of large tangential wave vector. Dependency of frequencies on the ratio of layer thicknesses is shown. Results are illustrated by a GaAs-GaP superlattice bounded by vacuum. (author)

  9. Superlattices: problems and new opportunities, nanosolids

    Directory of Open Access Journals (Sweden)

    Tsu Raphael

    2011-01-01

    Full Text Available Abstract Superlattices were introduced 40 years ago as man-made solids to enrich the class of materials for electronic and optoelectronic applications. The field metamorphosed to quantum wells and quantum dots, with ever decreasing dimensions dictated by the technological advancements in nanometer regime. In recent years, the field has gone beyond semiconductors to metals and organic solids. Superlattice is simply a way of forming a uniform continuum for whatever purpose at hand. There are problems with doping, defect-induced random switching, and I/O involving quantum dots. However, new opportunities in component-based nanostructures may lead the field of endeavor to new heights. The all important translational symmetry of solids is relaxed and local symmetry is needed in nanosolids.

  10. Development of high-capacity nickel-metal hydride batteries using superlattice hydrogen-absorbing alloys

    International Nuclear Information System (INIS)

    Yasuoka, Shigekazu; Magari, Yoshifumi; Murata, Tetsuyuki; Tanaka, Tadayoshi; Ishida, Jun; Nakamura, Hiroshi; Nohma, Toshiyuki; Kihara, Masaru; Baba, Yoshitaka; Teraoka, Hirohito

    2006-01-01

    New R-Mg-Ni (R: rare earths) superlattice alloys with higher-capacity and higher-durability than the conventional Mm-Ni alloys with CaCu 5 structure have been developed. The oxidation resistibility of the superlattice alloys has been improved by optimizing the alloy composition by such as substituting aluminum for nickel and optimizing the magnesium content in order to prolong the battery life. High-capacity nickel-metal hydride batteries for the retail market, the Ni-MH2500/900 series (AA size type 2500mAh, AAA size type 900mAh), have been developed and commercialized by using an improved superlattice alloy for negative electrode material. alized by using an improved superlattice alloy for negative electrode material. (author)

  11. Characterization by Raman scattering, x-ray diffraction, and transmission electron microscopy of (AlAs)m(InAs)m short period superlattices grown by migration enhanced epitaxy

    DEFF Research Database (Denmark)

    Bradshaw, J.; Song, X.J.; Shealy, J.R.

    1992-01-01

    We report growth of (InAs)1(AlAs)1 and (InAs)2(AlAs)2 strained layer superlattices by migration enhanced epitaxy. The samples were grown on InP (001) substrates and characterized by Raman spectroscopy, x-ray diffraction, and transmission electron microscopy. Satellite peaks in the x-ray data...... confirm the intended periodicity and indicate the presence of some disorder in the monolayer sample. The energies of the zone folded and quantum confined optic phonons are in reasonable agreement with calculations based on one-dimensional elastic continuum and linear chain models. Journal of Applied...

  12. Strain dependence of interfacial antiferromagnetic coupling in La0.7Sr0.3MnO3/SrRuO3 superlattices

    Science.gov (United States)

    Das, Sujit; Herklotz, Andreas; Pippel, Eckhard; Guo, Er-Jia; Rata, Diana; Dörr, Kathrin

    2015-03-01

    We have investigated the magnetic response of La0.7Sr0.3MnO3/SrRuO3 superlattices to biaxial in-plane strain applied in-situ. Superlattices grown on piezoelectric substrates of 0.72PbMg1/3Nb2/3O3-0.28PbTiO3(001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of μ0HAF = 1.8 T is found to change by μ0 ΔHAF / Δɛ ~ -520 mT %-1 under reversible biaxial strain (Δɛ) at 80 K in a [La0.7Sr0.3MnO3(22 Å)/SrRuO3(55 Å)]15 superlattice. This reveals a significant strain effect on interfacial coupling. The applied in-plane compression enhances the ferromagnetic order in the manganite layers which are under as-grown tensile strain. It is thus difficult to disentangle the contributions from strain-dependent antiferromagnetic Mn-O-Ru interface coupling and Mn-O-Mn ferromagnetic double exchange near the interface, since the enhanced magnetic order of Mn spins leads to a larger net coupling of SrRuO3 layers at the interface. We discuss our experimental findings taken into account both the strain-dependent orbital occupation in a single-ion picture and the enhanced Mn order at the interface. This work was supported by the DFG within the Collaborative Research Center SFB 762 ``Functionality of Oxide Interfaces.''

  13. Thermoelectric efficiency of IV-VI multiple quantumwell and superlattice systems

    International Nuclear Information System (INIS)

    Beyer, H.

    2001-09-01

    In this thesis the thermoelectric properties of PbTe-based multiple quantum well (MQW) and superlattice (SL) structures grown by molecular beam epitaxy (MBE) are investigated. The two approaches for an enhancement of the dimensionless thermoelectric figure of merit ZT = σS 2 T/λ (σ: electric conductivity, S: Seebeck coefficient, λ: thermal conductivity, T: temperature) in low dimensional structures are discussed: an enhancement of the two dimensional (2D) power factor σS 2 2D due to carrier confinement or a reduction of the thermal conductivity due to a modified phonon dispersion and stronger phonon scattering in layered structures. The novel PbTe/Pb 1-x Sr x Te-MQW system is investigated for a rise of the 2D power factor σS 2 2D . A detailed analysis of optical and electronic parameters based on infrared transmission spectra is performed. Only a small rise of the 2D power factor σS 2 2D for the MQW's compared to homogeneous epitaxial PbTe-layers is found. This smaller σS 2 2D -rise compared to first predictions for ideal MQW's is related to the finite barrier height and effects like lifting of the valley degeneracy or enhanced carrier scattering. However, a strong reduction of the thermal conductivity is found in MQW structures resulting in 2D ZT values between 0.5 and 1.0 at T = 300 K and up to 1.6 at T = 550 K. For the use in technical applications the three dimensional (3D) figure of merit of the whole structure ZT 3D must be raised. Different novel PbTe-based superlattice structures are investigated with respect to a 3D ZT-increase due to thermal conductivity reduction. ZT 3D -values of 0.33 for p-type PbTe-doping superlattices and of 0.45 at T = 300 K for PbTe/PbSe 20 Te 80 -SL's are determined (T = 300 K; n-PbTe: ZT max = 0.36, p-PbTe: ZT max = 0.30). Maximum ZT-values up to 1.25 for PbTe/PbSe 20 Te 80 -SL's and 1.20 for PbTe-doping SL's are estimated for temperatures between 450 K and 550 K. These values show the potential of these structures for

  14. Tunneling in quantum superlattices with variable lacunarity

    Energy Technology Data Exchange (ETDEWEB)

    Villatoro, Francisco R. [Departamento de Lenguajes y Ciencias de la Computacion, Universidad de Malaga, E-29071 Malaga (Spain); Monsoriu, Juan A. [Departamento de Fisica Aplicada, Universidad Politecnica de Valencia, E-46022 Valencia (Spain)], E-mail: jmonsori@fis.upv.es

    2008-05-19

    Fractal superlattices are composite, aperiodic structures comprised of alternating layers of two semiconductors following the rules of a fractal set. The scattering properties of polyadic Cantor fractal superlattices with variable lacunarity are determined. The reflection coefficient as a function of the particle energy and the lacunarity parameter present tunneling curves, which may be classified as vertical, arc, and striation nulls. Approximate analytical formulae for such curves are derived using the transfer matrix method. Comparison with numerical results shows good accuracy. The new results may be useful in the development of band-pass energy filters for electrons, semiconductor solar cells, and solid-state radiation sources up to THz frequencies.

  15. Interface properties of superlattices with artificially broken symmetry

    International Nuclear Information System (INIS)

    Lottermoser, Th.; Yamada, H.; Matsuno, J.; Arima, T.; Kawasaki, M.; Tokura, Y.

    2007-01-01

    We have used superlattices made of thin layers of transition metal oxides to design the so-called multiferroics, i.e. materials possessing simultaneously an electric polarization and a magnetic ordering. The polarization originates from the asymmetric stacking order accompanied by charge transfer effects, while the latter one also influences the magnetic properties of the interfaces. Due to the breaking of space and time-reversal symmetry by multiple ordering mechanism magnetic second harmonic generation is proven to be an ideal method to investigate the electric and magnetic properties of the superlattices

  16. Coherent, atomically thin transition-metal dichalcogenide superlattices with engineered strain

    Science.gov (United States)

    Xie, Saien; Tu, Lijie; Han, Yimo; Huang, Lujie; Kang, Kibum; Lao, Ka Un; Poddar, Preeti; Park, Chibeom; Muller, David A.; DiStasio, Robert A.; Park, Jiwoong

    2018-03-01

    Epitaxy forms the basis of modern electronics and optoelectronics. We report coherent atomically thin superlattices in which different transition metal dichalcogenide monolayers—despite large lattice mismatches—are repeated and laterally integrated without dislocations within the monolayer plane. Grown by an omnidirectional epitaxy, these superlattices display fully matched lattice constants across heterointerfaces while maintaining an isotropic lattice structure and triangular symmetry. This strong epitaxial strain is precisely engineered via the nanoscale supercell dimensions, thereby enabling broad tuning of the optical properties and producing photoluminescence peak shifts as large as 250 millielectron volts. We present theoretical models to explain this coherent growth and the energetic interplay governing the ripple formation in these strained monolayers. Such coherent superlattices provide building blocks with targeted functionalities at the atomically thin limit.

  17. Wave fronts, pulses and wave trains in photoexcited superlattices behaving as excitable or oscillatory media

    International Nuclear Information System (INIS)

    Arana, J I; Bonilla, L L; Grahn, H T

    2011-01-01

    Undoped and strongly photoexcited semiconductor superlattices with field-dependent recombination behave as excitable or oscillatory media with spatially discrete nonlinear convection and diffusion. Infinitely long, dc-current-biased superlattices behaving as excitable media exhibit wave fronts with increasing or decreasing profiles, whose velocities can be calculated by means of asymptotic methods. These superlattices can also support pulses of the electric field. Pulses moving downstream with the flux of electrons can be constructed from their component wave fronts, whereas pulses advancing upstream do so slowly and experience saltatory motion: they change slowly in long intervals of time separated by fast transitions during which the pulses jump to the previous superlattice period. Photoexcited superlattices can also behave as oscillatory media and exhibit wave trains. (paper)

  18. Control of phonon transport by the formation of the Al2O3 interlayer in Al2O3-ZnO superlattice thin films and their in-plane thermoelectric energy generator performance.

    Science.gov (United States)

    Park, No-Won; Ahn, Jay-Young; Park, Tae-Hyun; Lee, Jung-Hun; Lee, Won-Yong; Cho, Kwanghee; Yoon, Young-Gui; Choi, Chel-Jong; Park, Jin-Seong; Lee, Sang-Kwon

    2017-06-01

    Recently, significant progress has been made in increasing the figure-of-merit (ZT) of various nanostructured materials, including thin-film and quantum dot superlattice structures. Studies have focused on the size reduction and control of the surface or interface of nanostructured materials since these approaches enhance the thermopower and phonon scattering in quantum and superlattice structures. Currently, bismuth-tellurium-based semiconductor materials are widely employed for thermoelectric (TE) devices such as TE energy generators and coolers, in addition to other sensors, for use at temperatures under 400 K. However, new and promising TE materials with enhanced TE performance, including doped zinc oxide (ZnO) multilayer or superlattice thin films, are also required for designing solid-state TE power generating devices with the maximum output power density and for investigating the physics of in-plane TE generators. Herein, we report the growth of Al 2 O 3 /ZnO (AO/ZnO) superlattice thin films, which were prepared by atomic layer deposition (ALD), and the evaluation of their electrical and TE properties. All the in-plane TE properties, including the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ), of the AO/ZnO superlattice (with a 0.82 nm-thick AO layer) and AO/ZnO films (with a 0.13 nm-thick AO layer) were evaluated in the temperature range 40-300 K, and the measured S, σ, and κ were -62.4 and -17.5 μV K -1 , 113 and 847 (Ω cm) -1 , and 0.96 and 1.04 W m -1 K -1 , respectively, at 300 K. Consequently, the in-plane TE ZT factor of AO/ZnO superlattice films was found to be ∼0.014, which is approximately two times more than that of AO/ZnO films (ZT of ∼0.007) at 300 K. Furthermore, the electrical power generation efficiency of the TE energy generator consisting of four couples of n-AO/ZnO superlattice films and p-Bi 0.5 Sb 1.5 Te 3 (p-BST) thin-film legs on the substrate was demonstrated. Surprisingly, the output

  19. Molecular dynamics simulation of thermal conductivities of superlattice nanowires

    Institute of Scientific and Technical Information of China (English)

    YANG; Juekuan(杨决宽); CHEN; Yunfei(陈云飞); YAN; Jingping(颜景平)

    2003-01-01

    Nonequilibrium molecular dynamics simulations were carried out to investigate heat transfer in superlattice nanowires. Results show that for fixed period length superlattice nanowires, the ratio of the total interfacial thermal resistance to the total thermal resistance and the effective thermal conductivities are invariant with the changes in interface numbers. Increasing the period length leads to an increase in the average interfacial thermal resistance, which indicates that the interfacial thermal resistance depends not only on the materials that constitute the alternating segments of superlattice nanowires, but also on the lattice strain throughout the segments. The modification of the lattice structure due to the lattice mismatch should be taken into account in the acoustic mismatch model. Simulation results also demonstrated the size confinement effect on the thermal conductivities for low dimensional structures, i.e. the thermal conductivities and the interfacial thermal resistance increase as the nanowire cross-sectional area increases.

  20. Interface disorder and transport properties in HTC/CMR superlattices

    International Nuclear Information System (INIS)

    Haberkorn, N.; Guimpel, J.; Sirena, M.; Steren, L.B.; Campillo, G.; Saldarriaga, W.; Gomez, M.E.

    2004-01-01

    The physical properties of superlattices are affected by interface disorder, like roughness and interdiffusion. X-ray diffraction allows its measurement through modeling and structure refinement. The high-T c RBa 2 Cu 3 O 7 (RBCO) and colossal magnetoresistance La x A 1-x MnO 3 (LAMO) perovskites are interesting superlattice partners given their similar lattice parameters and because the combination of magnetic and superconducting properties is interesting for both basic and applied research. We have investigated the structural and transport properties of YBCO/La 2/3 Ca 1/3 MnO 3 and GdBCO/La 0.6 Sr 0.04 MnO 3 superlattices grown by sputtering on (1 0 0)MgO. We find a roughness of 1 RBCO unit cell and a 30% interdiffusion in the same length from the interfaces for all samples. The superconducting behavior is found strongly dependent on the LAMO layer thickness

  1. Rocksalt nitride metal/semiconductor superlattices: A new class of artificially structured materials

    Science.gov (United States)

    Saha, Bivas; Shakouri, Ali; Sands, Timothy D.

    2018-06-01

    Artificially structured materials in the form of superlattice heterostructures enable the search for exotic new physics and novel device functionalities, and serve as tools to push the fundamentals of scientific and engineering knowledge. Semiconductor heterostructures are the most celebrated and widely studied artificially structured materials, having led to the development of quantum well lasers, quantum cascade lasers, measurements of the fractional quantum Hall effect, and numerous other scientific concepts and practical device technologies. However, combining metals with semiconductors at the atomic scale to develop metal/semiconductor superlattices and heterostructures has remained a profoundly difficult scientific and engineering challenge. Though the potential applications of metal/semiconductor heterostructures could range from energy conversion to photonic computing to high-temperature electronics, materials challenges primarily had severely limited progress in this pursuit until very recently. In this article, we detail the progress that has taken place over the last decade to overcome the materials engineering challenges to grow high quality epitaxial, nominally single crystalline metal/semiconductor superlattices based on transition metal nitrides (TMN). The epitaxial rocksalt TiN/(Al,Sc)N metamaterials are the first pseudomorphic metal/semiconductor superlattices to the best of our knowledge, and their physical properties promise a new era in superlattice physics and device engineering.

  2. Feshbach shape resonance for high Tc pairing in superlattices of quantum stripes and quantum wells

    Directory of Open Access Journals (Sweden)

    A Bianconi

    2006-09-01

    Full Text Available   The Feshbach shape resonances in the interband pairing in superconducting superlattices of quantum wells or quantum stripes is shown to provide the mechanism for high Tc superconductivity. This mechanism provides the Tc amplification driven by the architecture of material: superlattices of quantum wells (intercalated graphite or diborides and superlattices of quantum stripes (doped high Tc cuprate perovskites where the chemical potential is tuned to a Van Hove-Lifshitz singularity (vHs in the electronic energy spectrum of the superlattice associated with the change of the Fermi surface dimensionality in one of the subbands.

  3. Electron dynamics in intentionally disordered semiconductor superlattices

    International Nuclear Information System (INIS)

    Diez, E.; Sanchez, A.; Dominguez-Adame, F.; Berman, G.P.

    1996-01-01

    We study the dynamical behavior of disordered quantum well-based semiconductor superlattices where the disorder is intentional and short-range correlated. We show that, whereas the transmission time of a particle grows exponentially with the number of wells in an usual disordered superlattice for any value of the incident particle energy, for specific values of the incident energy this time increases linearly when correlated disorder is included. As expected, those values of the energy coincide with a narrow subband of extended states predicted by the static calculations of Domacute inguez-Adame et al.[Phys. Rev. B 51, 14359 (1994)]; such states are seen in our dynamical results to exhibit a ballistic regime, very close to the WKB approximation of a perfect superlattice. Fourier transform of the output signal for an incident Gaussian wave packet reveals a dramatic filtering of the original signal, which makes us confident that devices based on this property may be designed and used for nanotechnological applications. This is more so in view of the possibility of controlling the output band using a dc-electric field, which we also discuss. In the conclusion we summarize our results and present an outlook for future developments arising from this work. copyright 1996 The American Physical Society

  4. Interface disorder and transport properties in HTC/CMR superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Haberkorn, N.; Guimpel, J.; Sirena, M.; Steren, L.B.; Campillo, G.; Saldarriaga, W.; Gomez, M.E

    2004-08-01

    The physical properties of superlattices are affected by interface disorder, like roughness and interdiffusion. X-ray diffraction allows its measurement through modeling and structure refinement. The high-T{sub c} RBa{sub 2}Cu{sub 3}O{sub 7} (RBCO) and colossal magnetoresistance La{sub x}A{sub 1-x}MnO{sub 3} (LAMO) perovskites are interesting superlattice partners given their similar lattice parameters and because the combination of magnetic and superconducting properties is interesting for both basic and applied research. We have investigated the structural and transport properties of YBCO/La{sub 2/3}Ca{sub 1/3}MnO{sub 3} and GdBCO/La{sub 0.6}Sr{sub 0.04}MnO{sub 3} superlattices grown by sputtering on (1 0 0)MgO. We find a roughness of 1 RBCO unit cell and a 30% interdiffusion in the same length from the interfaces for all samples. The superconducting behavior is found strongly dependent on the LAMO layer thickness.

  5. Emergent chirality in the electric polarization texture of titanate superlattices.

    Science.gov (United States)

    Shafer, Padraic; García-Fernández, Pablo; Aguado-Puente, Pablo; Damodaran, Anoop R; Yadav, Ajay K; Nelson, Christopher T; Hsu, Shang-Lin; Wojdeł, Jacek C; Íñiguez, Jorge; Martin, Lane W; Arenholz, Elke; Junquera, Javier; Ramesh, Ramamoorthy

    2018-01-30

    Chirality is a geometrical property by which an object is not superimposable onto its mirror image, thereby imparting a handedness. Chirality determines many important properties in nature-from the strength of the weak interactions according to the electroweak theory in particle physics to the binding of enzymes with naturally occurring amino acids or sugars, reactions that are fundamental for life. In condensed matter physics, the prediction of topologically protected magnetic skyrmions and related spin textures in chiral magnets has stimulated significant research. If the magnetic dipoles were replaced by their electrical counterparts, then electrically controllable chiral devices could be designed. Complex oxide BaTiO 3 /SrTiO 3 nanocomposites and PbTiO 3 /SrTiO 3 superlattices are perfect candidates, since "polar vortices," in which a continuous rotation of ferroelectric polarization spontaneously forms, have been recently discovered. Using resonant soft X-ray diffraction, we report the observation of a strong circular dichroism from the interaction between circularly polarized light and the chiral electric polarization texture that emerges in PbTiO 3 /SrTiO 3 superlattices. This hallmark of chirality is explained by a helical rotation of electric polarization that second-principles simulations predict to reside within complex 3D polarization textures comprising ordered topological line defects. The handedness of the texture can be topologically characterized by the sign of the helicity number of the chiral line defects. This coupling between the optical and novel polar properties could be exploited to encode chiral signatures into photon or electron beams for information processing.

  6. Enhanced radiative strength in the quasicontinuum of 117Sn.

    Science.gov (United States)

    Agvaanluvsan, U; Larsen, A C; Chankova, R; Guttormsen, M; Mitchell, G E; Schiller, A; Siem, S; Voinov, A

    2009-04-24

    The radiative strength function of 117Sn has been measured up to the neutron separation energy using the (3He, 3He' gamma) reaction. An increase in the slope of the strength function around E gamma=4.5 MeV indicates the onset of a resonancelike structure, giving a significant enhancement of the radiative strength function compared to standard models in the energy region 4.5

  7. Effect of layer composition on band spectrum of CdxHg1-xTe - CdyHg1-yTe-type superlattices

    International Nuclear Information System (INIS)

    Gerchikov, L.G.; Subashiev, A.V.; Salman Dalla

    1993-01-01

    Evolution of energy spectrum of Cd x Hg 1-x Te -Cd y Hg 1-y Te superlattices at variation of layer composition is considered. Transition from 3 type superlattice to 1 type superlattice occurring for y=0.16 is studied comprehensively. In this case, dependence of the width of superlattice forbidden zone on layer thickness is shown to become more smooth, than in CdTe - HgTe superlattice and it gives more possibilities to use such superlattices for making IR phototransistors. 10 refs., 4 figs

  8. Spontaneous Superlattice Formation in Nanorods through PartialCation Exchange

    Energy Technology Data Exchange (ETDEWEB)

    Robinson, Richard D.; Sadtler, Bryce; Demchenko, Denis O.; Erdonmez, Can K.; Wang, Lin-Wang; Alivisatos, A. Paul

    2007-03-14

    Lattice mismatch strains are widely known to controlnanoscale pattern formation in heteroepitaxy, but such effects have notbeen exploited in colloidal nanocrystal growth. We demonstrate acolloidal route to synthesizing CdS-Ag2S nanorod superlattices throughpartial cation exchange. Strain induces the spontaneous formation ofperiodic structures. Ab initio calculations of the interfacial energy andmodeling of strain energies show that these forces drive theself-organization. The nanorod superlattices exhibit high stabilityagainst ripening and phase mixing. These materials are tunablenear-infrared emitters with potential applications as nanometer-scaleoptoelectronic devices.

  9. Mechanical properties of ground state structures in substitutional ordered alloys: High strength, high ductility and high thermal stability

    International Nuclear Information System (INIS)

    Tawancy, H.M.; Aboelfotoh, M.O.

    2014-01-01

    We have studied the effect of atom arrangements in the ground state structures of substitutional ordered alloys on their mechanical properties using nickel–molybdenum-based alloys as model systems. Three alloys with nominal compositions of Ni–19.43 at% Mo, Ni–18.53 at% Mo–15.21 at% Cr and Ni–18.72 at% Mo–6.14 at% Nb are included in the study. In agreement with theoretical predictions, the closely related Pt 2 Mo-type, DO 22 and D1 a superlattices with similar energies are identified by electron diffraction of ground state structures, which can directly be derived from the parent disordered fcc structure by minor atom rearrangements on {420} fcc planes. The three superlattices are observed to coexist during the disorder–order transformation at 700 °C with the most stable superlattice being determined by the exact chemical composition. Although most of the slip systems in the parent disordered fcc structure are suppressed, many of the twinning systems remain operative in the superlattices favoring deformation by twinning, which leads to considerable strengthening while maintaining high ductility levels. Both the Pt 2 Mo-type and DO 22 superlattices are distinguished by high strength and high ductility due to their nanoscale microstructures, which have high thermal stability. However, the D1 a superlattice is found to exhibit poor thermal stability leading to considerable loss of ductility, which has been correlated with self-induced recrystallization by migration of grain boundaries

  10. Mechanical properties of ground state structures in substitutional ordered alloys: High strength, high ductility and high thermal stability

    Energy Technology Data Exchange (ETDEWEB)

    Tawancy, H.M., E-mail: tawancy@kfupm.edu.sa [Center for Engineering Research, Research Institute, King Fahd University of Petroleum and Minerals, KFUPM Box 1639, Dhahran 31261 (Saudi Arabia); Aboelfotoh, M.O., E-mail: oaboelfotoh@gmail.com [Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27606 (United States)

    2014-05-01

    We have studied the effect of atom arrangements in the ground state structures of substitutional ordered alloys on their mechanical properties using nickel–molybdenum-based alloys as model systems. Three alloys with nominal compositions of Ni–19.43 at% Mo, Ni–18.53 at% Mo–15.21 at% Cr and Ni–18.72 at% Mo–6.14 at% Nb are included in the study. In agreement with theoretical predictions, the closely related Pt{sub 2}Mo-type, DO{sub 22} and D1{sub a} superlattices with similar energies are identified by electron diffraction of ground state structures, which can directly be derived from the parent disordered fcc structure by minor atom rearrangements on {420}{sub fcc} planes. The three superlattices are observed to coexist during the disorder–order transformation at 700 °C with the most stable superlattice being determined by the exact chemical composition. Although most of the slip systems in the parent disordered fcc structure are suppressed, many of the twinning systems remain operative in the superlattices favoring deformation by twinning, which leads to considerable strengthening while maintaining high ductility levels. Both the Pt{sub 2}Mo-type and DO{sub 22} superlattices are distinguished by high strength and high ductility due to their nanoscale microstructures, which have high thermal stability. However, the D1{sub a} superlattice is found to exhibit poor thermal stability leading to considerable loss of ductility, which has been correlated with self-induced recrystallization by migration of grain boundaries.

  11. Lateral structure of (TiSe2)n(NbSe2)m superlattices

    International Nuclear Information System (INIS)

    Noh, M.; Shin, H.; Jeong, K.; Spear, J.; Johnson, D.C.; Kevan, S.D.; Warwick, T.

    1997-01-01

    The structures of a series of (TiSe 2 ) n (NbSe 2 ) m superlattices grown through controlled crystallization of designed multilayer reactants have been studied. X-ray diffraction of the data of the superlattices after crystallization show considerable preferred orientation, with the basal plane of the dichalcogenide structure parallel to the substrate to within 0.1 degree. Lattice refinement using the observed (00scr(l)) diffraction maxima yields lattice parameters along the c axis that are consistent with those expected based on the target superlattices and lattice parameters of the binary constituents. These (00scr(l)) diffraction data, however, contain no information about the crystalline structure in the ab plane of the superlattice associated with the preferred c-axis orientation. Off-specular x-ray diffraction (XRD), scanning electron microscopy, and scanning transmission x-ray microscopy (STXM) were used to explore the structure and homogeneity of the superlattices in the ab plane. XRD results rule out preferred long-range orientational order of the ab plane. Between grains, both the backscattered electron images and STXM images show grain domain structure in the ab plane with a characteristic grain domain size of approximately 50 μm. X-ray absorption microscopy in the STXM mode obtained at the Ti L 2,3 edge shows that the titanium in the superlattices is present as both octahedral Ti consistent with the TiSe 2 structure and metallic Ti. A comparison of the data obtained from these techniques highlights chemical information, which can be deduced on a submicrometer range from the space resolved spectra obtained using STXM. copyright 1997 American Institute of Physics

  12. Theory of transmission through disordered superlattices

    DEFF Research Database (Denmark)

    Wacker, Andreas; Hu, Ben Yu-Kuang

    1999-01-01

    We derive a theory for transmission through disordered finite superlattices in which the interface roughness scattering is treated by disorder averaging. This procedure permits efficient calculation of the transmission through samples with large cross sections. These calculations can be performed...

  13. Engineering the oxygen coordination in digital superlattices

    Science.gov (United States)

    Cook, Seyoung; Andersen, Tassie K.; Hong, Hawoong; Rosenberg, Richard A.; Marks, Laurence D.; Fong, Dillon D.

    2017-12-01

    The oxygen sublattice in complex oxides is typically composed of corner-shared polyhedra, with transition metals at their centers. The electronic and chemical properties of the oxide depend on the type and geometric arrangement of these polyhedra, which can be controlled through epitaxial synthesis. Here, we use oxide molecular beam epitaxy to create SrCoOx:SrTiO3 superlattices with tunable oxygen coordination environments and sublattice geometries. Using synchrotron X-ray scattering in combination with soft X-ray spectroscopy, we find that the chemical state of Co can be varied with the polyhedral arrangement, with higher Co oxidation states increasing the valence band maximum. This work demonstrates a new strategy for engineering unique electronic structures in the transition metal oxides using short-period superlattices.

  14. Observation of Significant Quantum Efficiency Enhancement from a Polarized Photocathode with Distributed Bragg Reflector

    Energy Technology Data Exchange (ETDEWEB)

    Zhang, Shukui [Thomas Jefferson National Accelerator Facility, Newport News, VA (United States); Poelker, Matthew [Thomas Jefferson National Accelerator Facility, Newport News, VA (United States); Stutzman, Marcy L. [Thomas Jefferson National Accelerator Facility, Newport News, VA (United States); Chen, Yiqiao [SVT Associates, Inc., Eden Prairie, MN (United States); Moy, Aaron [SVT Associates, Inc., Eden Prairie, MN (United States)

    2015-09-01

    Polarized photocathodes with higher Quantum efficiency (QE) would help to reduce the technological challenge associated with producing polarized beams at milliampere levels, because less laser light would be required, which simplifies photocathode cooling requirements. And for a given amount of available laser power, higher QE would extend the photogun operating lifetime. The distributed Bragg reflector (DBR) concept was proposed to enhance the QE of strained-superlattice photocathodes by increasing the absorption of the incident photons using a Fabry-Perot cavity formed between the front surface of the photocathode and the substrate that includes a DBR, without compromising electron polarization. Here we present recent results showing QE enhancement of a GaAs/GaAsP strained-superlattice photocathode made with a DBR structure. Typically, a GaAs/GaAsP strained-superlattice photocathode without DBR provides a QE of 1%, at a laser wavelength corresponding to peak polarization. In comparison, the GaAs/GaAsP strained-superlattice photocathodes with DBR exhibited an enhancement of over 2 when the incident laser wavelength was tuned to meet the resonant condition for the Fabry-Perot resonator.

  15. High-electric-field quantum transport theory for semiconductor superlattices

    International Nuclear Information System (INIS)

    Nguyen Hong Shon; Nazareno, H.N.

    1995-12-01

    Based on the Baym-Kadanoff-Keldysh nonequilibrium Green's functions technique, a quantum transport theory for semiconductor superlattices under high-electric field is developed. This theory is capable of considering collisional broadening, intra-collisional field effects and band transport and hopping regimes simultaneously. Numerical calculations for narrow-miniband superlattices in high electric field, when the hopping regime dominates are in reasonable agreement with experimental results and show a significant deviation from the Boltzmann theory. A semiphenomenological formula for current density in hopping regime is proposed. (author). 60 refs, 4 figs

  16. Non-random walk diffusion enhances the sink strength of semicoherent interfaces

    Science.gov (United States)

    Vattré, A.; Jourdan, T.; Ding, H.; Marinica, M.-C.; Demkowicz, M. J.

    2016-01-01

    Clean, safe and economical nuclear energy requires new materials capable of withstanding severe radiation damage. One strategy of imparting radiation resistance to solids is to incorporate into them a high density of solid-phase interfaces capable of absorbing and annihilating radiation-induced defects. Here we show that elastic interactions between point defects and semicoherent interfaces lead to a marked enhancement in interface sink strength. Our conclusions stem from simulations that integrate first principles, object kinetic Monte Carlo and anisotropic elasticity calculations. Surprisingly, the enhancement in sink strength is not due primarily to increased thermodynamic driving forces, but rather to reduced defect migration barriers, which induce a preferential drift of defects towards interfaces. The sink strength enhancement is highly sensitive to the detailed character of interfacial stresses, suggesting that `super-sink' interfaces may be designed by optimizing interface stress fields. Such interfaces may be used to create materials with unprecedented resistance to radiation-induced damage.

  17. Exploration of molecular interactions in cholesterol superlattices: effect of multibody interactions.

    Science.gov (United States)

    Huang, Juyang

    2002-08-01

    Experimental evidences have indicated that cholesterol may adapt highly regular lateral distributions (i.e., superlattices) in a phospholipid bilayer. We investigated the formations of superlattices at cholesterol mole fraction of 0.154, 0.25, 0.40, and 0.5 using Monte Carlo simulation. We found that in general, conventional pairwise-additive interactions cannot produce superlattices. Instead, a multibody (nonpairwise) interaction is required. Cholesterol superlattice formation reveals that although the overall interaction between cholesterol and phospholipids is favorable, it contains two large opposing components: an interaction favoring cholesterol-phospholipid mixing and an unfavorable acyl chain multibody interaction that increases nonlinearly with the number of cholesterol contacts. The magnitudes of interactions are in the order of kT. The physical origins of these interactions can be explained by our umbrella model. They most likely come from the requirement for polar phospholipid headgroups to cover the nonpolar cholesterol to avoid the exposure of cholesterol to water and from the sharp decreasing of acyl chain conformation entropy due to cholesterol contact. This study together with our previous work demonstrate that the driving force of cholesterol-phospholipid mixing is a hydrophobic interaction, and multibody interactions dominate others over a wide range of cholesterol concentration.

  18. Tuning the electrical and optical anisotropy of a monolayer black phosphorus magnetic superlattice

    Science.gov (United States)

    Li, X. J.; Yu, J. H.; Luo, K.; Wu, Z. H.; Yang, W.

    2018-04-01

    We investigate theoretically the effects of modulated periodic perpendicular magnetic fields on the electronic states and optical absorption spectrum in monolayer black phosphorus (phosphorene). We demonstrate that different phosphorene magnetic superlattice (PMS) orientations can give rise to distinct energy spectra, i.e. tuning the intrinsic electronic anisotropy. Rashba spin-orbit coupling (RSOC) develops a spin-splitting energy dispersion in this phosphorene magnetic superlattice. Anisotropic momentum-dependent carrier distributions along/perpendicular to the magnetic strips are demonstrated. The manipulations of these exotic electronic properties by tuning superlattice geometry, magnetic field and the RSOC term are addressed systematically. Accordingly, we find bright-to-dark transitions in the ground-state electron-hole pair transition rate spectrum and the PMS orientation-dependent anisotropic optical absorption spectrum. This feature offers us a practical way of modulating the electronic anisotropy in phosphorene by magnetic superlattice configurations and detecting this modulation capability by using an optical technique.

  19. Dynamic localization and negative absolute conductance in terahertz driven semiconductor superlattices

    International Nuclear Information System (INIS)

    Keay, B.J.; Allen, S.J.; Campman, K.L.

    1995-01-01

    We report the first observation of Negative Absolute Conductance (NAC), dynamic localization and multiphoton stimulated emission assisted tunneling in terahertz driven semiconductor superlattices. Theories predicting NAC in semiconductor superlattices subjected to AC electric fields have existed for twenty years, but have never been verified experimentally. Most theories are based upon semiclassical arguments and are only valid for superlattices in the miniband or coherent tunneling regime. We are not aware of models predicting NAC in superlattices in the sequential tunneling regime, although there has been recent theoretical work on double-barrier structures. Perhaps the most remarkable result is found in the power dependence of the current-voltage (I-V) characteristics near zero DC bias. As the laser power is increased the current decreases towards zero and then becomes negative. This result implies that the electrons are absorbing energy from the laser field, producing a net current in the direction opposite to the applied voltage. NAC around zero DC bias is a particularly surprising observation considering photon-assisted tunneling is not expected to be observable between the ground states of neighboring quantum wells in a semiconductor superlattice. Contrary to this believe our results are most readily attributable to photon absorption and multiphoton emission between ground states of neighboring wells. The I-V characteristics measured in the presence of terahertz radiation at low DC bias also contain steps and plateaus analogous to photon-assisted steps observed in superconducting junctions. As many as three steps have been clearly resolved corresponding to stimulated emission into the terahertz field by a three-photon process

  20. Stability and dynamic of strain mediated adatom superlattices on Cu

    Science.gov (United States)

    Kappus, Wolfgang

    2013-03-01

    Substrate strain mediated adatom equilibrium density distributions have been calculated for Cu surfaces using two complementing methods. A hexagonal adatom superlattice in a coverage range up to 0.045 ML is derived for repulsive short range interactions. For zero short range interactions a hexagonal superstructure of adatom clusters is derived in a coverage range about 0.08 ML. Conditions for the stability of the superlattice against formation of dimers or clusters and degradation are analyzed using simple neighborhood models. Such models are also used to investigate the dynamic of adatoms within their superlattice neighborhood. Collective modes of adatom diffusion are proposed from the analogy with bulk lattice dynamics and methods for measurement are suggested. The recently put forward explanation of surface state mediated interactions for superstructures found in scanning tunneling microscopy experiments is put in question and strain mediated interactions are proposed as an alternative.

  1. Moire superlattice effects in graphene/boron-nitride van der Waals heterostructures

    Energy Technology Data Exchange (ETDEWEB)

    Wallbank, John R.; Chen, Xi; Fal' ko, Vladimir I. [Department of Physics, Lancaster University, Lancaster (United Kingdom); Mucha-Kruczynski, Marcin [Department of Physics, University of Bath (United Kingdom)

    2015-06-15

    Van der Waals heterostructures of graphene and hexagonal boron nitride feature a moire superlattice for graphene's Dirac electrons. Here, we review the effects generated by this superlattice, including a specific miniband structure featuring gaps and secondary Dirac points, and a fractal spectrum of magnetic minibands known as Hofstadter's butterfly. (copyright 2015 by WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  2. Rectification of terahertz radiation in semiconductor superlattices in the absence of domains

    International Nuclear Information System (INIS)

    Isohätälä, J; Alekseev, K N

    2012-01-01

    We study theoretically the dynamical rectification of a terahertz AC electric field, i.e. the DC current and voltage response to the incident radiation, in strongly coupled semiconductor superlattices. We address the problem of stability against electric field domains: a spontaneous DC voltage is known to appear exactly for parameters for which a spatially homogeneous electron distribution is unstable. We show that by applying a weak direct current bias the rectifier can be switched from a state with zero DC voltage to one with a finite voltage in full absence of domains. The switching occurs near the conditions of dynamical symmetry breaking of an unbiased semiconductor superlattice. Therefore our scheme allows for the generation of DC voltages that would otherwise be unreachable due to domain instabilities. Furthermore, for realistic, highly doped wide miniband superlattices at room temperature, the generated DC field can be nearly quantized, that is, be approximately proportional to an integer multiple of ħω/ea where a is the superlattice period and ω is the AC field frequency. (paper)

  3. The effect of interfacial charge transfer on ferromagnetism in perovskite oxide superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Yang, F. [Univ. of California, Davis, CA (United States). Department of Chemical Engineering and Materials Science; Gu, M. [Univ. of California, Davis, CA (United States). Department of Chemical Engineering and Materials Science; Arenholz, E. [Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Advanced Light Source (ALS); Browning, N. D. [Univ. of California, Davis, CA (United States). Department of Molecular and Cellular Biology; Takamura, Y. [Univ. of California, Davis, CA (United States). Department of Chemical Engineering and Materials Science

    2012-01-05

    We investigate the structural, magnetic, and electrical properties of superlattices composed of the ferromagnetic/metal La0.7Sr0.3MnO3 and non-magnetic/metal La0.5Sr0.5TiO3 grown on (001)-oriented SrTiO3 substrates. Using a combination of bulk magnetometry, soft x-ray magnetic spectroscopy, and scanning transmission electron microscopy, we demonstrate that robust ferromagnetic properties can be maintained in this superlattice system where charge transfer at the interfaces is minimized. Thus, ferromagnetism can be controlled effectively through the chemical identity and the thickness of the individual superlattice layers.

  4. Magnetism and superconductivity in neodymium/lanthanum superlattices

    DEFF Research Database (Denmark)

    Goff, J.P.; Sarthour, R.S.; McMorrow, Desmond Francis

    1997-01-01

    bilayers. Magnetization studies reveal the onset of superconductivity at a temperature comparable to bulk DHCP La, and the results suggest coupling across the antiferromagnetic Nd layers. The magnetic structures, investigated using neutron diffraction techniques, resemble those found in bulk Nd....... For the cubic sites of the DHCP structure the magnetic order is confined to individual Nd blocks. However, the magnetic order on the Nd hexagonal sites propagates coherently through the La, even when it becomes superconducting. (C) 1998 Elsevier Science B.V. All rights reserved.......A single-crystal Nd30La10 superlattice grown using molecular beam epitaxy is found to consist of alternating antiferromagnetic and superconducting layers at low temperature. The superlattice has the DHCP crystal structure, and the stacking sequence of close-packed planes is coherent over many...

  5. Alternating current-driven graphene superlattices: Kinks, dissipative solitons, dynamic chaotization

    International Nuclear Information System (INIS)

    Kryuchkov, S. V.; Kukhar', E. I.

    2015-01-01

    The possibility of the solitary electromagnetic wave formation in graphene superlattice subjected to the electromagnetic radiation is discussed. The chaotic behavior of the electron subsystem in graphene superlattice is studied by Melnikov method. Dynamic chaos of electrons is shown to appear for certain intervals of frequencies of incident electromagnetic radiation. The frequency dependence of the radiation critical amplitude which determines the bound of chaos appearance is investigated. The values of radiation frequency at which the critical amplitude increases indefinitely were found

  6. Controlling Nanocrystal Superlattice Symmetry and Shape-Anisotropic Interactions through Variable Ligand Surface Coverage

    KAUST Repository

    Choi, Joshua J.; Bealing, Clive R.; Bian, Kaifu; Hughes, Kevin J.; Zhang, Wenyu; Smilgies, Detlef-M.; Hennig, Richard G.; Engstrom, James R.; Hanrath, Tobias

    2011-01-01

    The assembly of colloidal nanocrystals (NCs) into superstructures with long-range translational and orientational order is sensitive to the molecular interactions between ligands bound to the NC surface. We illustrate how ligand coverage on colloidal PbS NCs can be exploited as a tunable parameter to direct the self-assembly of superlattices with predefined symmetry. We show that PbS NCs with dense ligand coverage assemble into face-centered cubic (fcc) superlattices whereas NCs with sparse ligand coverage assemble into body-centered cubic (bcc) superlattices which also exhibit orientational ordering of NCs in their lattice sites. Surface chemistry characterization combined with density functional theory calculations suggest that the loss of ligands occurs preferentially on {100} than on reconstructed {111} NC facets. The resulting anisotropic ligand distribution amplifies the role of NC shape in the assembly and leads to the formation of superlattices with translational and orientational order. © 2011 American Chemical Society.

  7. Controlling Nanocrystal Superlattice Symmetry and Shape-Anisotropic Interactions through Variable Ligand Surface Coverage

    KAUST Repository

    Choi, Joshua J.

    2011-03-09

    The assembly of colloidal nanocrystals (NCs) into superstructures with long-range translational and orientational order is sensitive to the molecular interactions between ligands bound to the NC surface. We illustrate how ligand coverage on colloidal PbS NCs can be exploited as a tunable parameter to direct the self-assembly of superlattices with predefined symmetry. We show that PbS NCs with dense ligand coverage assemble into face-centered cubic (fcc) superlattices whereas NCs with sparse ligand coverage assemble into body-centered cubic (bcc) superlattices which also exhibit orientational ordering of NCs in their lattice sites. Surface chemistry characterization combined with density functional theory calculations suggest that the loss of ligands occurs preferentially on {100} than on reconstructed {111} NC facets. The resulting anisotropic ligand distribution amplifies the role of NC shape in the assembly and leads to the formation of superlattices with translational and orientational order. © 2011 American Chemical Society.

  8. Superlattice Microstructured Optical Fiber

    Science.gov (United States)

    Tse, Ming-Leung Vincent; Liu, Zhengyong; Cho, Lok-Hin; Lu, Chao; Wai, Ping-Kong Alex; Tam, Hwa-Yaw

    2014-01-01

    A generic three-stage stack-and-draw method is demonstrated for the fabrication of complex-microstructured optical fibers. We report the fabrication and characterization of a silica superlattice microstructured fiber with more than 800 rhomboidally arranged air-holes. A polarization-maintaining fiber with a birefringence of 8.5 × 10−4 is demonstrated. The birefringent property of the fiber is found to be highly insensitive to external environmental effects, such as pressure. PMID:28788693

  9. Nanoscale form dictates mesoscale function in plasmonic DNA–nanoparticle superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Ross, Michael B.; Ku, Jessie C.; Vaccarezza, Victoria M.; Schatz, George C.; Mirkin , Chad A. (NWU)

    2016-06-15

    The nanoscale manipulation of matter allows properties to be created in a material that would be difficult or even impossible to achieve in the bulk state. Progress towards such functional nanoscale architectures requires the development of methods to precisely locate nanoscale objects in three dimensions and for the formation of rigorous structure–function relationships across multiple size regimes (beginning from the nanoscale). Here, we use DNA as a programmable ligand to show that two- and three-dimensional mesoscale superlattice crystals with precisely engineered optical properties can be assembled from the bottom up. The superlattices can transition from exhibiting the properties of the constituent plasmonic nanoparticles to adopting the photonic properties defined by the mesoscale crystal (here a rhombic dodecahedron) by controlling the spacing between the gold nanoparticle building blocks. Furthermore, we develop a generally applicable theoretical framework that illustrates how crystal habit can be a design consideration for controlling far-field extinction and light confinement in plasmonic metamaterial superlattices.

  10. Single-crystal FCC and DHCP phases in Ce/Pr superlattices

    International Nuclear Information System (INIS)

    Lee, S.; Goff, J.P.; Ward, R.C.C.; Wells, M.R.; McIntyre, G.J.

    2002-01-01

    Cerium usually comprises a mixture of polycrystalline FCC and DHCP allotropes. Single-crystal Ce has been stabilised in Ce/Pr superlattices grown using molecular beam epitaxy. It is found that FCC or DHCP phases can be obtained depending on superlattice composition and growth conditions. Low-temperature neutron scattering was performed on Ce/Pr samples using the triple-axis spectrometer D10 at the ILL. Such measurements revealed one sample, [Ce 20 Pr 20 ] 60 , to be a single crystal with a DHCP unit cell; while another, [Ce 30 Pr 10 ] 56 , was a mixture of FCC and DHCP phases. Antiferromagnetic ordering of magnetic moments was observed in the DHCP sample (T N =11.1 K) with a magnetic structure similar to that found in bulk β-Ce. Surprisingly, the magnetic ordering was found to be confined to single Ce blocks. Furthermore, it was found that, at low temperatures, the lattice contraction observed for bulk FCC Ce was suppressed in Ce/Pr superlattices. (orig.)

  11. Modulating nanoparticle superlattice structure using proteins with tunable bond distributions

    International Nuclear Information System (INIS)

    McMillan, Janet R.; Brodin, Jeffrey D.; Millan, Jaime A.; Lee, Byeongdu; Olvera de la Cruz, Monica; Mirkin, Chad A.

    2017-01-01

    Here, we investigate the use of proteins with tunable DNA modification distributions to modulate nanoparticle superlattice structure. Using Beta-galactosidase (βgal) as a model system, we have employed the orthogonal chemical reactivities of surface amines and thiols to synthesize protein-DNA conjugates with 36 evenly distributed or 8 specifically positioned oligonucleotides. When assembled into crystalline superlattices with AuNPs, we find that the distribution of DNA modifications modulates the favored structure: βgal with uniformly distributed DNA bonding elements results in body-centered cubic crystals, whereas DNA functionalization of cysteines results in AB 2 packing. We probe the role of protein oligonucleotide number and conjugate size on this observation, which revealed the importance of oligonucleotide distribution and number in this observed assembly behavior. These results indicate that proteins with defined DNA-modification patterns are powerful tools to control the nanoparticle superlattices architecture, and establish the importance of oligonucleotide distribution in the assembly behavior of protein-DNA conjugates.

  12. Large negative differential resistance in graphene nanoribbon superlattices

    Science.gov (United States)

    Tseng, P.; Chen, C. H.; Hsu, S. A.; Hsueh, W. J.

    2018-05-01

    A graphene nanoribbon superlattice with a large negative differential resistance (NDR) is proposed. Our results show that the peak-to-valley ratio (PVR) of the graphene superlattices can reach 21 at room temperature with bias voltages between 90-220 mV, which is quite large compared with the one of traditional graphene-based devices. It is found that the NDR is strongly influenced by the thicknesses of the potential barrier. Therefore, the NDR effect can be optimized by designing a proper barrier thickness. The large NDR effect can be attributed to the splitting of the gap in transmission spectrum (segment of Wannier-Stark ladder) with larger thicknesses of barrier when the applied voltage increases.

  13. A possible radiation-resistant solar cell geometry using superlattices

    Science.gov (United States)

    Goradia, C.; Clark, R.; Brinker, D.

    1985-01-01

    A solar cell structure is proposed which uses a GaAs nipi doping superlattice. An important feature of this structure is that photogenerated minority carriers are very quickly collected in a time shorter than bulk lifetime in the fairly heavily doped n and p layers and these carriers are then transported parallel to the superlattice layers to selective ohmic contacts. Assuming that these already-separated carriers have very long recombination lifetimes, due to their across an indirect bandgap in real space, it is argued that the proposed structure may exhibit superior radiation tolerance along with reasonably high beginning-of-life efficiency.

  14. Magnetic profiles in ferromagnetic/superconducting superlattices.

    Energy Technology Data Exchange (ETDEWEB)

    te Velthuis, S. G. E.; Hoffmann, A.; Santamaria, J.; Materials Science Division; Univ. Complutense de Madrid

    2007-02-28

    The interplay between ferromagnetism and superconductivity has been of longstanding fundamental research interest to scientists, as the competition between these generally mutually exclusive types of long-range order gives rise to a rich variety of physical phenomena. A method of studying these exciting effects is by investigating artificially layered systems, i.e. alternating deposition of superconducting and ferromagnetic thin films on a substrate, which enables a straight-forward combination of the two types of long-range order and allows the study of how they compete at the interface over nanometer length scales. While originally studies focused on low temperature superconductors interchanged with metallic ferromagnets, in recent years the scope has broadened to include superlattices of high T{sub c} superconductors and colossal magnetoresistance oxides. Creating films where both the superconducting as well as the ferromagnetic layers are complex oxide materials with similar crystal structures (Figure 1), allows the creation of epitaxial superlattices, with potentially atomically flat and ordered interfaces.

  15. Organic p-n heterostructures and superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Kowarik, Stefan [Lawrence Berkeley National Laboratory, Berkeley, CA (United States); Hinderhofer, Alexander; Gerlach, Alexander; Schreiber, Frank [Institut fuer Angewandte Physik, Tuebingen (Germany); Osso, Oriol [MATGAS 2000 A.I.E., Esfera UAB, Barcelona (Spain); Wang, Cheng; Hexemer, Alexander [Advanced Light Source, Berkeley, CA (United States)

    2009-07-01

    For many applications of organic semiconductors two components such as e.g. n and p-type layers are required, and the morphology of such heterostructures is crucial for their performance. Pentacene (PEN) is one of the most promising p-type molecular semiconductors and recently perfluoro-pentacene (PFP) has been identified as a good electron conducting material for complementary circuits with PEN. We use soft and hard X-ray reflectivity measurements, scanning transmission X-ray microscopy (STXM) and atomic force microscopy for structural investigations of PFP-PEN heterostructures. The chemical contrast between PEN and PFP in STXM allows us to determine the lateral length scales of p and n domains in a bilayer. For a superlattice of alternating PFP and PEN layers grown by organic molecular beam deposition, X-ray reflectivity measurements demonstrate good structural order. We find a superlattice reflection that varies strongly when tuning the X-ray energy around the fluorine edge, demonstrating that there are indeed alternating PFP and PEN layers.

  16. Thermoelectric properties of IV–VI-based heterostructures and superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Borges, P.D., E-mail: pabloborges@ufv.br [Instituto de Ciências Exatas e Tec., Universidade Federal de Viçosa, Rio Paranaíba, MG (Brazil); Department of Physics, Texas State University, San Marcos, TX 78666 (United States); Petersen, J.E.; Scolfaro, L. [Department of Physics, Texas State University, San Marcos, TX 78666 (United States); Leite Alves, H.W. [Departamento de Ciências Naturais, Universidade Federal de São João Del Rei, Caixa Postal 110, São João Del Rei 36300-000, MG (Brazil); Myers, T.H. [Department of Physics, Texas State University, San Marcos, TX 78666 (United States)

    2015-07-15

    Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is a significant focus in thermoelectric research today. By solving the semiclassical Boltzmann transport equations in the constant scattering time (τ) approximation, in conjunction with ab initio electronic structure calculations, within Density Functional Theory, we compare the Seebeck coefficient (S) and figure of merit (ZT) of bulk PbTe to PbTe/SnTe/PbTe heterostructures and PbTe doping superlattices (SLs) with periodically doped planes. Bismuth and Thallium were used as the n- and p-type impurities, respectively. The effects of carrier concentration are considered via chemical potential variation in a rigid band approximation. The impurity bands near the Fermi level in the electronic structure of PbTe SLs are of Tl s- and Bi p-character, and this feature is independent of the doping concentration or the distance between impurity planes. We observe the impurity bands to have a metallic nature in the directions perpendicular to the doping planes, yet no improvement on the values of ZT is found when compared to bulk PbTe. For the PbTe/SnTe/PbTe heterostructures, the calculated S presents good agreement with recent experimental data, and an anisotropic behavior is observed for low carrier concentrations (n<10{sup 18} cm{sup −3}). A large value of ZT{sub ||} (parallel to the growth direction) of 3.0 is predicted for n=4.7×10{sup 18} cm{sup −3} and T=700 K, whereas ZT{sub p} (perpendicular to the growth direction) is found to peak at 1.5 for n=1.7×10{sup 17} cm{sup −3}. Both electrical conductivity enhancement and thermal conductivity reduction are analyzed. - Graphical abstract: Figure of merit for PbTe/SnTe/PbTe heterostructure along the [0 0 1] direction, P.D. Borges, J.E. Petersen, L. Scolfaro, H.W. Leite Alves, T.H. Myers, Improved thermoelectric properties of IV–VI-based heterostructures and superlattices. - Highlights: • Thermoelectric properties of IV

  17. A weakly coupled semiconductor superlattice as a harmonic hypersonic-electrical transducer

    International Nuclear Information System (INIS)

    Poyser, C L; Akimov, A V; Campion, R P; Kent, A J; Balanov, A G

    2015-01-01

    We study experimentally and theoretically the effects of high-frequency strain pulse trains on the charge transport in a weakly coupled semiconductor superlattice. In a frequency range of the order of 100 GHz such excitation may be considered as single harmonic hypersonic excitation. While travelling along the axis of the SL, the hypersonic acoustic wavepacket affects the electron tunnelling, and thus governs the electrical current through the device. We reveal how the change of current depends on the parameters of the hypersonic excitation and on the bias applied to the superlattice. We have found that the changes in the transport properties of the superlattices caused by the acoustic excitation can be largely explained using the current–voltage relation of the unperturbed system. Our experimental measurements show multiple peaks in the dependence of the transferred charge on the repetition rate of the strain pulses in the train. We demonstrate that these resonances can be understood in terms of the spectrum of the applied acoustic perturbation after taking into account the multiple reflections in the metal film serving as a generator of hypersonic excitation. Our findings suggest an application of the semiconductor superlattice as a hypersonic-electrical transducer, which can be used in various microwave devices. (paper)

  18. Calculation of Energy Diagram of Asymmetric Graded-Band-Gap Semiconductor Superlattices.

    Science.gov (United States)

    Monastyrskii, Liubomyr S; Sokolovskii, Bogdan S; Alekseichyk, Mariya P

    2017-12-01

    The paper theoretically investigates the peculiarities of energy diagram of asymmetric graded-band-gap superlattices with linear coordinate dependences of band gap and electron affinity. For calculating the energy diagram of asymmetric graded-band-gap superlattices, linearized Poisson's equation has been solved for the two layers forming a period of the superlattice. The obtained coordinate dependences of edges of the conduction and valence bands demonstrate substantial transformation of the shape of the energy diagram at changing the period of the lattice and the ratio of width of the adjacent layers. The most marked changes in the energy diagram take place when the period of lattice is comparable with the Debye screening length. In the case when the lattice period is much smaller that the Debye screening length, the energy diagram has the shape of a sawtooth-like pattern.

  19. Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices.

    Science.gov (United States)

    Ravichandran, Jayakanth; Yadav, Ajay K; Cheaito, Ramez; Rossen, Pim B; Soukiassian, Arsen; Suresha, S J; Duda, John C; Foley, Brian M; Lee, Che-Hui; Zhu, Ye; Lichtenberger, Arthur W; Moore, Joel E; Muller, David A; Schlom, Darrell G; Hopkins, Patrick E; Majumdar, Arun; Ramesh, Ramamoorthy; Zurbuchen, Mark A

    2014-02-01

    Elementary particles such as electrons or photons are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.

  20. Development of high-capacity nickel-metal hydride batteries using superlattice hydrogen-absorbing alloys

    Science.gov (United States)

    Yasuoka, Shigekazu; Magari, Yoshifumi; Murata, Tetsuyuki; Tanaka, Tadayoshi; Ishida, Jun; Nakamura, Hiroshi; Nohma, Toshiyuki; Kihara, Masaru; Baba, Yoshitaka; Teraoka, Hirohito

    New R-Mg-Ni (R: rare earths) superlattice alloys with higher-capacity and higher-durability than the conventional Mm-Ni alloys with CaCu 5 structure have been developed. The oxidation resistibility of the superlattice alloys has been improved by optimizing the alloy composition by such as substituting aluminum for nickel and optimizing the magnesium content in order to prolong the battery life. High-capacity nickel-metal hydride batteries for the retail market, the Ni-MH2500/900 series (AA size type 2500 mAh, AAA size type 900 mAh), have been developed and commercialized by using an improved superlattice alloy for negative electrode material.

  1. Superconducting properties and microstructure of YBa2Cu3O7-δ/PrBa2Cu3O7-δ superlattices

    International Nuclear Information System (INIS)

    Lowndes, D.H.; Norton, D.P.; Zheng, X.Y.; Zhu, Shen

    1991-10-01

    Epitaxial YBa 2 Cu 3 O 7-δ /PrBa 2 Cu 3 O 7-δ (YBCO/PBCO) superlattices are tools for systematic, fundamental studies of high-temperature superconductivity. The variation of T c in YBCO/PBCO superlattices can be understood as arising from changes in the interlayer phase coupling between YBCO layers that are highly two-dimensional when they are very thin (∼1--2 c-axis unit cells) and completely isolated from each other. Single-cell-thick YBCO layers, containing isolated pairs of CuO 2 planes, are found to be superconducting at T c ∼ 20 K, in a PBCO matrix. The resistance in the superconducting transition region scales with temperature as expected for the (flux flow) resistance produced by thermally generated 2D vortices, or for a 2D array of superconducting weak links. Relative to both thin-film and single-crystal HTSc specimens, the thin superconducting layers in YBCO/PBCO superlattices exhibit a greatly expanded temperature range over which characteristic 2D dissipation can be observed, as a consequence of the enhanced anisotropy and reduced dimensionality of the YBCO layers. Scanning tunneling microscope studies reveal that YBCO films and YBCO/PBCO superlattices grow unit cell-by-unit cell by a terraced-island growth mechanism. On miscut, near-(001) substrates the terraces are epitaxially aligned with the substrate crystal lattice and spiral growth structures (screw dislocation-mediated growth) are not seen. These observations explain the steps or ''kinks'' that are seen in cross-section Z-contrast TEM images of YBCO/PBCO superlattices. The kinks may correspond physically to regions where the supercurrent must tunnel along the c- axis, and thus may be weak-link barriers. 31 refs., 7 figs

  2. Low temperature synthesis of Mo2C/W2C superlattices via ultra-thin modulated reactants

    International Nuclear Information System (INIS)

    Johnson, C.D.; Johnson, D.C.

    1996-01-01

    The authors report here a synthesis method of preparing carbide superlattices using ultra-thin modulated reactants. Initial investigations into the synthesis of the binary systems, Mo 2 C and W 2 C using ultra-thin modulated reactants revealed that both can be formed at relatively low temperatures (500 and 600 C respectively). DSC and XRD data suggested a two step reaction pathway involving interdiffusion of the initial modulated reactant followed by crystallization of the final product, if the modulation length is on the order of 10 angstrom. This information was used to form Mo 2 C/W 2 C superlattices using the structure of the ultra-thin modulated reactant to control the final superlattice period. Relatively large superlattice modulations were kinetically trapped by having several repeat units of each binary within the total repeat of the initial reactant. DSC and XRD data again are consistent with a two step reaction pathway leading to the formation of carbide superlattices

  3. Thermoelectric transport in superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Reinecke, T L; Broido, D A

    1997-07-01

    The thermoelectric transport properties of superlattices have been studied using an exact solution of the Boltzmann equation. The role of heat transport along the barrier layers, of carrier tunneling through the barriers, of valley degeneracy and of the well width and energy dependences of the carrier-phonon scattering rates on the thermoelectric figure of merit are given. Calculations are given for Bi{sub 2}Te{sub 3} and for PbTe, and the results of recent experiments are discussed.

  4. RAMAN SCATTERING BY ACOUSTIC PHONONS AND STRUCTURAL PROPERTIES OF FIBONACCI, THUE-MORSE AND RANDOM SUPERLATTICES

    OpenAIRE

    Merlin , R.; Bajema , K.; Nagle , J.; Ploog , K.

    1987-01-01

    We report structural studies of incommensurate and random GaAs-AlAs superlattices using Raman scattering by acoustic phonons. Properties of the structure factor of Fibonacci and Thue-Morse superlattices are discussed in some detail.

  5. Enhancement in figure-of-merit with superlattices structures for thin-film thermoelectric devices

    Energy Technology Data Exchange (ETDEWEB)

    Venkatasubramanian, R; Colpitts, T

    1997-07-01

    Thin-film superlattice (SL) structures in thermoelectric materials are shown to be a promising approach to obtaining an enhanced figure-of-merit, ZT, compared to conventional, state-of-the-art bulk alloyed materials. In this paper the authors describe experimental results on Bi{sub 2}Te{sub 3}/Sb{sub 2}Te{sub 3} and Si/Ge SL structures, relevant to thermoelectric cooling and power conversion, respectively. The short-period Bi{sub 2}Te{sub 3} and Si/Ge SL structures appear to indicate reduced thermal conductivities compared to alloys of these materials. From the observed behavior of thermal conductivity values in the Bi{sub 2}Te{sub 3}/Sb{sub 2}Te{sub 3} SL structures, a distinction is made where certain types of periodic structures may correspond to an ordered alloy rather than an SL, and therefore, do not offer a significant reduction in thermal conductivity values. The study also indicates that SL structures, with little or weak quantum-confinement, also offer an improvement in thermoelectric power factor over conventional alloys. They present power factor and electrical transport data in the plane of the SL interfaces to provide preliminary support for the arguments on reduced alloy scattering and impurity scattering in Bi{sub 2}Te{sub 3}/Sb{sub 2}Te{sub 3} and Si/Ge SL structures. These results, though tentative due to the possible role of the substrate and the developmental nature of the 3-{omega} method used to determine thermal conductivity values, suggest that the short-period SL structures potentially offer factorial improvements in the three-dimensional figure-of-merit (ZT3D) compared to current state-of-the-art bulk alloys. An approach to a thin-film thermoelectric device called a Bipolarity-Assembled, Series-Inter-Connected Thin-Film Thermoelectric Device (BASIC-TFTD) is introduced to take advantage of these thin-film SL structures.

  6. Band Structure and Quantum Confined Stark Effect in InN/GaN superlattices

    DEFF Research Database (Denmark)

    Gorczyca, I.; Suski, T.; Christensen, Niels Egede

    2012-01-01

    InN/GaN superlattices offer an important way of band gap engineering in the blue-green range of the spectrum. This approach represents a more controlled method than the band gap tuning in quantum well systems by application of InGaN alloys. The electronic structures of short-period wurtzite InN/G...... wells and barriers one may tune band gaps over a wide spectral range, which provides flexibility in band gap engineering.......InN/GaN superlattices offer an important way of band gap engineering in the blue-green range of the spectrum. This approach represents a more controlled method than the band gap tuning in quantum well systems by application of InGaN alloys. The electronic structures of short-period wurtzite In......N/GaN(0001) superlattices are investigated, and the variation of the band gap with the thicknesses of the well and the barrier is discussed. Superlattices of the form mInN/nGaN with n ≥ m are simulated using band structure calculations in the Local Density Approximation with a semiempirical correction...

  7. Mg dopant distribution in an AlGaN/GaN p-type superlattice assessed using atom probe tomography, TEM and SIMS

    International Nuclear Information System (INIS)

    Bennett, S E; Kappers, M J; Barnard, J S; Humphreys, C J; Oliver, R A; Clifton, P H; Ulfig, R M

    2010-01-01

    P-type conducting layers are critical in GaN-based devices such as LEDs and laser diodes. Such layers are often produced by doping GaN with Mg, but the hole concentration can be enhanced using AlGaN/GaN p-type superlattices by exploiting the built-in polarisation fields. A Mg-doped AlGaN/GaN superlattice was studied using SIMS. Although the AlGaN and GaN were nominally doped to the same level, the SIMS data suggested a difference in doping density between the two materials. Atom probe tomography was then used to investigate the Mg distribution. The superlattice repeats were clearly visible, as expected and, in addition, significant Mg clustering was observed in both the GaN and AlGaN layers. There were many more Mg clusters in the AlGaN layers than the GaN layers, accounting for the difference in doping density suggested by SIMS. To evaluate the structural accuracy of the atom probe reconstruction, layer thicknesses from the atom probe were compared with STEM images. Finally, future work is proposed to investigate the Mg clusters in the TEM.

  8. Effects of anharmonic strain on the phase stability of epitaxial films and superlattices: Applications to noble metals

    International Nuclear Information System (INIS)

    Ozolins, V.; Wolverton, C.; Zunger, A.

    1998-01-01

    Epitaxial strain energies of epitaxial films and bulk superlattices are studied via first-principles total-energy calculations using the local-density approximation. Anharmonic effects due to large lattice mismatch, beyond the reach of the harmonic elasticity theory, are found to be very important in Cu/Au (lattice mismatch 12%), Cu/Ag (12%), and Ni/Au (15%). We find that left-angle 001 right-angle is the elastically soft direction for biaxial expansion of Cu and Ni, but it is left-angle 201 right-angle for large biaxial compression of Cu, Ag, and Au. The stability of superlattices is discussed in terms of the coherency strain and interfacial energies. We find that in phase separating systems such as Cu-Ag the superlattice formation energies decrease with superlattice period, and the interfacial energy is positive. Superlattices are formed easiest on (001) and hardest on (111) substrates. For ordering systems, such as Cu-Au and Ag-Au, the formation energy of superlattices increases with period, and interfacial energies are negative. These superlattices are formed easiest on (001) or (110) and hardest on (111) substrates. For Ni-Au we find a hybrid behavior: superlattices along left-angle 111 right-angle and left-angle 001 right-angle behave like phase separating systems, while for left-angle 110 right-angle they behave like ordering systems. Finally, recent experimental results on epitaxial stabilization of disordered Ni-Au and Cu-Ag alloys, immiscible in the bulk form, are explained in terms of destabilization of the phase separated state due to lattice mismatch between the substrate and constituents. copyright 1998 The American Physical Society

  9. Dramatically enhanced electrical breakdown strength in cellulose nanopaper

    Directory of Open Access Journals (Sweden)

    Jianwen Huang

    2016-09-01

    Full Text Available Electrical breakdown behaviors of nanopaper prepared from nanofibrillated cellulose (NFC were investigated. Compared to conventional insulating paper made from micro softwood fibers, nanopaper has a dramatically enhanced breakdown strength. Breakdown field of nanopaper is 67.7 kV/mm, whereas that of conventional paper is only 20 kV/mm. Air voids in the surface of conventional paper are observed by scanning electron microscope (SEM. Further analyses using mercury intrusion show that pore diameter of conventional paper is around 1.7 μm, while that of nanopaper is below 3 nm. Specific pore size of nanopaper is determined to be approximately 2.8 nm by the gas adsorption technique. In addition, theoretical breakdown strengths of nanopaper and conventional paper are also calculated to evaluate the effect of pore size. It turns out that theoretical values agree well with experimental data, indicating that the improved strength in nanopaper is mainly attributed to the decreased pore size. Due to its outstanding breakdown strength, this study indicates the suitability of nanopaper for electrical insulation in ultra-high voltage convert transformers and other electrical devices.

  10. Porous Hollow Superlattice NiMn2O4/NiCo2O4 Mesocrystals as a Highly Reversible Anode Material for Lithium-Ion Batteries

    Directory of Open Access Journals (Sweden)

    Lingjun Li

    2018-05-01

    Full Text Available As a promising high-capacity anode material for Li-ion batteries, NiMn2O4 always suffers from the poor intrinsic conductivity and the architectural collapse originating from the volume expansion during cycle. Herein, a combined structure and architecture modulation is proposed to tackle concurrently the two handicaps, via a facile and well-controlled solvothermal approach to synthesize NiMn2O4/NiCo2O4 mesocrystals with superlattice structure and hollow multi-porous architecture. It is demonstrated that the obtained NiCo1.5Mn0.5O4 sample is made up of a new mixed-phase NiMn2O4/NiCo2O4 compound system, with a high charge capacity of 532.2 mAh g−1 with 90.4% capacity retention after 100 cycles at a current density of 1 A g−1. The enhanced electrochemical performance can be attributed to the synergistic effects of the superlattice structure and the hollow multi-porous architecture of the NiMn2O4/NiCo2O4 compound. The superlattice structure can improve ionic conductivity to enhance charge transport kinetics of the bulk material, while the hollow multi-porous architecture can provide enough void spaces to alleviate the architectural change during cycling, and shorten the lithium ions diffusion and electron-transportation distances.

  11. Strain and Defect Engineering for Tailored Electrical Properties in Perovskite Oxide Thin Films and Superlattices

    Science.gov (United States)

    Hsing, Greg Hsiang-Chun

    Functional complex-oxides display a wide spectrum of physical properties, including ferromagnetism, piezoelectricity, ferroelectricity, photocatalytic and metal-insulating transition (MIT) behavior. Within this family, oxides with a perovskite structure have been widely studied, especially in the form of thin films and superlattices (heterostructures), which are strategically and industrially important because they offer a wide range of opportunities for electronic, piezoelectric and sensor applications. The first part of my thesis focuses on understanding and tuning of the built-in electric field found in PbTiO3/SrTiO3 (PTO/STO) ferroelectric superlattices and other ferroelectric films. The artificial layering in ferroelectric superlattices is a potential source of polarization asymmetry, where one polarization state is preferred over another. One manifestation of this asymmetry is a built-in electric field associated with shifted polarization hysteresis. Using off-axis RF-magnetron sputtering, we prepared several compositions of PTO/STO superlattice thin films; and for comparison PbTiO3/SrRuO 3 (PTO/SRO) superlattices, which have an additional intrinsic compositional asymmetry at the interface. Both theoretical modeling and experiments indicate that the layer-by-layer superlattice structure aligns the Pb-O vacancy defect dipoles in the c direction which contributes significantly to the built-in electric field; however the preferred polarization direction is different between the PTO/STO and PTO/SRO interface. By designing a hybrid superlattice that combines PTO/STO and PTO/SRO superlattices, we show the built-in electric field can be tuned to zero by changing the composition of the combo-superlattice. The second part of my thesis focuses on the epitaxial growth of SrCrO 3 (SCO) films. The inconsistent reports regarding its electrical and magnetic properties through the years stem from the compositionally and structurally ill-defined polycrystalline samples, but

  12. Creep strength and rupture ductility of creep strength enhanced ferritic steels

    Energy Technology Data Exchange (ETDEWEB)

    Kushima, Hideaki; Sawada, Kota; Kimura, Kazuhiro [National Inst. for Materials Science, Tsukuba, Ibaraki (Japan)

    2010-07-01

    Creep strength and rupture ductility of Creep Strength Enhanced Ferritic (CSEF) steels were investigated from a viewpoint of stress dependence in comparison with conventional low alloy ferritic creep resistant steels. Inflection of stress vs. time to rupture curve was observed at 50% of 0.2% offset yield stress for both CSEF and conventional ferritic steels. Creep rupture ductility tends to decrease with increase in creep exposure time, however, those of conventional low alloy steels indicate increase in the long-term. Creep rupture ductility of the ASME Grades 92 and 122 steels indicates drastic decrease with decrease in stress at 50% of 0.2% offset yield stress. Stress dependence of creep rupture ductility of the ASME Grades 92 and 122 steels is well described by stress ratio to 0.2% offset yield stress, regardless of temperature. Drop of creep rupture ductility is caused by inhomogeneous recovery at the vicinity of prior austenite grain boundary, and remarkable drop of creep rupture ductility of CSEF steels should be derived from those stabilized microstructure. (orig.)

  13. Free-Standing Metal Oxide Nanoparticle Superlattices Constructed with Engineered Protein Containers Show in Crystallo Catalytic Activity.

    Science.gov (United States)

    Lach, Marcel; Künzle, Matthias; Beck, Tobias

    2017-12-11

    The construction of defined nanostructured catalysts is challenging. In previous work, we established a strategy to assemble binary nanoparticle superlattices with oppositely charged protein containers as building blocks. Here, we show that these free-standing nanoparticle superlattices are catalytically active. The metal oxide nanoparticles inside the protein scaffold are accessible for a range of substrates and show oxidase-like and peroxidase-like activity. The stable superlattices can be reused for several reaction cycles. In contrast to bulk nanoparticle-based catalysts, which are prone to aggregation and difficult to characterize, nanoparticle superlattices based on engineered protein containers provide an innovative synthetic route to structurally defined heterogeneous catalysts with control over nanoparticle size and composition. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

  14. Field-effect transistor having a superlattice channel and high carrier velocities at high applied fields

    Science.gov (United States)

    Chaffin, R.J.; Dawson, L.R.; Fritz, I.J.; Osbourn, G.C.; Zipperian, T.E.

    1987-06-08

    A field effect transistor comprises a semiconductor having a source, a drain, a channel and a gate in operational relationship. The semiconductor is a strained layer superlattice comprising alternating quantum well and barrier layers, the quantum well layers and barrier layers being selected from the group of layer pairs consisting of InGaAs/AlGaAs, InAs/InAlGaAs, and InAs/InAlAsP. The layer thicknesses of the quantum well and barrier layers are sufficiently thin that the alternating layers constitute a superlattice which has a superlattice conduction band energy level structure in k-vector space. The layer thicknesses of the quantum well layers are selected to provide a superlattice L/sub 2D/-valley which has a shape which is substantially more two-dimensional than that of said bulk L-valley. 2 figs.

  15. Thermoelectric properties of strontium titanate superlattices incorporating niobium oxide nanolayers

    KAUST Repository

    Sarath Kumar, S. R.; Hedhili, Mohamed N.; Cha, Dong Kyu; Tritt, Terry M.; Alshareef, Husam N.

    2014-01-01

    A novel superlattice structure based on epitaxial nanoscale layers of NbOx and Nb-doped SrTiO3 is fabricated using a layer-by-layer approach on lattice matched LAO substrates. The absolute Seebeck coefficient and electrical conductivity of the [(NbOx) a/(Nb-doped SrTiO3)b]20 superlattices (SLs) were found to increase with decreasing layer thickness ratio (a/b ratio), reaching, at high temperatures, a power factor that is comparable to epitaxial Nb-doped SrTiO3 (STNO) films (∼0.7 W m-1 K-1). High temperature studies reveal that the SLs behave as n-type semiconductors and undergo an irreversible change at a varying crossover temperature that depends on the a/b ratio. By use of high resolution X-ray photoelectron spectroscopy and X-ray diffraction, the irreversible changes are identified to be due to a phase transformation from cubic NbO to orthorhombic Nb2O5, which limits the highest temperature of stable operation of the superlattice to 950 K. © 2014 American Chemical Society.

  16. Thermoelectric properties of strontium titanate superlattices incorporating niobium oxide nanolayers

    KAUST Repository

    Sarath Kumar, S. R.

    2014-04-22

    A novel superlattice structure based on epitaxial nanoscale layers of NbOx and Nb-doped SrTiO3 is fabricated using a layer-by-layer approach on lattice matched LAO substrates. The absolute Seebeck coefficient and electrical conductivity of the [(NbOx) a/(Nb-doped SrTiO3)b]20 superlattices (SLs) were found to increase with decreasing layer thickness ratio (a/b ratio), reaching, at high temperatures, a power factor that is comparable to epitaxial Nb-doped SrTiO3 (STNO) films (∼0.7 W m-1 K-1). High temperature studies reveal that the SLs behave as n-type semiconductors and undergo an irreversible change at a varying crossover temperature that depends on the a/b ratio. By use of high resolution X-ray photoelectron spectroscopy and X-ray diffraction, the irreversible changes are identified to be due to a phase transformation from cubic NbO to orthorhombic Nb2O5, which limits the highest temperature of stable operation of the superlattice to 950 K. © 2014 American Chemical Society.

  17. Bi-continuous Multi-component Nanocrystal Superlattices for Solar Energy Conversion

    Energy Technology Data Exchange (ETDEWEB)

    Kagan, Cherie [University of Pennsylvania; Murray, Christopher [University of Pennsylvania; Kikkawa, James [University of Pennsylvania; Engheta, Nader [University of Pennsylvania

    2017-06-14

    Our SISGR program studied an emerging class of nanomaterials wherein different combinations of semiconductor or semiconductor and plasmonic nanocrystals (NCs) are self-assembled into three-dimensional multi-component superlattices. The NC assemblies were designed to form bicontinuous semiconductor NC sublattices with type-II energy offsets to drive charge separation onto electron and hole transporting sublattices for collection and introduce plasmonic NCs to increase solar absorption and charge separation. Our group is expert in synthesizing and assembling an extraordinary variety of artificial systems by tailoring the NC building blocks and the superlattice unit cell geometry. Under this DOE BES Materials Chemistry program, we introduced chemical methods to control inter-particle distance and to dope NC assemblies, which enabled our demonstration of strong electronic communication between NCs and the use of NC thin films as electronic materials. We synthesized, assembled and structurally, spectroscopically, and electrically probed NC superlattices to understand and manipulate the flow of energy and charge toward discovering the design rules and optimizing these complex architectures to create materials that efficiently convert solar radiation into electricity.

  18. Waves in man-made materials: superlattice to metamaterials

    Science.gov (United States)

    Tsu, Raphael; Fiddy, Michael A.

    2014-07-01

    While artificial or man-made structures date back to Lord Rayleigh, the work started by Lewin in 1947, placing spheres onto cubic lattices, greatly enriched microwave materials and devices. It was very suggestive of both metamaterials and photonics crystals. Effective medium models were used to describe bulk properties with some success. The concept of metamaterials followed photonic crystals, and these both were introduced after the introduction of the man-made superlattices designed to enrich the class of materials for electronic devices. The work on serrated ridged waveguides by Kirschbaum and Tsu for the control of the refractive index of microwave lenses as well as microwave matching devices in 1959 used a combination of theory, such as Floquet's theory, Bloch theory in one dimension, as well as periodic lumped loading. There is much in common between metamaterials and superlattices, but in this paper, we discuss some practical limitations to both. It is pointed out that unlike superlattices where kl > 1 is the most important criterion, metamaterials try to avoid involve such restrictions. However, the natural random fluctuations that limit the properties of naturally occurring materials are shown to take a toll on the theoretical predictions of metamaterials. The question is how great that toll, i.e. how significant those fluctuations will be, in diminishing the unusual properties that metamaterials can exhibit.

  19. Band structure of ABC-trilayer graphene superlattice

    International Nuclear Information System (INIS)

    Uddin, Salah; Chan, K. S.

    2014-01-01

    We investigate the effect of one-dimensional periodic potentials on the low energy band structure of ABC trilayer graphene first by assuming that all the three layers have the same potential. Extra Dirac points having the same electron hole crossing energy as that of the original Dirac point are generated by superlattice potentials with equal well and barrier widths. When the potential height is increased, the numbers of extra Dirac points are increased. The dispersions around the Dirac points are not isotropic. It is noted that the dispersion along the k y direction for k x  = 0 oscillates between a non-linear dispersion and a linear dispersion when the potential height is increased. When the well and barrier widths are not identical, the symmetry of the conduction and valence bands is broken. The extra Dirac points are shifted either upward or downward depending on the barrier and well widths from the zero energy, while the position of the central Dirac point oscillates with the superlattice potential height. By considering different potentials for different layers, extra Dirac points are generated not from the original Dirac points but from the valleys formed in the energy spectrum. Two extra Dirac points appear from each pair of touched valleys, so four Dirac points appeared in the spectrum at particular barrier height. By increasing the barrier height of superlattice potential two Dirac points merge into the original Dirac point. This emerging and merging of extra Dirac points is different from the equal potential case

  20. Energy minibands degeneration induced by magnetic field effects in graphene superlattices

    Science.gov (United States)

    Reyes-Villagrana, R. A.; Carrera-Escobedo, V. H.; Suárez-López, J. R.; Madrigal-Melchor, J.; Rodríguez-Vargas, I.

    2017-12-01

    Energy minibands are a basic feature of practically any superlattice. In this regard graphene superlattices are not the exception and recently miniband transport has been reported through magneto-transport measurements. In this work, we compute the energy miniband and transport characteristics for graphene superlattices in which the energy barriers are generated by magnetic and electric fields. The transfer matrix approach and the Landauer-Büttiker formalism have been implemented to calculate the energy minibands and the linear-regime conductance. We find that energy minibands are very sensitive to the magnetic field and become degenerate by rising it. We were also able to correlate the evolution of the energy minibands as a function of the magnetic field with the transport characteristics, finding that miniband transport can be destroyed by magnetic field effects. Here, it is important to remark that although magnetic field effects have been a key element to unveil miniband transport, they can also destroy it.

  1. Propagation and generation of Josephson radiation in superconductor/insulator superlattices

    International Nuclear Information System (INIS)

    Auvil, P.R.; Ketterson, J.B.

    1987-01-01

    The wave propagation and generation characteristics of a metal-insulator superlattice are calculated in a low-field Landau--Ginzburg model, including Josephson coupling through the insulating layers. It is shown that a significant increase in the phase velocity of the electromagnetic waves propagating in the superlattice occurs when the thickness of the superconducting layers becomes much less than the London penetration depth, suggesting that increased output of Josephson radiation may be achieved from such structures. Wave generation via the ac Josephson effect (in the presence of applied dc electric and magnetic fields) is studied for both parallel and series driven multilayer structures

  2. THz laser based on quasi-periodic AlGaAs superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Malyshev, K V [N.E. Bauman Moscow State Technical University, Moscow (Russian Federation)

    2013-06-30

    The use of quasi-periodic AlGaAs superlattices as an active element of a quantum cascade laser of terahertz range is proposed and theoretically investigated. A multi-colour emission, having from three to six peaks of optical gain, is found in Fibonacci, Thue-Morse, and figurate superlattices in electric fields of intensity F = 11 - 13 kV cm{sup -1} in the frequency range f = 2 - 4 THz. The peaks depend linearly on the electric field, retain the height of 20 cm{sup -1}, and strongly depend on the thickness of the AlGaAs-layers. (lasers)

  3. Photoacoustic transformation of Bessel light beams in magnetoactive superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Mityurich, G. S., E-mail: George-mityurich@mail.ru [Belarusian Trade and Economics University of Consumer Cooperatives (Belarus); Chernenok, E. V.; Sviridova, V. V.; Serdyukov, A. N. [Gomel State University (Belarus)

    2015-03-15

    Photoacoustic transformation of the TE mode of a Bessel light beam (BLB) has been studied for piezoelectric detection in short-period superlattices formed by magnetoactive crystals of bismuth germanate (Bi{sub 12}GeO{sub 20}) and bismuth silicate (Bi{sub 12}SiO{sub 20}) types. It is shown that the resulting signal amplitude can be controlled using optical schemes of BLB formation with a tunable cone angle. A resonant increase in the signal amplitude has been found in the megahertz range of modulation frequencies and its dependences on the BLB modulation frequency, geometric sizes of the two-layer structure and piezoelectric transducer, radial coordinate of the polarization BLB mode, and dissipative superlattice parameters are analyzed.

  4. Investigation of InAs/GaSb-based superlattices by diffraction methods

    Energy Technology Data Exchange (ETDEWEB)

    Ashuach, Y.; Kauffmann, Y.; Lakin, E. [Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000 (Israel); Zolotoyabko, E., E-mail: zloto@tx.technion.ac.i [Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000 (Israel); Grossman, S.; Klin, O.; Weiss, E. [SCD, SemiConductor Devices, P. O. Box 2250, Haifa 31021 (Israel)

    2010-02-15

    We use high-resolution X-ray diffraction and high-resolution transmission electron microscopy in order to study the strain state, atomic intermixing and layer thicknesses in the MBE-grown GaSb/InSb/InAs/InSb superlattices. Simple and fast metrology procedure is developed, which allows us to obtain the most important technological parameters, such as the thicknesses of the GaSb, InAs and ultra-thin InSb sub-layers, the superlattice period and the fraction of atomic substitutions in the InSb sub-layers.

  5. Quantum dynamical phenomena of independent electrons in semiconductor superlattices subject to a uniform electric field

    International Nuclear Information System (INIS)

    Bouchard, A.M.

    1994-01-01

    This report discusses the following topics: Bloch oscillations and other dynamical phenomena of electrons in semiconductor superlattices; solvable dynamical model of an electron in a one-dimensional aperiodic lattice subject to a uniform electric field; and quantum dynamical phenomena of electrons in aperiodic semiconductor superlattices

  6. Strain-tunable half-metallicity in hybrid graphene-hBN monolayer superlattices

    International Nuclear Information System (INIS)

    Meng, Fanchao; Zhang, Shiqi; Lee, In-Ho; Jun, Sukky; Ciobanu, Cristian V.

    2016-01-01

    Highlights: • Armchair superlattices have a bandgap modulated by the deformed domain widths. • Strain and domain width lead to novel spin-dependent behavior for zigzag boundaries. • Limits for spin-dependent bandgap and half-metallic behavior have been charted. - Abstract: As research in 2-D materials evolves toward combinations of different materials, interesting electronic and spintronic properties are revealed and may be exploited in future devices. A way to combine materials is the formation of spatially periodic domain boundaries in an atom-thick monolayer: as shown in recent reports, when these domains are made of graphene and hexagonal boron nitride, the resulting superlattice has half-metallic properties in which one spin component is (semi)metallic and the other is semiconductor. We explore here the range of spin-dependent electronic properties that such superlattices can develop for different type of domain boundaries, domain widths, and values of tensile strain applied to the monolayer. We show evidence of an interplay between strain and domain width in determining the electronic properties: while for armchair boundaries the bandgap is the same for both spin components, superlattices with zigzag boundaries exhibit rich spin-dependent behavior, including different bandgaps for each spin component, half-metallicity, and reversal of half-metallicity. These findings can lead to new ways of controlling the spintronic properties in hybrid-domain monolayers, which may be exploited in devices based on 2-D materials.

  7. Microstructure evolution during the precipitation and growth of fully coherent DO{sub 22} superlattice in an Ni-Cr-W alloy

    Energy Technology Data Exchange (ETDEWEB)

    Gao, Xiangyu [Stake Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi' an 710072 (China); Hu, Rui, E-mail: rhu@nwpu.edu.cn [Stake Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi' an 710072 (China); Li, Xiaolin [Stake Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819 (China); Luo, Gongliao [Stake Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi' an 710072 (China)

    2016-08-15

    The ordering transformation occurring in a model Ni-Cr-W superalloy during prolonged exposure to proper temperature has been investigated systematically. It is demonstrated that nanometer-sized precipitates with a DO{sub 22} structure can precipitate in the Ni-Cr-W alloy by means of simple aging treatment at 650–700 °C. The mechanism of transformation to DO{sub 22} superlattice has been determined to be continuous ordering based on the results of high resolution transmission electron microscopy investigation and variation trend in Vickers microhardness. Different variants of DO{sub 22} phase can coexist in the matrix with no signs of overaging as aging time increases, indicating it has a high thermal stability. The precipitates of DO{sub 22} superlattice has been found to be of ellipsoidal shape which results in the greatest reduction of strain energy. The interfaces between DO{sub 22} precipitates and matrix have been revealed to be coherent at the atomic scale, resulting in considerable coherency strain attributing to the lattice misfit between DO{sub 22} particle and matrix. Because of the high-density nanometer-sized DO{sub 22} phase, the microhardness of the alloy has been improved remarkably after aging treatment. - Graphical abstract: Different variants of the DO{sub 22} superlattice can coexist in the matrix, and the interface between precipitate and the matrix remain coherence at the atomic scale. The three dimensional form of the DO{sub 22} precipitates constructed from three mutually perpendicular projections is an ellipsoidal stick, and the directions of elongations are along the longest axis of the unit cell for DO{sub 22} phase. - Highlights: •The DO{sub 22} phase precipitated in the Ni-Cr-W alloy has a high thermal stability. •The morphology of DO{sub 22} superlattice has been determined to be ellipsoid. •The interface between DO{sub 22} phase and matrix are fully coherent at the atomic scale. •Different variants of DO{sub 22} phase occur

  8. InAs/GaSb type-II superlattice infrared detectors: three decades of development

    Science.gov (United States)

    Rogalski, A.; Kopytko, M.; Martyniuk, P.

    2017-02-01

    Recently, there has been considerable progress towards III-V antimonide-based low dimensional solids development and device design innovations. From a physics point of view, the type-II InAs/GaSb superlattice is an extremely attractive proposition. Their development results from two primary motivations: the perceived challenges of reproducibly fabricating high-operability HgCdTe FPAs at reasonable cost and theoretical predictions of lower Auger recombination for type-II superlattice (T2SL) detectors compared to HgCdTe. Lower Auger recombination should be translated into a fundamental advantage for T2SL over HgCdTe in terms of lower dark current and/or higher operating temperature, provided other parameters such as Shockley-Read-Hall lifetime are equal. Based on these promising results it is obvious now that the InAs/GaSb superlattice technology is competing with HgCdTe third generation detector technology with the potential advantage of standard III-V technology to be more competitive in costs and as a consequence series production pricing. Comments to the statement whether the superlattice IR photodetectors can outperform the "bulk" narrow gap HgCdTe detectors is one of the most important questions for the future of IR photodetectors presented by Rogalski at the April 2006 SPIE meeting in Orlando, Florida, are more credible today and are presented in this paper. It concerns the trade-offs between two most competing IR material technologies: InAs/GaSb type-II superlattices and HgCdTe ternary alloy system.

  9. InGaAs/InAlAs superlattice detector for THz radiation

    CERN Document Server

    Schomburg, E; Kratschmer, M; Vollnhals, A; Scheuerer, R; Renk, K F; Ustinov, V; Zhukov, A; Kovsh, A

    2002-01-01

    We report the use of an InGaAs/InAlAs superlattice for detection of THz radiation pulses generated by a free-electron-laser (FELIX). The detector showed a response corresponding to a reduction of the direct current through the superlattice. The current reduction is attributed to the THz-field induced modulation of Bloch oscillations performed by miniband electrons. The detector response was measured in a frequency range between 4 and 12 THz and showed strong minima at the frequencies of infrared active transverse optic phonons. (10 refs).

  10. Hybrid inorganic–organic superlattice structures with atomic layer deposition/molecular layer deposition

    Energy Technology Data Exchange (ETDEWEB)

    Tynell, Tommi; Yamauchi, Hisao; Karppinen, Maarit, E-mail: maarit.karppinen@aalto.fi [Department of Chemistry, Aalto University, FI-00076 Aalto (Finland)

    2014-01-15

    A combination of the atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques is successfully employed to fabricate thin films incorporating superlattice structures that consist of single layers of organic molecules between thicker layers of ZnO. Diethyl zinc and water are used as precursors for the deposition of ZnO by ALD, while three different organic precursors are investigated for the MLD part: hydroquinone, 4-aminophenol and 4,4′-oxydianiline. The successful superlattice formation with all the organic precursors is verified through x-ray reflectivity studies. The effects of the interspersed organic layers/superlattice structure on the electrical and thermoelectric properties of ZnO are investigated through resistivity and Seebeck coefficient measurements at room temperature. The results suggest an increase in carrier concentration for small concentrations of organic layers, while higher concentrations seem to lead to rather large reductions in carrier concentration.

  11. Intraband dynamics and terahertz emission in biased semiconductor superlattices coupled to double far-infrared pulses

    International Nuclear Information System (INIS)

    Min, Li; Xian-Wu, Mi

    2009-01-01

    This paper studies both the intraband polarization and terahertz emission of a semiconductor superlattice in combined dc and ac electric fields by using the superposition of two identical time delayed and phase shifted optical pulses. By adjusting the delay between these two optical pulses, our results show that the intraband polarization is sensitive to the time delay. The peak values appear again for the terahertz emission intensity due to the superposition of two optical pulses. The emission lines of terahertz blueshift and redshift in different ac electric fields and dynamic localization appears. The emission lines of THz only appear to blueshift when the biased superlattice is driven by a single optical pulse. Due to excitonic dynamic localization, the terahertz emission intensity decays with time in different dc and ac electric fields. These are features of this superlattice which distinguish it from a superlattice generated by a single optical pulse to drive it. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

  12. Two-dimensional thermoelectric Seebeck coefficient of SrTiO3-based superlattices

    International Nuclear Information System (INIS)

    Ohta, Hiromichi

    2008-01-01

    This review provides the origin of the unusually large thermoelectric Seebeck coefficient vertical stroke S vertical stroke of a two-dimensional electron gas confined within a unit cell layer thickness (∝0.4 nm) of a SrTi 0.8 Nb 0.2 O 3 layer of artificial superlattices of SrTiO 3 /SrTi 0.8 Nb 0.2 O 3 [H. Ohta et al., Nature Mater. 6, 129 (2007)]. The vertical stroke S vertical stroke 2D values of the[(SrTiO 3 ) 17 /(SrTi 0.8 Nb 0.2 O 3 ) y ] 20 superlattice increase proportional to y -0.5 , and reach 290 μV K -1 (y=1) at room temperature, which is ∝5 times larger than that of the SrTi 0.8 Nb 0.2 O 3 bulk (vertical stroke S vertical stroke 3D =61 μVK -1 ), proving that the density of states in the ground state for SrTiO 3 increases in inverse proportion to y. The critical barrier thickness for quantum electron confinement is also clarified to be 6.25 nm (16 unit cells of SrTiO 3 ). Significant structural changes are not observed in the superlattice after annealing at 900 K in a vacuum. The value of vertical stroke S vertical stroke 2D of the superlattice gradually increases with temperature and reaches 450 μVK -1 at 900 K, which is ∝3 times larger than that of bulk SrTi 0.8 Nb 0.2 O 3 . These observations provide clear evidence that the [(SrTiO 3 ) 17 /(SrTi 0.8 Nb 0.2 O 3 ) 1 ] 20 superlattice is stable and exhibits a giant vertical stroke S vertical stroke even at high temperature. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  13. ) m /SrVO3 ( m = 5, 6) Superlattices

    KAUST Repository

    Dai, Qingqing

    2018-05-04

    The (LaV3+O3)m/SrV4+O3 (m = 5, 6) superlattices are investigated by first principles calculations. While bulk LaVO3 is a C‐type antiferromagnetic semiconductor and bulk SrVO3 is a paramagnetic metal, semiconducting A‐type antiferromagnetic states for both superlattices are found due to epitaxial strain. At the interfaces, however, the V spins couple antiferromagnetically for m = 5 and ferromagnetically for m = 6 (m‐dependence of the magnetization). Electronic reconstruction in form of charge ordering is predicted to occur with V3+ and V4+ states arranged in a checkerboard pattern on both sides of the SrO layer. As compared to bulk LaVO3, the presence of V4+ ions introduces in‐gap states that strongly reduce the bandgap and influence the orbital occupation and ordering.

  14. Photon strength and the low-energy enhancement

    Energy Technology Data Exchange (ETDEWEB)

    Wiedeking, M. [iThemba LABS, P.O. Box 722, Somerset West 7129 (South Africa); Bernstein, L. A.; Bleuel, D. L.; Burke, J. T.; Hatarik, R.; Lesher, S. R.; Scielzo, N. D. [Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94551 (United States); Krtička, M. [Faculty of Mathematics and Physics, Charles University, V Holešovickách 2, Prague 8 (Czech Republic); Allmond, J. M. [Department of Physics, University of Richmond, Virginia 23173 (United States); Basunia, M. S.; Fallon, P.; Firestone, R. B.; Lake, P. T.; Lee, I-Y.; Paschalis, S.; Petri, M.; Phair, L. [Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (United States); Goldblum, B. L. [Department of Nuclear Engineering, University of California, Berkeley, California 94720 (United States)

    2014-08-14

    Several measurements in medium mass nuclei have reported a low-energy enhancement in the photon strength function. Although, much effort has been invested in unraveling the mysteries of this effect, its physical origin is still not conclusively understood. Here, a completely model-independent experimental approach to investigate the existence of this enhancement is presented. The experiment was designed to study statistical feeding from the quasi-continuum (below the neutron separation energy) to individual low-lying discrete levels in {sup 95}Mo produced in the (d, p) reaction. A key aspect to successfully study gamma decay from the region of high-level density is the detection and extraction of correlated particle-gamma-gamma events which was accomplished using an array of Clover HPGe detectors and large area annular silicon detectors. The entrance channel excitation energy into the residual nucleus produced in the reaction was inferred from the detected proton energies in the silicon detectors. Gating on gamma-transitions originating from low-lying discrete levels specifies the state fed by statistical gamma-rays. Any particle-gamma-gamma event in combination with specific energy sum requirements ensures a clean and unambiguous determination of the initial and final state of the observed gamma rays. With these requirements the statistical feeding to individual discrete levels is extracted on an event-by-event basis. The results are presented and compared to {sup 95}Mo photon strength function data measured at the University of Oslo.

  15. Development of Strained-Layer Superlattice (SLS) IR Detector Camera

    Data.gov (United States)

    National Aeronautics and Space Administration — Strained Layer Superlattice (SLS) detectors are a new class of detectors which may be the next generation of band-gap engineered, large format infrared detector...

  16. Interface-Induced Nucleation, Orientational Alignment and Symmetry Transformations in Nanocube Superlattices

    KAUST Repository

    Choi, Joshua J.

    2012-09-12

    The self-assembly of colloidal nanocrystals into ordered superstructures depends critically on the shape of the nanocrystal building blocks. We investigated the self-assembly of cubic PbSe nanocrystals from colloidal suspensions in real-time using in situ synchrotron-based X-ray scattering. We combined small-angle and wide-angle scattering to investigate the translational ordering of nanocrystals and their orientational ordering in the lattice sites, respectively. We found that cubic PbSe nanocrystals assembled into a face-up (i.e., 〈100〉 normal to the interface) configuration at the liquid/substrate interface whereas nanocubes at the liquid/air interface assume a corner-up (i.e., 〈111〉 normal to the interface) configuration. The latter nanocrystal superlattice displays polymorphism as a function inter-NC separation distance. We explain the observed superlattice structure polymorphs in terms of the interactions directing the self-assembly. Insights into the directed self-assembly of superlattices gained from this study have important implication on the future development of nanocrystals as building blocks in artificial solids. © 2012 American Chemical Society.

  17. Kinetic Monte Carlo simulation of phase-precipitation versus instability behavior in short period FeCr superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Rodríguez-Martínez, F.J. [UCAM, Universidad Católica de Murcia, Campus de los Jerónimos, 30107 Guadalupe (Murcia) (Spain); Castejón-Mochón, J.F., E-mail: jfcastejon@ucam.edu [UCAM, Universidad Católica de Murcia, Campus de los Jerónimos, 30107 Guadalupe (Murcia) (Spain); Castrillo, P.; Berenguer-Vidal, R. [UCAM, Universidad Católica de Murcia, Campus de los Jerónimos, 30107 Guadalupe (Murcia) (Spain); Dopico, I.; Martin-Bragado, I. [IMDEA Materials Institute, Eric Kandel 2, 28906 Getafe (Madrid) (Spain)

    2017-02-15

    The structural evolution of FeCr superlattices has been studied using a quasi-atomistic Object Kinetic Monte Carlo model. Superlattices with different spatial periods have been simulated for anneal durations from few hours to several months at 500 °C. Relatively-long period superlattices stabilize into Fe-rich and Cr-rich layers with compositions close to those of bulk α and α′ phases. In contrast, superlattices with very short periods (4, 5, 6 nm) are observed to undergo instability and, for long annealing times, evolve into three-dimensionally decomposed regions, in qualitative agreement to recent experimental observations. The instability onset is delayed as the spatial period increases, and it occurs via interface roughness. This evolution can be explained as a minimization of the free-energy associated to the α/α′ interfaces. A comprehensive description of the evolution dynamics of FeCr-based structures is obtained with our model.

  18. Tunable Noncollinear Antiferromagnetic Resistive Memory through Oxide Superlattice Design

    Science.gov (United States)

    Hoffman, Jason D.; Wu, Stephen M.; Kirby, Brian J.; Bhattacharya, Anand

    2018-04-01

    Antiferromagnets (AFMs) have recently gathered a large amount of attention as a potential replacement for ferromagnets (FMs) in spintronic devices due to their lack of stray magnetic fields, invisibility to external magnetic probes, and faster magnetization dynamics. Their development into a practical technology, however, has been hampered by the small number of materials where the antiferromagnetic state can be both controlled and read out. We show that by relaxing the strict criterion on pure antiferromagnetism, we can engineer an alternative class of magnetic materials that overcome these limitations. This is accomplished by stabilizing a noncollinear magnetic phase in LaNiO3 /La2 /3Sr1 /3MnO3 superlattices. This state can be continuously tuned between AFM and FM coupling through varying the superlattice spacing, strain, applied magnetic field, or temperature. By using this alternative "knob" to tune magnetic ordering, we take a nanoscale materials-by-design approach to engineering ferromagneticlike controllability into antiferromagnetic synthetic magnetic structures. This approach can be used to trade-off between the favorable and unfavorable properties of FMs and AFMs when designing realistic resistive antiferromagnetic memories. We demonstrate a memory device in one such superlattice, where the magnetic state of the noncollinear antiferromagnet is reversibly switched between different orientations using a small magnetic field and read out in real time with anisotropic magnetoresistance measurements.

  19. Lateral surface superlattices in strained InGaAs layers

    International Nuclear Information System (INIS)

    Milton, B.

    2000-08-01

    Lateral Surface Superlattices were fabricated by etching in strained InGaAs layers above a GaAs/AlGaAs 2DEG channel. These were etched both by dry plasma wet chemical etching to produce periods of 100nm, 200nm and 300nm. These superlattices were fabricated on Hall bars to allow four terminal measurement and a blanket gate was placed on top, to allow variations in the carrier concentration. The magnetoresistance effects of these superlattices were studied at varying values of gate voltage, which varies the carrier concentration and the electrostatic periodic potential and at temperatures down to 45mK in a dilution refrigerator. From the oscillations observed in the magnetoresistance trace's it is possible to calculate the magnitude of the periodic potential. This showed that the etched, strained InGaAs was producing an anisotropic piezoelectric potential, along with an isotropic electrostatic potential. The variation in period allowed a study of the change of this piezoelectric potential with the period as well as a study of the interactions between the electrostatic and piezoelectric potentials. Further, at the lowest temperatures a strong interaction was observed between the Commensurability Oscillations, caused by the periodic potential, and the Shubnikov-de Haas Oscillations due to the Landau. Levels. This interaction was studied as it varied with temperature and carrier concentration. (author)

  20. Design of band pass filter in a modulated magnetic graphene superlattice

    International Nuclear Information System (INIS)

    Lu, Wei-Tao; Li, Wen

    2015-01-01

    Electronic transport of graphene through a modulated magnetic superlattice where the barrier heights present Gaussian profile is studied. It is found that the incident electron could be completely transmitted in the miniband regions and be completely reflected in the bandgap regions. The results suggest an application of the structure as an effectively band pass filter, which can be controlled by the structural parameters. It is concluded that the positions of miniband and bandgap are robust to the Gaussian variation of barrier heights. The effect of this modulated magnetic superlattice is also available for the conventional electrons described by Schrödinger equation

  1. Raman scattering from In0.2Ga0.8N/GaN superlattices

    International Nuclear Information System (INIS)

    Kisoda, Kenji; Hirakura, Kohji; Harima, Hiroshi

    2006-01-01

    We have performed Raman scattering experiments on high quality In 0.2 Ga 0.8 N/GaN superlattices(SLs). The A 1 LO phonon mode from the In 0.2 Ga 0.8 N layer was observed in the Mg doped SL. This was attributable to manifestation of a resonance enhancement via acceptor levels formed by magnesium doping. The peak frequency of the A 1 LO mode shifted to high frequency side with the excitation energy. The frequency shift suggested that the composition of indium was fluctuated along the growth direction in the InGaN layer. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  2. Surface magnetic phase transitions in Dy/Lu superlattices

    International Nuclear Information System (INIS)

    Goff, J.P.; Sarthour, R.S.; Micheletti, C.; Langridge, S.; Wilkins, C.J.T.; Ward, R.C.C.; Wells, M.R.

    1999-01-01

    Dy/Lu superlattices comprising ferromagnetic Dy blocks coupled antiferromagnetically across the Lu blocks may be modelled as a chain of XY spins with antiferromagnetic exchange and six-fold anisotropy. We have calculated the stable magnetic phases for the cases of large anisotropy and a field applied along an easy direction. For an infinite chain an intermediate phase (1, 5,...) is predicted, where the notation gives the angle between the moment and the applied field in units of π/3. Furthermore, the effects of surface reconstruction are determined for finite chains. A [Dy 20 Lu 12 ] 20 superlattice has been studied using bulk magnetization and polarized neutron reflectivity. The (1, 5,...) phase has been identified and the results provide direct evidence in support of the theoretical predictions. Dipolar forces are shown to account for the magnitude of the observed exchange coupling. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)

  3. Broadband mid-infrared superlattice light-emitting diodes

    Science.gov (United States)

    Ricker, R. J.; Provence, S. R.; Norton, D. T.; Boggess, T. F.; Prineas, J. P.

    2017-05-01

    InAs/GaSb type-II superlattice light-emitting diodes were fabricated to form a device that provides emission over the entire 3-5 μm mid-infrared transmission window. Variable bandgap emission regions were coupled together using tunnel junctions to emit at peak wavelengths of 3.3 μm, 3.5 μm, 3.7 μm, 3.9 μm, 4.1 μm, 4.4 μm, 4.7 μm, and 5.0 μm. Cascading the structure recycles the electrons in each emission region to emit several wavelengths simultaneously. At high current densities, the light-emitting diode spectra broadened into a continuous, broadband spectrum that covered the entire mid-infrared band. When cooled to 77 K, radiances of over 1 W/cm2 sr were achieved, demonstrating apparent temperatures above 1000 K over the 3-5 μm band. InAs/GaSb type-II superlattices are capable of emitting from 3 μm to 30 μm, and the device design can be expanded to include longer emission wavelengths.

  4. Strain-Mediated Inverse Photoresistivity in SrRuO3/La0.7Sr0.3MnO3Superlattices

    KAUST Repository

    Liu, Heng-Jui

    2015-12-09

    © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. In the pursuit of novel functionalities by utilizing the lattice degree of freedom in complex oxide heterostructure, the control mechanism through direct strain manipulation across the interfaces is still under development, especially with various stimuli, such as electric field, magnetic field, light, etc. In this study, the superlattices consisting of colossal-magnetoresistive manganites La0.7Sr0.3MnO3 (LSMO) and photostrictive SrRuO3 (SRO) have been designed to investigate the light-dependent controllability of lattice order in the corresponding functionalities and rich interface physics. Two substrates, SrTiO3 (STO) and LaAlO3 (LAO), have been employed to provide the different strain environments to the superlattice system, in which the LSMO sublayers exhibit different orbital occupations. Subsequently, by introducing light, we can modulate the strain state and orbital preference of LSMO sublayers through light-induced expansion of SRO sublayers, leading to surprisingly opposite changes in photoresistivity. The observed photoresistivity decreases in the superlattice grown on STO substrate while increases in the superlattice grown on LAO substrate under light illumination. This work has presented a model system that demonstrates the manipulation of orbital-lattice coupling and the resultant functionalities in artificial oxide superlattices via light stimulus. A fascinating model system of optic-driven functionalities has been achieved by artificial superlattices consisting of manganite La0.7Sr0.3MnO3 (LSMO) and photostrictive SrRuO3 (SRO). With design of different initial strain and orbital states in superlattices, we can even control the photoresistivity of the superlattices in an opposite trend that cannot be achieved in pure single film.

  5. Effect of anisotropy on the magnon energy gap in a two-layer ferromagnetic superlattice

    International Nuclear Information System (INIS)

    Qiu Rongke; Liang Jing; Li Qingfeng; Zhang Zhidong; Song Panpan; Hong Xiaomin

    2009-01-01

    The magnon energy bands or spectra in a two-layer ferromagnetic superlattice are studied. It is found that a modulated energy gap exists in the magnon energy band along K x direction perpendicular to the superlattice plane, which is different from the optical magnon gap at K x =0. The anisotropy, the spin quantum numbers and the interlayer exchange couplings all affect the magnon energy gap. If the anisotropy exists, there will be no acoustic energy branch in the system. There is a competition effect of the anisotropy and the spin quantum number on the magnon energy gap. The competition achieves a balance at the zero energy gap, at which the symmetry of the system is higher. The two energy spectra of the two-layer ferromagnetic superlattice are lowered with increasing temperature.

  6. Theory and simulation of photogeneration and transport in Si-SiOx superlattice absorbers

    Directory of Open Access Journals (Sweden)

    Aeberhard Urs

    2011-01-01

    Full Text Available Abstract Si-SiOx superlattices are among the candidates that have been proposed as high band gap absorber material in all-Si tandem solar cell devices. Owing to the large potential barriers for photoexited charge carriers, transport in these devices is restricted to quantum-confined superlattice states. As a consequence of the finite number of wells and large built-in fields, the electronic spectrum can deviate considerably from the minibands of a regular superlattice. In this article, a quantum-kinetic theory based on the non-equilibrium Green's function formalism for an effective mass Hamiltonian is used for investigating photogeneration and transport in such devices for arbitrary geometry and operating conditions. By including the coupling of electrons to both photons and phonons, the theory is able to provide a microscopic picture of indirect generation, carrier relaxation, and inter-well transport mechanisms beyond the ballistic regime.

  7. ) m /SrVO3 ( m = 5, 6) Superlattices

    KAUST Repository

    Dai, Qingqing; Lü ders, Ulrike; Fré sard, Raymond; Eckern, Ulrich; Schwingenschlö gl, Udo

    2018-01-01

    The (LaV3+O3)m/SrV4+O3 (m = 5, 6) superlattices are investigated by first principles calculations. While bulk LaVO3 is a C‐type antiferromagnetic semiconductor and bulk SrVO3 is a paramagnetic metal, semiconducting A‐type antiferromagnetic states

  8. Free-Standing Bilayered Nanoparticle Superlattice Nanosheets with Asymmetric Ionic Transport Behaviors.

    Science.gov (United States)

    Rao, Siyuan; Si, Kae Jye; Yap, Lim Wei; Xiang, Yan; Cheng, Wenlong

    2015-11-24

    Natural cell membranes can directionally and selectively regulate the ion transport, which is critical for the functioning of living cells. Here, we report on the fabrication of an artificial membrane based on an asymmetric nanoparticle superlattice bilayered nanosheet, which exhibits similar ion transport characteristics. The superlattice nanosheets were fabricated via a drying-mediated self-assembly of polystyrene-capped gold nanoparticles at the liquid-air interface. By adopting a layer-by-layer assembly process, an asymmetric nanomembrane could be obtained consisting of two nanosheets with different nanoparticle size. The resulting nanomembranes exhibit an asymmetric ion transport behavior, and diode-like current-voltage curves were observed. The asymmetric ion transport is attributed to the cone-like nanochannels formed within the membranes, upon which a simulation map was established to illustrate the relationship between the channel structure and the ionic selectivity, in consistency with our experimental results. Our superlattice nanosheet-based design presents a promising strategy for the fabrication of next-generation smart nanomembranes for rationally and selectively regulating the ion transport even at a large ion flux, with potential applications in a wide range of fields, including biosensor devices, energy conversion, biophotonics, and bioelectronics.

  9. Quantum ratchets for quantum communication with optical superlattices

    International Nuclear Information System (INIS)

    Romero-Isart, Oriol; Garcia-Ripoll, Juan Jose

    2007-01-01

    We propose to use a quantum ratchet to transport quantum information in a chain of atoms trapped in an optical superlattice. The quantum ratchet is created by a continuous modulation of the optical superlattice which is periodic in time and in space. Though there is zero average force acting on the atoms, we show that indeed the ratchet effect permits atoms on even and odd sites to move along opposite directions. By loading the optical lattice with two-level bosonic atoms, this scheme permits us to perfectly transport a qubit or entangled state imprinted in one or more atoms to any desired position in the lattice. From the quantum computation point of view, the transport is achieved by a smooth concatenation of perfect swap gates. We analyze setups with noninteracting and interacting particles and in the latter case we use the tools of optimal control to design optimal modulations. We also discuss the feasibility of this method in current experiments

  10. Shape-Anisotropy Driven Symmetry Transformations in Nanocrystal Superlattice Polymorphs

    KAUST Repository

    Bian, Kaifu; Choi, Joshua J.; Kaushik, Ananth; Clancy, Paulette; Smilgies, Detlef-M.; Hanrath, Tobias

    2011-01-01

    Despite intense research efforts by research groups worldwide, the potential of self-assembled nanocrystal superlattices (NCSLs) has not been realized due to an incomplete understanding of the fundamental molecular interactions governing the self-assembly process. Because NCSLs reside naturally at length-scales between atomic crystals and colloidal assemblies, synthetic control over the properties of constituent nanocrystal (NC) building blocks and their coupling in ordered assemblies is expected to yield a new class of materials with remarkable optical, electronic, and vibrational characteristics. Progress toward the formation of suitable test structures and subsequent development of NCSL-based technologies has been held back by the limited control over superlattice spacing and symmetry. Here we show that NCSL symmetry can be controlled by manipulating molecular interactions between ligands bound to the NC surface and the surrounding solvent. Specifically, we demonstrate solvent vapor-mediated NCSL symmetry transformations that are driven by the orientational ordering of NCs within the lattice. The assembly of various superlattice polymorphs, including face-centered cubic (fcc), body-centered cubic (bcc), and body-centered tetragonal (bct) structures, is studied in real time using in situ grazing incidence small-angle X-ray scattering (GISAXS) under controlled solvent vapor exposure. This approach provides quantitative insights into the molecular level physics that controls solvent-ligand interactions and assembly of NCSLs. Computer simulations based on all-atom molecular dynamics techniques confirm several key insights gained from experiment. © 2011 American Chemical Society.

  11. Shape-Anisotropy Driven Symmetry Transformations in Nanocrystal Superlattice Polymorphs

    KAUST Repository

    Bian, Kaifu

    2011-04-26

    Despite intense research efforts by research groups worldwide, the potential of self-assembled nanocrystal superlattices (NCSLs) has not been realized due to an incomplete understanding of the fundamental molecular interactions governing the self-assembly process. Because NCSLs reside naturally at length-scales between atomic crystals and colloidal assemblies, synthetic control over the properties of constituent nanocrystal (NC) building blocks and their coupling in ordered assemblies is expected to yield a new class of materials with remarkable optical, electronic, and vibrational characteristics. Progress toward the formation of suitable test structures and subsequent development of NCSL-based technologies has been held back by the limited control over superlattice spacing and symmetry. Here we show that NCSL symmetry can be controlled by manipulating molecular interactions between ligands bound to the NC surface and the surrounding solvent. Specifically, we demonstrate solvent vapor-mediated NCSL symmetry transformations that are driven by the orientational ordering of NCs within the lattice. The assembly of various superlattice polymorphs, including face-centered cubic (fcc), body-centered cubic (bcc), and body-centered tetragonal (bct) structures, is studied in real time using in situ grazing incidence small-angle X-ray scattering (GISAXS) under controlled solvent vapor exposure. This approach provides quantitative insights into the molecular level physics that controls solvent-ligand interactions and assembly of NCSLs. Computer simulations based on all-atom molecular dynamics techniques confirm several key insights gained from experiment. © 2011 American Chemical Society.

  12. Excitation on breather (bion) in superlattice

    International Nuclear Information System (INIS)

    Mensah, S.Y.; Allotey, F.K.A.; Mensah, N.G.

    1999-09-01

    Soliton breather excitation in superlattice has been studied in this paper. It is observed that under certain conditions, the vector potential equation for the electromagnetic wave propagating through the superlattice assumes the sine-Gordon(sG) equation. The solution of which does not give only a soliton but also a soliton breather. The binding energy of the breather is calculated to be E b = 16γ(1 - sin ν), γ = (1 - u 2 /v 0 2 ) -1/2 where u is the velocity of the breather and v 0 is the velocity of the electromagnetic wave in the absence of electrons. As can be seen, when ν → π/2 the binding energy tends to zero, hence, the breather disintegrates into a soliton and antisoliton. It was further observed that the binding energy decreases with an increase in Δ (the half miniband width) for a given value of d (SL period). Similarly it also decreases with increase in d for a given value of Δ. Comparing the breather's rest energy E b to that of soliton E s i.e E b = 2E s sin ν. We noted that the breather's rest energy is less than that required to excite a soliton. (author)

  13. InGaNAs/GaAs multi-quantum wells and superlattices solar cells

    International Nuclear Information System (INIS)

    Courel Piedrahita, Maykel; Rimada Herrera, Julio Cesar; Hernandez Garcia, Luis

    2011-01-01

    A theoretical study of the GaAs/InGaNAs solar cells based on a multi-quantum wells (MQWSC) and superlattices (SLSC) configuration is presented for the first time. The conversion efficiency as a function of wells width and depth is modeled. The photon absorption increases with the well levels incorporation and therefore the photocurrent as well. It is shown that the MQWSC efficiency overcomes the solar cells without wells about 25%. A study of the SLSC viability is also presented. The conditions for resonant tunneling are established by the matrix transfer method for a superlattice with variable quantum wells width. The effective density of states and the absorption coefficients for SL structure are calculated in order to determinate the JV characteristic. The influence of the superlattice or cluster width in the cell efficiency is researched showing a better performance when width and the number of cluster are increased. The SLSC efficiency is compared with the optimum efficiency obtained for the MQWSC showing that it is reached an amazing increment of 27%. (author)

  14. Formation mechanism of dot-line square superlattice pattern in dielectric barrier discharge

    Energy Technology Data Exchange (ETDEWEB)

    Liu, Weibo; Dong, Lifang, E-mail: donglfhbu@163.com, E-mail: pyy1616@163.com; Wang, Yongjie; Zhang, Xinpu [College of Physics Science and Technology, Hebei University, Baoding 071002 (China); College of Quality and Technical Supervision, Hebei University, Baoding 071002 (China); Pan, Yuyang, E-mail: donglfhbu@163.com, E-mail: pyy1616@163.com [College of Quality and Technical Supervision, Hebei University, Baoding 071002 (China)

    2014-11-15

    We investigate the formation mechanism of the dot-line square superlattice pattern (DLSSP) in dielectric barrier discharge. The spatio-temporal structure studied by using the intensified-charge coupled device camera shows that the DLSSP is an interleaving of three different subpatterns in one half voltage cycle. The dot square lattice discharges first and, then, the two kinds of line square lattices, which form square grid structures discharge twice. When the gas pressure is varied, DLSSP can transform from square superlattice pattern (SSP). The spectral line profile method is used to compare the electron densities, which represent the amounts of surface charges qualitatively. It is found that the amount of surface charges accumulated by the first discharge of DLSSP is less than that of SSP, leading to a bigger discharge area of the following discharge (lines of DLSSP instead of halos of SSP). The spatial distribution of the electric field of the surface charges is simulated to explain the formation of DLSSP. This paper may provide a deeper understanding for the formation mechanism of complex superlattice patterns in DBD.

  15. Transport properties of YBa2Cu3O7/PrBa2Cu3O7 superlattices

    International Nuclear Information System (INIS)

    Jakob, G.; Hahn, T.; Stoelzel, C.; Tome-Rosa, C.; Adrian, H.

    1992-01-01

    We investigated the transport properties of high-quality YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 superlattices. The exceptional structural order of the superlattices resulted in satellite peaks up to the ninth order in X-ray diffraction diagrams and high Tc values. We find high superconducting critical transport current densities j c even for ultrafine modulated superlattices which proves the existence of nearly continuous YBa 2 Cu 3 O 7 layers. The activation energy U is found to be constant or to have a linear temperatures dependence over a wide temperature range. (orig.)

  16. Simulation of electron transport in GaAs/AlAs superlattices with a small number of periods for the THz frequency range

    International Nuclear Information System (INIS)

    Pavelyev, D. G.; Vasilev, A. P.; Kozlov, V. A.; Koschurinov, Yu. I.; Obolenskaya, E. S.; Obolensky, S. V.; Ustinov, V. M.

    2016-01-01

    The electron transport in superlattices based on GaAs/AlAs heterostructures with a small number of periods (6 periods) is calculated by the Monte Carlo method. These superlattices are used in terahertz diodes for the frequency stabilization of quantum cascade lasers in the range up to 4.7 THz. The band structure of superlattices with different numbers of AlAs monolayers is considered and their current–voltage characteristics are calculated. The calculated current–voltage characteristics are compared with the experimental data. The possibility of the efficient application of these superlattices in the THz frequency range is established both theoretically and experimentally.

  17. Development of Strained-Layer Superlattice (SLS) IR Detector Camera Project

    Data.gov (United States)

    National Aeronautics and Space Administration — Strained Layer Superlattice (SLS) detectors are a new class of detectors which may be the next generation of band-gap engineered, large format infrared detector...

  18. Green’s function theory of ferromagnetic resonance in magnetic superlattices with damping

    International Nuclear Information System (INIS)

    Qiu, R.K.; Guo, F.F.; Zhang, Z.D.

    2016-01-01

    We explore a quantum Green’s-function method to study the resonance absorption of magnetic materials. The relationship between the resonance magnon (spin wave) density and the resonance frequency of a superlattice consisting of two magnetic layers with damping and antiferromagnetic interlayer exchange coupling is studied. The effects of temperature, interlayer coupling, anisotropy, external magnetic field and damping on the the resonance frequency and resonance magnon density are investigated. The resonance excitation probability for a magnon is proportional to the resonance magnon density. In the classic methods, the imaginary part of magnetic permeability represents the resonance absorption in magnetic materials. In the quantum approach, the resonance magnon density can be used to estimate the strength of the resonance absorption. In the present work, a quantum approach is developed to study resonance absorption of magnetic materials and the results show the method to obtain a magnetic multilayered materials with both high resonance frequency and high resonance absorption.

  19. Green’s function theory of ferromagnetic resonance in magnetic superlattices with damping

    Energy Technology Data Exchange (ETDEWEB)

    Qiu, R.K., E-mail: rkqiu@163.com [Shenyang University of Technology, Shenyang 110870 (China); Guo, F.F. [Shenyang University of Technology, Shenyang 110870 (China); Zhang, Z.D. [Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016 (China)

    2016-02-01

    We explore a quantum Green’s-function method to study the resonance absorption of magnetic materials. The relationship between the resonance magnon (spin wave) density and the resonance frequency of a superlattice consisting of two magnetic layers with damping and antiferromagnetic interlayer exchange coupling is studied. The effects of temperature, interlayer coupling, anisotropy, external magnetic field and damping on the the resonance frequency and resonance magnon density are investigated. The resonance excitation probability for a magnon is proportional to the resonance magnon density. In the classic methods, the imaginary part of magnetic permeability represents the resonance absorption in magnetic materials. In the quantum approach, the resonance magnon density can be used to estimate the strength of the resonance absorption. In the present work, a quantum approach is developed to study resonance absorption of magnetic materials and the results show the method to obtain a magnetic multilayered materials with both high resonance frequency and high resonance absorption.

  20. Characterization of band structure for transverse acoustic phonons in Fibonacci superlattices by a bandedge formalism

    International Nuclear Information System (INIS)

    Hsueh, W J; Chen, R F; Tang, K Y

    2008-01-01

    We present a divergence-free method to determine the characteristics of band structures and projected band structures of transverse acoustic phonons in Fibonacci superlattices. A set of bandedge equations is formulated to solve the band structures for the phonon instead of using the traditional dispersion relation. Numerical calculations show band structures calculated by the present method for the Fibonacci superlattice without numerical instability, which may occur in traditional methods. Based on the present formalism, the band structure for the acoustic phonons has been characterized by closure points and the projected bandgaps of the forbidden bands. The projected bandgaps are determined by the projected band structure, which is characterized by the cross points of the projected bandedges. We observed that the band structure and projected band structure and their characteristics were quite different for different generation orders and the basic layers for the Fibonacci superlattice. In this study, concise rules to determine these characteristics of the band structure and the projected band structure, including the number and the location of closure points of forbidden bands and those of projected bandgaps, in Fibonacci superlattices with arbitrary generation order and basic layers are proposed.

  1. Superlattice electroabsorption radiation detector

    International Nuclear Information System (INIS)

    Cooke, B.J.

    1993-06-01

    This paper provides a preliminary investigation of a new class of superlattice electroabsorption radiation detectors that employ direct optical modulation for high-speed, two-dimensional (2-D), high-resolution imaging. Applications for the detector include nuclear radiation measurements, tactical guidance and detection (laser radar), inertial fusion plasma studies, and satellite-based sensors. Initial calculations discussed in this paper indicate that a 1.5-μm (GaAlAs) multi-quantum-well (MQW) Fabry-Perot detector can respond directly to radiation of energies 1 eV to 10 KeV, and indirectly (with scattering targets) up through gamma, with 2-D sample rates on the order of 20 ps

  2. Hot electrons in superlattices: quantum transport versus Boltzmann equation

    DEFF Research Database (Denmark)

    Wacker, Andreas; Jauho, Antti-Pekka; Rott, S.

    1999-01-01

    A self-consistent solution of the transport equation is presented for semiconductor superlattices within different approaches: (i) a full quantum transport model based on nonequilibrium Green functions, (ii) the semiclassical Boltzmann equation for electrons in a miniband, and (iii) Boltzmann...

  3. Quantum Transport: The Link between Standard Approaches in Superlattices

    DEFF Research Database (Denmark)

    Wacker, Andreas; Jauho, Antti-Pekka

    1998-01-01

    Theories describing electrical transport in semiconductor superlattices can essentially be divided in three disjoint categories: (i) transport in a miniband; (ii) hopping between Wannier-Stark ladders; and (iii) sequential tunneling. We present a quantum transport model, based on nonequilibrium G...

  4. Nearly Efficiency-Droop-Free AlGaN-Based Ultraviolet Light-Emitting Diodes with a Specifically Designed Superlattice p-Type Electron Blocking Layer for High Mg Doping Efficiency

    Science.gov (United States)

    Zhang, Zi-Hui; Huang Chen, Sung-Wen; Chu, Chunshuang; Tian, Kangkai; Fang, Mengqian; Zhang, Yonghui; Bi, Wengang; Kuo, Hao-Chung

    2018-04-01

    This work reports a nearly efficiency-droop-free AlGaN-based deep ultraviolet light-emitting diode (DUV LED) emitting in the peak wavelength of 270 nm. The DUV LED utilizes a specifically designed superlattice p-type electron blocking layer (p-EBL). The superlattice p-EBL enables a high hole concentration in the p-EBL which correspondingly increases the hole injection efficiency into the multiple quantum wells (MQWs). The enhanced hole concentration within the MQW region can more efficiently recombine with electrons in the way of favoring the radiative recombination, leading to a reduced electron leakage current level. As a result, the external quantum efficiency for the proposed DUV LED structure is increased by 100% and the nearly efficiency-droop-free DUV LED structure is obtained experimentally.

  5. Characterization Of Graphene-Ferroelectric Superlattice Hybrid Devices

    Science.gov (United States)

    Yusuf, Mohammed; Du, Xu; Dawber, Matthew

    2013-03-01

    Ferroelectric materials possess a spontaneous electrical polarization, which can be controlled by an electric field. A good interface between ferroelectric surface and graphene sheets can introduce a new generation of multifunctional devices, in which the ferroelectric material can be used to control the properties of graphene. In our approach, problems encountered in previous efforts to combine ferroelectric/carbon systems are overcome by the use of artificially layered superlattice materials grown in the form of epitaxial thin films. In these materials the phase transition temperature and dielectric response of the material can be tailored, allowing us to avoid polarization screening by surface absorbates, whilst maintaining an atomically smooth surface and optimal charge doping properties. Using ferroelectric PbTiO3/SrTiO3 superlattices, we have shown ultra-low-voltage operation of graphene field effect devices within +/- 1 V at room temperature. The switching of the graphene field effect transistors is characterized by pronounced resistance hysteresis, suitable for ultra-fast non-volatile electronics. Low temperature characterization confirmed that the coercive field required for the ferroelectric domain switching increases significantly with decreasing temperatures. National Science Foundation (NSF) (grant number 1105202)

  6. Electron drag by solitons in superlattices in an external magnetic field

    International Nuclear Information System (INIS)

    Vyazovskii, M.V.; Syrodoev, G.A.

    1996-01-01

    The soliton-electric effect accompanying the propagation of an electromagnetic soliton along an axis of a superlattice in an external magnetic field directed along the magnetic field of the soliton is studied. It is assumed that the duration γ-1 of the soliton pulse is much shorter than the free flight time of an electron. It is shown that in the absence of a constant magnetic field the drag current varies as sin(αsech2γt) (α is a constant determined by the parameters of the superlattice). In the presence of a constant magnetic field of intensity H0>>Hs, where Hs is the amplitude of the soliton field, the drag current oscillates

  7. Electronic structure and optical properties of (BeTen/(ZnSem superlattices

    Directory of Open Access Journals (Sweden)

    Caid M.

    2016-03-01

    Full Text Available The structural, electronic and optical properties of (BeTen/(ZnSem superlattices have been computationally evaluated for different configurations with m = n and m≠n using the full-potential linear muffin-tin method. The exchange and correlation potentials are treated by the local density approximation (LDA. The ground state properties of (BeTen/(ZnSem binary compounds are determined and compared with the available data. It is found that the superlattice band gaps vary depending on the layers used. The optical constants, including the dielectric function ε(ω, the refractive index n(ω and the refractivity R(ω, are calculated for radiation energies up to 35 eV.

  8. Analysis of polariton dispersion in metal nanocomposite based novel superlattice system

    Science.gov (United States)

    DoniPon, V.; Joseph Wilson, K. S.; Malarkodi, A.

    2018-06-01

    The influence of metal nanoparticles in tuning the polaritonic gap in a novel piezoelectric superlattice is studied. Dielectric function of the metal nanoparticles is analyzed using Kawabata-Kubo effect and Drude's theory. The effective dielectric function of the nanocomposite system is studied using Maxwell Garnett approximation. Nanocomposite based LiTaO3 novel superlattice is formed by arranging the nanocomposite systems in such a way that their orientations are in the opposite direction. Hence there are two additional modes of propagation. The top most modes reflect the metal behavior of the nanoparticles. It is found that these modes of propagation vary with the filling factor. These additional modes of propagations can be exploited in the field of communication.

  9. Transfer matrix theory of monolayer graphene/bilayer graphene heterostructure superlattice

    International Nuclear Information System (INIS)

    Wang, Yu

    2014-01-01

    We have formulated a transfer matrix method to investigate electronic properties of graphene heterostructure consisting of monolayer graphene and bilayer counterpart. By evaluating transmission, conductance, and band dispersion, we show that, irrespective of the different carrier chiralities in monolayer graphene and bilayer graphene, superlattice consisting of biased bilayer graphene barrier and monolayer graphene well can mimic the electronic properties of conventional semiconductor superlattice, displaying the extended subbands in the quantum tunneling regime and producing anisotropic minigaps for the classically allowed transport. Due to the lateral confinement, the lowest mode has shifted away from the charge neutral point of monolayer graphene component, opening a sizeable gap in concerned structure. Following the gate-field and geometry modulation, all electronic states and gaps between them can be externally engineered in an electric-controllable strategy.

  10. Direct observation of two-step crystallization in nanoparticle superlattice formation

    Energy Technology Data Exchange (ETDEWEB)

    Park, Jungwon; Zheng, Haimei; Lee, Won Chul; Geissler, Phillip L.; Rabani, Eran; Alivisatos, A. Paul

    2011-10-06

    Direct imaging of nanoparticle solutions by liquid phase transmission electron microscopy has enabled unique in-situ studies of nanoparticle motion and growth. In the present work, we report on real-time formation of two-dimensional nanoparticle arrays in the very low diffusive limit, where nanoparticles are mainly driven by capillary forces and solvent fluctuations. We find that superlattice formation appears to be segregated into multiple regimes. Initially, the solvent front drags the nanoparticles, condensing them into an amorphous agglomerate. Subsequently, the nanoparticle crystallization into an array is driven by local fluctuations. Following the crystallization event, superlattice growth can also occur via the addition of individual nanoparticles drawn from outlying regions by different solvent fronts. The dragging mechanism is consistent with simulations based on a coarse-grained lattice gas model at the same limit.

  11. Effect of retardation on the reflectance properties of the metallic Fibonacci quasi-superlattice

    International Nuclear Information System (INIS)

    Feng Weiguo; Yao Hesheng; Xu Xiang

    1989-12-01

    Based on the hydrodynamic model theory and the transfer matrix method, we have re-examined the reflection properties by taking account of the retardation effect to the system of the metallic Fibonacci quasi-superlattice. For the normal incident S-polarized Soft X-rays and extreme ultraviolet, we find that the self-similar reflecting spectrum will be restrained with the increasing of the retardation, but for the higher frequency region or at the smaller grazing angle, the self similarity will still exist for the lower generation quasi-superlattice. (author). 19 refs, 2 figs, 1 tab

  12. Increased residual force enhancement in older adults is associated with a maintenance of eccentric strength.

    Directory of Open Access Journals (Sweden)

    Geoffrey A Power

    Full Text Available Despite an age-related loss of voluntary isometric and concentric strength, muscle strength is well maintained during lengthening muscle actions (i.e., eccentric strength in old age. Additionally, in younger adults during lengthening of an activated skeletal muscle, the force level observed following the stretch is greater than the isometric force at the same muscle length. This feature is termed residual force enhancement (RFE and is believed to be a combination of active and passive components of the contractile apparatus. The purpose of this study was to provide an initial assessment of RFE in older adults and utilize aging as a muscle model to explore RFE in a system in which isometric force production is compromised, but structural mechanisms of eccentric strength are well-maintained. Therefore, we hypothesised that older adults will experience greater RFE compared with young adults. Following a reference maximal voluntary isometric contraction (MVC of the dorsiflexors in 10 young (26.1 ± 2.7 y and 10 old (76.0 ± 6.5 y men, an active stretch was performed at 15°/s over a 30° ankle joint excursion ending at the same muscle length as the reference MVCs (40° of plantar flexion. Any additional torque compared with the reference MVC therefore represented RFE. In older men RFE was ~2.5 times greater compared to young. The passive component of force enhancement contributed ~37% and ~20% to total force enhancement, in old and young respectively. The positive association (R(2 = 0.57 between maintained eccentric strength in old age and RFE indicates age-related mechanisms responsible for the maintenance of eccentric strength likely contributed to the observed elevated RFE. Additionally, as indicated by the greater passive force enhancement, these mechanisms may be related to increased muscle series elastic stiffness in old age.

  13. Structural study of multilayered vanadium/nickel superlattices

    International Nuclear Information System (INIS)

    Homma, H.; Lepetre, Y.; Murduck, J.M.; Schuller, I.K.; Majkrzak, C.F.

    1985-07-01

    We have studied the microstructure of V/Ni metallic superlattice, using x-ray and neutron diffraction. We find a sharp and broad rocking curves around the first-order Bragg peak, and attribute them to a columnar structure which gives rise to two modulation structures; one the ordinary layered structure within the columns and the other the averaged modulation structure which produces the sharp rocking peak

  14. Experimental investigations of superconductivity in quasi-two-dimensional epitaxial copper oxide superlattices and trilayers

    International Nuclear Information System (INIS)

    Lowndes, D.H.; Norton, D.P.

    1993-01-01

    Epitaxial trilayer and superlattice structures grown by pulsed laser ablation have been used to study the superconducting-to-normal transition of ultrathin (one and two c-axis unit cells) YBa 2 Cu 3 O 7-x layers. The normalized flux-flow resistances for several epitaxial structures containing two-cell-thick YBa 2 Cu 3 O 7-x films collapse onto the ''universal'' curve of the Ginzburg-Landau Coulomb Gas (GLCG) model. Analysis of normalized resistance data for a series of superlattices containing one-cell-thick YBa 2 Cu 3 O 7-x layers also is consistent with the behavior expected for quasi-two-dimensional layers in a highly anisotropic, layered three-dimensional superconductor. Current-voltage measurements for one of the trilayer structures also are consistent with the normalized resistance data, and with the GLCG model. Scanning tunneling microscopy, transmission electron microscopy, and electrical transport studies show that growth-related steps in ultrathin YBa 2 Cu 3 O 7-x layers affect electrical continuity over macroscopic distances, acting as weak links. However , the perturbation of the superconducting order parameter can be minimized by utilizing hole-doped buffer and cap layers, on both sides of the YBa 2 Cu 3 O 7-x layer, in trilayers and superlattices. These results demonstrate the usefulness of epitaxial trilayer and superlattice structures as tools for systematic, fundamental studies of high-temperature superconductivity

  15. Formation mechanism of gas bubble superlattice in UMo metal fuels: Phase-field modeling investigation

    Energy Technology Data Exchange (ETDEWEB)

    Hu, Shenyang, E-mail: shenyang.hu@pnnl.gov; Burkes, Douglas E.; Lavender, Curt A.; Senor, David J.; Setyawan, Wahyu; Xu, Zhijie

    2016-10-15

    Nano-gas bubble superlattices are often observed in irradiated UMo nuclear fuels. However, the formation mechanism of gas bubble superlattices is not well understood. A number of physical processes may affect the gas bubble nucleation and growth; hence, the morphology of gas bubble microstructures including size and spatial distributions. In this work, a phase-field model integrating a first-passage Monte Carlo method to investigate the formation mechanism of gas bubble superlattices was developed. Six physical processes are taken into account in the model: 1) heterogeneous generation of gas atoms, vacancies, and interstitials informed from atomistic simulations; 2) one-dimensional (1-D) migration of interstitials; 3) irradiation-induced dissolution of gas atoms; 4) recombination between vacancies and interstitials; 5) elastic interaction; and 6) heterogeneous nucleation of gas bubbles. We found that the elastic interaction doesn’t cause the gas bubble alignment, and fast 1-D migration of interstitials along 〈110〉 directions in the body-centered cubic U matrix causes the gas bubble alignment along 〈110〉 directions. It implies that 1-D interstitial migration along [110] direction should be the primary mechanism of a fcc gas bubble superlattice which is observed in bcc UMo alloys. Simulations also show that fission rates, saturated gas concentration, and elastic interaction all affect the morphology of gas bubble microstructures.

  16. Ionic strength-induced formation of smectite quasicrystals enhances nitroaromatic compound sorption.

    Science.gov (United States)

    Li, Hui; Pereira, Tanya R; Teppen, Brian J; Laird, David A; Johnston, Cliff T; Boyd, Stephen A

    2007-02-15

    Sorption of organic contaminants by soils is a determinant controlling their transport and fate in the environment. The influence of ionic strength on nitroaromatic compound sorption by K+- and Ca2+ -saturated smectite was examined. Sorption of 1,3-dinitrobenzene by K-smectite increased as KCl ionic strength increased from 0.01 to 0.30 M. In contrast, sorption by Ca-smectite at CaCl2 ionic strengths of 0.015 and 0.15 M remained essentially the same. The "salting-out" effect on the decrease of 1,3-dinitrobenzene aqueous solubility within this ionic strength range was smectite with increasing KCl ionic strength. X-ray diffraction patterns and light absorbance of K-clay suspensions indicated the aggregation of clay particles and the formation of quasicrystal structures as KCI ionic strength increased. Sorption enhancement is attributed to the formation of better-ordered K-clay quasicrystals with reduced interlayer distances rather than to the salting-out effect. Dehydration of 1,3-dinitrobenzene is apparently a significant driving force for sorption, and we show for the first time that sorption of small, planar, neutral organic molecules, namely, 1,3-dinitrobenzene, causes previously expanded clay interlayers to dehydrate and collapse in aqueous suspension.

  17. Efficient spin filtering in a disordered semiconductor superlattice in the presence of Dresselhaus spin-orbit coupling

    International Nuclear Information System (INIS)

    Khayatzadeh Mahani, Mohammad Reza; Faizabadi, Edris

    2008-01-01

    The influence of the Dresselhaus spin-orbit coupling on spin polarization by tunneling through a disordered semiconductor superlattice was investigated. The Dresselhaus spin-orbit coupling causes the spin polarization of the electron due to transmission possibilities difference between spin up and spin down electrons. The electron tunneling through a zinc-blende semiconductor superlattice with InAs and GaAs layers and two variable distance In x Ga (1-x) As impurity layers was studied. One hundred percent spin polarization was obtained by optimizing the distance between two impurity layers and impurity percent in disordered layers in the presence of Dresselhaus spin-orbit coupling. In addition, the electron transmission probability through the mentioned superlattice is too much near to one and an efficient spin filtering was recommended

  18. Ge/SiGe superlattices for nanostructured thermoelectric modules

    International Nuclear Information System (INIS)

    Chrastina, D.; Cecchi, S.; Hague, J.P.; Frigerio, J.; Samarelli, A.; Ferre–Llin, L.; Paul, D.J.; Müller, E.; Etzelstorfer, T.; Stangl, J.; Isella, G.

    2013-01-01

    Thermoelectrics are presently used in a number of applications for both turning heat into electricity and also for using electricity to produce cooling. Mature Si/SiGe and Ge/SiGe heteroepitaxial growth technology would allow highly efficient thermoelectric materials to be engineered, which would be compatible and integrable with complementary metal oxide silicon micropower circuits used in autonomous systems. A high thermoelectric figure of merit requires that electrical conductivity be maintained while thermal conductivity is reduced; thermoelectric figures of merit can be improved with respect to bulk thermoelectric materials by fabricating low-dimensional structures which enhance the density of states near the Fermi level and through phonon scattering at heterointerfaces. We have grown and characterized Ge-rich Ge/SiGe/Si superlattices for nanofabricated thermoelectric generators. Low-energy plasma-enhanced chemical vapor deposition has been used to obtain nanoscale-heterostructured material which is several microns thick. Crystal quality and strain control have been investigated by means of high resolution X-ray diffraction. High-resolution transmission electron microscopy images confirm the material and interface quality. Electrical conductivity has been characterized by the mobility spectrum technique. - Highlights: ► High-quality Ge/SiGe multiple quantum wells for thermoelectric applications ► Mobility spectra of systems featuring a large number of parallel conduction channels ► Competitive thermoelectric properties measured in single devices

  19. Electronic states in tunneling semiconductor superlattices: Technical progress report for the period September 15, 1987-September 14, 1988

    International Nuclear Information System (INIS)

    Ulloa, S.E.

    1988-01-01

    This research project funded by DOE has concentrated in the systematic study of the effects of a gate voltage on the electronic structure of a tunneling superlattice system. The effects of strong magnetic fields and other various parameters on energy levels of tunneling superlattices have been investigated

  20. Morphological Instability in InAs/GaSb Superlattices due to Interfacial Bonds

    International Nuclear Information System (INIS)

    Li, J.H.; Moss, S.C.; Stokes, D.W.; Caha, O.; Bassler, K.E.; Ammu, S.L.; Bai, J.

    2005-01-01

    Synchrotron x-ray diffraction is used to compare the misfit strain and composition in a self-organized nanowire array in an InAs/GaSb superlattice with InSb interfacial bonds to a planar InAs/GaSb superlattice with GaAs interfacial bonds. It is found that the morphological instability that occurs in the nanowire array results from the large misfit strain that the InSb interfacial bonds have in the nanowire array. Based on this result, we propose that tailoring the type of interfacial bonds during the epitaxial growth of III-V semiconductor films provides a novel approach for producing the technologically important morphological instability in anomalously thin layers

  1. Epitaxial (100)-oriented Mo/V superlattice grown on MgO(100) by dcMS and HiPIMS

    International Nuclear Information System (INIS)

    Shayestehaminzadeh, S.; Magnusson, R.L.; Gislason, H.P.; Olafsson, S.

    2013-01-01

    Epitaxial (100)-oriented Mo/V superlattices have been grown by High Power Impulse Magnetron Sputtering (HiPIMS) and dc Magnetron Sputtering (dcMS) on single-crystalline MgO(100) substrates at growth temperatures ranging from 30 °C to 600 °C. Superlattice bilayer period of Mo/V around 12/12 monolayers and 15 repeat periods was studied. This study aims to investigate the effect of the HiPIMS process on reducing the growth temperature of Mo/V superlattices using the high energy ionized Mo, V species in the HiPIMS plasma. In one case, the Mo layer was only grown with the HiPIMS process and V layer grown using the dcMS process while in another both layers were grown with the HiPIMS process. The as-deposited films were characterized by X-ray reflection and diffraction techniques. The dcMS process was found to give superior superlattice growth at high growth temperatures while a mixed Mo HiPIMS and V dcMS process gives better result at lower growth temperatures (300 °C). Room temperature growth reveals that neither the mixed Mo HiPIMS and V dcMS process nor the pure HiPIMS for both materials can produce better result compared to the pure dcMS process, which gives a relatively better result. - Highlights: • Epitaxial (100)-oriented Mo/V superlattices have been grown by HiPIMS and dcMS on MgO(100) for various temperatures. • The study was aimed to investigate the effect of ionized HiPIMS process onlowering the growth temperature. • The dcMS process was found to give superior superlattice growth at high growth temperature. • The mixed Mo HiPIMS and V dcMS process gives best result at lower growth temperatures

  2. Strain distribution and band structure of InAs/GaAs quantum ring superlattice

    Science.gov (United States)

    Mughnetsyan, Vram; Kirakosyan, Albert

    2017-12-01

    The elastic strain distribution and the band structure of InAs/GaAs one-layer quantum ring superlattice with square symmetry has been considered in this work. The Green's function formalism based on the method of inclusions has been implied to calculate the components of the strain tensor, while the combination of Green's function method with the Fourier transformation to momentum space in Pikus-Bir Hamiltonian has been used for obtaining the miniband energy dispersion surfaces via the exact diagonalization procedure. The dependencies of the strain tensor components on spatial coordinates are compared with ones for single quantum ring and are in good agreement with previously obtained results for cylindrical quantum disks. It is shown that strain significantly affects the miniband structure of the superlattice and has contribution to the degeneracy lifting effect due to heavy hole-light hole coupling. The demonstrated method is simple and provides reasonable results for comparatively small Hamiltonian matrix. The obtained results may be useful for further investigation and construction of novel devices based on quantum ring superlattices.

  3. Localization in superlattices with randomness in layer thickness

    International Nuclear Information System (INIS)

    Yuan Jian; Tsai Chienhua.

    1987-08-01

    The localization length for electrons in superlattices with randomness in layer thickness is studied in both the commensurate and the incommensurate cases. It is demonstrated that disorder limits the electrons to see only structures within the extent of their wave functions and to be hardly effected by any long range correlation. (author). 4 refs, 6 figs

  4. Future device applications of low-dimensional carbon superlattice structures

    Science.gov (United States)

    Bhattacharyya, Somnath

    2005-03-01

    We observe superior transport properties in low-dimensional amorphous carbon (a-C) and superlattice structures fabricated by a number of different techniques. Low temperature conductivity of these materials is explained using argument based on the crossover of dimensionality of weak localization and electron-electron interactions along with a change of sign of the magneto-resistance. These trends are significantly different from many other well characterized ordered or oriented carbon structures, and, show direct evidence of high correlation length, mobility and an effect of the dimensionality in low-dimensional a-C films. We show routes to prepare bespoke features by tuning the phase relaxation time in order to make high-speed devices over large areas. The artificially grown multi-layer superlattice structures of diamond-like amorphous carbon films show high-frequency resonance and quantum conductance suggesting sufficiently high values of phase coherence length in the present disordered a-C system that could lead to fast switching multi-valued logic.

  5. Optical transmission through multi-component generalized Thue-Morse superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Zhang Guogang [MOE Key Laboratory of Laser Life Science and Institute of Laser Life Science, South China Normal University, Guangzhou 510631 (China); Yang Xiangbo, E-mail: xbyang@scnu.edu.c [MOE Key Laboratory of Laser Life Science and Institute of Laser Life Science, South China Normal University, Guangzhou 510631 (China); Li Yuhong; Song Huanhuan [MOE Key Laboratory of Laser Life Science and Institute of Laser Life Science, South China Normal University, Guangzhou 510631 (China)

    2010-09-01

    In this paper, by the three kinds of basic components (BCs) of three-component Thue-Morse (3CTM) sequence we construct a type of interesting optical basic-structural-units (BSUs) and propose multi-component generalized Thue-Morse (mCGTM) model. Based on the conventional electromagnetic wave theory we investigate the optical transmission vertically through the one-dimensional (1D) mCGTM superlattices. It is found that the optical transmission possesses an interesting pseudo-constant characteristic at the central wavelength. mCGTM sequence exhibits a cantor-set structure which results in the system possessing certain kinds of effective component pairs (ECPs), and each kind of ECP brings about certain contribution towards the optical transmission. The cantor-set structure is the reason that mCGTM multilayers exhibit the optical transmission pseudo-constant property. For the pseudo-constant optical transmission of mCGTM superlattices, there would be a potential application in the designing of some complex optical devices.

  6. Comparison of the Cc and R3c space groups for the superlattice phase of Pb(Zr0.52Ti0.48)O3

    International Nuclear Information System (INIS)

    Ranjan, Rajeev; Singh, Akhilesh Kumar; Ragini; Pandey, Dhananjai

    2005-01-01

    Recent controversy about the space group of the low temperature superlattice phase of Pb(Zr 0.52 Ti 0.48 )O 3 is settled. It is shown that the R3c space group for the superlattice phase cannot correctly account for the peak positions of the superlattice reflections present in the neutron diffraction patterns. The correct space group is reconfirmed to be Cc. A comparison of the atomic coordinates of Cc and Cm space groups is also presented to show that in the absence of superlattice reflections, as is the case with x-ray diffraction data, one would land up in the Cm space group. This superlattice phase is found to coexist with another monoclinic phase of the Cm space group

  7. Energy band and transport properties in magnetic aperiodic graphene superlattices of Thue-Morse sequence

    Science.gov (United States)

    Yin, Yiheng; Niu, Yanxiong; Zhang, Huiyun; Zhang, Yuping; Liu, Haiyue

    2016-02-01

    Utilizing the transfer matrix method, we develop the electronic band structure and transport properties in Thue-Morse aperiodic graphene superlattices with magnetic barriers. It is found that the normal transmission is blocked and the position of the Dirac point can be shifted along the wavevector axis by changing the height and width ratio of magnetic barriers, which is intrinsic different from electronic field modulated superlattices. In addition, the angular threshold property of the transmission spectra and the oscillatory property of the conductance have been studied.

  8. Enhanced reconstruction of weighted networks from strengths and degrees

    International Nuclear Information System (INIS)

    Mastrandrea, Rossana; Fagiolo, Giorgio; Squartini, Tiziano; Garlaschelli, Diego

    2014-01-01

    Network topology plays a key role in many phenomena, from the spreading of diseases to that of financial crises. Whenever the whole structure of a network is unknown, one must resort to reconstruction methods that identify the least biased ensemble of networks consistent with the partial information available. A challenging case, frequently encountered due to privacy issues in the analysis of interbank flows and Big Data, is when there is only local (node-specific) aggregate information available. For binary networks, the relevant ensemble is one where the degree (number of links) of each node is constrained to its observed value. However, for weighted networks the problem is much more complicated. While the naïve approach prescribes to constrain the strengths (total link weights) of all nodes, recent counter-intuitive results suggest that in weighted networks the degrees are often more informative than the strengths. This implies that the reconstruction of weighted networks would be significantly enhanced by the specification of both strengths and degrees, a computationally hard and bias-prone procedure. Here we solve this problem by introducing an analytical and unbiased maximum-entropy method that works in the shortest possible time and does not require the explicit generation of reconstructed samples. We consider several real-world examples and show that, while the strengths alone give poor results, the additional knowledge of the degrees yields accurately reconstructed networks. Information-theoretic criteria rigorously confirm that the degree sequence, as soon as it is non-trivial, is irreducible to the strength sequence. Our results have strong implications for the analysis of motifs and communities and whenever the reconstructed ensemble is required as a null model to detect higher-order patterns

  9. Charge dynamics in graphene and graphene superlattices under a high-frequency electric field: a semiclassical approach

    International Nuclear Information System (INIS)

    Kryuchkov, S V; Kukhar’, E I; Zav’yalov, D V

    2013-01-01

    The semiclassical theory of the dynamics of the charge carriers in graphene and in graphene superlattices exposed to a high-frequency electric field is developed. The dispersion law of the solid averaged over the period of the high-frequency electric field is found with the Kapitza method. The band gap in graphene is shown to arise under a high-frequency electric field polarized circularly. The effective mass of charge carriers in the center of the Brillouin band of the graphene superlattice is found to change sign under certain values of the amplitude of the high-frequency field. These values are shown to determine the bounds of the regions of the electromagnetic 2π-pulse stability. The dynamics of the π-pulse in a graphene superlattice is studied. (paper)

  10. Hierarchical self-assembly of hexagonal single-crystal nanosheets into 3D layered superlattices with high conductivity

    Science.gov (United States)

    Tao, Yulun; Shen, Yuhua; Yang, Liangbao; Han, Bin; Huang, Fangzhi; Li, Shikuo; Chu, Zhuwang; Xie, Anjian

    2012-05-01

    While the number of man-made nano superstructures realized by self-assembly is growing in recent years, assemblies of conductive polymer nanocrystals, especially for superlattices, are still a significant challenge, not only because of the simplicity of the shape of the nanocrystal building blocks and their interactions, but also because of the poor control over these parameters in the fabrication of more elaborate nanocrystals. Here, we firstly report a facile and general route to a new generation of 3D layered superlattices of polyaniline doped with CSA (PANI-CSA) and show how PANI crystallize and self-assemble, in a suitable single solution environment. In cyclohexane, 1D amorphous nanofibers transformed to 1D nanorods as building blocks, and then to 2D single-crystal nanosheets with a hexagonal phase, and lastly to 3D ordered layered superlattices with the narrowest polydispersity value (Mw/Mn = 1.47). Remarkably, all the instructions for the hierarchical self-assembly are encoded in the layered shape in other non-polar solvents (hexane, octane) and their conductivity in the π-π stacking direction is improved to about 50 S cm-1, which is even higher than that of the highest previously reported value (16 S cm-1). The method used in this study is greatly expected to be readily scalable to produce superlattices of conductive polymers with high quality and low cost.While the number of man-made nano superstructures realized by self-assembly is growing in recent years, assemblies of conductive polymer nanocrystals, especially for superlattices, are still a significant challenge, not only because of the simplicity of the shape of the nanocrystal building blocks and their interactions, but also because of the poor control over these parameters in the fabrication of more elaborate nanocrystals. Here, we firstly report a facile and general route to a new generation of 3D layered superlattices of polyaniline doped with CSA (PANI-CSA) and show how PANI crystallize and

  11. Recent results on heterojunctions and superlattices: transport and optics

    International Nuclear Information System (INIS)

    Voos, M.

    1983-01-01

    Recent experimental results obtained on two-dimensional semiconductor structures, namely heterojunctions and superlattices are presented. This review, which includes both optical and transport experiments, is not exhaustive, but describes briefly some investigations which are thought to be important from the point of view of fundamental physics. (Author) [pt

  12. Chemically Triggered Formation of Two-Dimensional Epitaxial Quantum Dot Superlattices

    NARCIS (Netherlands)

    Walravens, Willem; De Roo, Jonathan; Drijvers, Emile; Ten Brinck, Stephanie; Solano, Eduardo; Dendooven, Jolien; Detavernier, Christophe; Infante, Ivan; Hens, Zeger

    2016-01-01

    Two dimensional superlattices of epitaxially connected quantum dots enable size-quantization effects to be combined with high charge carrier mobilities, an essential prerequisite for highly performing QD devices based on charge transport. Here, we demonstrate that surface active additives known to

  13. Thermoelectric properties of In-rich InGaN and InN/InGaN superlattices

    Directory of Open Access Journals (Sweden)

    James (Zi-Jian Ju

    2016-04-01

    Full Text Available The thermoelectric properties of n-type InGaN alloys with high In-content and InN/InGaN thin film superlattices (SL grown by molecular beam epitaxy are investigated. Room-temperature measurements of the thermoelectric properties reveal that an increasing Ga-content in ternary InGaN alloys (0 < x(Ga < 0.2 yields a more than 10-fold reduction in thermal conductivity (κ without deteriorating electrical conductivity (σ, while the Seebeck coefficient (S increases slightly due to a widening band gap compared to binary InN. Employing InN/InGaN SLs (x(Ga = 0.1 with different periods, we demonstrate that confinement effects strongly enhance electron mobility with values as high as ∼820 cm2/V s at an electron density ne of ∼5×1019 cm−3, leading to an exceptionally high σ of ∼5400 (Ωcm−1. Simultaneously, in very short-period SL structures S becomes decoupled from ne, κ is further reduced below the alloy limit (κ < 9 W/m-K, and the power factor increases to 2.5×10−4 W/m-K2 by more than a factor of 5 as compared to In-rich InGaN alloys. These findings demonstrate that quantum confinement in group-III nitride-based superlattices facilitates improvements of thermoelectric properties over bulk-like ternary nitride alloys.

  14. Molecular beam epitaxy growth of [CrGe/MnGe/FeGe] superlattices: Toward artificial B20 skyrmion materials with tunable interactions

    Science.gov (United States)

    Ahmed, Adam S.; Esser, Bryan D.; Rowland, James; McComb, David W.; Kawakami, Roland K.

    2017-06-01

    Skyrmions are localized magnetic spin textures whose stability has been shown theoretically to depend on material parameters including bulk Dresselhaus spin orbit coupling (SOC), interfacial Rashba SOC, and magnetic anisotropy. Here, we establish the growth of a new class of artificial skyrmion materials, namely B20 superlattices, where these parameters could be systematically tuned. Specifically, we report the successful growth of B20 superlattices comprised of single crystal thin films of FeGe, MnGe, and CrGe on Si(1 1 1) substrates. Thin films and superlattices are grown by molecular beam epitaxy and are characterized through a combination of reflection high energy electron diffraction, X-ray diffraction, and cross-sectional scanning transmission electron microscopy (STEM). X-ray energy dispersive spectroscopy (XEDS) distinguishes layers by elemental mapping and indicates good interface quality with relatively low levels of intermixing in the [CrGe/MnGe/FeGe] superlattice. This demonstration of epitaxial, single-crystalline B20 superlattices is a significant advance toward tunable skyrmion systems for fundamental scientific studies and applications in magnetic storage and logic.

  15. New spintronic superlattices composed of half-metallic compounds with zinc-blende structure

    International Nuclear Information System (INIS)

    Fong, C Y; Qian, M C

    2004-01-01

    The successful growth of zinc-blende half-metallic compounds, namely CrAs and CrSb, in thin film forms offers a new direction to search for novel spintronic materials. By using a well documented first-principles algorithm, the VASP code, we predict the electronic and magnetic properties of superlattices made of these exciting half-metallic materials. Not only are the superlattices constructed with two of the half-metallic compounds (CrAs/MnAs) but also they are modelled to combine with both a III-V (GaAs-MnAs/CrAs/GaAs) and a IV-IV (MnC/SiC) semiconductor. We investigate variable thicknesses for the combinations. For every case, we find the equilibrium lattice constant as well as the lattice constant at which the superlattice exhibits the half-metallic properties. For CrAs/MnAs, the half-metallic properties are presented and the magnetic moments are shown to be the sum of the moments for MnAs and CrAs. The half-metallic properties of GaAs-MnAs/CrAs/GaAs are found to be crucially dependent on the completion of the d-p hybridization. The magnetic properties of MnC/SiC are discussed with respect to the properties of MnC

  16. Stability and electronic structure of superlattices (IIIV)n/(IV2)n

    International Nuclear Information System (INIS)

    Casagrande, D.; Ferraz, A.C.

    1996-01-01

    Theoretical investigations of atomic relaxation and electronic states have been made for ultrathin superlattices (GaP) n /(Ge 2 ) n , (GaP) n /(Si 2 ) n , (In P) n /(Ge 2 ) n and (In P) n /(Si 2 ) n with period n ≤ 3 in growth directions (001) and (110). The calculations were performed within the momentum-space formalism of the self-consistent ab-initio pseudopotential method and the molecular dynamics approach as proposed by Car and Parrinello. The structures were found to be unstable with respect to the phase separation into the constituent bulk materials. The results for the enthalpy show a metastability as increasing the superlattice period n. The density of nonoctet wrong-bonds play an important role to determine the stability of the structures. (author). 13 refs., 2 figs., 3 tabs

  17. Electronic structure of a graphene superlattice with massive Dirac fermions

    International Nuclear Information System (INIS)

    Lima, Jonas R. F.

    2015-01-01

    We study the electronic and transport properties of a graphene-based superlattice theoretically by using an effective Dirac equation. The superlattice consists of a periodic potential applied on a single-layer graphene deposited on a substrate that opens an energy gap of 2Δ in its electronic structure. We find that extra Dirac points appear in the electronic band structure under certain conditions, so it is possible to close the gap between the conduction and valence minibands. We show that the energy gap E g can be tuned in the range 0 ≤ E g  ≤ 2Δ by changing the periodic potential. We analyze the low energy electronic structure around the contact points and find that the effective Fermi velocity in very anisotropic and depends on the energy gap. We show that the extra Dirac points obtained here behave differently compared to previously studied systems

  18. Charge driven metal-insulator transitions in LaMnO3|SrTiO3 (111) superlattices

    KAUST Repository

    Cossu, Fabrizio; Tahini, Hassan Ali; Singh, Nirpendra; Schwingenschlö gl, Udo

    2017-01-01

    Interfaces of perovskite oxides, due to the strong interplay between the lattice, charge and spin degrees of freedom, can host various phase transitions, which is particularly interesting if these transitions can be tuned by external fields. Recently, ferromagnetism was found together with a seemingly insulating state in superlattices of manganites and titanates. We therefore study the (111) oriented $(\\text{LaMnO}_3)_{6-x}\\vert(\\text{SrTiO}_3)_{6+x}~(x = -0.5, 0, 0.5)$ superlattices by means of ab initio calculations, predicting a ferromagnetic ground state due to double exchange in all cases. We shed light on the ferromagnetic coupling in the LaMnO3 region and at the interfaces. The insulating states of specific superlattices can be understood on the basis of Jahn-Teller modes and electron/hole doping.

  19. Charge driven metal-insulator transitions in LaMnO3|SrTiO3 (111) superlattices

    KAUST Repository

    Cossu, Fabrizio

    2017-08-01

    Interfaces of perovskite oxides, due to the strong interplay between the lattice, charge and spin degrees of freedom, can host various phase transitions, which is particularly interesting if these transitions can be tuned by external fields. Recently, ferromagnetism was found together with a seemingly insulating state in superlattices of manganites and titanates. We therefore study the (111) oriented $(\\\\text{LaMnO}_3)_{6-x}\\\\vert(\\\\text{SrTiO}_3)_{6+x}~(x = -0.5, 0, 0.5)$ superlattices by means of ab initio calculations, predicting a ferromagnetic ground state due to double exchange in all cases. We shed light on the ferromagnetic coupling in the LaMnO3 region and at the interfaces. The insulating states of specific superlattices can be understood on the basis of Jahn-Teller modes and electron/hole doping.

  20. Unexpected enhancements and reductions of rf spin resonance strengths

    Directory of Open Access Journals (Sweden)

    M. A. Leonova

    2006-05-01

    Full Text Available We recently analyzed all available data on spin-flipping stored beams of polarized protons, electrons, and deuterons. Fitting the modified Froissart-Stora equation to the measured polarization data after crossing an rf-induced spin resonance, we found 10–20-fold deviations from the depolarizing resonance strength equations used for many years. The polarization was typically manipulated by linearly sweeping the frequency of an rf dipole or rf solenoid through an rf-induced spin resonance; spin-flip efficiencies of up to 99.9% were obtained. The Lorentz invariance of an rf dipole’s transverse ∫Bdl and the weak energy dependence of its spin resonance strength E together imply that even a small rf dipole should allow efficient spin flipping in 100 GeV or even TeV storage rings; thus, it is important to understand these large deviations. Therefore, we recently studied the resonance strength deviations experimentally by varying the size and vertical betatron tune of a 2.1  GeV/c polarized proton beam stored in COSY. We found no dependence of E on beam size, but we did find almost 100-fold enhancements when the rf spin resonance was near an intrinsic spin resonance.

  1. Defect enhanced diffusion process and hydrogen delayed fracture in high strength steels

    International Nuclear Information System (INIS)

    Lung, C.W.; Mu Zaiqin.

    1985-10-01

    A defect enhanced diffusion model for hydrogen delayed fracture in high strength steels is suggested. It is shown that the rate of crack growth is dependent on the square or higher power of the stress intensity factor which is consistent with recent experiments. (author)

  2. Formation of uniform magnetic structures and epitaxial hydride phases in Nd/Pr superlattices

    DEFF Research Database (Denmark)

    Goff, J.P.; Bryn-Jacobsen, C.; McMorrow, D.F.

    1997-01-01

    , and that the stacking sequence is coherent over many bilayer repeats. The neutron measurements show that for the hexagonal sites of the dhcp structure, the Nd magnetic order propagates coherently through the Pr, whereas the order on the cubic sites is either suppressed or confined to single Nd blocks. It is also shown...... that the singlet ground state of Pr is perturbed to produce a local moment on the hexagonal sites, so that in some cases there is a uniform magnetic structure throughout the superlattice. These results cast new light on the theory of magnetic interactions in rare-earth superlattices. Within a few months of growth...

  3. Space-time evolution of Gaussian wave packets through superlattices containing left-handed layers

    Energy Technology Data Exchange (ETDEWEB)

    Pereyra, P; Romero-Serrano, M [Departamento de Ciencias Basicas, Universidad Autonoma Metropolitana-Azcapotzalco, Mexico DF (Mexico); Robledo-Martinez, A, E-mail: ppereyra@correo.azc.uam.m, E-mail: a.robledo@mailaps.or [Departamento de EnergIa, Universidad Autonoma Metropolitana-Azcapotzalco, Mexico DF (Mexico)

    2009-05-01

    We study the space-time evolution of Gaussian electromagnetic wave packets moving through (L/R){sup n} superlattices, containing alternating layers of left and right-handed materials. We show that the time spent by the wave packet moving through arbitrary (L/R){sup n} superlattices are well described by the phase time. We show that in the particular case where the thicknesses d{sub L,R} and indices n{sub l,r} of the layers satisfy the condition d{sub L}|n{sub L}| = d{sub R}n{sub R}, the usual band structure becomes a sequence of isolated and equidistant peaks with negative phase times.

  4. Transport and spin effects in homogeneous magnetic superlattice

    International Nuclear Information System (INIS)

    Cardoso, J.L.; Pereyra, P.; Anzaldo-Meneses, A.

    2000-09-01

    Homogeneous semiconductors under spacially periodic external magnetic fields exhibit spin-band splitting and displacements, more clearly defined than in diluted magnetic semiconductor superlattices. We study the influence of the geometrical parameters and the spin-field interaction on the electronic transport properties. We show that by varying the external magnetic field, one can easily block the transmission of either the spin-up or the spin-down electrons. (author)

  5. Generation of three-mode continuous-variable entanglement by cascaded nonlinear interactions in a quasiperiodic superlattice

    International Nuclear Information System (INIS)

    Yu, Y. B.; Xie, Z. D.; Yu, X. Q.; Li, H. X.; Xu, P.; Yao, H. M.; Zhu, S. N.

    2006-01-01

    The generation of three-mode continuous-variable entanglement in a quasiperiodically optical superlattice is studied theoretically in this paper. This work is based on the previous experiment result in which three-color light generated from a quasiperiodically optical superlattice through a stimulated parametric down-conversion cascaded with a sum-frequency process. The degree of quadrature phase amplitude correlations, a nonclassical characteristic, among the three mode was discussed by a sufficient inseparability criterion for continuous-variable entanglement, which was proposed by van Loock and Furusawa

  6. Synchrotron spectroscopy of confined carriers in CdF{sub 2}-CaF{sub 2} superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Ivanovskikh, K. V. [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); Institute of Physics and Technology, Ural Federal University, Ekaterinburg 620002 (Russian Federation); Hughes-Currie, R. B. [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); Reid, M. F.; Reeves, R. J. [MacDiarmid Institute for Advanced Materials and Nanotechnology, P.O. Box 600, Wellington 6140 (New Zealand); Dodd-Walls Centre for Photonic and Quantum Technologies and Department of Physics and Astronomy, University of Canterbury, PB4800, Christchurch 8140 (New Zealand); Wells, J.-P. R., E-mail: jon-paul.wells@canterbury.ac.nz [Dodd-Walls Centre for Photonic and Quantum Technologies and Department of Physics and Astronomy, University of Canterbury, PB4800, Christchurch 8140 (New Zealand); Sokolov, N. S. [Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg (Russian Federation)

    2016-03-14

    Luminescence spectroscopic and temporal dynamic properties of high energy elementary excitations in CdF{sub 2}-CaF{sub 2} superlattices have been studied utilising excitation with vacuum ultraviolet and X-ray synchrotron radiation while comparing the results with those obtained for CdF{sub 2} and CaF{sub 2} bulk crystals. It is shown that the optical properties of the superlattice structures are determined by exciton emission in the CdF{sub 2} monolayers. The experimental manifestations of exciton confinement phenomena are discussed.

  7. Solvent-driven symmetry of self-assembled nanocrystal superlattices-A computational study

    KAUST Repository

    Kaushik, Ananth P.; Clancy, Paulette

    2012-01-01

    used solvents, toluene and hexane. System sizes in the 400,000-500,000-atom scale followed for nanoseconds are required for this computationally intensive study. The key questions addressed here concern the thermodynamic stability of the superlattice

  8. Interwell and intrawell magnetoexcitons in GaAs/AlGaAs superlattices

    DEFF Research Database (Denmark)

    Timofeev, V. B.; Filin, A. I.; Tartakovskii, A. I.

    1997-01-01

    The formation of spatially indirect (interwell) excitons in superlattices (SLs) with different barrier widths (different tunneling coupling) is experimentally investigated in a strong enough magnetic field with the use of photoluminescence (PL), photoluminescence excitation (PLE), reflectance spec...

  9. Effects of underlying InGaN/GaN superlattice structures on the structural and optical properties of InGaN LEDs

    Energy Technology Data Exchange (ETDEWEB)

    Tsai, Chia-Lung, E-mail: cltsai@mail.cgu.edu.tw

    2016-06-15

    This study proposes the use of InGaN/GaN superlattices grown beneath InGaN multiple quantum wells (MQWs) and designed with different well widths to act as an electron emitter layer (EEL). Cross-sectional transmission electron microscopy reveals strong indium segregation in the underlying superlattices with a 5-nm-thick In{sub 0.1}Ga{sub 0.9}N well, thus corrupting the crystalline perfection of the resulting LEDs, and also increasing their leakage current. It was also found that the depth of the localized states increases with the well width of the underlying superlattices. In the proposed LEDs, variation in the biaxial strains of the superlattice EELs with different well widths results in an increase in indium incorporation of InGaN MQWs, thus obtaining a redshifted photoluminescence emission with respect to that of normal LED. Furthermore, the presence of relatively strong carrier localization and the alleviation of electron leakage from the InGaN MQWs results in improved light output performance from the proposed LEDs grown with a narrow In{sub 0.1}Ga{sub 0.9}N well in the underlying superlattices. Although growth in a wide In{sub 0.1}Ga{sub 0.9}N well (~3.5 nm) containing underlying superlattices suffers from poor crystalline quality due to partial strain relaxation, it resulted in improved roll-off behavior in terms of light intensity. This may be due to the improved hot electron cooling capacity mitigating the extent of carrier leakage. - Highlights: • In{sub 0.1}Ga{sub 0.9}N/GaN superlattices are used as an electron emitter layer. • Improved LED performance can be achieved using a narrow In{sub 0.1}Ga{sub 0.9}N well. • A wider well can further reduce carrier leakage despite poor quality is presented.

  10. Superlattice structure of Ce{sup 3+}-doped BaMgF{sub 4} fluoride crystals - x-ray diffraction, electron spin-resonance, and optical investigations

    Energy Technology Data Exchange (ETDEWEB)

    Yamaga, M.; Hattori, K. [Department of Electrical and Electronic Engineering, Faculty of Engineering, Gifu University, Gifu (Japan); Kodama, N. [Department of Materials Science and Engineering, Faculty of Engineering and Resource Science, Akita University, Akita (Japan); Ishizawa, N. [Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama (Japan); Honda, M. [Faculty of Science, Naruto University of Education, Naruto (Japan); Shimamura, K.; Fukuda, T. [Institute for Materials Research, Tohoku University, Sendai (Japan)

    2001-09-14

    The x-ray diffraction patterns for Ce{sup 3+}-doped BaMgF{sub 4} (BMF) crystals suggest the existence of superlattice structure. The superlattice model is consistent with the characterization of the 4f{sup 1} ground state of Ce{sup 3+} as a probe ion using the electron spin-resonance (ESR) technique. The distinct Ce{sup 3+} luminescence spectra with different peak energies and lifetimes also support the superlattice model. Although the detailed superlattice structure could not be analysed using the diffraction spots, a model has been proposed, taking into account the eight Ce{sup 3+} polyhedra with different anion coordinations in the unit cell of the BMF crystal obtained from the ESR experiments. (author)

  11. Two-dimensional collective electron magnetotransport, oscillations, and chaos in a semiconductor superlattice.

    Science.gov (United States)

    Bonilla, L L; Carretero, M; Segura, A

    2017-12-01

    When quantized, traces of classically chaotic single-particle systems include eigenvalue statistics and scars in eigenfuntions. Since 2001, many theoretical and experimental works have argued that classically chaotic single-electron dynamics influences and controls collective electron transport. For transport in semiconductor superlattices under tilted magnetic and electric fields, these theories rely on a reduction to a one-dimensional self-consistent drift model. A two-dimensional theory based on self-consistent Boltzmann transport does not support that single-electron chaos influences collective transport. This theory agrees with existing experimental evidence of current self-oscillations, predicts spontaneous collective chaos via a period doubling scenario, and could be tested unambiguously by measuring the electric potential inside the superlattice under a tilted magnetic field.

  12. Two-dimensional collective electron magnetotransport, oscillations, and chaos in a semiconductor superlattice

    Science.gov (United States)

    Bonilla, L. L.; Carretero, M.; Segura, A.

    2017-12-01

    When quantized, traces of classically chaotic single-particle systems include eigenvalue statistics and scars in eigenfuntions. Since 2001, many theoretical and experimental works have argued that classically chaotic single-electron dynamics influences and controls collective electron transport. For transport in semiconductor superlattices under tilted magnetic and electric fields, these theories rely on a reduction to a one-dimensional self-consistent drift model. A two-dimensional theory based on self-consistent Boltzmann transport does not support that single-electron chaos influences collective transport. This theory agrees with existing experimental evidence of current self-oscillations, predicts spontaneous collective chaos via a period doubling scenario, and could be tested unambiguously by measuring the electric potential inside the superlattice under a tilted magnetic field.

  13. Large enhancement of radiative strength for soft transitions in the quasicontinuum.

    Science.gov (United States)

    Voinov, A; Algin, E; Agvaanluvsan, U; Belgya, T; Chankova, R; Guttormsen, M; Mitchell, G E; Rekstad, J; Schiller, A; Siem, S

    2004-10-01

    Radiative strength functions (RSFs) for the (56,57)Fe nuclei below the separation energy are obtained from the 57Fe(3He,alphagamma)56Fe and 57Fe(3He,3He'gamma)57Fe reactions, respectively. An enhancement of more than a factor of 10 over common theoretical models of the soft (E(gamma) less than or approximately equal 2 MeV) RSF for transitions in the quasicontinuum (several MeV above the yrast line) is observed. Two-step cascade intensities with soft primary transitions from the 56Fe(n,2gamma)57Fe reaction confirm the enhancement.

  14. The role of commitment strength in enhancing safe water consumption: mediation analysis of a cluster-randomized trial.

    Science.gov (United States)

    Inauen, Jennifer; Tobias, Robert; Mosler, Hans-Joachim

    2014-11-01

    The objectives of this study were to investigate the importance of commitment strength in the theory of planned behaviour (TPB) and to test whether behaviour change techniques (BCTs) aimed at increasing commitment strength indeed promote switching to arsenic-safe wells by changing commitment strength. A cluster-randomized controlled trial with four arms was conducted to compare an information-only intervention to information plus one, two, or three commitment-enhancing BCTs. Randomly selected households (N = 340) of Monoharganj, Bangladesh, in seven geographically separate areas, whose members were drinking arsenic-contaminated water at baseline and had access to arsenic-safe wells, participated in this trial. The areas were randomly allocated to the four intervention arms. Water consumption behaviour, variables of the TPB, commitment strength, and socio-demographic characteristics were assessed at baseline and at 3-month follow-up by structured face-to-face interviews. Mediation analysis was used to investigate the mechanisms of behaviour change. Changes in commitment strength significantly increased the explanatory power of the TPB to predict well-switching. Commitment-enhancing BCTs - public self-commitment, implementation intentions, and reminders - increased the behaviour change effects of information by up to 50%. Mediation analyses confirmed that the BCTs indeed increased well-switching by increasing commitment strength. Unexpectedly, however, mediation via changes in behavioural intentions was the strongest mechanism of the intervention effects. Commitment is an important construct to consider in water- and health-related behaviour change and may be for other health behaviours as well. BCTs that alter behavioural intentions and commitment strength proved highly effective at enhancing the behaviour change effects of information alone. Statement of contribution What is already known on this subject? Millions of people drink contaminated water even if they

  15. Stratigraphy of a diamond epitaxial three-dimensional overgrowth using doping superlattices

    Science.gov (United States)

    Lloret, F.; Fiori, A.; Araujo, D.; Eon, D.; Villar, M. P.; Bustarret, E.

    2016-05-01

    The selective doped overgrowth of 3D mesa patterns and trenches has become an essential fabrication step of advanced monolithic diamond-based power devices. The methodology here proposed combines the overgrowth of plasma-etched cylindrical mesa structures with the sequential growth of doping superlattices. The latter involve thin heavily boron doped epilayers separating thicker undoped epilayers in a periodic fashion. Besides the classical shape analysis under the scanning electron microscope relying on the appearance of facets corresponding to the main crystallographic directions and their evolution toward slow growing facets, the doping superlattices were used as markers in oriented cross-sectional lamellas prepared by focused ion beam and observed by transmission electron microscopy. This stratigraphic approach is shown here to be applicable to overgrown structures where faceting was not detectable. Intermediate growth directions were detected at different times of the growth process and the periodicity of the superlattice allowed to calculate the growth rates and parameters, providing an original insight into the planarization mechanism. Different configurations of the growth front were obtained for different sample orientations, illustrating the anisotropy of the 3D growth. Dislocations were also observed along the lateral growth fronts with two types of Burger vector: b 01 1 ¯ = /1 2 [ 01 1 ¯ ] and b 112 = /1 6 [ 112 ] . Moreover, the clustering of these extended defects in specific regions of the overgrowth prompted a proposal of two different dislocation generation mechanisms.

  16. Tunneling time and Hartman effect in a ferromagnetic graphene superlattice

    Directory of Open Access Journals (Sweden)

    Farhad Sattari

    2012-03-01

    Full Text Available Using transfer-matrix and stationary phase methods, we study the tunneling time (group delay time in a ferromagnetic monolayer graphene superlattice. The system we peruse consists of a sequence of rectangular barriers and wells, which can be realized by putting a series of electronic gates on the top of ferromagnetic graphene. The magnetization in the two ferromagnetic layers is aligned parallel. We find out that the tunneling time for normal incident is independent of spin state of electron as well as the barrier height and electron Fermi energy while for the oblique incident angles the tunneling time depends on the spin state of electron and has an oscillatory behavior. Also the effect of barrier width on tunneling time is also investigated and shown that, for normal incident, the Hartman effect disappears in a ferromagnetic graphene superlattice but it appears for oblique incident angles when the x component of the electron wave vector in the barrier is imaginary.

  17. Tunable electronic and magnetism of SrTiO{sub 3}/BiFeO{sub 3} (001) superlattice: For electrochemical applications

    Energy Technology Data Exchange (ETDEWEB)

    Xu, Qiang; Sopiha, Kostiantyn; Sobhan, Mushtaq; Anariba, Franklin; Wu, Ping, E-mail: wuping@sutd.edu.sg [Entropic Interface Group (EIG), Engineering Product Development, Singapore University of Technology and Design (SUTD), 8 Somapah Road, Singapore 487372 (Singapore); Ong, Khuong Phuong; Zheng, Jian Wei [Institute of High Performance Computing (IHPC), Agency for Science, Technology and Research - A*Star, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632 (Singapore)

    2016-01-04

    Practical strategy in tuning the conductivity and magnetism of SrTiO{sub 3}/BiFeO{sub 3} (STO/BFO) (001) superlattice is investigated using the first-principles method based on density functional theory. Our calculated results show that both the conductivity and magnetism of this superlattice can be tuned via a control of its interface terminations. The STO layers maintain semiconducting, while the BFO layers demonstrate metallic character. Therefore, the conductivity of STO/BFO is controlled by the BFO layers. Furthermore, a magnetic STO/BFO (001) superlattice can be found in n-type TiO{sub 2}/BiO interface but with heavy electron carriers. The thickness of BFO does not change the electronic structure and character of STO/BFO (001) superlattice. This study provides a fundamental understanding of the chemically turned conductivity and magnetism of BFO thin films, which may further advance electrochemical applications like magnetic-field aided chemical gas sensing, solar cells, and photo-catalytic chemical reactions.

  18. Enhancing psychological capital and personal growth initiative: working on strengths or deficiencies.

    Science.gov (United States)

    Meyers, Maria Christina; van Woerkom, Marianne; de Reuver, Renee S M; Bakk, Zsuzsa; Oberski, Daniel L

    2015-01-01

    Personal growth initiative (PGI), defined as being proactive about one's personal development, is critical to graduate students' academic success. Prior research has shown that students' PGI can be enhanced through interventions that focus on stimulating developmental activities. Within this study, we aimed to investigate whether an intervention that stimulates development in the area of one's personal strengths (strengths intervention) has more beneficial effects on students' PGI than an intervention that stimulates development in the area of individual deficiencies (deficiency intervention). We conducted 2 longitudinal field experiments to investigate the effects of the 2 interventions on students' PGI (Experiment 1) and the potential mediating role of psychological capital (PsyCap) in this regard (Experiment 2). In Experiment 1, 105 (N = 105) university students participated in either a strengths intervention or a deficiency intervention. Results indicated that the strengths intervention increased the students' PGI in the short but not in the long term, whereas the deficiency intervention did not affect PGI. Ninety students (N = 90) participated in Experiment 2, in which we slightly refined both interventions by putting a stronger emphasis on the ongoing development of strengths (strengths intervention) or correction of deficiencies (deficiency intervention) by adding posttraining assignments. Results suggested that participating in both interventions led to increases in PGI over a 3-month period, but that these increases were bigger for the strengths intervention group. Furthermore, the relationship between the strengths intervention and PGI was mediated by hope as one component of PsyCap. PsycINFO Database Record (c) 2015 APA, all rights reserved.

  19. Revisiting the Local Structure in Ge-Sb-Te based Chalcogenide Superlattices

    NARCIS (Netherlands)

    Casarin, Barbara; Caretta, Antonio; Momand, Jamo; Kooi, Bart J.; Verheijen, Marcel A.; Bragaglia, Valeria; Calarco, Raffaella; Chukalina, Marina; Yu, Xiaoming; Robertson, John; Lange, Felix R. L.; Wuttig, Matthias; Redaelli, Andrea; Varesi, Enrico; Parmigiani, Fulvio; Malvestuto, Marco

    2016-01-01

    The technological success of phase-change materials in the field of data storage and functional systems stems from their distinctive electronic and structural peculiarities on the nanoscale. Recently, superlattice structures have been demonstrated to dramatically improve the optical and electrical

  20. Revisiting the local structure in Ge-Sb-Te based chalcogenide superlattices

    NARCIS (Netherlands)

    Casarin, B.; Caretta, A.; Momand, J.; Kooi, B. J.; Verheijen, M.A.; Bragaglia, V.; Calarco, R.; Chukalina, M.; Yu, X.; Robertson, J.; Lange, F.R.L.; Wuttig, M.; Redaelli, A.; Varesi, E.; Parmigiani, F.; Malvestuto, M.

    2016-01-01

    The technological success of phase-change materials in the field of data storage and functional systems stems from their distinctive electronic and structural peculiarities on the nanoscale. Recently, superlattice structures have been demonstrated to dramatically improve the optical and electrical

  1. Fish-oil supplementation enhances the effects of strength training in elderly women.

    Science.gov (United States)

    Rodacki, Cintia L N; Rodacki, André L F; Pereira, Gleber; Naliwaiko, Katya; Coelho, Isabela; Pequito, Daniele; Fernandes, Luiz Cléudio

    2012-02-01

    Muscle force and functional capacity generally decrease with aging in the older population, although this effect can be reversed, attenuated, or both through strength training. Fish oil (FO), which is rich in n-3 (omega-3) PUFAs, has been shown to play a role in the plasma membrane and cell function of muscles, which may enhance the benefits of training. The effect of strength training and FO supplementation on the neuromuscular system of the elderly has not been investigated. The objective was to investigate the chronic effect of FO supplementation and strength training on the neuromuscular system (muscle strength and functional capacity) of older women. Forty-five women (aged 64 ± 1.4 y) were randomly assigned to 3 groups. One group performed strength training only (ST group) for 90 d, whereas the others performed the same strength-training program and received FO supplementation (2 g/d) for 90 d (ST90 group) or for 150 d (ST150 group; supplemented 60 d before training). Muscle strength and functional capacity were assessed before and after the training period. No differences in the pretraining period were found between groups for any of the variables. The peak torque and rate of torque development for all muscles (knee flexor and extensor, plantar and dorsiflexor) increased from pre- to posttraining in all groups. However, the effect was greater in the ST90 and ST150 groups than in the ST group. The activation level and electromechanical delay of the muscles changed from pre- to posttraining only for the ST90 and ST150 groups. Chair-rising performance in the FO groups was higher than in the ST group. Strength training increased muscle strength in elderly women. The inclusion of FO supplementation caused greater improvements in muscle strength and functional capacity.

  2. ICP etching for InAs-based InAs/GaAsSb superlattice long wavelength infrared detectors

    Science.gov (United States)

    Huang, Min; Chen, Jianxin; Xu, Jiajia; Wang, Fangfang; Xu, Zhicheng; He, Li

    2018-05-01

    In this work, we study and report the dry etching processes for InAs-based InAs/GaAsSb strain-free superlattice long wavelength infrared (LWIR) detectors. The proper etching parameters were first obtained through the parametric studies of Inductively Coupled Plasma (ICP) etching of both InAs and GaSb bulk materials in Cl2/N2 plasmas. Then an InAs-based InAs/GaAsSb superlattice LWIR detector with PπN structure was fabricated by using the optimized etching parameters. At 80 K, the detector exhibits a 100% cut-off wavelength of 12 μm and a responsivity of 1.5 A/W. Moreover, the dark current density of the device under a bias of -200 mV reaches 5.5 × 10-4 A/cm2, and the R0A is 15 Ω cm2. Our results pave the way towards InAs-based superlattice LWIR detectors with better performances.

  3. Spin wave relaxation and magnetic properties in [M/Cu] super-lattices; M=Fe, Co and Ni

    International Nuclear Information System (INIS)

    Fahmi, A.; Qachaou, A.

    2009-01-01

    In this work, we study the elementary excitations and magnetic properties of the [M/Cu] super-lattices with: M=Fe, Co and Ni, represented by a Heisenberg ferromagnetic system with N atomic planes. The nearest neighbour (NN), next nearest neighbour (NNN) exchange, dipolar interactions and surface anisotropy effects are taken into account and the Hamiltonian is studied in the framework of the linear spin wave theory. In the presence of the exchange alone, the excitation spectrum E(k) and the magnetization z >/S analytical expressions are obtained using the Green's function formalism. The obtained relaxation time of the magnon populations is nearly the same in the Fe and Co-based super-lattices, while these magnetic excitations would last much longer in the Ni-based super lattice. A numerical study of the surface anisotropy and long-ranged dipolar interaction combined effects are also reported. The exchange integral values deduced from a comparison with experience for the three super-lattices are coherent.

  4. Rutile IrO2/TiO2 superlattices: A hyperconnected analog to the Ruddelsden-Popper structure

    Science.gov (United States)

    Kawasaki, Jason K.; Baek, David; Paik, Hanjong; Nair, Hari P.; Kourkoutis, Lena F.; Schlom, Darrell G.; Shen, Kyle M.

    2018-05-01

    Dimensionality and connectivity among octahedra play important roles in determining the properties, electronic structure, and phase transitions of transition-metal oxides. Here we demonstrate the epitaxial growth of (110)-oriented alternating layers of IrO2 and TiO2, both of which have the rutile structure. These (IrO2)n/(TiO2)2 superlattices consist of IrO6 and TiO6 octahedra tiled in a hyperconnected, edge- and corner-sharing network. Despite the large lattice mismatch between constituent layers (Δ d∥=-2.1 % and Δ c =+6.6 % ), our reactive molecular-beam epitaxy-grown superlattices show high structural quality as determined by x-ray diffraction and sharp interfaces as observed by transmission electron microscopy. The large strain at the interface is accommodated by an ordered interfacial reconstruction. The superlattices show persistent metallicity down to n =3 atomic layers, and angle-resolved photoemission spectroscopy measurements reveal quantized sub-bands with signatures of IrO2-IrO2 interlayer coupling.

  5. Anisotropic critical fields in superconducting superlattices

    International Nuclear Information System (INIS)

    Banerjee, I.; Yang, Q.S.; Falco, C.M.; Schuller, I.K.

    1983-01-01

    The temperature and angular dependence of critical fields (H/sub c/) have been studied as a function of layer thickness for superconducting Nb/Cu superlattices. For layer thicknesses between 100 and 300 A, dimensional crossover has been observed in the temperature dependence of H/sub c/. Associated with the crossover we find a change in the angular dependence of H/sub c/ to that given by the effective-mass theory. This is the first time that a relationship has been found between dimensional crossover observed in the temperature dependence and that in the angular dependence of critical fields

  6. Magnetoelectric control of valley and spin in a silicene nanoribbon modulated by the magnetic superlattices

    Energy Technology Data Exchange (ETDEWEB)

    An, Xing-Tao, E-mail: anxt@hku.hk

    2015-03-20

    The control of valley and spin degrees of freedom and the transport properties of electrons in a zigzag silicene nanoribbon modulated by the magnetic superlattices are investigated theoretically. Due to the valley–spin locking effect in silicene, the valley degree of freedom can be controlled by magnetic means. The valley or/and spin selection induced by the exchange field result in the perfect spin–valley filter and tunneling magnetoresistance effect in the double ferromagnetic barriers on the surface of the silicene nanoribbon. It is more interesting that there are valley-resolved minigaps and minibands in the zigzag silicene nanoribbon modulated by the magnetic superlattices which give rise to the periodically modulated spin (or/and valley) polarization and tunneling magnetoresistance. The results obtained may have certain practical significance in applications for future valleytronic and spintronic devices. - Highlights: • The valley can be controlled by a magnetic field in silicene. • The valley-resolved miniband transport is studied in the silicene superlattices. • There are the perfect spin–valley filter and tunneling magnetoresistance effect.

  7. Electrochemical growth of highly oriented organic-inorganic superlattices using solid-supported multilamellar membranes as templates.

    Science.gov (United States)

    Xing, Li-Li; Li, Da-Peng; Hu, Shu-Xin; Jing, Huai-Yu; Fu, Honglan; Mai, Zhen-Hong; Li, Ming

    2006-02-08

    Controllable depositing of relatively thick inorganic sublayers into organic templates to fabricate organic-inorganic superlattices is of great importance. We report a novel approach to fabricating phospholipid/Ni(OH)(2) superlattices by electrochemical deposition of the inorganic component into solid-supported multilamellar templates. The well-ordered and highly oriented multilamellar templates are produced by spreading small drops of lipid solution on silicon surfaces and letting the solvent evaporate slowly. The templates which are used as working electrodes preserve the lamellar structure in the electrolyte solution. The resulting superlattices are highly oriented. The thickness of the nickel hydroxide is controlled by the concentration of nickel ions in the electrolyte bath. The electron density profiles derived from the X-ray diffraction data reveal that the thickness of the nickel hydroxide sublayers increases from 15 to 27 A as the concentration of nickel nitrate increases from 0.005 mol/L to 0.08 mol/L. We expect that the new method can be extended to depositing a variety of inorganic components including metals, oxides, and semiconductors.

  8. Capacitance-Voltage (CV) Measurement of Type-2 Superlattice Photodiodes

    Science.gov (United States)

    2016-01-05

    Department of Defense position, policy, or decision. CQD Contents 1. Background and Motivation ...1. Background and Motivation 1.1. Development of Type-II superalttice Type-II InAs/GaSb superlattices (T2SLs) were first proposed by Sai-Halasz et...equals the ionized impurity concentration. In such case, the semiconductor is under extrinsic regime, and the dynamic of mobile carriers depends on

  9. IS ENHANCED-ECCENTRIC RESISTANCE TRAINING SUPERIOR TO TRADITIONAL TRAINING FOR INCREASING ELBOW FLEXOR STRENGTH?

    Directory of Open Access Journals (Sweden)

    Thomas W. Kaminski

    2003-06-01

    Full Text Available Protocols for strengthening muscle are important for fitness, rehabilitation, and the prevention of myotendinous injuries. In trained individuals, the optimal method of increasing strength remains unclear. The purpose of this study was to compare the effects of a traditional method of strengthening with a method that allowed for enhanced-eccentric training, on changes in elbow flexor strength in trained subjects. Thirty-nine (8 male, 31 female trained subjects with normal elbow function participated in this study. Subjects were rank-ordered according to isometric force production and randomly assigned to one of three training groups: control (CONT, traditional concentric/eccentric (TRAD, and concentric/enhanced-eccentric (NEG. The training groups completed 24 training sessions. An evaluator blinded to training group performed all testing. Mixed model ANOVA techniques were used to determine if differences existed in concentric one repetition maximum strength, and isometric force production among groups. Changes in peak and average isokinetic force production were also compared. Type 1 error was maintained at 5%. While both groups improved concentric one repetition maximum (NEG = 15.5%, TRAD = 13.8% neither training group statistically differed from changes demonstrated by the CONT group. Nor did either training group show significant improvements in isometric or isokinetic force production over the CONT group. These results do not support the superiority of enhanced-eccentric training for increasing force production in trained subjects.

  10. Electrocrystallization and scanning probe microscopy of ceramic thin films and superlattices

    Science.gov (United States)

    Hung, Chen-Jen

    This dissertation presents an investigation of the electrocrystallization and scanning probe microscopy of ceramic thin films and superlattices. All of the films were deposited from aqueous solution at room temperature with no subsequent heat treatment needed to effect crystallization. Thallium(III) oxide defect chemistry superlattices were electrodeposited by pulsing the applied overpotential during deposition. The defect chemistry of the oxide is dependent on the applied overpotential. High overpotentials favor oxygen vacancies, while low overpotentials favor cation interstitials. Nanometer-scale holes were formed in thin thallium(III) oxide films using the scanning tunneling microscope in humid ambient conditions. Both cathodic and anodic etching reactions were performed on this metal oxide surface. The hole formation was attributed to localized electrochemical etching reactions beneath the STM tip. The scanning tunneling microscope (STM) was also used to both induce local surface modifications and image cleaved Pb-Tl-O superlattices. A trench of 100 nm in width, 32 nm in depth, and over 1 μm in length was formed after sweeping a bias voltage of ±2.5 V for 1 minute using a fixed STM tip. It has been suggested that STM results obtained under ambient conditions must be evaluated with great care because of the possibility of localized electrochemcial reactions. A novel synthesis method for the production of Cu(II) oxide from an alkaline solution containing Cu(II) tartrate was developed. Rietveld refinement of the cupric oxide films reveals pure Cu(II) oxide with no Cu(I) oxide present in the film.

  11. Soliton excitation in superlattice

    International Nuclear Information System (INIS)

    Mensah, S.Y.; Allotey, F.K.A.; Mensah, N.G.; Twum, A.K.

    1995-10-01

    Excitation of soliton in superlattice has been investigated theoretically. It is noted that the soliton velocity u and the length L depend on the amplitude E 0 and that an increase in the amplitude causes soliton width L to approach zero and the velocity u to that of light V in homogeneous medium. The characteristic parameters of soliton u, L and E 0 are related by expression u/L E 0 = ed/2(h/2π) which is constant depending only on the SL period d. It is observed also that the soliton has both energy E = 8V 2 (1 - u 2 /V 2 ) -1/2 and momentum P = u/V 2 E which makes it behave as relativistic free particle with rest energy 8V 2 . Its interaction with electrons can cause the soliton electric effect in SL. (author). 27 refs

  12. An experimental study on flexural strength enhancement of concrete by means of small steel fibers

    Directory of Open Access Journals (Sweden)

    Abdoullah Namdar

    2013-10-01

    Full Text Available Cost effective improvement of the mechanical performances of structural materials is an important goal in construction industry. To improve the flexural strength of plain concrete so as to reduce construction costs, the addition of fibers to the concrete mixture can be adopted. The addition of small steel fibers with different lengths and proportion have experimentally been analyzed in terms of concrete flexural strength enhancement. The main objectives of the present study are related to the evaluation of the influence of steel fibers design on the increase of concrete flexural characteristics and on the mode of failure. Two types of beams have been investigated. The force level, deflection and time to failure of beams have been measured. The shear crack, flexural crack and intermediate shear-flexural crack have been studied. The steel fiber content controlled crack morphology. Flexural strength and time to failure of fiber reinforce concrete could be further enhanced if, instead of smooth steel fibers, corrugated fibers were used.

  13. Thermoelectric cross-plane properties on p- and n-Ge/Si{sub x}Ge{sub 1-x} superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Ferre Llin, L.; Samarelli, A. [University of Glasgow, School of Engineering, Oakfield Avenue, Glasgow G12 8LT (United Kingdom); Cecchi, S.; Chrastina, D.; Isella, G. [L-NESS, Politecnico di Milano, Via Anzani 42, 22100 Como (Italy); Müller Gubler, E. [ETH, Electron Microscopy ETH Zurich, Wolgang-Pauli-Str. Ch-8093 Zurich (Switzerland); Etzelstorfer, T.; Stangl, J. [Johannes Kepler Universität, Institute of Semiconductor and Solid State Physics, A-4040 Linz (Austria); Paul, D.J., E-mail: Douglas.Paul@glasgow.ac.uk [University of Glasgow, School of Engineering, Oakfield Avenue, Glasgow G12 8LT (United Kingdom)

    2016-03-01

    Silicon and germanium materials have demonstrated an increasing attraction for energy harvesting, due to their sustainability and integrability with complementary metal oxide semiconductor and micro-electro-mechanical-system technology. The thermoelectric efficiencies for these materials, however, are very poor at room temperature and so it is necessary to engineer them in order to compete with telluride based materials, which have demonstrated at room temperature the highest performances in literature [1]. Micro-fabricated devices consisting of mesa structures with integrated heaters, thermometers and Ohmic contacts were used to extract the cross-plane values of the Seebeck coefficient and the thermal conductivity from p- and n-Ge/Si{sub x}Ge{sub 1-x} superlattices. A second device consisting in a modified circular transfer line method structure was used to extract the electrical conductivity of the materials. A range of p-Ge/Si{sub 0.5}Ge{sub 0.5} superlattices with different doping levels was investigated in detail to determine the role of the doping density in dictating the thermoelectric properties. A second set of n-Ge/Si{sub 0.3}Ge{sub 0.7} superlattices was fabricated to study the impact that quantum well thickness might have on the two thermoelectric figures of merit, and also to demonstrate a further reduction of the thermal conductivity by scattering phonons at different wavelengths. This technique has demonstrated to lower the thermal conductivity by a 25% by adding different barrier thicknesses per period. - Highlights: • Growth of epitaxial Ge/SiGe superlattices on Si substrates as energy harvesters • Study of cross-plane thermoelectric properties of Ge/SiGe superlattices at 300 K • Thermoelectric figures of merit studied as a function of doping density • Phonon scattering at different wavelengths to reduce thermal transport.

  14. Identification of an organic semiconductor superlattice structure of pentacene and perfluoro-pentacene through resonant and non-resonant X-ray scattering

    Energy Technology Data Exchange (ETDEWEB)

    Kowarik, S.; Weber, C. [Humboldt-Universität zu Berlin, Institut für Physik, Newtonstr. 15, 12489 Berlin (Germany); Hinderhofer, A.; Gerlach, A.; Schreiber, F. [Universität Tübingen, Institut für Angewandte Physik, Auf der Morgenstelle 10, 72076 Tübingen (Germany); Wang, C.; Hexemer, A. [Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (United States); Leone, S. R. [Departments of Chemistry and Physics, University of California, and Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (United States)

    2015-11-15

    Highly crystalline and stable molecular superlattices are grown with the smallest possible stacking period using monolayers (MLs) of the organic semiconductors pentacene (PEN) and perfluoro-pentacene (PFP). Superlattice reflections in X-ray reflectivity and their energy dependence in resonant soft X-ray reflectivity measurements show that PFP and PEN MLs indeed alternate even though the coherent ordering is lost after ∼ 4 ML. The observed lattice spacing of 15.9 Å in the superlattice is larger than in pure PEN and PFP films, presumably because of more upright standing molecules and lack of interdigitation between the incommensurate crystalline PEN and PFP layers. The findings are important for the development of novel organic quantum optoelectronic devices.

  15. Identification of an organic semiconductor superlattice structure of pentacene and perfluoro-pentacene through resonant and non-resonant X-ray scattering

    Directory of Open Access Journals (Sweden)

    S. Kowarik

    2015-11-01

    Full Text Available Highly crystalline and stable molecular superlattices are grown with the smallest possible stacking period using monolayers (MLs of the organic semiconductors pentacene (PEN and perfluoro-pentacene (PFP. Superlattice reflections in X-ray reflectivity and their energy dependence in resonant soft X-ray reflectivity measurements show that PFP and PEN MLs indeed alternate even though the coherent ordering is lost after ∼ 4 ML. The observed lattice spacing of 15.9 Å in the superlattice is larger than in pure PEN and PFP films, presumably because of more upright standing molecules and lack of interdigitation between the incommensurate crystalline PEN and PFP layers. The findings are important for the development of novel organic quantum optoelectronic devices.

  16. Effects of electron correlations application to Ti atoms on physical properties of (LaMnO{sub 3}){sub m}/(SrTiO{sub 3}){sub n} superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Aezami, A., E-mail: a.aezami@gmail.com; Abolhassani, M.; Elahi, M.

    2016-05-15

    Magnetic structures and Curie temperatures of the (LaMnO{sub 3}){sub m}/(SrTiO{sub 3}){sub n} superlattices (SLm–n) with m=1, 2, 3 and n=1, 2, 3, 8 were investigated, using density functional theory implemented in Quantum-Espresso open source code. By applying on-site coulomb interaction (Hubbard term U) to Ti atoms for all of these superlattices, using Stoner–Wolfarth model, it was found that the magnetic order of interfacial atoms of these superlattices changed to ferromagnetic by implying U=5 eV on Ti atoms. The inclusion of electron–electron correlation with U=5 eV on the Ti atoms for all of the superlattices made the two dimensional electron gas (2DEG) formed at the interfaces, half-metallic. The obtained values of Curie temperature, calculated within mean field approximation with U=5 eV on the Ti atoms, are in good agreement with the experimental results. - Highlights: • Calculated the magnetic structure and Curie temperature of the (LaMnO{sub 3}){sub m}/(SrTiO{sub 3}){sub n} superlattices with m=1, 2, 3 and n=1, 2, 3, 8 by mean field approximation. • By implying U=5 eV on the Ti atoms, the magnetic order of interfacial atoms of these superlattices has changed to ferromagnetic. • The 2DEG formed at the interface half-metallic have made in these superlattices by the inclusion of electron-electron correlation with U=5 eV on the Ti atoms for all of the superlattices.

  17. Structure and properties of (Sr, Ca)CuO2-BaCuO2 superlattices grown by pulsed laser interval deposition

    NARCIS (Netherlands)

    Koster, Gertjan; Verbist, Karen; Rijnders, Augustinus J.H.M.; Rogalla, Horst; van Tendeloo, Gustaav; Blank, David H.A.

    2001-01-01

    We report on the preparation of CuBa2(SrxCa1¿x)nCun¿1Oy compounds by fabrication of (Ba,Sr,Ca)CuO2 superlattices with pulsed laser deposition (PLD). A technique called interval deposition is used to suppress multi-level or island growth resulting in high-quality superlattice structures. Both, the

  18. The exercise-induced biochemical milieu enhances collagen content and tensile strength of engineered ligaments.

    Science.gov (United States)

    West, Daniel W D; Lee-Barthel, Ann; McIntyre, Todd; Shamim, Baubak; Lee, Cassandra A; Baar, Keith

    2015-10-15

    Exercise stimulates a dramatic change in the concentration of circulating hormones, such as growth hormone (GH), but the biological functions of this response are unclear. Pharmacological GH administration stimulates collagen synthesis; however, whether the post-exercise systemic milieu has a similar action is unknown. We aimed to determine whether the collagen content and tensile strength of tissue-engineered ligaments is enhanced by serum obtained post-exercise. Primary cells from a human anterior cruciate ligament (ACL) were used to engineer ligament constructs in vitro. Blood obtained from 12 healthy young men 15 min after resistance exercise contained GH concentrations that were ∼7-fold greater than resting serum (P Ligament constructs were treated for 7 days with medium supplemented with serum obtained at rest (RestTx) or 15 min post-exercise (ExTx), before tensile testing and collagen content analysis. Compared with RestTx, ExTx enhanced collagen content (+19%; 181 ± 33 vs. 215 ± 40 μg per construct P = 0.001) and ligament mechanical properties - maximal tensile load (+17%, P = 0.03 vs. RestTx) and ultimate tensile strength (+10%, P = 0.15 vs. RestTx). In a separate set of engineered ligaments, recombinant IGF-1, but not GH, enhanced collagen content and mechanics. Bioassays in 2D culture revealed that acute treatment with post-exercise serum activated mTORC1 and ERK1/2. In conclusion, the post-exercise biochemical milieu, but not recombinant GH, enhances collagen content and tensile strength of engineered ligaments, in association with mTORC1 and ERK1/2 activation. © 2015 The Authors. The Journal of Physiology © 2015 The Physiological Society.

  19. Optical properties of graphene superlattices.

    Science.gov (United States)

    Le, H Anh; Ho, S Ta; Nguyen, D Chien; Do, V Nam

    2014-10-08

    In this work, the optical responses of graphene superlattices, i.e. graphene subjected to a periodic scalar potential, are theoretically reported. The optical properties were studied by investigating the optical conductivity, which was calculated using the Kubo formalism. It was found that the optical conductivity becomes dependent on the photon polarization and is suppressed in the photon energy range of (0, Ub), where Ub is the potential barrier height. In the higher photon energy range, i.e. Ω > Ub, the optical conductivity is, however, almost identical to that of pristine graphene. Such behaviors of the optical conductivity are explained microscopically through the analysis of the elements of optical matrices and effectively through a simple model, which is based on the Pauli blocking mechanism.

  20. Competing interactions in ferromagnetic/antiferromagnetic perovskite superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Takamura, Y.; Biegalski, M.B.; Christen, H.M.

    2009-10-22

    Soft x-ray magnetic dichroism, magnetization, and magnetotransport measurements demonstrate that the competition between different magnetic interactions (exchange coupling, electronic reconstruction, and long-range interactions) in La{sub 0.7}Sr{sub 0.3}FeO{sub 3}(LSFO)/La{sub 0.7}Sr{sub 0.3}MnO{sub 3}(LSMO) perovskite oxide superlattices leads to unexpected functional properties. The antiferromagnetic order parameter in LSFO and ferromagnetic order parameter in LSMO show a dissimilar dependence on sublayer thickness and temperature, illustrating the high degree of tunability in these artificially layered materials.

  1. Structure of highly perfect semiconductor strained-layer superlattices

    International Nuclear Information System (INIS)

    Vandenberg, J.M.

    1989-01-01

    High-resolution x-ray diffraction (HRXRD) measurements of strained-layer superlattices (SLS's) have been carried out using a four-crystal monochromator. A wide asymmetric range of sharp higher-order x-ray satellite peaks is observed indicating well-defined periodic structures. Using a kinematical diffraction step model very good agreement between measured and simulated x-ray satellite patterns could be achieved. These results show that this x- ray method is a powerful tool to evaluate the crystal quality of SLS's

  2. Enhancement of broadband optical absorption in photovoltaic devices by band-edge effect of photonic crystals.

    Science.gov (United States)

    Tanaka, Yoshinori; Kawamoto, Yosuke; Fujita, Masayuki; Noda, Susumu

    2013-08-26

    We numerically investigate broadband optical absorption enhancement in thin, 400-nm thick microcrystalline silicon (µc-Si) photovoltaic devices by photonic crystals (PCs). We realize absorption enhancement by coupling the light from the free space to the large area resonant modes at the photonic band-edge induced by the photonic crystals. We show that multiple photonic band-edge modes can be produced by higher order modes in the vertical direction of the Si photovoltaic layer, which can enhance the absorption on multiple wavelengths. Moreover, we reveal that the photonic superlattice structure can produce more photonic band-edge modes that lead to further optical absorption. The absorption average in wavelengths of 500-1000 nm weighted to the solar spectrum (AM 1.5) increases almost twice: from 33% without photonic crystal to 58% with a 4 × 4 period superlattice photonic crystal; our result outperforms the Lambertian textured structure.

  3. Low-energy enhancement of nuclear γ strength and its impact on astrophysical reaction rates

    Directory of Open Access Journals (Sweden)

    Larsen A. C.

    2014-03-01

    Full Text Available An unexpected enhancement in the low-energy part of the γ-strength function for light and medium-mass nuclei has been discovered at the Oslo Cyclotron Laboratory. This enhancement could lead to an increase in the neutron-capture rates up to two orders of magnitude for very exotic, neutron-rich nuclei. However, it is still an open question whether this structure persists when approaching the neutron drip line.

  4. Silicon carbide whiskers with superlattice structure: A precursor for a new type of nanoreactor

    International Nuclear Information System (INIS)

    Lutsenko, Vadym G.

    2008-01-01

    Silicon carbide whiskers exhibit growth predominantly in the direction. The high level of impurities, stacking faults and nanosized twins govern the formation of homojunctions and heterojunctions in crystals. The structure of the whiskers comprises a hybrid superlattice, i.e. contains elements of doped and composite superlattices. An individual SiC whisker can contain hundreds of quantum wells with anomalous chemical properties. This paper shows that it is possible to selectively etch quantum wells and to construct whiskers with quasi-regularly distributed slit-like nanopores (nanoreactors), which are bordered by polar planes {1 1 1}, {0 0 0 1} or a combination of them, and also to produce flat SiC nanocrystals bordered by polar planes

  5. Structural control of metamaterial oscillator strength and electric field enhancement at terahertz frequencies

    DEFF Research Database (Denmark)

    Keiser, G. R.; Seren, H. R.; Strikwerda, Andrew C.

    2014-01-01

    The design of artificial nonlinear materials requires control over internal resonant charge densities and local electric field distributions. We present a MM design with a structurally controllable oscillator strength and local electric field enhancement at terahertz frequencies. The MM consists...... of a split ring resonator (SRR) array stacked above an array of closed conducting rings. An in-plane, lateral shift of a half unit cell between the SRR and closed ring arrays results in an increase of the MM oscillator strength by a factor of 4 and a 40% change in the amplitude of the resonant electric field...

  6. Correlating interfacial octahedral rotations with magnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices.

    Science.gov (United States)

    Zhai, Xiaofang; Cheng, Long; Liu, Yang; Schlepütz, Christian M; Dong, Shuai; Li, Hui; Zhang, Xiaoqiang; Chu, Shengqi; Zheng, Lirong; Zhang, Jing; Zhao, Aidi; Hong, Hawoong; Bhattacharya, Anand; Eckstein, James N; Zeng, Changgan

    2014-07-09

    Lattice distortion due to oxygen octahedral rotations have a significant role in mediating the magnetism in oxides, and recently attracts a lot of interests in the study of complex oxides interface. However, the direct experimental evidence for the interrelation between octahedral rotation and magnetism at interface is scarce. Here we demonstrate that interfacial octahedral rotation are closely linked to the strongly modified ferromagnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices. The maximized ferromagnetic moment in the N=6 superlattice is accompanied by a metastable structure (space group Imcm) featuring minimal octahedral rotations (a(-)a(-)c(-), α~4.2°, γ~0.5°). Quenched ferromagnetism for Nmagnetism. Our study demonstrates that engineering superlattices with controllable interfacial structures can be a feasible new route in realizing functional magnetic materials.

  7. Bottom-up meets top-down: tailored raspberry-like Fe3O4-Pt nanocrystal superlattices.

    Science.gov (United States)

    Qiu, Fen; Vervuurt, René H J; Verheijen, Marcel A; Zaia, Edmond W; Creel, Erin B; Kim, Youngsang; Urban, Jeffrey J; Bol, Ageeth A

    2018-03-29

    Supported catalysts are widely used in industry and can be optimized by tuning the composition, chemical structure, and interface of the nanoparticle catalyst and oxide support. Here we firstly combine a bottom up colloidal synthesis method with a top down atomic layer deposition (ALD) process to achieve a raspberry-like Pt-decorated Fe3O4 (Fe3O4-Pt) nanoparticle superlattices. This nanocomposite ensures the precision of the catalyst/support interface, improving the catalytic efficiency of the Fe3O4-Pt nanocomposite system. The morphology of the hybrid nanocomposites resulting from different cycles of ALD was monitored by scanning transmission electron microscopy, giving insight into the nucleation and growth mechanism of the ALD process. X-ray photoelectron spectroscopy studies confirm the anticipated electron transfer from Fe3O4 to Pt through the nanocomposite interface. Photocurrent measurement further suggests that Fe3O4 superlattices with controlled decoration of Pt have substantial promise for energy-efficient photoelectrocatalytic oxygen evolution reaction. This work opens a new avenue for designing supported catalyst architectures via precisely controlled decoration of single component superlattices with noble metals.

  8. Anisotropic magnetoresistance and spin polarization of La0.7Sr0.3MnO3/SrTiO3 superlattices

    International Nuclear Information System (INIS)

    Wang, L.M.; Guo, C.-C.

    2005-01-01

    The crystalline structure, anisotropic magnetoresistance (AMR), and magnetization of La 0.7 Sr 0.3 MnO 3 /SrTiO 3 (LSMO/STO) superlattices grown by a rf sputtering system are systematically analyzed to study the spin polarization of manganite at interfaces. The presence of positive low-temperature AMR in LSMO/STO superlattices implies that two bands of majority and minority character contribute to the transport properties, leading to a reduced spin polarization. Furthermore, the magnetization of superlattices follows the T 3/2 law and decays more quickly as the thickness ratio d STO /d LSMO increases, corresponding to a reduced exchange coupling. The results clearly show that the spin polarization is strongly correlated with the influence of interface-induced strain on the structure

  9. The Luttinger liquid in superlattice structures: atomic gases, quantum dots and the classical Ising chain

    International Nuclear Information System (INIS)

    Bhattacherjee, Aranya B; Jha, Pradip; Kumar, Tarun; Mohan, Man

    2011-01-01

    We study the physical properties of a Luttinger liquid in a superlattice that is characterized by alternating two tunneling parameters. Using the bosonization approach, we describe the corresponding Hubbard model by the equivalent Tomonaga-Luttinger model. We analyze the spin-charge separation and transport properties of the superlattice system. We suggest that cold Fermi gases trapped in a bichromatic optical lattice and coupled quantum dots offer the opportunity to measure these effects in a convenient manner. We also study the classical Ising chain with two tunneling parameters. We find that the classical two-point correlator decreases as the difference between the two tunneling parameters increases.

  10. Theoretical modelling of electron transport in InAs/GaAs quantum dot superlattices

    International Nuclear Information System (INIS)

    Vukmirovic, Nenad; Ikonic, Zoran; Savic, Ivana; Indjin, Dragan; Harrison, Paul

    2006-01-01

    A theoretical model describing the electron transport in InAs/GaAs quantum dot infrared photodetectors, modelled as ideal quantum dot superlattices, is presented. The carrier wave functions and energy levels were evaluated using the strain dependent 8-band k.p Hamiltonian and used to calculate all intra- and inter-period transition rates due to interaction with phonons and electromagnetic radiation. The interaction with longitudinal acoustic phonons and electromagnetic radiation was treated perturbatively within the framework of Fermi's golden rule, while the interaction with longitudinal optical phonons was considered taking into account their strong coupling to electrons. The populations of energy levels were then found from a system of rate equations, and the electron current in the superlattice was subsequently extracted. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  11. Low-bias flat band-stop filter based on velocity modulated gaussian graphene superlattice

    Science.gov (United States)

    Sattari-Esfahlan, S. M.; Shojaei, S.

    2018-05-01

    Transport properties of biased planar Gaussian graphene superlattice (PGGSL) with Fermi velocity barrier is investigated by transfer matrix method (TMM). It is observed that enlargement of bias voltage over miniband width breaks the miniband to WSLs leads to suppressing resonant tunneling. Transmission spectrum shows flat wide stop-band property controllable by external bias voltage with stop-band width of near 200 meV. The simulations demonstrate that strong velocity barriers prevent tunneling of Dirac electrons leading to controllable enhancement of stop-band width. By increasing ratio of Fermi velocity in barriers to wells υc stop-band width increase. As wide transmission stop-band width (BWT) of filter is tunable from 40 meV to 340 meV is obtained by enhancing ratio of υc from 0.2 to 1.5, respectively. Proposed structure suggests easy tunable wide band-stop electronic filter with a modulated flat stop-band characteristic by height of electrostatic barrier and structural parameters. Robust sensitivity of band width to velocity barrier intensity in certain bias voltages and flat band feature of proposed filter may be opens novel venue in GSL based flat band low noise filters and velocity modulation devices.

  12. Non-linear spin transport in magnetic semiconductor superlattices

    International Nuclear Information System (INIS)

    Bejar, Manuel; Sanchez, David; Platero, Gloria; MacDonald, A.H.

    2004-01-01

    The electronic spin dynamics in DC-biased n-doped II-VI semiconductor multiquantum wells doped with magnetic impurities is presented. Under certain range of electronic doping, conventional semiconductor superlattices present self-sustained oscillations. Magnetically doped wells (Mn) present large spin splittings due to the exchange interaction. The interplay between non-linear interwell transport, the electron-electron interaction and the exchange between electrons and the magnetic impurities produces interesting time-dependent features in the spin polarization current tuned by an external magnetic field

  13. A proposed GaAs-based superlattice solar cell structure with high efficiency and high radiation tolerance

    Science.gov (United States)

    Goradia, Chandra; Clark, Ralph; Brinker, David

    1985-01-01

    A solar cell structure is proposed which uses a GaAs nipi doping superlattice. An important feature of this structure is that photogenerated minority carriers are very quickly collected in a time shorter than bulk lifetime in the fairly heavily doped n and p layers and these carriers are then transported parallel to the superlattice layers to selective ohmic contacts. Assuming that these already-separated carriers have very long recombination lifetimes, due to their being across an indirect bandgap in real space, it is argued that the proposed structure may exhibit superior radiation tolerance along with reasonably high beginning-of-life efficiency.

  14. Microwave properties of YBa2Cu3O7-δ/PrBa2Cu3O7-δ superlattices

    International Nuclear Information System (INIS)

    Carlos, W.E.; Kaplan, R.; Lowndes, D.H.; Norton, D.P.

    1992-01-01

    We have used non-resonant microwave absorption to study c-axis YBa 2 Cu 3 O 7-δ /PrBa 2 Cu 3 O 7-δ superlattices and compare the response to a film of similarly grown YBa 2 Cu 3 O 7-δ (YBCO). Near the respective transition temperatures, the response of the superlattice samples and the YBCO film have similar amplitudes and orientation dependences. This is consistent with the microwave loss being related to magnetic flux penetration at (110) slip planes. At lower temperatures, the response of the superlattices is much stronger than that of the YBCO film and, while both responses are hysteretic at low temperatures, the widths of the hysteresis have opposite orientation dependences, which we attribute to the role of the PrBa 2 Cu 3 O 7-δ layers. (orig.)

  15. Phonon dispersion relations in monoatomic superlattices: a transfer matrix theory

    International Nuclear Information System (INIS)

    Albuquerque, E.L. de; Fulco, P.

    1986-01-01

    We present a lattice dynamical theory for monoatomic superlattices consisting of alternating layers of two different materials. Using a transfer matrix method we obtain explicit the equation for dispersion of the phonon's bulk modes, including the well known result in the long wave-length limit which can be obtained by elasticity theory. An illustation is shown and its features discussed. (Author) [pt

  16. A CPA study of the phonon structure of disordered superlattices

    International Nuclear Information System (INIS)

    Shijie Xiong; Gendi Pang; Chienhua Tsai.

    1985-08-01

    The phonon structure of superlattices or modulated alloys with substitutional disorder is studied in the Coherent Phase Approximation (CPA). We consider first the case with diagonal disorder only, by adopting a virtual crystal approximation for the force constants. Then we treat the more complicated case with inclusion of off-diagonal disorder. Numerical examples are also studied in both cases. (author)

  17. Improved structural and electrical properties in native Sb2Te3/GexSb2Te3+x van der Waals superlattices due to intermixing mitigation

    Directory of Open Access Journals (Sweden)

    Stefano Cecchi

    2017-02-01

    Full Text Available Superlattices made of Sb2Te3/GeTe phase change materials have demonstrated outstanding performance with respect to GeSbTe alloys in memory applications. Recently, epitaxial Sb2Te3/GeTe superlattices were found to feature GexSb2Te3+x blocks as a result of intermixing between constituting layers. Here we present the epitaxy and characterization of Sb2Te3/GexSb2Te3+x van der Waals superlattices, where GexSb2Te3+x was intentionally fabricated. X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, and lateral electrical transport data are reported. The intrinsic 2D nature of both sublayers is found to mitigate the intermixing in the structures, significantly improving the interface sharpness and ultimately the superlattice structural and electrical properties.

  18. Optical properties of graphene superlattices

    International Nuclear Information System (INIS)

    Le, H Anh; Do, V Nam; Ho, S Ta; Nguyen, D Chien

    2014-01-01

    In this work, the optical responses of graphene superlattices, i.e. graphene subjected to a periodic scalar potential, are theoretically reported. The optical properties were studied by investigating the optical conductivity, which was calculated using the Kubo formalism. It was found that the optical conductivity becomes dependent on the photon polarization and is suppressed in the photon energy range of (0, U b ), where U b is the potential barrier height. In the higher photon energy range, i.e. Ω > U b , the optical conductivity is, however, almost identical to that of pristine graphene. Such behaviors of the optical conductivity are explained microscopically through the analysis of the elements of optical matrices and effectively through a simple model, which is based on the Pauli blocking mechanism. (paper)

  19. Passivation of MBE grown InGaSb/InAs superlattice photodiodes

    Science.gov (United States)

    Hill, Cory J.; Keo, Sam S.; Mumolo, Jason M.; Gunapala, Sarath D.

    2005-01-01

    We have performed wet chemical passivation tests on InGaSb/InAs superlattice photodiode structures grown molecular beam epitaxy. The details of the devices growth and characterization as well as the results of chemical passivation involving RuCl3 and H2SO4 with SiO2 dielectric depositions are presented.

  20. Transport properties of YBa2Cu3O7/PrBa2Cu3O7-superlattices

    International Nuclear Information System (INIS)

    El Tahan, Ayman Mohamed Moussa

    2010-01-01

    The understanding of the coupling between superconducting YBa 2 Cu 3 O 7 (YBCO) layers decoupled by non superconducting PrBa 2 Cu 3 O 7 (PBCO) layers in c-axis oriented superlattices was the aim of this thesis. For this purpose two conceptually different kind of transport experiments have been performed. In the first type of transport experiments the current is flowing parallel to the layers. Here the coupling is probed indirectly using magnetic vortex lines, which are penetrating the superlattice. Movement of the vortex segments in neighbouring YBCO layers is more or less coherent depending on the thickness of both the superconducting and non superconducting layers. This in-plane transport was measured either by sending an external current through bridges patterned in the superlattice or by an induced internal current. The vortex-creep activation energy U was determined by analysis of the in-plane resistive transition in an external magnetic field B oriented along the c-axis. The activation energies for two series of superlattices were investigated. In one series the thickness of the YBCO layers was constant (n Y =4 unit cells) and the number of the PBCO unit cells was varied, while in the other the number of PBCO layers was constant (n P =4) and n Y varied. The correlation length of the vortex system was determined to be 80 nm along the c-axis direction. It was found that even a single PBCO unit cell in a superlattice effectively cuts the flux lines into shorter weakly coupled segments, and the coupling of the vortex systems in neighbouring layers is negligible already for a thickness of four unit cells of the PBCO layers. A characteristic variation of the activation energy for the two series of superlattices was found, where U 0 is proportional to the YBCO thickness. A change in the variation of U 0 with the current I in the specimen was observed. The analysis of standard dc magnetization relaxation data obtained for a series superlattices revealed the

  1. Improved performance of In0.83Ga0.17As/InP photodetectors through modifying the position of In0.66Ga0.34As/InAs superlattice electron barrier

    Science.gov (United States)

    Shi, Yan-hui; Zhang, Yong-gang; Ma, Ying-jie; Gu, Yi; Chen, Xing-you; Gong, Qian; Du, Ben; Zhang, Jian; Zhu, Yi

    2018-03-01

    The performance of wavelength extended In0.83Ga0.17As/InP photodetectors has been improved notably through modifying the position of electron barriers in absorption layer. In order to fully utilize the diffusion component of the photocurrent, the In0.66Ga0.34As/InAs superlattice electron barrier is moved to the edge of the depletion region. Enhanced peak photo responsivity up to 0.84 A/W is realized, which raises 24% compared to that of a reference detector with the superlattice barrier in the middle of the absorber. The dark current slightly increases by 25% at room temperature while decreases by more than an order of magnitude at 150 K, resulting in about 10% or more than twofold improvements for the detectivity, respectively. The results suggest that optimized barrier position is a necessity for barrier-type photodetectors to achieve better performances.

  2. Matter-Wave Solitons In Optical Superlattices

    International Nuclear Information System (INIS)

    Louis, Pearl J. Y.; Ostrovskaya, Elena A.; Kivshar, Yuri S.

    2006-01-01

    In this work we show that the properties of both bright and dark Bose-Einstein condensate (BEC) solitons trapped in optical superlattices can be controlled by changing the shape of the trapping potential whilst maintaining a constant periodicity and lattice height. Using this method we can control the properties of bright gap solitons by dispersion management. We can also control the interactions between dark lattice solitons. In addition we demonstrate a method for controlled generation of matter-wave gap solitons in stationary optical lattices by interfering two condensate wavepackets, producing a single wavepacket at a gap edge with properties similar to a gap soliton. As this wavepacket evolves, it forms a bright gap soliton

  3. Phase transitions of a spin-one Ising ferromagnetic superlattice

    International Nuclear Information System (INIS)

    Saber, A.

    2001-09-01

    Using the effective field theory with a probability distribution technique, the magnetic properties in an infinite superlattice consisting of two different ferromagnets are studied in a spin-one Ising model. The dependence of the Curie temperatures are calculated as a function of two slabs in one period and as a function of the intra- and interlayer exchange interactions. A critical value of the exchange reduced interaction above which the interface magnetism appears is found. (author)

  4. Sb2Te3 and Its Superlattices: Optimization by Statistical Design.

    Science.gov (United States)

    Behera, Jitendra K; Zhou, Xilin; Ranjan, Alok; Simpson, Robert E

    2018-05-02

    The objective of this work is to demonstrate the usefulness of fractional factorial design for optimizing the crystal quality of chalcogenide van der Waals (vdW) crystals. We statistically analyze the growth parameters of highly c axis oriented Sb 2 Te 3 crystals and Sb 2 Te 3 -GeTe phase change vdW heterostructured superlattices. The statistical significance of the growth parameters of temperature, pressure, power, buffer materials, and buffer layer thickness was found by fractional factorial design and response surface analysis. Temperature, pressure, power, and their second-order interactions are the major factors that significantly influence the quality of the crystals. Additionally, using tungsten rather than molybdenum as a buffer layer significantly enhances the crystal quality. Fractional factorial design minimizes the number of experiments that are necessary to find the optimal growth conditions, resulting in an order of magnitude improvement in the crystal quality. We highlight that statistical design of experiment methods, which is more commonly used in product design, should be considered more broadly by those designing and optimizing materials.

  5. Theoretical modelling of electron transport in InAs/GaAs quantum dot superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Vukmirovic, Nenad; Ikonic, Zoran; Savic, Ivana; Indjin, Dragan; Harrison, Paul [School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT (United Kingdom)

    2006-07-01

    A theoretical model describing the electron transport in InAs/GaAs quantum dot infrared photodetectors, modelled as ideal quantum dot superlattices, is presented. The carrier wave functions and energy levels were evaluated using the strain dependent 8-band k.p Hamiltonian and used to calculate all intra- and inter-period transition rates due to interaction with phonons and electromagnetic radiation. The interaction with longitudinal acoustic phonons and electromagnetic radiation was treated perturbatively within the framework of Fermi's golden rule, while the interaction with longitudinal optical phonons was considered taking into account their strong coupling to electrons. The populations of energy levels were then found from a system of rate equations, and the electron current in the superlattice was subsequently extracted. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  6. Photoinduced Domain Pattern Transformation in Ferroelectric-Dielectric Superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Ahn, Youngjun; Park, Joonkyu; Pateras, Anastasios; Rich, Matthew B.; Zhang, Qingteng; Chen, Pice; Yusuf, Mohammed H.; Wen, Haidan; Dawber, Matthew; Evans, Paul G.

    2017-07-01

    The nanodomain pattern in ferroelectric/dielectric superlattices transforms to a uniform polarization state under above-bandgap optical excitation. X-ray scattering reveals a disappearance of domain diffuse scattering and an expansion of the lattice. The reappearance of the domain pattern occurs over a period of seconds at room temperature, suggesting a transformation mechanism in which charge carriers in long-lived trap states screen the depolarization field. A Landau-Ginzburg-Devonshire model predicts changes in lattice parameter and a critical carrier concentration for the transformation.

  7. Maximum Entropy Closure of Balance Equations for Miniband Semiconductor Superlattices

    Directory of Open Access Journals (Sweden)

    Luis L. Bonilla

    2016-07-01

    Full Text Available Charge transport in nanosized electronic systems is described by semiclassical or quantum kinetic equations that are often costly to solve numerically and difficult to reduce systematically to macroscopic balance equations for densities, currents, temperatures and other moments of macroscopic variables. The maximum entropy principle can be used to close the system of equations for the moments but its accuracy or range of validity are not always clear. In this paper, we compare numerical solutions of balance equations for nonlinear electron transport in semiconductor superlattices. The equations have been obtained from Boltzmann–Poisson kinetic equations very far from equilibrium for strong fields, either by the maximum entropy principle or by a systematic Chapman–Enskog perturbation procedure. Both approaches produce the same current-voltage characteristic curve for uniform fields. When the superlattices are DC voltage biased in a region where there are stable time periodic solutions corresponding to recycling and motion of electric field pulses, the differences between the numerical solutions produced by numerically solving both types of balance equations are smaller than the expansion parameter used in the perturbation procedure. These results and possible new research venues are discussed.

  8. Raman analysis of phonon modes in a short period AlN/GaN superlattice

    Science.gov (United States)

    Sarkar, Ketaki; Datta, Debopam; Gosztola, David J.; Shi, Fengyuan; Nicholls, Alan; Stroscio, Michael A.; Dutta, Mitra

    2018-03-01

    AlN/GaN-based optoelectronic devices have been the subject of intense research underlying the commercialization of efficient devices. Areas of considerable interest are the study of their lattice dynamics, phonon transport, and electron-phonon interactions specific to the interface of these heterostructures which results in additional optical phonon modes known as interface phonon modes. In this study, the framework of the dielectric continuum model (DCM) has been used to compare and analyze the optical phonon modes obtained from experimental Raman scattering measurements on AlN/GaN short-period superlattices. We have observed the localized E2(high), A1(LO) and the E1(TO) modes in superlattice measurements at frequencies shifted from their bulk values. To the best of our knowledge, the nanostructures used in these studies are among the smallest yielding useful Raman signatures for the interface modes. In addition, we have also identified an additional spread of interface phonon modes in the TO range resulting from the superlattice periodicity. The Raman signature contribution from the underlying AlxGa1-xN ternary has also been observed and analyzed. A temperature calibration was done based on Stokes/anti-Stokes ratio of A1(LO) using Raman spectroscopy in a broad operating temperature range. Good agreement between the experimental results and theoretically calculated calibration plot predicted using Bose-Einstein statistics was obtained.

  9. The electronic band parameters calculated by the Kronig-Penney method for Cd1-xZnxS quantum dot superlattices

    International Nuclear Information System (INIS)

    Sakly, A.; Safta, N.; Mejri, H.; Lamine, A. Ben

    2009-01-01

    This work reports on a theoretical study of superlattices based on Cd 1-x Zn x S quantum dots embedded in an insulating material. We show, in particular, how this system can be assumed to a series of flattened cylindrical quantum dots with a finite barrier height at the boundary. In this paper, are also reviewed the approximations needed to calculate the band edges of the Cd 1-x Zn x S superlattices with use of the Kronig-Penney model. The electronic states and the electron effective masses of both Γ 1 - and Γ 2 -minibands have been computed as a function of zinc composition for different inter-quantum dot separations. As is found, the CdS system is appropriate to give rise a superlattice behavior for conduction electrons in a relatively large range of inter-sheet separations. An attempt to explain the electron band parameters calculated will be presented.

  10. Theoretical Study of the Transverse Dielectric Constant of Superlattices and Their Alloys. Ph.D Thesis

    Science.gov (United States)

    Kahen, K. B.

    1986-01-01

    The optical properties of III to V binary and ternary compounds and GaAs-Al(x)Ga(1-x)As superlattices are determined by calculating the real and imaginary parts of the transverse dielectric constant. Emphasis is given to determining the influence of different material and superlattice parameters on the values of the index of refraction and absorption coefficient. In order to calculate the optical properties of a material, it is necessary to compute its electronic band structure. This was accomplished by introducing a partition band structure approach based on a combination of the vector k x vector p and nonlocal pseudopotential techniques. The advantages of this approach are that it is accurate, computationally fast, analytical, and flexible. These last two properties enable incorporation of additional effects into the model, such as disorder scattering, which occurs for alloy materials and excitons. Furthermore, the model is easily extended to more complex structures, for example multiple quantum wells and superlattices. The results for the transverse dielectric constant and absorption coefficient of bulk III to V compounds compare well with other one-electron band structure models and the calculations show that for small frequencies, the index of refraction is determined mainly by the contibution of the outer regions of the Brillouin zone.

  11. Optical and vibrational properties of (ZnO){sub k} In{sub 2}O{sub 3} natural superlattice nanostructures

    Energy Technology Data Exchange (ETDEWEB)

    Margueron, Samuel [Laboratoire Matériaux Optiques, Photonique et Systèmes, Université de Lorraine et CentraleSupélec, 57070 Metz (France); John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Maryland 02138 (United States); Pokorny, Jan; Skiadopoulou, Stella; Kamba, Stanislav [Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21 Prague 8 (Czech Republic); Liang, Xin [School of Materials Science and Engineering, Changzhou University, Changzhou, Jiangsu Province 213164 (China); Clarke, David R. [John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Maryland 02138 (United States)

    2016-05-21

    A thermodynamically stable series of superlattices, (ZnO){sub k}In{sub 2}O{sub 3}, form in the ZnO-In{sub 2}O{sub 3} binary oxide system for InO{sub 1.5} concentrations from about 13 up to about 33 mole percent (m/o). These natural superlattices, which consist of a periodic stacking of single, two-dimensional sheets of InO{sub 6} octahedra, are found to give rise to systematic changes in the optical and vibrational properties of the superlattices. Low-frequency Raman scattering provides the evidence for the activation of acoustic phonons due to the folding of Brillouin zone. New vibrational modes at 520 and 620 cm{sup −1}, not present in either ZnO or In{sub 2}O{sub 3}, become Raman active. These new modes are attributed to collective plasmon oscillations localized at the two-dimensional InO{sub 1.5} sheets. Infrared reflectivity experiments, and simulations taking into account a negative dielectric susceptibility due to electron carriers in ZnO and interface modes of the dielectric layer of InO{sub 2}, explain the occurrence of these new modes. We postulate that a localized electron gas forms at the ZnO/InO{sub 2} interface due to the electron band alignment and polarization effects. All our observations suggest that there are quantum contributions to the thermal and electrical conductivity in these natural superlattices.

  12. Level densities of iron isotopes and lower-energy enhancement of y-strength function

    International Nuclear Information System (INIS)

    Voinov, A V; Grimes, S M; Agvaanluvsan, U; Algin, E; Belgya, T; Brune, C R; Guttormsen, M; Hornish, M J; Massey, T N; Mitchell, G; Rekstad, J; Schiller, A; Siem, S

    2005-01-01

    The neutron spectrum from the 55 Mn(d,n) 56 Fe reaction has been measured at E d = 7 MeV. The level density of 56 Fe obtained from neutron evaporation spectrum has been compared to the level density from Oslo-type 57 Fe( 3 He, aγ) 56 Fe experiment [1]. The good agreement supports the recent results [1, 8] including an availability of a low-energy enhancement in the γ-strength function for iron isotopes. The new level density function allowed us to investigate an excitation energy dependence of this enhancement, which is shown to increase with increasing excitation energy

  13. Unconventional superconductivity in magic-angle graphene superlattices

    Science.gov (United States)

    Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo

    2018-04-01

    The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity—which cannot be explained by weak electron–phonon interactions—in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°—the first ‘magic’ angle—the electronic band structure of this ‘twisted bilayer graphene’ exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature–carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 1011 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high

  14. Full Polarization Analysis of Resonant Superlattice and Forbidden x-ray Reflections in Magnetite

    International Nuclear Information System (INIS)

    Wilkins, S.B.; Bland, S.R.; Detlefs, B.; Beale, T.A.W.; Mazzoli, C.; Joly, Y.; Hatton, P.D.; Lorenzo, J.E.; Brabers, V.A.M.

    2009-01-01

    Despite being one of the oldest known magnetic materials, and the classic mixed valence compound, thought to be charge ordered, the structure of magnetite below the Verwey transition is complex and the presence and role of charge order is still being debated. Here, we present resonant x-ray diffraction data at the iron K-edge on forbidden (0, 0, 2n+1) C and superlattice (0, 0, 2n+1/2)C reflections. Full linear polarization analysis of the incident and scattered light was conducted in order to explore the origins of the reflections. Through simulation of the resonant spectra we have confirmed that a degree of charge ordering takes place, while the anisotropic tensor of susceptibility scattering is responsible for the superlattice reflections below the Verwey transition. We also report the surprising result of the conversion of a significant proportion of the scattered light from linear to nonlinear polarization.

  15. THz elastic dynamics in finite-size CoFeB-MgO phononic superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Ulrichs, Henning, E-mail: hulrich@gwdg.de; Meyer, Dennis; Müller, Markus; Wittrock, Steffen; Mansurova, Maria [I. Physical Institute, Georg-August University of Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen (Germany); Walowski, Jakob; Münzenberg, Markus [Institute of Physics, Ernst-Moritz-Arndt University of Greifswald, Felix-Hausdorff-Str. 6, 17489 Greifswald (Germany)

    2016-10-14

    In this article, we present the observation of coherent elastic dynamics in a nano-scale phononic superlattice, which consists of only 4 bilayers. We demonstrate how ultra-short light pulses with a length of 40 fs can be utilized to excite a coherent elastic wave at 0.535 THz, which persist over about 20 ps. In later steps of the elastic dynamics, modes with frequency of 1.7 THz and above appear. All these modes are related to acoustic band gaps. Thus, the periodicity strongly manifests in the wave physics, although the system under investigation has only a small number of spatial periods. To further illustrate this, we show how by breaking the translational invariance of the superlattice, these features can be suppressed. Discussed in terms of phonon blocking and radiation, we elucidate in how far our structures can be considered as useful building blocks for phononic devices.

  16. Simultaneously enhanced strength and ductility for 3D-printed stainless steel 316L by selective laser melting

    Science.gov (United States)

    Sun, Zhongji; Tan, Xipeng; Tor, Shu Beng; Chua, Chee Kai

    2018-04-01

    Laser-based powder-bed fusion additive manufacturing or three-dimensional printing technology has gained tremendous attention due to its controllable, digital, and automated manufacturing process, which can afford a refined microstructure and superior strength. However, it is a major challenge to additively manufacture metal parts with satisfactory ductility and toughness. Here we report a novel selective laser melting process to simultaneously enhance the strength and ductility of stainless steel 316L by in-process engineering its microstructure into a crystallographic texture. We find that the tensile strength and ductility of SLM-built stainless steel 316L samples could be enhanced by 16% and 40% respectively, with the engineered textured microstructure compared to the common textured microstructure. This is because the favorable nano-twinning mechanism was significantly more activated in the textured stainless steel 316L samples during plastic deformation. In addition, kinetic simulations were performed to unveil the relationship between the melt pool geometry and crystallographic texture. The new additive manufacturing strategy of engineering the crystallographic texture can be applied to other metals and alloys with twinning-induced plasticity. This work paves the way to additively manufacture metal parts with high strength and high ductility.

  17. Structural simulation of superlattices in lithium aluminates; Simulacion estructural de superredes en aluminatos de litio

    Energy Technology Data Exchange (ETDEWEB)

    Carrera G, L.M.; Basurto S, R. [Instituto Nacional de Investigaciones Nucleares, A.P. 18-1027, 11801 Mexico D.F. (Mexico)

    1997-07-01

    Among the materials to be used on the tritium generator cover of the future fusion reactors the lithium aluminate ({gamma} - LiAlO{sub 2}) is one of the more studied. In this work it is presented the superlattice structural simulation that presents to {gamma} - LiAlO{sub 2} as main phase and to {alpha} - LiAlO{sub 2} as the secondary phase. The simulation is developed considering that as the two phases present different symmetry ({gamma} - LiAlO{sub 2} is tetrahedral and {alpha} - LiAlO{sub 2} is hexahedral) it is had a superlattice LUCS type (Layered Ultrathin Coherent Structure) that is it presents an structure in coherent ultrathin layers since it is what implicates a lesser energy of formation. (Author)

  18. Optical properties of metallic Fibonacci quasi-superlattice

    International Nuclear Information System (INIS)

    Feng Weiguo; Liu Nianhua; Wu Xiang

    1990-06-01

    Within the approximation of hydrodynamic model, the optical properties of the metallic Fibonacci quasi-superlattice have been studied for the region of s-polarized soft x-rays and extreme ultraviolet. By using the transfer-matrix method and taking account of damping effects, we have discussed the electromagnetic normal modes for the quasisuperlattice in the rational approximation. The related dispersion curves explain the reflection spectra well, and we found that similar to the reflectivities, both real part and imagine part of the dispersion relation pattern has a rich structure of self-similarity. With the increasing of the generation number, the electromagnetic modes all become critical. (author). 13 refs, 3 figs

  19. Engineering the oxygen coordination in digital superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Cook, Seyoung [Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA; Department of Materials Science, Northwestern University, Evanston, Illinois 60202, USA; Andersen, Tassie K. [Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA; Department of Materials Science, Northwestern University, Evanston, Illinois 60202, USA; Hong, Hawoong [X-Ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA; Rosenberg, Richard A. [X-Ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA; Marks, Laurence D. [Department of Materials Science, Northwestern University, Evanston, Illinois 60202, USA; Fong, Dillon D. [Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

    2017-12-01

    The oxygen sublattice in the complex oxides is typically composed of corner-shared polyhedra, with transition metals at their centers. The electronic and chemical properties of the oxide depend on the type and geometric arrangement of these polyhedra, which can be controlled through epitaxial synthesis. Here, we use oxide molecular beam epitaxy to create SrCoOx:SrTiO3 superlattices with tunable oxygen coordination environments and sublattice geometries. Using soft X-ray spectroscopy, we find that the chemical state of Co can be varied with the polyhedral arrangement, demonstrating a new strategy for achieving unique electronic properties in the transition metal oxides.

  20. Novel Si-Ge-C Superlattices for More than Moore CMOS

    Science.gov (United States)

    2016-03-31

    Keywords: Silicon ; Germanium; Carbon; Superlattices; Direct Band-Gaps; Silicon - Photonics ; Image Sensors. Introduction Materials with direct band-gaps and...Wang Z., B. Tian, M. Pantouvaki, et al., “Room- temperature InP distributed feedback laser array directly grown on silicon ”, Nature Photonics 9, 2015...Creek Blvd. Suite 284 San Jose, CA 95129 Contact author: Lynn.Forester@Quantumsemi.com Abstract: The search for Silicon -based direct band-gap

  1. Enhanced fluidity liquid chromatography of inulin fructans using ternary solvent strength and selectivity gradients.

    Science.gov (United States)

    Bennett, Raffeal; Olesik, Susan V

    2018-01-25

    The value of exploring selectivity and solvent strength ternary gradients in enhanced fluidity liquid chromatography (EFLC) is demonstrated for the separation of inulin-type fructans from chicory. Commercial binary pump systems for supercritical fluid chromatography only allow for the implementation of ternary solvent strength gradients which can be restrictive for the separation of polar polymeric analytes. In this work, a custom system was designed to extend the capability of EFLC to allow tuning of selectivity or solvent strength in ternary gradients. Gradient profiles were evaluated using the Berridge function (RF 1 ), normalized resolution product (NRP), and gradient peak capacity (P c ). Selectivity gradients provided the separation of more analytes over time. The RF 1 function showed favor to selectivity gradients with comparable P c to that of solvent strength gradients. NRP did not strongly correlate with P c or RF 1 score. EFLC with the hydrophilic interaction chromatography, HILIC, separation mode was successfully employed to separate up to 47 fructan analytes in less than 25 min using a selectivity gradient. Copyright © 2017 Elsevier B.V. All rights reserved.

  2. Effects of diode laser welding with dye-enhanced glue on tensile strength of sutures commonly used in urology.

    Science.gov (United States)

    Kirsch, A J; Chang, D T; Kayton, M L; Libutti, S K; Connor, J P; Hensle, T W

    1996-01-01

    Tissue welding using laser-activated protein solders may soon become an alternative to sutured tissue approximation. In most cases, approximating sutures are used both to align tissue edges and provide added tensile strength. Collateral thermal injury, however, may cause disruption of tissue alignment and weaken the tensile strength of sutures. The objective of this study was to evaluate the effect of laser welding on the tensile strength of suture materials used in urologic surgery. Eleven types of sutures were exposed to diode laser energy (power density = 15.9 W/cm2) for 10, 30, and 60 seconds. Each suture was compared with and without the addition of dye-enhanced albumin-based solder. After exposure, each suture material was strained (2"/min) until ultimate breakage on a tensometer and compared to untreated sutures using ANOVA. The strength of undyed sutures were not significantly affected; however, violet and green-dyed sutures were in general weakened by laser exposure in the presence of dye-enhanced glue. Laser activation of the smallest caliber, dyed sutures (7-0) in the presence of glue caused the most significant loss of tensile strength of all sutures tested. These results indicate that the thermal effects of laser welding using our technique decrease the tensile strength of dyed sutures. A thermally resistant suture material (undyed or clear) may prevent disruption of wounds closed by laser welding techniques.

  3. Growth and characterization of In1-xGaxAs/InAs0.65Sb0.35 strained layer superlattice infrared detectors

    Science.gov (United States)

    Ariyawansa, G.; Duran, J. M.; Reyner, C. J.; Steenbergen, E. H.; Yoon, N.; Wasserman, D.; Scheihing, J. E.

    2017-02-01

    Type-II strained layer superlattices (SLS) are an active research topic in the infrared detector community and applications for SLS detectors continue to grow. SLS detector technology has already reached the commercial market due to improvements in material quality, device design, and device fabrication. Despite this progress, the optimal superlattice design has not been established, and at various times has been believed to be InAs/GaSb, InAs/InGaSb, or InAs/InAsSb. Building on these, we investigate the properties of a new mid-wave infrared SLS material: InGaAs/InAsSb SLS. The ternary InGaAs/InAsSb SLS has three main advantages over other SLS designs: greater support for strain compensation, enhanced absorption due to increased electron-hole wavefunction overlap, and improved vertical hole mobility due to reduced hole effective mass. Here, we compare three ternary SLSs, with approximately the same bandgap (0.240 eV at 150 K), comprised of Ga fractions of 5%, 10%, and 20% to a reference sample with 0% Ga. Enhanced absorption is both theoretically predicted and experimentally realized. Furthermore, the characteristics of ternary SLS infrared detectors based on an nBn architecture are reported and exhibit nearly state-of-the-art dark current performance with minimal growth optimization. We report standard material and device characterization information, including dark current and external quantum efficiency, and provide further analysis that indicates improved quantum efficiency and vertical hole mobility. Finally, a 320×256 focal plane array built based on the In0.8Ga0.2As/InAs0.65Sb0.35 SLS design is demonstrated with promising performance.

  4. Fe/V and Fe/Co (0 0 1) superlattices: growth, anisotropy, magnetisation and magnetoresistance

    International Nuclear Information System (INIS)

    Nordblad, P.; Broddefalk, A.; Mathieu, R.; Blomqvist, P.; Eriksson, O.; Waeppling, R.

    2003-01-01

    Some physical properties of BCC Fe/V and Fe/Co (0 0 1) superlattices are reviewed. The dependence of the magnetic anisotropy on the in-plane strain introduced by the lattice mismatch between Fe and V is measured and compared to a theoretical derivation. The dependence of the magnetic anisotropy (and saturation magnetisation) on the layer thickness ratio Fe/Co is measured and a value for the anisotropy of BCC Co is derived from extrapolation. The interlayer exchange coupling of Fe/V superlattices is studied as a function of the V layer thickness (constant Fe thickness) and layer thickness of Fe (constant V thickness). A region of antiferromagnetic coupling and GMR is found for V thicknesses 12-14 monolayers. However, surprisingly, a 'cutoff' of the antiferromagnetic coupling and GMR is found when the iron layer thickness exceeds about 10 monolayers

  5. Transmission spectra of electrons through the Thue-Morse graphene superlattice

    International Nuclear Information System (INIS)

    Korol', A.M.

    2014-01-01

    The transmission spectra of the Thue-Morse superlattice (SL) based on a monolayer gapped graphene are investigated. The SL consists of rectangular barriers located along the Ox axis. The Thue-Morse aperiodic modulation is proposed to be realized due to the difference in values of the gap width in different SL elements. It is shown that the effective splitting of the allowed bands (and thereby the arising of a series of gaps) is observed under the influence of the aperiodic factor in the case of normal incidence of the electron wave on the SL as well as in the case of oblique incidence. The spectra are periodical with potential barrier height. In some regions of the spectra, the splitting of bands is subjected to the Fibonacci inflation rule in every new Thue-Morse generation. As in the periodical graphene-based SL, in every Thue-Morse sequence a superlattice Dirac point is created. The width of the gap associated with this point depends on SL parameters substantially; at the same time the energy location of this gap depends weakly on mass term in the Hamiltonian and it does not depend on SL period. The spectra dependence on the angle of electron wave incidence is not substantial

  6. Indium-bump-free antimonide superlattice membrane detectors on silicon substrates

    Energy Technology Data Exchange (ETDEWEB)

    Zamiri, M., E-mail: mzamiri@chtm.unm.edu, E-mail: skrishna@chtm.unm.edu; Klein, B.; Schuler-Sandy, T.; Dahiya, V.; Cavallo, F. [Center for High Technology Materials, Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, New Mexico 87106 (United States); Myers, S. [SKINfrared, LLC, Lobo Venture Lab, 801 University Blvd., Suite 10, Albuquerque, New Mexico 87106 (United States); Krishna, S., E-mail: mzamiri@chtm.unm.edu, E-mail: skrishna@chtm.unm.edu [Center for High Technology Materials, Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, New Mexico 87106 (United States); SKINfrared, LLC, Lobo Venture Lab, 801 University Blvd., Suite 10, Albuquerque, New Mexico 87106 (United States)

    2016-02-29

    We present an approach to realize antimonide superlattices on silicon substrates without using conventional Indium-bump hybridization. In this approach, PIN superlattices are grown on top of a 60 nm Al{sub 0.6}Ga{sub 0.4}Sb sacrificial layer on a GaSb host substrate. Following the growth, the individual pixels are transferred using our epitaxial-lift off technique, which consists of a wet-etch to undercut the pixels followed by a dry-stamp process to transfer the pixels to a silicon substrate prepared with a gold layer. Structural and optical characterization of the transferred pixels was done using an optical microscope, scanning electron microscopy, and photoluminescence. The interface between the transferred pixels and the new substrate was abrupt, and no significant degradation in the optical quality was observed. An Indium-bump-free membrane detector was then fabricated using this approach. Spectral response measurements provided a 100% cut-off wavelength of 4.3 μm at 77 K. The performance of the membrane detector was compared to a control detector on the as-grown substrate. The membrane detector was limited by surface leakage current. The proposed approach could pave the way for wafer-level integration of photonic detectors on silicon substrates, which could dramatically reduce the cost of these detectors.

  7. Theory of coherent time-dependent transport in one-dimensional multiband semiconductor super-lattices

    DEFF Research Database (Denmark)

    Rotvig, J.; Smith, H.; Jauho, Antti-Pekka

    1996-01-01

    We present an analytical study of one-dimensional semiconductor superlattices in external electric fields, which may be time dependent. A number of general results for the (quasi)energies and eigenstates are derived. An equation of motion for the density matrix is obtained for a two-band model...

  8. Dualband MW/LW Strained Layer Superlattice Focal Plane Arrays For Satellite-Based Wildfire Detection, Phase I

    Data.gov (United States)

    National Aeronautics and Space Administration — Dualband focal plane arrays (FPAs) based on gallium-free Type-II strained layer superlattice (SLS) photodiodes have recently experienced significant advances. We...

  9. Illumination-induced changes of the Fermi surface topology in three-dimensional superlattices

    Czech Academy of Sciences Publication Activity Database

    Goncharuk, Natalya; Smrčka, Ludvík; Svoboda, Pavel; Vašek, Petr; Kučera, Jan; Krupko, Yu.; Wegscheider, W.

    2007-01-01

    Roč. 75, č. 24 (2007), 245322/1-245322/7 ISSN 1098-0121 R&D Projects: GA MŠk LC510; GA AV ČR KAN400100652 Institutional research plan: CEZ:AV0Z10100521 Keywords : persistent photoconductivity * superlattice * Fermi surface Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.172, year: 2007

  10. Visualizing period fluctuations in strained-layer superlattices with scanning tunneling microscopy

    Science.gov (United States)

    Kanedy, K.; Lopez, F.; Wood, M. R.; Gmachl, C. F.; Weimer, M.; Klem, J. F.; Hawkins, S. D.; Shaner, E. A.; Kim, J. K.

    2018-01-01

    We show how cross-sectional scanning tunneling microscopy (STM) may be used to accurately map the period fluctuations throughout epitaxial, strained-layer superlattices based on the InAs/InAsSb and InGaAs/InAlAs material systems. The concept, analogous to Bragg's law in high-resolution x-ray diffraction, relies on an analysis of the [001]-convolved reciprocal-space satellite peaks obtained from discrete Fourier transforms of individual STM images. Properly implemented, the technique enables local period measurements that reliably discriminate vertical fluctuations localized to within ˜5 superlattice repeats along the [001] growth direction and orthogonal, lateral fluctuations localized to within ˜40 nm along directions in the growth plane. While not as accurate as x-ray, the inherent, single-image measurement error associated with the method may be made as small as 0.1%, allowing the vertical or lateral period fluctuations contributing to inhomogeneous energy broadening and carrier localization in these structures to be pinpointed and quantified. The direct visualization of unexpectedly large, lateral period fluctuations on nanometer length scales in both strain-balanced systems supports a common understanding in terms of correlated interface roughness.

  11. Predicting Chiral Nanostructures, Lattices and Superlattices in Complex Multicomponent Nanoparticle Self-Assembly

    KAUST Repository

    Hur, Kahyun

    2012-06-13

    "Bottom up" type nanoparticle (NP) self-assembly is expected to provide facile routes to nanostructured materials for various, for example, energy related, applications. Despite progress in simulations and theories, structure prediction of self-assembled materials beyond simple model systems remains challenging. Here we utilize a field theory approach for predicting nanostructure of complex and multicomponent hybrid systems with multiple types of short- and long-range interactions. We propose design criteria for controlling a range of NP based nanomaterial structures. In good agreement with recent experiments, the theory predicts that ABC triblock terpolymer directed assemblies with ligand-stabilized NPs can lead to chiral NP network structures. Furthermore, we predict that long-range Coulomb interactions between NPs leading to simple NP lattices, when applied to NP/block copolymer (BCP) assemblies, induce NP superlattice formation within the phase separated BCP nanostructure, a strategy not yet realized experimentally. We expect such superlattices to be of increasing interest to communities involved in research on, for example, energy generation and storage, metamaterials, as well as microelectronics and information storage. © 2012 American Chemical Society.

  12. Electronic band structure of magnetic bilayer graphene superlattices

    International Nuclear Information System (INIS)

    Pham, C. Huy; Nguyen, T. Thuong; Nguyen, V. Lien

    2014-01-01

    Electronic band structure of the bilayer graphene superlattices with δ-function magnetic barriers and zero average magnetic flux is studied within the four-band continuum model, using the transfer matrix method. The periodic magnetic potential effects on the zero-energy touching point between the lowest conduction and the highest valence minibands of pristine bilayer graphene are exactly analyzed. Magnetic potential is shown also to generate the finite-energy touching points between higher minibands at the edges of Brillouin zone. The positions of these points and the related dispersions are determined in the case of symmetric potentials.

  13. Tunable electronic transmission gaps in a graphene superlattice

    International Nuclear Information System (INIS)

    Lu Weitao; Wang Shunjin; Li Wen; Wang Yonglong; Jiang Hua

    2012-01-01

    The transmission in graphene superlattices with adjustable barrier height is investigated using transfer-matrix method. It is found that one could control the angular range of transmission by changing the ratio of incidence energy and barrier height. The transmission as a function of incidence energy has more than one gaps, due to the appearance of evanescent waves in different barriers. Accordingly, more than one conductivity minimums are induced. The transmission gaps could be controlled by adjusting the incidence angle, the barrier height, and the barrier number, which gives the possibility to construct an energy-dependent wavevector filter.

  14. Dualband MW/LW Strained Layer Superlattice Focal Plane Arrays for Satellite-Based Wildfire Detection, Phase II

    Data.gov (United States)

    National Aeronautics and Space Administration — Infrared focal plane arrays (FPAs) based on Type-II strained layer superlattice (SLS) photodiodes have recently experienced significant advances. In Phase I we...

  15. Lattice relaxation and ferromagnetic character of (LaVO3)m/SrVO3 superlattices

    KAUST Repository

    Schuster, Cosima B.

    2013-08-01

    The experimental observation that vanadate superlattices (LaVO 3)m/SrVO3 show ferromagnetism up to room temperature (Lüders U. et al., Phys. Rev. B, 80 (2009) 241102(R)) is investigated by means of density functional theory, and the band structure for m = 5 and 6 is calculated. A buckling of the interface VO2 layers is found in both cases, but subtle differences in bond length lead to very different properties for even and odd values of m: in the even case, the two interface VO2 layers effectively decouple from the adjacent LaO layers due to a strong bond length enhancement. This results into a local inversion of the orbital occupancy and to the confinement of the charge carriers. In the odd case, the amplitude of the bond length variation is smaller, so that the charge carriers spill into the deeper-lying VO2 layers, and spin-polarised interfaces are obtained. © Copyright EPLA, 2013.

  16. Enhanced Flexural Strength of Tellurium Nanowires/epoxy Composites with the Reinforcement Effect of Nanowires

    Science.gov (United States)

    Balguri, Praveen Kumar; Harris Samuel, D. G.; Aditya, D. B.; Vijaya Bhaskar, S.; Thumu, Udayabhaskararao

    2018-02-01

    Investigating the mechanical properties of polymer nanocomposite materials has been greatly increased in the last decade. In particular, flexural strength plays a major role in resisting bending and shear loads of a composite material. Here, one dimensional (1D) tellurium nanowires (TeNWs) reinforced epoxy composites have been prepared and the flexural properties of resulted TeNWs/epoxy nanocomposites are studied. The diameter and length of the TeNWs used to make TeNWs/epoxy nanocomposites are 21±2.5 nm and 697±87 nm, respectively. Plain and TeNWs/epoxy nanocomposites are characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and differential thermal analysis (DTA). Furthermore, significant enhancement in the flexural strength of TeNWs/epoxy nanocomposite is observed in comparison to plain epoxy composite, i.e. flexural strength is increased by 65% with the addition of very little amount of TeNWs content (0.05 wt.%) to epoxy polymer. Structural details of plain and TeNWs/epoxy at micrometer scale were examined by scanning electron microscopy (SEM). We believe that our results provide a new type of semiconductor nanowires based high strength epoxy polymer nanocomposites.

  17. Fibroblast implantation enhances wound healing as indicated by breaking strength determinations

    Energy Technology Data Exchange (ETDEWEB)

    Krueger, W W; Goepfert, H; Romsdahl, M; Hersen, J; Withers, R H; Jesse, R H

    1978-09-01

    Irradiation of normal tissues at the dose/time factor employed in the treatment of solid tumors impairs the subsequent healing of surgical wounds made in those tissues. Irreversible radiation damage to regional fibroblasts is one cause of impared healing. This study was conducted to determine whether syngeneic guinea pig fibroblasts is one cause of impared healing. This study was conducted to determine whether syngeneic guinea pig fibroblasts, harvested from tissue culture when injected into irradiated guinea pig skin at the time of wound closure, could improve wound healing. Breaking strength determinations indicate that irradiated wounds demonstrate enhanced wound healing if implanted with fibroblasts.

  18. High mobility half-metallicity in the (LaMnO3)2/(SrTiO3)8 superlattice

    KAUST Repository

    Cossu, Fabrizio

    2013-01-28

    First principles calculations have been performed to investigate the LaMnO3/SrTiO3 superlattice. Structural relaxation within the generalized gradient approximation results in no significant tiltings or rotations of oxygen octahedra, but in distinct distortions in the SrTiO3 region. Taking into account the onsite Coulomb interaction, we find that the Mn spins order ferromagnetically, in contrast to the antiferromagnetic state of bulk LaMnO3. Most importantly, the interface strain combined with charge transfer across the interface induces half-metallicity within the MnO2 layers. The superlattice is particulary interesting for spintronics applications because the half-metallic states are characterized by an extraordinary high mobility.

  19. High mobility half-metallicity in the (LaMnO3)2/(SrTiO3)8 superlattice

    KAUST Repository

    Cossu, Fabrizio; Schwingenschlö gl, Udo; Singh, Nirpendra

    2013-01-01

    First principles calculations have been performed to investigate the LaMnO3/SrTiO3 superlattice. Structural relaxation within the generalized gradient approximation results in no significant tiltings or rotations of oxygen octahedra, but in distinct distortions in the SrTiO3 region. Taking into account the onsite Coulomb interaction, we find that the Mn spins order ferromagnetically, in contrast to the antiferromagnetic state of bulk LaMnO3. Most importantly, the interface strain combined with charge transfer across the interface induces half-metallicity within the MnO2 layers. The superlattice is particulary interesting for spintronics applications because the half-metallic states are characterized by an extraordinary high mobility.

  20. Absence of confinement in (SrTiO3)/( SrTi0.8Nb0.2O3 ) superlattices

    Science.gov (United States)

    Bouzerar, G.; Thébaud, S.; Bouzerar, R.; Pailhès, S.; Adessi, Ch.

    2018-03-01

    The reduction of dimensionality is considered an efficient pathway to boost the performances of thermoelectric materials. Quantum confinement of the carriers is expected to induce large Seebeck coefficients (S ) and it also suppresses the thermal conductivity by increasing the phonon scattering processes. However, quantum confinement in superlattices is not always easy to achieve and needs to be carefully validated. In the past decade, large values of S have been measured in (SrTiO3)/(SrTi0.8Nb0.2O3 ) superlattices [H. Ohta et al., Nat. Mater. 6, 129 (2007), 10.1038/nmat1821; Y. Mune et al., Appl. Phys. Lett. 91, 192105 (2007), 10.1063/1.2809364]. In the δ -doped compound, the reported S was almost six times larger than that of the bulk material. This huge increase has been attributed to the two-dimensional carrier confinement in the doped regions. Here, we demonstrate that the experimental data are well explained quantitatively assuming delocalized electrons in both in-plane and growth directions. Moreover, we rule out the confined electron hypothesis whose signature would be the suppression of the Seebeck coefficient. This strongly suggests that the presupposed confinement picture in these superlattices is unlikely.

  1. Improved structural and electrical properties in native Sb2Te3/GexSb2Te3+x van der Waals superlattices due to intermixing mitigation

    NARCIS (Netherlands)

    Cecchi, Stefano; Zallo, Eugenio; Momand, Jamo; Wang, Ruining; Kooi, Bart J.; Verheijen, Marcel A.; Calarco, Raffaella

    Superlattices made of Sb2Te3/GeTe phase change materials have demonstrated outstanding performance with respect to GeSbTe alloys in memory applications. Recently, epitaxial Sb2Te3/GeTe superlattices were found to feature GexSb2Te3+x blocks as a result of intermixing between constituting layers.

  2. Improved structural and electrical properties in native Sb2Te3/GexSb2Te3+x van der Waals superlattices due to intermixing mitigation

    NARCIS (Netherlands)

    Cecchi, S.; Zallo, E.; Momand, J.; Wang, R.; Kooi, B.J.; Verheijen, M.A.; Calarco, R.

    Superlattices made of Sb2Te3/GeTe phase change materials have demonstrated outstanding performance with respect to GeSbTe alloys in memory applications. Recently, epitaxial Sb2Te3/GeTe superlattices were found to feature GexSb2Te3+x blocks as a result of intermixing between constituting layers. Here

  3. Enhancing paper strength by optimizing defect configuration

    Science.gov (United States)

    J.M. Considine; W. Skye; W. Chen; D. Matthys; David W. Vahey; K. Turner; R. Rowlands

    2009-01-01

    Poor formation in paper, as denoted by large local variation of mass, tends to reduce maximum tensile strength but has not been well characterized. The effect of grammage variation on tensile strength was studied by introducing carefully placed holes in tensile specimens made of three different paper materials. Previous researchers demonstrated that the point-stress...

  4. The hyperfine properties of a hydrogenated Fe/V superlattice

    Energy Technology Data Exchange (ETDEWEB)

    Elzain, M., E-mail: elzain@squ.edu.om; Al-Barwani, M.; Gismelseed, A.; Al-Rawas, A.; Yousif, A.; Widatallah, H.; Bouziane, K.; Al-Omari, I. [Sultan Qaboos University, Department of Physics, College of Science (Oman)

    2012-03-15

    We study the effect of hydrogen on the electronic, magnetic and hyperfine structures of an iron-vanadium superlattice consisting of three Fe monolayers and nine V monolayers. The contact charge density ({rho}), the contact hyperfine field (B{sub hf}) and the electronic field gradient (EFG) at the Fe sites for different H locations and H fillings are calculated using the first principle full-potential linear-augmented-plane-wave (FP-LAPW) method. It is found that sizeable changes in the hyperfine properties are obtained only when H is in the interface region.

  5. Moiré superlattice-level stick-slip instability originated from geometrically corrugated graphene on a strongly interacting substrate

    Science.gov (United States)

    Shi, Ruoyu; Gao, Lei; Lu, Hongliang; Li, Qunyang; Ma, Tian-Bao; Guo, Hui; Du, Shixuan; Feng, Xi-Qiao; Zhang, Shuai; Liu, Yanmin; Cheng, Peng; Hu, Yuan-Zhong; Gao, Hong-Jun; Luo, Jianbin

    2017-06-01

    Two dimensional (2D) materials often exhibit novel properties due to various coupling effects with their supporting substrates. Here, using friction force microscopy (FFM), we report an unusual moiré superlattice-level stick-slip instability on monolayer graphene epitaxially grown on Ru(0 0 0 1) substrate. Instead of smooth friction modulation, a significant long-range stick-slip sawtooth modulation emerges with a period coinciding with the moiré superlattice structure, which is robust against high external loads and leads to an additional channel of energy dissipation. In contrast, the long-range stick-slip instability reduces to smooth friction modulation on graphene/Ir(1 1 1) substrate. The moiré superlattice-level slip instability could be attributed to the large sliding energy barrier, which arises from the morphological corrugation of graphene on Ru(0 0 0 1) surface as indicated by density functional theory (DFT) calculations. The locally steep humps acting as obstacles opposing the tip sliding, originates from the strong interfacial electronic interaction between graphene and Ru(0 0 0 1). This study opens an avenue for modulating friction by tuning the interfacial atomic interaction between 2D materials and their substrates.

  6. Thermal transport through Ge-rich Ge/Si superlattices grown on Ge(0 0 1)

    Science.gov (United States)

    Thumfart, L.; Carrete, J.; Vermeersch, B.; Ye, N.; Truglas, T.; Feser, J.; Groiss, H.; Mingo, N.; Rastelli, A.

    2018-01-01

    The cross-plane thermal conductivities of Ge-rich Si/Ge superlattices have been measured using both time-domain thermoreflectance and the differential 3ω method. The superlattices were grown by molecular beam epitaxy on Ge(0 0 1) substrates. Crystal quality and structural information were investigated by x-ray diffractometry and transmission electron microscopy. The influence of segregation during growth on the composition profiles was modeled using the experimental growth temperatures and deposition rates. Those profiles were then employed to obtain parameter-free theoretical estimates of the thermal conductivity by combining first-principles calculations, Boltzmann transport theory and phonon Green’s functions. Good agreement between theory and experiment is observed. The thermal conductivity shows a strong dependence on the composition and the thickness of the samples. Moreover, the importance of the composition profile is reflected in the fact that the thermal conductivity of the superlattices is considerably lower than predicted values for alloys with the same average composition and thickness. Measurement on different samples with the same Si layer thickness and number of periods, but different Ge layer thickness, show that the thermal resistance is only weakly dependent on the Ge layers. We analyze this phenomenon based on the first-principles mode, and build an approximate parametrization showing that, in this regime, the resistivity of a SL is roughly linear on the amount of Si.

  7. Investigation of the structural and electronic properties of quantum well superlattices made of CuCl, CuBr and CuI

    International Nuclear Information System (INIS)

    Badi, F.; Louhibi, S.; Aced, M.R.; Mehnane, N.; Sekkal, N.

    2008-05-01

    This paper reports first principle investigations of structural and electronic properties of both bulk and superlattice (SL) systems of cuprous halides. The obtained structural data for bulks compares well with literature while data for SL is not compared because unavailable (to our knowledge). The electronic structure of bulks and also of (001) and (110) growth axis superlattices (SLs) are calculated and it is shown that at the reverse of the IV-IV and III-V systems, the (110) growth axis impacts the electronic structure which indicates a great influence of the d-orbital. (author)

  8. Tuning the electronic structure of SrTiO3/SrFeO3−x superlattices via composition and vacancy control

    Directory of Open Access Journals (Sweden)

    Robert F. Berger

    2014-04-01

    Full Text Available Using density functional theory-based calculations, we explore the effects of oxygen vacancies and epitaxial layering on the atomic, magnetic, and electronic structure of (SrTiO3n(SrFeO3−x1 superlattices. While structures without oxygen vacancies (x = 0 possess small or non-existent band gaps and ferromagnetic ordering in their iron layers, those with large vacancy concentrations (x = 0.5 have much larger gaps and antiferromagnetic ordering. Though the computed gaps depend numerically on the delicate energetic balance of vacancy ordering and on the value of Hubbard U eff used in the calculations, we demonstrate that changes in layering can tune the band gaps of these superlattices below that of SrTiO3 (3.2 eV by raising their valence band maxima. This suggests the possibility that these superlattices could absorb in the solar spectrum, and could serve as water-splitting photocatalysts.

  9. Coherent light from E-field induced quantum coupling of exciton states in superlattice-like quantum wells

    DEFF Research Database (Denmark)

    Lyssenko, V. G.; Østergaard, John Erland; Hvam, Jørn Märcher

    1999-01-01

    Summary form only given. We focus on the ability to control the electronic coupling in coupled quantum wells with external E-fields leading to a strong modification of the coherent light emission, in particular at a bias where a superlattice-like miniband is formed. More specifically, we investig......Summary form only given. We focus on the ability to control the electronic coupling in coupled quantum wells with external E-fields leading to a strong modification of the coherent light emission, in particular at a bias where a superlattice-like miniband is formed. More specifically, we...... investigate a MBE-grown GaAs sample with a sequence of 15 single quantum wells having a successive increase of 1 monolayer in width ranging from 62 A to 102 A and with AlGaAs barriers of 17 Å....

  10. Effect of disorders on topological phases in one-dimensional optical superlattices

    International Nuclear Information System (INIS)

    Wang Zhizhou; Wu Yidong; Du Huijing; Jing Xili

    2016-01-01

    In a recent paper, Lang et al. proposed that edge states and topological phases can be observed in one-dimensional optical superlattices. They showed that the topological phases can be revealed by observing the density profile of a trapped fermion system, which displays plateaus with their positions. However, disorders are not considered in their model. To study the effect of disorders on the topological phases, we introduce random potentials to the model for optical superlattcies. Our calculations show that edge states are robust against the disorders. We find the edge states are very sensitive to the number of the sites in the optical superlattice and we propose a simple rule to describe the relationship between the edge states and the number of sites. The density plateaus are also robust against weak disorders provided that the average density is calculated over a long interval. The widths of the plateaus are proportional to the widths of the bulk energy gaps when there are disorders. The disorders can diminish the bulk energy gaps. So the widths of the plateaus decrease with the increase of disorders and the density plateaus disappear when disorders are too strong. The results in our paper can be used to guide the experimental detection of topological phases in one-dimensional systems. (paper)

  11. Spectroscopic ellipsometry characterization of interface reactivity in GaAs-based superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Losurdo, M.; Giuva, D.; Giangregorio, M.M.; Bruno, G.; Brown, A.S

    2004-05-01

    Pseudodielectric function spectra of GaAs/GaSb{sub 1-y}As{sub y}, GaSb/GaAs{sub y}Sb{sub 1-y} and GaAs/GaP{sub y}As{sub 1-y} superlattices have been measured by spectroscopic ellipsometry in the 0.75-5.5 eV photon energy range. The analysis of the E{sub 1} interband critical point and modeling of spectra has been carried out to investigate the chemistry of the anion exchange reaction and abruptness of interface composition in the superlattices. It has been found that a ternary compound GaP{sub y}As{sub 1-y} forms in the case of the P-for-As anion exchange reaction. In the case of As-for-Sb anion exchange reaction for (GaSb/GaAs{sub y}Sb{sub 1-y}){sub 20} SLs, SE data show that this anion exchange results in the formation not only of a ternary alloy GaAs{sub y}Sb{sub 1-y}, but also in the formation of isoelectronic compounds AsSb{sub x} that segregate at the GaSb/GaAs interface. In the case of Sb-for-As anion exchange for (GaAs/GaSbyAs{sub 1-y}){sub 20} SLs, Sb segregates at the GaAs surface.

  12. XYO{sub 3} (X = K, Na; Y = Nb, Ta) based superlattices for photocatalysis

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Guang-Zhao; Chen, Xiao-Rui; Yuan, Hong-Kuan; Kuang, An-Long [School of Physical Science and Technology, Southwest University, Chongqing 400715 (China); Chen, Hong [School of Physical Science and Technology, Southwest University, Chongqing 400715 (China); Key Laboratory of Luminescent and Real-Time Analytical Chemistry, Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715 (China)

    2017-05-15

    The photocatalytic activities of XYO{sub 3} (X = K, Na; Y = Ta, Nb) and XYO{sub 3}/X1Y1O{sub 3} (X, X1 = K, Na; Y, Y1 = Ta, Nb) systems are investigated by using hybrid density functional. All the XYO{sub 3} and XYO{sub 3}/X1Y1O{sub 3} systems are indirect band gap semiconductors, and the band gap of KNbO{sub 3}/KTaO{sub 3} is smaller than those of KNbO{sub 3} and KTaO{sub 3}, while the band gaps of KNbO{sub 3}/NaNbO{sub 3}, KNbO{sub 3}/NaTaO{sub 3}, KTaO{sub 3}/NaNbO{sub 3}, KTaO{sub 3}/NaTaO{sub 3}, and NaNbO{sub 3}/NaTaO{sub 3} are respectively between the band gaps of these two crystals which make up these superlattices. The electronic structure of KNbO{sub 3}/NaTaO{sub 3} is the same as that of KTaO{sub 3}/NaNbO{sub 3} since both have the same component and similar crystal structure. The band edges of all the considered superlattices are thermodynamically allowed for the water reduction and oxidation processes, and therefore, they could be used for photocatalytic water splitting. Band structures for (i) KNbO{sub 3}/KTaO{sub 3}, (ii) KNbO{sub 3}/NaNbO{sub 3}, (iii) KNbO{sub 3}/NaTaO{sub 3}, (iv) KTaO{sub 3}/NaNbO{sub 3}, (v) KTaO{sub 3}/NaTaO{sub 3}, and (vi) NaNbO{sub 3}/NaTaO{sub 3} superlattices. The horizontal dashed lines represent the Fermi levels. (copyright 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  13. Superconducting superlattices. Les super reseaux de supraconducteurs

    Energy Technology Data Exchange (ETDEWEB)

    Triscone, J M; Fischer, O [Geneva Univ. (Switzerland)

    1993-03-01

    By piling up ultra-thin layers of discrete materials, physicists now have a choice method for the study of superconductivity at high temperature. These superlattices are prepared by successive layers of YBaCuO and PrBaCuO deposited by cathode sputtering to study the variation of superconductivity with layer thickness. The transition temperature decreases rapidly when the distance between two layers increases. Current vortices are created, without a magnetic field, widening the transition temperature. The variation of resistivity near critical temperature in a magnetic field shows that the energy required to displace vortices is increasing with the thickness of the YBaCuO layer, with thin layers anisotropy is high and energy dissipation is important. (G.R.). refs., figs.

  14. Response functions of a superlattice with a basis: A model for oxide superconductors

    International Nuclear Information System (INIS)

    Griffin, A.

    1988-01-01

    The new high-T/sub c/ oxide superconductors appear to be superlattice structures with a basis composed of metallic sheets as well as metallic chains. Using a simple free-electron-gas model for the sheets and chains, we obtain the dielectric function ε(q,ω) of such a multilayer system within the random-phase approximation (RPA). We give results valid for arbitrary wave vector q appropriate to sheets and chains (as in the orthorhombic phase of Y-Ba-Cu-O) as well as for two different kinds of sheets (such as may be present in the Bi-Ca-Sr-Cu-O superconductors). The occurrence of acoustic plasmons is a general phenomenon in such superlattices, as shown by an alternative formulation based on the exact response functions for the individual sheets and chains, in which only the interchain (sheet) Coulomb interaction is treated in the RPA. These results generalize the long-wavelength expressions recently given in the literature. We also briefly discuss the analogous results for two arrays of mutually perpendicular chains, such as found in Hg chain compounds

  15. Nonlinear elasticity in wurtzite GaN/AlN planar superlattices and quantum dots

    International Nuclear Information System (INIS)

    Lepkowski, S.P.; Majewski, J.A.; Jurczak, G.

    2005-01-01

    The elastic stiffness tensor for wurtzite GaN and AlN show a significant hydrostatic pressure dependence, which id the evidence of nonlinear elasticity of these compounds. We have examined how the pressure dependence of elastic constants for wurtzite nitrides influences elastic and piezoelectric properties of GaN/AlN planar superlattices and quantum dots. Particularly we show that built-in hydrostatic pressure, present in both quantum wells of the GaN/AlN superlattices and GaN/AlN quantum dots, increases significantly by 0.3-0.7 GPa when nonlinear elasticity is used. Consequently, the compressive volumetric strain in quantum wells and quantum dots decreases in comparison to the case of the linear elastic theory, However, the-component of the built-in electric field in the quantum wells and quantum dots increases considerably when nonlinear elasticity is taken into account. Both effects, i.e., a decrease in the compressive volumetric strain as well as an increase in the built-in electric field, decrease the band-to-band transition energies in the quantum wells and quantum dots. (author)

  16. Phase coexistence and electric-field control of toroidal order in oxide superlattices.

    Science.gov (United States)

    Damodaran, A R; Clarkson, J D; Hong, Z; Liu, H; Yadav, A K; Nelson, C T; Hsu, S-L; McCarter, M R; Park, K-D; Kravtsov, V; Farhan, A; Dong, Y; Cai, Z; Zhou, H; Aguado-Puente, P; García-Fernández, P; Íñiguez, J; Junquera, J; Scholl, A; Raschke, M B; Chen, L-Q; Fong, D D; Ramesh, R; Martin, L W

    2017-10-01

    Systems that exhibit phase competition, order parameter coexistence, and emergent order parameter topologies constitute a major part of modern condensed-matter physics. Here, by applying a range of characterization techniques, and simulations, we observe that in PbTiO 3 /SrTiO 3 superlattices all of these effects can be found. By exploring superlattice period-, temperature- and field-dependent evolution of these structures, we observe several new features. First, it is possible to engineer phase coexistence mediated by a first-order phase transition between an emergent, low-temperature vortex phase with electric toroidal order and a high-temperature ferroelectric a 1 /a 2 phase. At room temperature, the coexisting vortex and ferroelectric phases form a mesoscale, fibre-textured hierarchical superstructure. The vortex phase possesses an axial polarization, set by the net polarization of the surrounding ferroelectric domains, such that it possesses a multi-order-parameter state and belongs to a class of gyrotropic electrotoroidal compounds. Finally, application of electric fields to this mixed-phase system permits interconversion between the vortex and the ferroelectric phases concomitant with order-of-magnitude changes in piezoelectric and nonlinear optical responses. Our findings suggest new cross-coupled functionalities.

  17. Ferroic properties in bi-component perovskites: artificial superlattices and naturally forming compounds

    International Nuclear Information System (INIS)

    Saha-Dasgupta, T

    2014-01-01

    The use of four different metal cations in a bi-component perovskite ABO 3 structure with 50 : 50 substitution at A sublattice as well as B sublattice, opens up the door for materials designing, with the aim to improve ferroic properties. This can be achieved following two different routes; one using the concept of artificially grown superlattices with alternating layers of ABO 3 and A′B′O 3 perovskites in a periodic set-up and another, through synthesis of naturally grown bulk double perovskites with ordered arrangement of A and A′ cations, simultaneously with that of B and B′ cations. The tremendous progress in layered deposition techniques as well as advances in solid state chemistry methods, has made both routes equally plausible and an area of much activity. This review summarizes some of the recent progress in this field, with a special emphasis on two computational studies, (i) one on ultra-thin 1–1 superlattices built out of paraelectric and ferroelectric components, showing tunable piezoelectric properties, and (ii) another on CrOs-based double perovskites which show multiferroic behavior, achieved through layered ordering of A and A′ cations. (topical review)

  18. Phase coexistence and electric-field control of toroidal order in oxide superlattices

    International Nuclear Information System (INIS)

    Damodaran, A. R.; Clarkson, J. D.; Hong, Z.

    2017-01-01

    Systems that exhibit phase competition, order parameter coexistence, and emergent order parameter topologies constitute a major part of modern condensed-matter physics. Here, by applying a range of characterization techniques, and simulations, we observe that in PbTiO 3 /SrTiO 3 superlattices all of these effects can be found. By exploring superlattice period-, temperature- and field-dependent evolution of these structures, we observe several new features. First, it is possible to engineer phase coexistence mediated by a first-order phase transition between an emergent, low-temperature vortex phase with electric toroidal order and a high-temperature ferroelectric a 1 /a 2 phase. At room temperature, the coexisting vortex and ferroelectric phases form a mesoscale, fibre-textured hierarchical superstructure. The vortex phase possesses an axial polarization, set by the net polarization of the surrounding ferroelectric domains, such that it possesses a multi-order-parameter state and belongs to a class of gyrotropic electrotoroidal compounds. Finally, application of electric fields to this mixed-phase system permits interconversion between the vortex and the ferroelectric phases concomitant with order-of-magnitude changes in piezoelectric and nonlinear optical responses. Here, our findings suggest new cross-coupled functionalities.

  19. Coherent dynamics of interwell excitons in GaAs/AlxGa1-xAs superlattices

    DEFF Research Database (Denmark)

    Mizeikis, V.; Birkedal, Dan; Langbein, Wolfgang Werner

    1997-01-01

    Coherent exciton dynamics in a GaAs/AlxGa1-xAs narrow-miniband superlattice is studied by spectrally resolved transient four-wave mixing. Coherent optical properties of the investigated structure are found to be strongly affected by the existence of two different heavy-hole excitonic states. One...

  20. Perspectives from ab-initio and tight-binding: Applications to transition metal compounds and superlattices

    Science.gov (United States)

    Venkataraman, Vijay Shankar

    The experimental and theoretical study of transition metal compounds have occupied condensed matter physicists for the best part of the last century. The rich variety of physical behaviour exhibited by these compounds owes its origin to the subtle balance of the energy scales at play for the d orbitals. In this thesis, we study three different systems comprised of transition metal atoms from the third, the fourth, and the fifth group of the periodic table using a combination of ab-initio density functional theory (DFT) computations and effective tight-binding models for the electronic properties. We first consider the electronic properties of artificially fabricated perovskite superlattices of the form [(SrIrO3)m / SrTiO3] with integer m denoting the number of layers of SrIrO3. After discussing the results of experiments undertaken by our collaborators, we present the results of our DFT calculations and build tight-binding models for the m = 1 and m = 2 superlattices. The active ingredient is found to be the 5d orbitals with significant spin-orbit coupling. We then study the energies of magnetic ground states within DFT and compare and contrast our results with those obtained for the bulk Ruddlesden-Popper iridates. Together with experimental measurements, our results suggest that these superlattices are an exciting venue to probe the magnetism and metal-insulator transitions that occur from the intricate balance of the spin-orbit coupling and electron interactions, as has been reported for their bulk counterparts. Next, we consider alpha-RuCl3, a honeycomb lattice compound. We first show using DFT calculations in conjunction with experiments performed by our collaborators, how spin-orbit coupling in the 4d orbitals of Ru is essential to understand the insulating state realized in this compound. Then, in the latter half of the chapter, we study the magnetic ground states of a two-dimensional analogue of alpha-RuCl3 in weak and strong-coupling regimes obtained from

  1. Caffeine ingestion acutely enhances muscular strength and power but not muscular endurance in resistance-trained men.

    Science.gov (United States)

    Grgic, Jozo; Mikulic, Pavle

    2017-09-01

    The goal of this randomized, double-blind, cross-over study was to assess the acute effects of caffeine ingestion on muscular strength and power, muscular endurance, rate of perceived exertion (RPE), and pain perception (PP) in resistance-trained men. Seventeen volunteers (mean ± SD: age = 26 ± 6 years, stature = 182 ± 9 cm, body mass = 84 ± 9 kg, resistance training experience = 7 ± 3 years) consumed placebo or 6 mg kg -1 of anhydrous caffeine 1 h before testing. Muscular power was assessed with seated medicine ball throw and vertical jump exercises, muscular strength with one-repetition maximum (1RM) barbell back squat and bench press exercises, and muscular endurance with repetitions of back squat and bench press exercises (load corresponding to 60% of 1RM) to momentary muscular failure. RPE and PP were assessed immediately after the completion of the back squat and bench press exercises. Compared to placebo, caffeine intake enhanced 1RM back squat performance (+2.8%; effect size [ES] = 0.19; p = .016), which was accompanied by a reduced RPE (+7%; ES = 0.53; p = .037), and seated medicine ball throw performance (+4.3%, ES = 0.32; p = .009). Improvements in 1RM bench press were not noted although there were significant (p = .029) decreases in PP related to this exercise when participants ingested caffeine. The results point to an acute benefit of caffeine intake in enhancing lower-body strength, likely due to a decrease in RPE; upper-, but not lower-body power; and no effects on muscular endurance, in resistance-trained men. Individuals competing in events in which strength and power are important performance-related factors may consider taking 6 mg kg -1 of caffeine pre-training/competition for performance enhancement.

  2. Light-hole conduction in InGaAs/GaAs strained-layer superlattices

    International Nuclear Information System (INIS)

    Schirber, J.E.; Fritz, I.J.; Dawson, L.R.

    1985-01-01

    We report the first observation of light-hole band carriers in In/sub 0.2/Ga/sub 0.8/As/GaAs strained-layer superlattices by direct measurements of their effective mass (m*m/sub o/ = 0.14) using oscillatory magnetoresistance data. Preferential population of light-hole states, due to splitting of the degenerate bulk valence bands by built-in strain, allows this direct observation

  3. Advances in the characterization of InAs/GaSb superlattice infrared photodetectors

    Science.gov (United States)

    Wörl, A.; Daumer, V.; Hugger, T.; Kohn, N.; Luppold, W.; Müller, R.; Niemasz, J.; Rehm, R.; Rutz, F.; Schmidt, J.; Schmitz, J.; Stadelmann, T.; Wauro, M.

    2016-10-01

    This paper reports on advances in the electro-optical characterization of InAs/GaSb short-period superlattice infrared photodetectors with cut-off wavelengths in the mid-wavelength and long-wavelength infrared ranges. To facilitate in-line monitoring of the electro-optical device performance at different processing stages we have integrated a semi-automated cryogenic wafer prober in our process line. The prober is configured for measuring current-voltage characteristics of individual photodiodes at 77 K. We employ it to compile a spatial map of the dark current density of a superlattice sample with a cut-off wavelength around 5 μm patterned into a regular array of 1760 quadratic mesa diodes with a pitch of 370 μm and side lengths varying from 60 to 350 μm. The different perimeter-to-area ratios make it possible to separate bulk current from sidewall current contributions. We find a sidewall contribution to the dark current of 1.2×10-11 A/cm and a corrected bulk dark current density of 1.1×10-7 A/cm2, both at 200 mV reverse bias voltage. An automated data analysis framework can extract bulk and sidewall current contributions for various subsets of the test device grid. With a suitable periodic arrangement of test diode sizes, the spatial distribution of the individual contributions can thus be investigated. We found a relatively homogeneous distribution of both bulk dark current density and sidewall current contribution across the sample. With the help of an improved capacitance-voltage measurement setup developed to complement this technique a residual carrier concentration of 1.3×1015 cm-3 is obtained. The work is motivated by research into high performance superlattice array sensors with demanding processing requirements. A novel long-wavelength infrared imager based on a heterojunction concept is presented as an example for this work. It achieves a noise equivalent temperature difference below 30 mK for realistic operating conditions.

  4. Hydrogenation of Very Long Wavelength Infrared Focal Plane Arrays Based on Type II Superlattices, Phase I

    Data.gov (United States)

    National Aeronautics and Space Administration — We propose to advance the Ga-free InAs/InAsSb type II superlattice (T2SL) materials technology for very long wavelength infrared (VLWIR) focal plane arrays (FPAs) by...

  5. Magnetic quantum oscillations of diagonal conductivity in a two-dimensional conductor with a weak square superlattice modulation under conditions of the integer quantum Hall effect

    International Nuclear Information System (INIS)

    Gvozdikov, V M; Taut, M

    2009-01-01

    We report on analytical and numerical studies of the magnetic quantum oscillations of the diagonal conductivity σ xx in a two-dimensional conductor with a weak square superlattice modulation under conditions of the integer quantum Hall (IQHE) effect. The quantum Hall effect in such a system differs from the conventional IQHE, in which the finite width of the Landau bands is due to disorder only. The superlattice modulation potential yields a fractal splitting of the Landau levels into Hofstadter minibands. For rational flux through a unit cell, the minibands have a finite width and intrinsic dispersion relations. We consider a regime, now accessible experimentally, in which disorder does not wash out the fractal internal gap structure of the Landau bands completely. We found the following distinctions from the conventional IQHE produced by the superlattice: (i) the peaks in diagonal conductivity are split due to the Hofstadter miniband structure of Landau bands; (ii) the number of split peaks in the bunch, their positions and heights depend irregularly on the magnetic field and the Fermi energy; (iii) the gaps between the split Landau bands (and related quantum Hall plateaus) become narrower with the superlattice modulation than without it.

  6. Dopant-free twinning superlattice formation in InSb and InP nanowires

    Energy Technology Data Exchange (ETDEWEB)

    Yuan, Xiaoming [School of Physics and Electronics, Hunan Key Laboratory for Supermicrostructure and Ultrafast Process, Central South University, Changsha, Hunan (China); Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT (Australia); Guo, Yanan; Caroff, Philippe; Tan, Hark Hoe; Jagadish, Chennupati [Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT (Australia); He, Jun [School of Physics and Electronics, Hunan Key Laboratory for Supermicrostructure and Ultrafast Process, Central South University, Changsha, Hunan (China)

    2017-11-15

    Periodic arrangement of twin planes creates a controllable polytype that can affect both the electronic and optical properties of nanowires. The approach that is most used for inducing twinning superlattice (TSL) formation in III-V nanowires is introducing impurity dopants during growth. Here, we demonstrate that controlling the growth parameters is sufficient to produce regular twinning planes in Au-catalysed InSb and InP nanowires. Our results show that TSL formation in InSb nanowires only exists in a very narrow growth window. We suggest that growth conditions induce a high concentration of In (or Sb) in the Au droplet, which plays a similar role to that of surfactant impurities such as Zn, and increases the droplet wetting angle to yield a geometry that is favorable for TSL formation. The demonstration of TSL structure in InSb and InP nanowires by controlling the input of In (or Sb) further enhances fundamental understanding of TSL formation in III-V nanowires and allows us to tune the properties of these nanowires by crystal phase engineering. (copyright 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  7. Dopant-free twinning superlattice formation in InSb and InP nanowires

    International Nuclear Information System (INIS)

    Yuan, Xiaoming; Guo, Yanan; Caroff, Philippe; Tan, Hark Hoe; Jagadish, Chennupati; He, Jun

    2017-01-01

    Periodic arrangement of twin planes creates a controllable polytype that can affect both the electronic and optical properties of nanowires. The approach that is most used for inducing twinning superlattice (TSL) formation in III-V nanowires is introducing impurity dopants during growth. Here, we demonstrate that controlling the growth parameters is sufficient to produce regular twinning planes in Au-catalysed InSb and InP nanowires. Our results show that TSL formation in InSb nanowires only exists in a very narrow growth window. We suggest that growth conditions induce a high concentration of In (or Sb) in the Au droplet, which plays a similar role to that of surfactant impurities such as Zn, and increases the droplet wetting angle to yield a geometry that is favorable for TSL formation. The demonstration of TSL structure in InSb and InP nanowires by controlling the input of In (or Sb) further enhances fundamental understanding of TSL formation in III-V nanowires and allows us to tune the properties of these nanowires by crystal phase engineering. (copyright 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  8. Valley-chiral quantum Hall state in graphene superlattice structure

    Science.gov (United States)

    Tian, H. Y.; Tao, W. W.; Wang, J.; Cui, Y. H.; Xu, N.; Huang, B. B.; Luo, G. X.; Hao, Y. H.

    2016-05-01

    We theoretically investigate the quantum Hall effect in a graphene superlattice (GS) system, in which the two valleys of graphene are coupled together. In the presence of a perpendicular magnetic field, an ordinary quantum Hall effect is found with the sequence σxy=ν e^2/h(ν=0,+/-1,+/-2,\\cdots) . At the zeroth Hall platform, a valley-chiral Hall state stemming from the single K or K' valley is found and it is localized only on one sample boundary contributing to the longitudinal conductance but not to the Hall conductivity. Our findings may shed light on the graphene-based valleytronics applications.

  9. Minority Carrier Lifetime Studies of Narrow Bandgap Antimonide Superlattices

    Science.gov (United States)

    Hoglund, Linda; Ting, David Z.; Khoshakhlagh, Arezou; Soibel, Alexander; Hill, Cory J.; Fisher, Anita; Keo, Sam; Gunapala, Sarath D.

    2014-01-01

    In this study optical modulation response and photoluminescence spectroscopy were used to study mid-wave Ga-free InAs/InAsSb superlattices. The minority carrier lifetimes in the different samples varied from 480 ns to 4700 ns, partly due to different background doping concentrations. It was shown that the photoluminescence intensity can be used as a fast non-destructive tool to predict the material quality. It was also demonstrated that it is crucial to use a low excitation power in the photoluminescence measurements in order to get a good correlation between the photoluminescence intensity and the minority carrier lifetime.

  10. Atomic stacking and van-der-Waals bonding in GeTe-Sb2Te3 superlattices

    NARCIS (Netherlands)

    Momand, J.; Lange, F.R.L.; Wang, R.; Boschker, J.E.; Verheijen, M.A.; Calarco, R.; Wuttig, M.; Kooi, B.J.

    2016-01-01

    GeTe–Sb2Te3 superlattices have attracted major interest in the field of phase-change memories due to their improved properties compared with their mixed counterparts. However, their crystal structure and resistance-switching mechanism are currently not clearly understood. In this work epitaxial

  11. Ferroelectricity driven magnetism at domain walls in LaAlO3/PbTiO3 superlattices

    Science.gov (United States)

    Zhou, P. X.; Dong, S.; Liu, H. M.; Ma, C. Y.; Yan, Z. B.; Zhong, C. G.; Liu, J. -M.

    2015-01-01

    Charge dipole moment and spin moment rarely coexist in single-phase bulk materials except in some multiferroics. Despite the progress in the past decade, for most multiferroics their magnetoelectric performance remains poor due to the intrinsic exclusion between charge dipole and spin moment. As an alternative approach, the oxide heterostructures may evade the intrinsic limits in bulk materials and provide more attractive potential to realize the magnetoelectric functions. Here we perform a first-principles study on LaAlO3/PbTiO3 superlattices. Although neither of the components is magnetic, magnetic moments emerge at the ferroelectric domain walls of PbTiO3 in these superlattices. Such a twist between ferroelectric domain and local magnetic moment, not only manifests an interesting type of multiferroicity, but also is possible useful to pursuit the electrical-control of magnetism in nanoscale heterostructures. PMID:26269322

  12. First-principles modeling of titanate/ruthenate superlattices

    Science.gov (United States)

    Junquera, Javier

    2013-03-01

    The possibility to create highly confined two-dimensional electron gases (2DEG) at oxide interfaces has generated much excitement during the last few years. The most widely studied system is the 2DEG formed at the LaO/TiO2 polar interface between LaAlO3 and SrTiO3, where the polar catastrophe at the interface has been invoked as the driving force. More recently, partial or complete delta doping of the Sr or Ti cations at a single layer of a SrTiO3 matrix has also been used to generate 2DEG. Following this recipe, we report first principles characterization of the structural and electronic properties of (SrTiO3)5/(SrRuO3)1 superlattices, where all the Ti of a given layer have been replaced by Ru. We show that the system exhibits a spin-polarized two-dimensional electron gas extremely confined to the 4 d orbitals of Ru in the SrRuO3 layer, a fact that is independent of the level of correlation included in the simulations. For hybrid functionals or LDA+U, every interface in the superlattice behaves as minority-spin half-metal ferromagnet, with a magnetic moment of μ = 2.0 μB/SrRuO3 unit. The shape of the electronic density of states, half metallicity and magnetism are explained in terms of a simplified tight-binding model, considering only the t2 g orbitals plus (i) the bi-dimensionality of the system, and (ii) strong electron correlations. Possible applications are discussed, from their eventual role in thermoelectric applications to the possible tuning of ferromagnetic properties of the 2DEG with the polarization of the dielectric. Work done in collaboration with P. García, M. Verissimo-Alves, D. I. Bilc, and Ph. Ghosez. Financial support provided by MICINN Grant FIS2009-12721-C04-02, and by the European Union Grant No. CP-FP 228989-2 ``OxIDes.'' The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the BSC/RES.

  13. 2D Electron Gas with 100% Spin-Polarization in the (LaMnO3)2/(SrTiO3)2 Superlattice under Uniaxial Strain

    KAUST Repository

    Cossu, Fabrizio

    2014-07-28

    By first-principles calculations we investigate the structural, electronic, and magnetic properties of the (LaMnO3)2/(SrTiO3)2 superlattice. We find that a monoclinic C2h symmetry is energetically favorable and that the spins order ferromagnetically. Under both compressive and tensile uniaxial strain the electronic structure of the superlattice shows a half-metallic character. In particular, a fully spin-polarized two-dimensional electron gas, which traces back to the Ti 3dxy orbitals, is achieved under compressive uniaxial strain. The (LaMnO3)2/(SrTiO3)2 superlattice is analysed with respect to its structure, magnetism, and electronic properties. Our results demonstrate that uniaxial strain in an experimentally accessible range, both tensile and compressive, can be used to induce half-metallicity. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  14. O deficient LaAlO3/SrTiO3(110) and (001) superlattices under hydrostatic pressure: a comparative first principles study

    KAUST Repository

    Albar, Arwa; Schwingenschlö gl, Udo

    2017-01-01

    We compare the electronic properties of O deficient LaAlO3/SrTiO3 superlattices oriented along the (001) and (110) directions, taking into account the effect of hydrostatic compression and tension. Interfacial O vacancies turn out to be less likely in the case of the (110) orientation, with compression (tension) reducing (enhancing) the energy cost for both orientations. The presence of O vacancies results in the formation of a two-dimensional electron gas, for which we observe a distinct spatial pattern of carrier density that depends strongly on the amount of applied pressure. We clarify the interrelation between structural features and the properties of this electron gas (confinement, carrier density, and mobility).

  15. O deficient LaAlO3/SrTiO3(110) and (001) superlattices under hydrostatic pressure: a comparative first principles study

    KAUST Repository

    Albar, Arwa

    2017-03-17

    We compare the electronic properties of O deficient LaAlO3/SrTiO3 superlattices oriented along the (001) and (110) directions, taking into account the effect of hydrostatic compression and tension. Interfacial O vacancies turn out to be less likely in the case of the (110) orientation, with compression (tension) reducing (enhancing) the energy cost for both orientations. The presence of O vacancies results in the formation of a two-dimensional electron gas, for which we observe a distinct spatial pattern of carrier density that depends strongly on the amount of applied pressure. We clarify the interrelation between structural features and the properties of this electron gas (confinement, carrier density, and mobility).

  16. Enhancement of mechanical strength in hot-pressed TiB2 composites by the addition of Fe and Ni

    International Nuclear Information System (INIS)

    Yen, C.F.; Yust, C.S.; Clark, G.W.

    1978-01-01

    Improvement in the fracture strength of TiB 2 composites through the addition of Fe and Ni is reported. Beam specimens containing up to 20 wt % of the metal component were fabricated by hot pressing above the eutectic temperature. Four point bending results indicated that the observed enhancement in strength was proportional to the vol % of the metal present. The results also revealed that TiB 2 --Ni composites have slightly better strength than TiB 2 --Fe samples at elevated temperatures, and that their strengths were consistently about twice that of pure TiB 2 . They also did not exhibit any serious degradation in strength up to 700 0 C. The fracture behavior was characterized using optical and scanning electron microscopy. The strengthening appears to be related to the presence of a metallic grain boundary phase between TiB 2 particles. The results further suggested that the formation of a special eutectic-like grain microstructure could render additional strengthening

  17. Magnetoresistance oscillations in GaAs/AlGaAs superlattices subject to in-plane magnetic fields

    Czech Academy of Sciences Publication Activity Database

    Smrčka, Ludvík; Vašek, Petr; Svoboda, Pavel; Goncharuk, Natalya; Pacherová, Oliva; Krupko, Yuriy; Sheikin, Y.; Wegscheider, W.

    2006-01-01

    Roč. 34, - (2006), s. 632-635 ISSN 1386-9477 R&D Projects: GA AV ČR(CZ) IAA1010408 Institutional research plan: CEZ:AV0Z10100521 Keywords : superlattice * Fermi surface * magnetoresistance Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.084, year: 2006

  18. Hydrostatic pressure and strain effects in short period InN/GaN superlattices

    DEFF Research Database (Denmark)

    Gorczyca, I.; Suski, T.; Christensen, Niels Egede

    2012-01-01

    The electronic structures of short-period pseudomorphically grown superlattices (SLs) of the form mInN/nGaN are calculated and the band gap variation with the well and the barrier thicknesses is discussed including hydrostatic pressure effects. The calculated band gap shows a strong dependence...... strongly on the strain conditions and SL geometry, but weakly on the applied external hydrostatic pressure....

  19. Raman Scattering and Surface Photovoltage Spectroscopy Studies of InGaAs/GaAs Radial Superlattices

    Science.gov (United States)

    Angelova, T.; Cros, A.; Ivanov, Ts.; Donchev, V.; Cantarero, A.; Shtinkov, N.; Deneke, Ch.; Schmidt, O. G.

    2011-12-01

    In this work we get insight into the multilayer structure of rolled-up microtube radial superlattices (RSLs) by the study of the optical and folded acoustic phonon modes of individual microtubes. Raman results show shifts of the InGaAs and GaAs related longitudinal optical modes that can be related to the strain state of the tubes. The folding of the acoustic modes has been related with the periodicity of the artificial superlattice formed by the multiple turns of the heterostructures. Information on the electronic structure and optical transitions of RSLs has been obtained by surface photovoltage spectroscopy. Room temperature spectra reveal several electronic transitions with energies below 1.3 eV. These transitions have been identified as originating from defect levels at the interfaces, as well as from the RSLs and the In0.215Ga0.785As/GaAs quantum well in the unfolded regions of the sample.

  20. The phase diagrams and the order parameters of the diluted superlattice with antiferromagnetic interface coupling

    International Nuclear Information System (INIS)

    Oubelkacem, A.; El Aouad, N.; Bentaleb, M.; Laaboudi, B.; Saber, M.

    2004-01-01

    Using the effective field theory with a probability distribution technique that accounts for the self-spin correlation functions, the magnetic properties of the diluted Ising superlattice consisting of two ferromagnetic materials A and B, with L a layers of diluted spins S a =((1)/(2)) and L b layers of diluted spins S b =1 with antiferromagnetic interface coupling are examined. For fixed values of the reduced exchange interactions and the concentration c of magnetic atoms, the phase diagrams, the two sublattice magnetizations and the total magnetization for the superlattice with the same spin S a =S b =((1)/(2)) and for S a =((1)/(2)), S b =1 are studied as a function of the temperature. We find a number of characteristic phenomena. In particular, the effect of the concentration c of magnetic atoms, the interlayer coupling and the layer thickness on both the compensation temperature and the magnetization profiles are clarified

  1. Modelling and measurement of bandgap behaviour in medium-wavelength IR InAs/InAs0.815Sb0.185 strained-layer superlattices

    Science.gov (United States)

    Letka, Veronica; Keen, James; Craig, Adam; Marshall, Andrew R. J.

    2017-10-01

    InAs/InAs1-xSbx type-II strained-layer superlattices (SLS) are a structure with potential infrared detection applications, owing to its tunable bandgap and suppressed Auger recombination. A series of medium-wavelength infrared (MWIR) InAs/InAs0.815Sb0.185 SLS structures, grown as undoped absorption epilayers on GaAs, were fabricated using molecular beam epitaxy in order to study the dependence of the ground state transitions on temperature and superlattice period thickness. Photoluminescence peaks at 4 K were obtained with the use of a helium-cooled micro-PL system and an InSb detector, and temperature-dependent absorption spectra were measured in the range 77 K - 300 K on a Fourier Transform Infrared (FTIR) spectrometer, equipped with a 1370 K blackbody source and a DTGS detector. An nBn device sample with the absorber structure identical to one of the undoped samples was also grown and processed with the goal of measuring temperature-dependent spectral response. A model for superlattice band alignment was also devised, incorporating the Bir-Pikus transformation results for uniaxial and biaxial strain, and the Einstein oscillator model for bandgap temperature dependence. Absorption coefficients of several 1000 cm-1 throughout the entire MWIR range are found for all samples, and temperature dependence of the bandgaps is extracted and compared to the model. This and photoluminescence data also demonstrate bandgap shifts consistent with the different superlattice periods of the three samples.

  2. Charge superlattice effects on the electronic structure of a model acceptor graphite intercalation compound

    International Nuclear Information System (INIS)

    Campagnoli, G.; Tosatti, E.

    1981-08-01

    In the present attempt we have considered a model ordered situation (a super-superlattice) where starting from a basic stoichiometry C 8 X, a fraction 1/3 of the molecules acquire one electron, the remaining 2/3 being left neutral. We have performed an electronic structure calculation using tight-binding plus electrostatic (Hartree) self-consistency

  3. Influence of substrate quality on structural properties of AlGaN/GaN superlattices grown by molecular beam epitaxy

    Energy Technology Data Exchange (ETDEWEB)

    Schubert, F. [NaMLab gGmbH, Nöthnitzer Straße 64, 01187 Dresden (Germany); Merkel, U.; Schmult, S. [TU Dresden, Institute of Semiconductors and Microsystems, Nöthnitzer Straße 64, 01187 Dresden (Germany); Mikolajick, T. [NaMLab gGmbH, Nöthnitzer Straße 64, 01187 Dresden (Germany); TU Dresden, Institute of Semiconductors and Microsystems, Nöthnitzer Straße 64, 01187 Dresden (Germany)

    2014-02-28

    Short-period AlGaN/GaN superlattices were established as versatile test structures to investigate the structural properties of molecular beam epitaxy (MBE)-grown GaN and AlGaN layers and their dependence on the GaN substrate quality. X-ray diffractometry data of the investigated superlattices allow access to relevant structural parameters such as aluminum mole fraction and layer thicknesses. The occurrence of theoretically predicted intense high-order satellite peaks and pronounced interface fringes in the diffraction pattern reflects abrupt interfaces and perfect 2-dimensional growth resulting in smooth surfaces. The data unambiguously demonstrate that the structural quality of the MBE grown layers is limited by the structural properties of the GaN substrate.

  4. Electron Backscatter Diffraction Studies on the Formation of Superlattice Metal Hydride Alloys

    Directory of Open Access Journals (Sweden)

    Shuli Yan

    2017-12-01

    Full Text Available Microstructures of a series of La-Mg-Ni-based superlattice metal hydride alloys produced by a novel method of interaction of a LaNi5 alloy and Mg vapor were studied using a combination of X-ray energy dispersive spectroscopy and electron backscatter diffraction. The conversion rate of LaNi5 increased from 86.8% into 98.2%, and the A2B7 phase abundance increased from 42.5 to 45.8 wt % and reduced to 39.2 wt % with the increase in process time from four to 32 h. During the first stage of reaction, Mg formed discrete grains with the same orientation, which was closely related to the orientation of the host LaNi5 alloy. Mg then diffused through the ab-phase of LaNi5 and formed the AB2, AB3, and A2B7 phases. Diffusion of Mg stalled at the grain boundary of the host LaNi5 alloy. Good alignments in the c-axis between the newly formed superlattice phases and LaNi5 were observed. The density of high-angle grain boundary decreased with the increase in process time and was an indication of lattice cracking.

  5. Intrinsic to extrinsic phonon lifetime transition in a GaAs–AlAs superlattice

    International Nuclear Information System (INIS)

    Hofmann, F; Garg, J; Chen, G; Maznev, A A; Nelson, K A; Jandl, A; Bulsara, M; Fitzgerald, E A

    2013-01-01

    We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon–phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the ‘interfacial atomic disorder’ model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness. (paper)

  6. Intrinsic to extrinsic phonon lifetime transition in a GaAs-AlAs superlattice.

    Science.gov (United States)

    Hofmann, F; Garg, J; Maznev, A A; Jandl, A; Bulsara, M; Fitzgerald, E A; Chen, G; Nelson, K A

    2013-07-24

    We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon-phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the 'interfacial atomic disorder' model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness.

  7. MSM optical detector on the basis of II-type ZnSe/ZnTe superlattice

    Energy Technology Data Exchange (ETDEWEB)

    Kuznetzov, P. I., E-mail: pik218@ire216.msk.su; Averin, S. V., E-mail: sva278@ire216.msk.su; Zhitov, V. A.; Zakharov, L. Yu.; Kotov, V. M. [Russian Academy of Sciences, Kotel’nikov Institute of Radioengineering and Electronics (Fryazino Branch) (Russian Federation)

    2017-02-15

    On the basis of a type-II ZnSe/ZnTe superlattice, a MSM (metal—semiconductor–metal) photodetector is fabricated and investigated. The detector features low dark currents and a high sensitivity. The spectral characteristic of the detector provides the possibility of the selective detection of three separate spectral portions of visible and near-infrared radiation.

  8. Superlattice formation lifting degeneracy protected by nonsymmorphic symmetry through a metal-insulator transition in RuAs

    Science.gov (United States)

    Kotegawa, Hisashi; Takeda, Keiki; Kuwata, Yoshiki; Hayashi, Junichi; Tou, Hideki; Sugawara, Hitoshi; Sakurai, Takahiro; Ohta, Hitoshi; Harima, Hisatomo

    2018-05-01

    A single crystal of RuAs obtained with the Bi-flux method shows obvious successive metal-insulator transitions at TMI 1˜255 K and TMI 2˜195 K. The x-ray diffraction measurement reveals the formation of a superlattice of 3 ×3 ×3 of the original unit cell below TMI 2, accompanied by a change of the crystal system from the orthorhombic structure to the monoclinic one. Simple dimerization of the Ru ions is not seen in the ground state. The multiple As sites observed in the nuclear quadrupole resonance spectrum also demonstrate the formation of the superlattice in the ground state, which is clarified to be nonmagnetic. The divergence in 1 /T1 at TMI 1 shows that a symmetry lowering by the metal-insulator transition is accompanied by strong critical fluctuations of some degrees of freedom. Using the structural parameters in the insulating state, the first-principles calculation reproduces successfully the reasonable size of nuclear quadrupole frequencies νQ for the multiple As sites, ensuring the high validity of the structural parameters. The calculation also gives a remarkable suppression in the density of states near the Fermi level, although the gap opening is insufficient. A coupled modulation of the calculated Ru d -electron numbers and the crystal structure proposes the formation of a charge density wave in RuAs. Some lacking factors remain, but it is shown that a lifting of degeneracy protected by the nonsymmorphic symmetry through the superlattice formation is a key ingredient for the metal-insulator transition in RuAs.

  9. Enhancement of impact strength of poly (methyl methacrylate) with surface fine-tuned nano-silica

    International Nuclear Information System (INIS)

    Wen, Bin; Dong, Yixiao; Wu, Lili; Long, Chao; Zhang, Chaocan

    2015-01-01

    Highly dispersible nanoparticles in organic solvent always receive wide interests due to their compatibility with polymer materials. This paper reported a kind of isopropanol alcohol silica dispersion which obtained using a method of azeotropic distillation. The isopropanol alcohol dispersed silica (IPADS) were treated with coupling agents to fine-tune their surface properties. Polymethyl methacrylate (PMMA) was then used as a research object to test the compatibility between IPADS and polymer. UV-vis spectra indicate that IPADS would reach its high compatibility with PMMA if coupling with trimethoxypropylsilane (PTMS). Followed experiments on PMMA proved that the high compatibility can prominently enhance the impact strength about 30%. The results may provide reference both for nano-silica modification and better understanding of nano-enhanced materials. (paper)

  10. Enhancement of impact strength of poly (methyl methacrylate) with surface fine-tuned nano-silica

    Science.gov (United States)

    Wen, Bin; Dong, Yixiao; Wu, Lili; Long, Chao; Zhang, Chaocan

    2015-07-01

    Highly dispersible nanoparticles in organic solvent always receive wide interests due to their compatibility with polymer materials. This paper reported a kind of isopropanol alcohol silica dispersion which obtained using a method of azeotropic distillation. The isopropanol alcohol dispersed silica (IPADS) were treated with coupling agents to fine-tune their surface properties. Polymethyl methacrylate (PMMA) was then used as a research object to test the compatibility between IPADS and polymer. UV-vis spectra indicate that IPADS would reach its high compatibility with PMMA if coupling with trimethoxypropylsilane (PTMS). Followed experiments on PMMA proved that the high compatibility can prominently enhance the impact strength about 30%. The results may provide reference both for nano-silica modification and better understanding of nano-enhanced materials.

  11. FY98 Final Report Initial Interfacial Chemical Control for Enhancement of Composite Material Strength; TOPICAL

    International Nuclear Information System (INIS)

    GE Fryxell; KL Alford; KL Simmons; RD Voise; WD Samuels

    1999-01-01

    The U.S. Army Armament Research Development and Engineering Center (ARDEC) sponsored this research project to support the development of new self-assembled monolayer fiber coatings. These coatings can greatly increase the bond strength between the fiber and the resin matrix of a composite material. Composite ammunition components molded from such materials will exhibit higher strength than current materials, and will provide a major improvement in the performance of composites in military applications. Use of composite materials in military applications is desirable because of the lighter weight of the materials and their high strengths. The FY97 project investigated initial interfacial chemical control for enhancement of composite material strength. The core of the project was to modify the covalent interface of glass fibers (or other reinforcing fibers) to induce strong, uniform, defect-free adhesion between the fibers' surfaces and the polymer matrix. Installing a self-assembled monolayer tailored to the specific matrix resin accomplished this. Simply, the self-assembled monolayer modifies the fiber to make it appear to have the same chemical composition as the resin matrix. The self-assembled monolayer creates a receptive, hydrophobic interface that the thermoset resin (or polymer precursors) would wet more effectively, leading to a higher contact surface area and more efficient adhesion. The FY97 work phase demonstrated that it is possible to increase the adhesive strength, as well as increase the heat deflection temperature through the use of self-assembled monolayer

  12. Manipulation of resonant tunneling by substrate-induced inhomogeneous energy band gaps in graphene with square superlattice potentials

    International Nuclear Information System (INIS)

    Li, Guanqiang; Chen, Guangde; Peng, Ping; Cao, Zhenzhou; Ye, Honggang

    2013-01-01

    We investigate the resonant transmission of Dirac electrons through inhomogeneous band gap graphene with square superlattice potentials by transfer matrix method. The effects of the incident angle of the electrons, Fermi energy and substrate-induced Dirac gaps on the transmission are considered. It is found that the Dirac gap of graphene adds another degree of freedom with respect to the incident angle, the Fermi energy and the parameters of periodic superlattice potentials (i.e., the number, width and height of the barriers) for the transmission. In particular, the inhomogeneous Dirac gap induced by staggered substrates can be used to manipulate the transmission. The properties of the conductance and Fano factor at the resonant peaks are found to be affected by the gaps significantly. The results may be helpful for the practical application of graphene-based electronic devices

  13. Laser induced structural transformation in chalcogenide based superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Zallo, Eugenio, E-mail: zallo@pdi-berlin.de; Wang, Ruining; Bragaglia, Valeria; Calarco, Raffaella [Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin (Germany)

    2016-05-30

    Superlattices made of alternating layers of nominal GeTe and Sb{sub 2}Te{sub 3} have been studied by micro-Raman spectroscopy. A structural irreversible transformation into ordered GeSbTe alloy is induced by high power laser light exposure. The intensity ratio of anti-Stokes and Stokes scattering under laser illumination gives a maximum average temperature in the sample of 177 °C. The latter is lower than the growth temperature and of 400 °C necessary by annealing to transform the structure in a GeSbTe alloy. The absence of this configuration after in situ annealing even up to 300 °C evidences an electronic excitation induced-transition which brings the system into a different and stable crystalline state.

  14. Laser induced structural transformation in chalcogenide based superlattices

    International Nuclear Information System (INIS)

    Zallo, Eugenio; Wang, Ruining; Bragaglia, Valeria; Calarco, Raffaella

    2016-01-01

    Superlattices made of alternating layers of nominal GeTe and Sb 2 Te 3 have been studied by micro-Raman spectroscopy. A structural irreversible transformation into ordered GeSbTe alloy is induced by high power laser light exposure. The intensity ratio of anti-Stokes and Stokes scattering under laser illumination gives a maximum average temperature in the sample of 177 °C. The latter is lower than the growth temperature and of 400 °C necessary by annealing to transform the structure in a GeSbTe alloy. The absence of this configuration after in situ annealing even up to 300 °C evidences an electronic excitation induced-transition which brings the system into a different and stable crystalline state.

  15. Quantification of flexoelectricity in PbTiO3/SrTiO3 superlattice polar vortices using machine learning and phase-field modeling.

    Science.gov (United States)

    Li, Q; Nelson, C T; Hsu, S-L; Damodaran, A R; Li, L-L; Yadav, A K; McCarter, M; Martin, L W; Ramesh, R; Kalinin, S V

    2017-11-13

    Flexoelectricity refers to electric polarization generated by heterogeneous mechanical strains, namely strain gradients, in materials of arbitrary crystal symmetries. Despite more than 50 years of work on this effect, an accurate identification of its coupling strength remains an experimental challenge for most materials, which impedes its wide recognition. Here, we show the presence of flexoelectricity in the recently discovered polar vortices in PbTiO 3 /SrTiO 3 superlattices based on a combination of machine-learning analysis of the atomic-scale electron microscopy imaging data and phenomenological phase-field modeling. By scrutinizing the influence of flexocoupling on the global vortex structure, we match theory and experiment using computer vision methodologies to determine the flexoelectric coefficients for PbTiO 3 and SrTiO 3 . Our findings highlight the inherent, nontrivial role of flexoelectricity in the generation of emergent complex polarization morphologies and demonstrate a viable approach to delineating this effect, conducive to the deeper exploration of both topics.

  16. Coherent Phonon Dynamics in Short-Period InAs/GaSb Superlattices

    OpenAIRE

    Noe, G. T.; Haugan, H. J.; Brown, G. J.; Sanders, G. D.; Stanton, C. J.; Kono, J.

    2011-01-01

    We have performed ultrafast pump-probe spectroscopy studies on a series of InAs/GaSb-based short-period superlattice (SL) samples with periods ranging from 46 \\AA to 71 \\AA. We observe two types of oscillations in the differential reflectivity with fast ($\\sim$ 1- 2 ps) and slow ($\\sim$ 24 ps) periods. The period of the fast oscillations changes with the SL period and can be explained as coherent acoustic phonons generated from carriers photoexcited within the SL. This mode provides an accura...

  17. Attitude Strength.

    Science.gov (United States)

    Howe, Lauren C; Krosnick, Jon A

    2017-01-03

    Attitude strength has been the focus of a huge volume of research in psychology and related sciences for decades. The insights offered by this literature have tremendous value for understanding attitude functioning and structure and for the effective application of the attitude concept in applied settings. This is the first Annual Review of Psychology article on the topic, and it offers a review of theory and evidence regarding one of the most researched strength-related attitude features: attitude importance. Personal importance is attached to an attitude when the attitude is perceived to be relevant to self-interest, social identification with reference groups or reference individuals, and values. Attaching personal importance to an attitude causes crystallizing of attitudes (via enhanced resistance to change), effortful gathering and processing of relevant information, accumulation of a large store of well-organized relevant information in long-term memory, enhanced attitude extremity and accessibility, enhanced attitude impact on the regulation of interpersonal attraction, energizing of emotional reactions, and enhanced impact of attitudes on behavioral intentions and action. Thus, important attitudes are real and consequential psychological forces, and their study offers opportunities for addressing behavioral change.

  18. Theory of the negative differential conductivity effect in semiconductor superlattices

    International Nuclear Information System (INIS)

    Vo Hong Anh; Nguyen Hong Shon; Le Vu Ky

    1990-01-01

    A new mechanism of the negative differential conductivity (NDC) effect in semiconductor superlattices (SL) is proposed and analysed that is due to the conduction electron trapping by donor centers. It is shown that the NDC effect occurs for sufficently high (but reasonable) impurity concentration and not too large value of the τ ε /τ c ratio (where τ ε is the electron energy relaxation time and τ c the electron life time in the conduction band) when the applied d.c. electric field reaches certain critical value defined by the physical parameters of the sample. (author). 8 refs, 2 figs

  19. Emergence of the sub-THz central peak at phase transitions in artificial BaTiO3/(Ba,Sr)TiO3 superlattices

    International Nuclear Information System (INIS)

    Tikhonov, Yu.A.; Razumnaya, A.G.; Torgashev, V.I.; Zakharchenko, I.N.; Yuzyuk, Yu.I.; El Marssi, M.; Ortega, N.; Kumar, A.; Katiyar, R.S.

    2015-01-01

    A prominent central peak in the sub-THz frequency range was observed in the Raman spectra of BaTiO 3 /(Ba,Sr)TiO 3 (BT/BST) superlattice grown on (001)MgO substrate. Both soft and central mode show an anomaly around 200 K and 280 K, which can be correlated with orthorhombic to monoclinic phase transition of BST and BT, respectively. The observed temperature dependence of the central mode enabled us to explain rather broad temperature dependence of the dielectric permittivity previously observed in BT/BST superlattices. (copyright 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  20. The phase diagrams and the order parameters of the diluted transverse superlattice with antiferromagnetic interface coupling

    International Nuclear Information System (INIS)

    Oubelkacem, A.; El Aouad, N.; Benaboud, A.; Saber, M.

    2004-01-01

    Using the effective field theory with a probability distribution technique that accounts for the self-spin correlation functions, the magnetic properties of the Ising superlattice consisting of two ferromagnetic materials A and B, with L a layers of diluted spins S a =((1)/(2)) and L b layers of diluted spins S b =1 in an applied transverse field Ω with antiferromagnetic interface coupling are examined. For fixed values of the reduced exchange interactions and the concentration c of magnetic atoms, the phase diagrams and the total magnetization for the superlattice are studied as a function of the transverse field and the temperature. We find a number of characteristic phenomena. In particular, the effect of the concentration c of magnetic atoms, the interlayer coupling and the transverse field on both the compensation temperature and the magnetization profiles are clarified. Some of them may be related to the experimental works of rare-earth (RE)/transition metal (TM) multilayer films

  1. Spin echo dynamics under an applied drift field in graphene nanoribbon superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Prabhakar, Sanjay, E-mail: sprabhakar@wlu.ca [M 2NeT Laboratory, Wilfrid Laurier University, 75 University Avenue West, Waterloo, Ontario N2L 3C5 (Canada); Melnik, Roderick [M 2NeT Laboratory, Wilfrid Laurier University, 75 University Avenue West, Waterloo, Ontario N2L 3C5 (Canada); Gregorio Millan Institute, Universidad Carlos III de Madrid, 28911 Leganes (Spain); Bonilla, Luis L. [Gregorio Millan Institute, Universidad Carlos III de Madrid, 28911 Leganes (Spain); Raynolds, James E. [Drinker Biddle and Reath LLP, Washington, DC 20005 (United States)

    2013-12-02

    We investigate the evolution of spin dynamics in graphene nanoribbon superlattices (GNSLs) with armchair and zigzag edges in the presence of a drift field. We determine the exact evolution operator and show that it exhibits spin echo phenomena due to rapid oscillations of the quantum states along the ribbon. The evolution of the spin polarization is accompanied by strong beating patterns. We also provide detailed analysis of the band structure of GNSLs with armchair and zigzag edges.

  2. Spin echo dynamics under an applied drift field in graphene nanoribbon superlattices

    International Nuclear Information System (INIS)

    Prabhakar, Sanjay; Melnik, Roderick; Bonilla, Luis L.; Raynolds, James E.

    2013-01-01

    We investigate the evolution of spin dynamics in graphene nanoribbon superlattices (GNSLs) with armchair and zigzag edges in the presence of a drift field. We determine the exact evolution operator and show that it exhibits spin echo phenomena due to rapid oscillations of the quantum states along the ribbon. The evolution of the spin polarization is accompanied by strong beating patterns. We also provide detailed analysis of the band structure of GNSLs with armchair and zigzag edges

  3. Evidence of negative electrorefraction in type-II GaAs/GaAlAs short-period superlattice

    International Nuclear Information System (INIS)

    Shchukin, V A; Ledentsov, N N; Karachinsky, L Ya; Novikov, I I; Egorov, A Yu; Blokhin, S A; Maximov, M V; Gordeev, N Yu; Kulagina, M M; Ustinov, V M

    2015-01-01

    A type-II GaAs/GaAlAs short-period superlattice (SPSL) used as an electro-optic medium for the spectral range 820–850 nm is studied in a vertical microcavity geometry. SPSL is sandwiched between two GaAlAs distributed Bragg reflectors. Optical power reflectance (OR) spectra are measured as a function of applied reverse bias at different tilt angles and temperatures. All spectra reveal a blue shift of the reflectivity dip upon applied voltage which evidences a negative electrorefraction of the electro-optic medium. The shift enhances up to ∼0.6 nm once the exciton resonance is brought close to the wavelength of the reflectivity dip. As opposed to those modulators based on quantum–confined Stark effect, no increased absorption is observed at an applied bias, because the integrated intensity of the reflectivity dip in the OR spectra is virtually constant. This indicates a low absorption loss with applied bias and consequently a high potential for the increased dynamic range of the related modulator. (paper)

  4. Tuning the electronic transport anisotropy in α-phase phosphorene through superlattice design

    Science.gov (United States)

    He, Yuanyuan; Xiong, Shiyun; Xia, Feifei; Shao, Zhibin; Zhao, Jianwei; Zhang, Xiujuan; Jie, Jiansheng; Zhang, Xiaohong

    2018-02-01

    Rational tuning the anisotropic electronic properties of monolayer phosphorene is essential to their applications in electronic and optoelectronic devices. By combining the density functional theory and the nonequilibrium Green's function method, we developed a strategy to tune the anisotropic transport properties of phosphorene by designing stable arsenic-phosphorene (A sxP1 -x ) superlattice (SL). It was found that, with a careful design of As:P ratio and atomic arrangement, the anisotropic transport properties could be tuned in a wide range. The transport current along the zigzag direction, which is very low in pristine phosphorene, was gradually enhanced by increasing the As:P ratio, and even became larger than that along armchair direction when the As:P ratio achieved 1:1 under a given arrangement of As atoms in A sxP1 -x SL. The tunable anisotropic transport properties of A sxP1 -x SL are attributed to the interplay between the different scattering rates related to the number and orientation of As-P interfaces. This finding demonstrates that the A sxP1 -x SL design could be an effective approach to tune the anisotropic electronic properties of monolayer phosphorene, which is important for the development of high-performance electronic and optoelectronic devices based on phosphorene.

  5. A new effect of retained austenite on ductility enhancement in high strength bainitic steel

    Energy Technology Data Exchange (ETDEWEB)

    Wang Ying; Zhang Ke; Guo Zhenghong; Chen Nailu [School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240 (China); Rong Yonghua, E-mail: yhrong@sjtu.edu.cn [School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240 (China)

    2012-08-30

    Highlights: Black-Right-Pointing-Pointer A new DARA effect in the bainitic steel is proposed. Black-Right-Pointing-Pointer The conditions of DARA effect are proposed. Black-Right-Pointing-Pointer The mechanism of retained austenite on ductility enhancement is clarified. - Abstract: A designed high strength bainitic steel with considerable amount of retained austenite is presented in order to study the effect of retained austenite on the ductility enhancement in bainitic steels. Transformation induced plasticity (TRIP) effect is verified by both X-ray diffraction (XRD) measurement of retained austenite fraction in various deformation stages and transmission electron microscopy observation of the deformed twin-type martensite. Results from XRD line profile analysis reveal that the average dislocation density in bainite during the deformation is lower than that before deformation, and such a phenomenon can be explained by a new effect, dislocations absorption by retained austenite (DARA) effect, based on our previous investigation of martensitic steels. DARA effect availably enhances the compatibility of deformation ability of bainite with retained austenite. In view of microstructure similarity of bainitic steels with martensitic steels, the conditions of DARA effect are proposed. The effects of retained austenite on the ductility enhancement in bainitic steels are clarified.

  6. Ionic Potential and Band Narrowing as a Source of Orbital Polarization in Nickelate/Insulator Superlattices

    Science.gov (United States)

    Georgescu, Alexandru B.; Disa, Ankit S.; Kumah, Divine P.; Ismail-Beigi, Sohrab; Walker, Frederick J.; Ahn, Charles H.

    Nickelate interfaces display complex, interacting electronic properties such as thickness dependent metal-insulator transitions. One large body of effort involving nickelates has aimed to split the energies of the Ni 3d orbitals (orbital polarization) to make the resulting band structure resemble that of cuprate superconductors. The most commonly studied interfacial system involves superlattices of alternating nickelate and insulating perovksite-structure layers; the resulting orbital polarization at the nickelate-insulator interface is understood as being due to confinement or structural symmetry breaking. By using first principles theory on the NdNiO3/NdAlO3 superlattice, we show that another important source of orbital polarization stems from electrostatic effects: the more ionic nature of the cations in the insulator (when compared to the nickelate) can shift the relative orbital energies of the Ni. We use density functional theory (DFT) and add electronic correlations via slave-bosons to describe the effect of correlation-induced band narrowing on the orbital polarization. Work supported by NSF Grant MRSEC DMR-1119826.

  7. Oblique surface waves at an interface between a metal-dielectric superlattice and an isotropic dielectric

    International Nuclear Information System (INIS)

    Vuković, Slobodan M; Miret, Juan J; Zapata-Rodriguez, Carlos J; Jakšić, Zoran

    2012-01-01

    We investigate the existence and dispersion characteristics of surface waves that propagate at an interface between a metal-dielectric superlattice and an isotropic dielectric. Within the long-wavelength limit, when the effective-medium (EM) approximation is valid, the superlattice behaves like a uniaxial plasmonic crystal with the main optical axes perpendicular to the metal-dielectric interfaces. We demonstrate that if such a semi-infinite plasmonic crystal is cut normally to the layer interfaces and brought into contact with a semi-infinite dielectric, a new type of surface mode can appear. Such modes can propagate obliquely to the optical axes if favorable conditions regarding the thickness of the layers and the dielectric permittivities of the constituent materials are met. We show that losses within the metallic layers can be substantially reduced by making the layers sufficiently thin. At the same time, a dramatic enlargement of the range of angles for oblique propagation of the new surface modes is observed. This can lead, however, to field non-locality and consequently to failure of the EM approximation.

  8. The electronic states calculated using the sinusoidal potential for Cd1-xZnxS quantum dot superlattices

    International Nuclear Information System (INIS)

    Sakly, A.; Safta, N.; Mejri, H.; Lamine, A. Ben

    2011-01-01

    Research highlights: → This paper is dedicated to structures based on Cd 1-x Zn x S. - Abstract: The present work reports on a theoretical investigation of superlattices based on Cd 1-x Zn x S quantum dots embedded in an insulating material. The system to model is assumed to be a series of flattened cylindrical quantum dots with a finite barrier at the boundary and is studied using a sinusoidal potential. The electronic states of both Γ 1 - (ground) and Γ 2 - (first excited) minibands have been computed as a function of inter-quantum dot separation and Zn composition. An analysis of the results shows that the widths of Γ 1 - and Γ 2 - minibands decrease as the superlattice period and Zn content increase separately. Moreover, the sinusoidal shape of the confining potential accounts for the coupling between quantum dots quantitatively less than the Kronig-Penney potential model.

  9. Nanocomposites biodegradable coating on BOPET films to enhance hot seal strength properties

    International Nuclear Information System (INIS)

    Barbaro, G.; Galdi, M. R.; Di Maio, L.; Incarnato, L.

    2015-01-01

    The coating technology is a strategic solution to improve the properties of flexible packaging films. Indeed, additional functional layers are often designed and added as coating on the substrate, in order to improve the characteristic of the flexible packaging and to meet the requirements for the desired gas or vapour barrier, for adhesion and sealing, or for improving the film printability, its aesthetics and durability. Moreover, this technology allows to functionalize a polymeric substrate applying materials with different chemistry, rheology, thermal and structural characteristics. BOPET films are widely used for food packaging applications thanks to their good gas barrier and mechanical properties, high transparency and for the excellent printability. In regard to sealing performance, BOPET films show poor sealing properties so they are mostly submitted to lamination processes with polyethylene. Nevertheless, this solution compromises the PET recyclability and influences the gas permeability of the multilayer PET based structures. The aim of this work is to investigate on the effect of nanocomposite biodegradable coatings for BOPET substrates in enhancing the heat sealing strength of eco-compatible PET/PLA films. At this regards, different percentages of Cloisite C30B (0%, 2% and 4% wt/wt ) have been added to PLA by solution intercalation technique and the nanocomposite biodegradable materials produced have been applied on BOPET commercial films by casting. The BOPET coated films have been characterized in order to evaluate the heat sealing strength and the mechanical, gas permeability and surface properties. The results have shown that the addition of nanoclay in PLA coating significantly enhance the hot tack properties of the PET/PLA system produced, while the oxygen and water vapour permeability are slightly increased if compared to pure BOPET films

  10. Nanocomposites biodegradable coating on BOPET films to enhance hot seal strength properties

    Energy Technology Data Exchange (ETDEWEB)

    Barbaro, G., E-mail: giovannibarbaro@email.it; Galdi, M. R., E-mail: mrgaldi@unisa.it; Di Maio, L., E-mail: ldimaio@unisa.it; Incarnato, L., E-mail: lincarnato@unisa.it [Industrial Engineering Department, University of Salerno, Via Giovanni Paolo II 132, 84084 Fisciano (Italy)

    2015-12-17

    The coating technology is a strategic solution to improve the properties of flexible packaging films. Indeed, additional functional layers are often designed and added as coating on the substrate, in order to improve the characteristic of the flexible packaging and to meet the requirements for the desired gas or vapour barrier, for adhesion and sealing, or for improving the film printability, its aesthetics and durability. Moreover, this technology allows to functionalize a polymeric substrate applying materials with different chemistry, rheology, thermal and structural characteristics. BOPET films are widely used for food packaging applications thanks to their good gas barrier and mechanical properties, high transparency and for the excellent printability. In regard to sealing performance, BOPET films show poor sealing properties so they are mostly submitted to lamination processes with polyethylene. Nevertheless, this solution compromises the PET recyclability and influences the gas permeability of the multilayer PET based structures. The aim of this work is to investigate on the effect of nanocomposite biodegradable coatings for BOPET substrates in enhancing the heat sealing strength of eco-compatible PET/PLA films. At this regards, different percentages of Cloisite C30B (0%, 2% and 4%{sub wt/wt}) have been added to PLA by solution intercalation technique and the nanocomposite biodegradable materials produced have been applied on BOPET commercial films by casting. The BOPET coated films have been characterized in order to evaluate the heat sealing strength and the mechanical, gas permeability and surface properties. The results have shown that the addition of nanoclay in PLA coating significantly enhance the hot tack properties of the PET/PLA system produced, while the oxygen and water vapour permeability are slightly increased if compared to pure BOPET films.

  11. Nanocomposites biodegradable coating on BOPET films to enhance hot seal strength properties

    Science.gov (United States)

    Barbaro, G.; Galdi, M. R.; Di Maio, L.; Incarnato, L.

    2015-12-01

    The coating technology is a strategic solution to improve the properties of flexible packaging films. Indeed, additional functional layers are often designed and added as coating on the substrate, in order to improve the characteristic of the flexible packaging and to meet the requirements for the desired gas or vapour barrier, for adhesion and sealing, or for improving the film printability, its aesthetics and durability. Moreover, this technology allows to functionalize a polymeric substrate applying materials with different chemistry, rheology, thermal and structural characteristics. BOPET films are widely used for food packaging applications thanks to their good gas barrier and mechanical properties, high transparency and for the excellent printability. In regard to sealing performance, BOPET films show poor sealing properties so they are mostly submitted to lamination processes with polyethylene. Nevertheless, this solution compromises the PET recyclability and influences the gas permeability of the multilayer PET based structures. The aim of this work is to investigate on the effect of nanocomposite biodegradable coatings for BOPET substrates in enhancing the heat sealing strength of eco-compatible PET/PLA films. At this regards, different percentages of Cloisite C30B (0%, 2% and 4%wt/wt) have been added to PLA by solution intercalation technique and the nanocomposite biodegradable materials produced have been applied on BOPET commercial films by casting. The BOPET coated films have been characterized in order to evaluate the heat sealing strength and the mechanical, gas permeability and surface properties. The results have shown that the addition of nanoclay in PLA coating significantly enhance the hot tack properties of the PET/PLA system produced, while the oxygen and water vapour permeability are slightly increased if compared to pure BOPET films.

  12. Bulk-like-phonon polaritons in one-dimensional photonic superlattices

    Science.gov (United States)

    Gómez-Urrea, H. A.; Duque, C. A.; Mora-Ramos, M. E.

    2017-05-01

    We investigate the properties of a one-dimensional photonic superlattice made of alternating layers of air and wurtzite aluminum nitride. The Maxwell equations are solved for any admissible values of the angle of incidence by means of the transfer matrix formalism. The band structure of the frequency spectrum is obtained, as well as the density of states and transmittance associated to both the TM and TE modes. The dispersion relations indicate that for oblique incidence and TM modes there is a component of the electric field oriented along the growth direction of the structure that couples with the longitudinal optical phonon oscillations of the aluminum nitride thus leading to the appearance of longitudinal phonon polaritons in the system.

  13. Sequential magnetic switching in Fe/MgO(001) superlattices

    Science.gov (United States)

    Magnus, F.; Warnatz, T.; Palsson, G. K.; Devishvili, A.; Ukleev, V.; Palisaitis, J.; Persson, P. O. Å.; Hjörvarsson, B.

    2018-05-01

    Polarized neutron reflectometry is used to determine the sequence of magnetic switching in interlayer exchange coupled Fe/MgO(001) superlattices in an applied magnetic field. For 19.6 Å thick MgO layers we obtain a 90∘ periodic magnetic alignment between adjacent Fe layers at remanence. In an increasing applied field the top layer switches first followed by its second-nearest neighbor. For 16.4 Å MgO layers, a 180∘ periodic alignment is obtained at remanence and with increasing applied field the layer switching starts from the two outermost layers and proceeds inwards. This sequential tuneable switching opens up the possibility of designing three-dimensional magnetic structures with a predefined discrete switching sequence.

  14. Tunable negative differential resistance in planar graphene superlattice resonant tunneling diode

    Science.gov (United States)

    Sattari-Esfahlan, S. M.; Fouladi-Oskuei, J.; Shojaei, S.

    2017-04-01

    Here, we study the negative differential resistance (NDR) of Dirac electrons in biased planar graphene superlattice (PGSL) and investigate the transport characteristics by adopted transfer matrix method within Landauer-Buttiker formalism. Our model device is based on one-dimensional Kronig-Penney type electrostatic potential in monolayer graphene deposited on a substrate, where the bias voltage is applied by two electrodes in the left and right. At Low bias voltages, we found that NDR appears due to breaking of minibands to Wannier-Stark ladders (WSLs). At the critical bias voltage, delocalization appeared by WS states leads to tunneling peak current in current-voltage (I-V) characteristics. With increasing bias voltage, crossing of rungs from various WSL results in multi-peak NDR. The results demonstrate that the structure parameters like barrier/well thickness and barrier height have remarkable effect on I-V characteristics of PGSL. In addition, Dirac gap enhances peak to valley (PVR) value due to suppressing Klein tunneling. Our results show that the tunable PVR in PGSL resonant tunneling diode can be achievable by structure parameters engineering. NDR at ultra-low bias voltages, such as 100 mV, with giant PVR of 20 is obtained. In our device, the multiple same NDR peaks with ultra-low bias voltage provide promising prospect for multi-valued memories and the low power nanoelectronic tunneling devices.

  15. Superlattices assembled through shape-induced directional binding

    Science.gov (United States)

    Lu, Fang; Yager, Kevin G.; Zhang, Yugang; Xin, Huolin; Gang, Oleg

    2015-04-01

    Organization of spherical particles into lattices is typically driven by packing considerations. Although the addition of directional binding can significantly broaden structural diversity, nanoscale implementation remains challenging. Here we investigate the assembly of clusters and lattices in which anisotropic polyhedral blocks coordinate isotropic spherical nanoparticles via shape-induced directional interactions facilitated by DNA recognition. We show that these polyhedral blocks--cubes and octahedrons--when mixed with spheres, promote the assembly of clusters with architecture determined by polyhedron symmetry. Moreover, three-dimensional binary superlattices are formed when DNA shells accommodate the shape disparity between nanoparticle interfaces. The crystallographic symmetry of assembled lattices is determined by the spatial symmetry of the block's facets, while structural order depends on DNA-tuned interactions and particle size ratio. The presented lattice assembly strategy, exploiting shape for defining the global structure and DNA-mediation locally, opens novel possibilities for by-design fabrication of binary lattices.

  16. Ordered patterns and structures via interfacial self-assembly: superlattices, honeycomb structures and coffee rings.

    Science.gov (United States)

    Ma, Hongmin; Hao, Jingcheng

    2011-11-01

    Self-assembly is now being intensively studied in chemistry, physics, biology, and materials engineering and has become an important "bottom-up" approach to create intriguing structures for different applications. Self-assembly is not only a practical approach for creating a variety of nanostructures, but also shows great superiority in building hierarchical structures with orders on different length scales. The early work in self-assembly focused on molecular self-assembly in bulk solution, including the resultant dye aggregates, liposomes, vesicles, liquid crystals, gels and so on. Interfacial self-assembly has been a great concern over the last two decades, largely because of the unique and ingenious roles of this method for constructing materials at interfaces, such as self-assembled monolayers, Langmuir-Blodgett films, and capsules. Nanocrystal superlattices, honeycomb films and coffee rings are intriguing structural materials with more complex features and can be prepared by interfacial self-assembly on different length scales. In this critical review, we outline the recent development in the preparation and application of colloidal nanocrystal superlattices, honeycomb-patterned macroporous structures by the breath figure method, and coffee-ring-like patterns (247 references). This journal is © The Royal Society of Chemistry 2011

  17. Trends in (LaMnO3)n/(SrTiO3)m superlattices with varying layer thicknesses

    KAUST Repository

    Jilili, J.; Cossu, Fabrizio; Schwingenschlö gl, Udo

    2015-01-01

    We investigate the thickness dependence of the structural, electronic, and magnetic properties of (LaMnO3)n/(SrTiO3)m (n, m = 2, 4, 6, 8) superlattices using density functional theory. The electronic structure turns out to be highly sensitive

  18. Intraband absorption in GaAs-(Ga,Al)As variably spaced semiconductor superlattices under crossed electric and magnetic fields

    Science.gov (United States)

    Reyes-Gómez, E.; Raigoza, N.; Oliveira, L. E.

    2013-11-01

    A theoretical study of the intraband absorption properties of GaAs-Ga1-xAlxAs variably spaced semiconductor superlattices under crossed magnetic and electric fields is presented. Calculations are performed for the applied electric field along the growth-axis direction, whereas the magnetic field is considered parallel to the heterostructure layers. By defining a critical electric field so that the heterostructure energy levels are aligned in the absence of the applied magnetic fields, one finds that, in the weak magnetic-field regime, an abrupt red shift of the absorption coefficient maxima is obtained at fields equal to or larger than the critical electric field, a fact which may be explained from the localization properties of the electron wave functions. Results in the strong magnetic-field regime reveal a rich structure on the intraband absorption coefficient which may be explained from the strong dispersion exhibited by both the energy levels and transition strengths as functions of the generalized orbit-center position. Moreover, the possibility of occurrence of absorption in a wide frequency range is also demonstrated. Present calculated results may be of interest for future design and improvement of multilayered-based photovoltaic and solar-cell devices.

  19. Deep levels in silicon–oxygen superlattices

    International Nuclear Information System (INIS)

    Simoen, E; Jayachandran, S; Delabie, A; Caymax, M; Heyns, M

    2016-01-01

    This work reports on the deep levels observed in Pt/Al 2 O 3 /p-type Si metal-oxide-semiconductor capacitors containing a silicon–oxygen superlattice (SL) by deep-level transient spectroscopy. It is shown that the presence of the SL gives rise to a broad band of hole traps occurring around the silicon mid gap, which is absent in reference samples with a silicon epitaxial layer. In addition, the density of states of the deep layers roughly scales with the number of SL periods for the as-deposited samples. Annealing in a forming gas atmosphere reduces the maximum concentration significantly, while the peak energy position shifts from close-to mid-gap towards the valence band edge. Based on the flat-band voltage shift of the Capacitance–Voltage characteristics it is inferred that positive charge is introduced by the oxygen atomic layers in the SL, indicating the donor nature of the underlying hole traps. In some cases, a minor peak associated with P b dangling bond centers at the Si/SiO 2 interface has been observed as well. (paper)

  20. Influence of the interface on growth rates in AlN/GaN short period superlattices via metal organic vapor phase epitaxy

    Science.gov (United States)

    Rodak, L. E.; Korakakis, D.

    2011-11-01

    AlN/GaN short period superlattices are well suited for a number of applications including, but not limited to, digital alloys, intersubband devices, and emitters. In this work, AlN/GaN superlattices with periodicities ranging from 10 to 20 Å have been grown via metal organic vapor phase epitaxy in order to investigate the influence of the interface on the binary alloy growth rates. The GaN growth rate at the interface was observed to decrease with increasing GaN thickness while the AlN growth rate remained constant. This has been attributed to a decrease in the decomposition rate of GaN at the hetero-interface as seen in other III-V hetero-structures.

  1. Advanced characterization techniques in understanding the roles of nickel in enhancing strength and toughness of submerged arc welding high strength low alloy steel multiple pass welds in the as-welded condition

    Science.gov (United States)

    Sham, Kin-Ling

    Striving for higher strength along with higher toughness is a constant goal in material properties. Even though nickel is known as an effective alloying element in improving the resistance of a steel to impact fracture, it is not fully understood how nickel enhances toughness. It was the goal of this work to assist and further the understanding of how nickel enhanced toughness and maintained strength in particular for high strength low alloy (HSLA) steel submerged arc welding multiple pass welds in the as-welded condition. Using advanced analytical techniques such as electron backscatter diffraction, x-ray diffraction, electron microprobe, differential scanning calorimetry, and thermodynamic modeling software, the effect of nickel was studied with nickel varying from one to five wt. pct. in increments of one wt. pct. in a specific HSLA steel submerged arc welding multiple pass weldment. The test matrix of five different nickel compositions in the as-welded and stress-relieved condition was to meet the targeted mechanical properties with a yield strength greater than or equal to 85 ksi, a ultimate tensile strength greater than or equal to 105 ksi, and a nil ductility temperature less than or equal to -140 degrees F. Mechanical testing demonstrated that nickel content of three wt. pct and greater in the as-welded condition fulfilled the targeted mechanical properties. Therefore, one, three, and five wt. pct. nickel in the as-welded condition was further studied to determine the effect of nickel on primary solidification mode, nickel solute segregation, dendrite thickness, phase transformation temperatures, effective ferrite grain size, dislocation density and strain, grain misorientation distribution, and precipitates. From one to five wt. pct nickel content in the as-welded condition, the primary solidification was shown to change from primary delta-ferrite to primary austenite. The nickel partitioning coefficient increased and dendrite/cellular thickness was

  2. Topological hierarchy matters — topological matters with superlattices of defects

    International Nuclear Information System (INIS)

    He Jing; Kou Su-Peng

    2016-01-01

    Topological insulators/superconductors are new states of quantum matter with metallic edge/surface states. In this paper, we review the defects effect in these topological states and study new types of topological matters — topological hierarchy matters. We find that both topological defects (quantized vortices) and non topological defects (vacancies) can induce topological mid-gap states in the topological hierarchy matters after considering the superlattice of defects. These topological mid-gap states have nontrivial topological properties, including the nonzero Chern number and the gapless edge states. Effective tight-binding models are obtained to describe the topological mid-gap states in the topological hierarchy matters. (topical review)

  3. Leaky electronic states for photovoltaic photodetectors based on asymmetric superlattices

    Science.gov (United States)

    Penello, Germano Maioli; Pereira, Pedro Henrique; Pires, Mauricio Pamplona; Sivco, Deborah; Gmachl, Claire; Souza, Patricia Lustoza

    2018-01-01

    The concept of leaky electronic states in the continuum is used to achieve room temperature operation of photovoltaic superlattice infrared photodetectors. A structural asymmetric InGaAs/InAlAs potential profile is designed to create states in the continuum with the preferential direction for electron extraction and, consequently, to obtain photovoltaic operation at room temperature. Due to the photovoltaic operation and virtual increase in the bandoffset, the device presents both low dark current and low noise. The Johnson noise limited specific detectivity reaches values as high as 1.4 × 1011 Jones at 80 K. At 300 K, the detectivity obtained is 7.0 × 105 Jones.

  4. RBS studies of the lattice damage caused by 1 MeV Si+ implantation into Al0.3Ga0.7As/GaAs superlattices at elevated temperature

    International Nuclear Information System (INIS)

    Xu Tianbing; Zhu Peiran; Zhou Junsi; Li Daiqing; Gong Baoan; Wan Ya; Mu Shanming; Zhao Qingtai; Wang Zhonglie

    1994-01-01

    The lattice damage accumulation in GaAs and Al 0.3 Ga 0.7 As/GaAs superlattices by 1 MeV Si + irradiation at room temperature and 350 C has been studied. For irradiations at 350 C, at lower doses the samples were almost defect-free after irradiation, while a large density of accumulated defects was induced at a higher dose. The critical dose above which the damage accumulation is more efficient is estimated to be 2 x 10 15 Si/cm 2 for GaAs, and is 5 x 10 15 Si/cm 2 for Al 0.8 Ga 0.7 As/GaAs superlattice for implantation with 1.0 MeV Si ions at 350 C. The damage accumulation rate for 1 MeV Si ion implantation in Al 0.3 Ga 0.7 As/GaAs superlattice is less than that in GaAs. (orig.)

  5. The orientation dependence of the hydrogen distribution within Mo/V (110) and (001) multilayered artificial superlattices

    CERN Document Server

    Hjörvarsson, B; Olafsson, S; Karlsson, E B; Birch, J; Sundgren, J E

    1997-01-01

    We have investigated the hydrogen distribution within multilayered Mo/V (110) and (001) artificial superlattices. The hydrogen concentration, determined by the sup 1 H( sup 1 sup 5 N, alpha gamma) sup 1 sup 2 C nuclear resonance reaction, is found to decrease with decreasing layer thickness. The decrease is attributed to an interface region with reduced hydrogen content. The extension of this interface region is found to be two monolayers in Mo/V (110), compared to the previously reported three monolayers in Mo/V (001) superlattices. In the (110) oriented samples the relative amount of H in the interface region decreases with increasing average H content (H/V (atomicratio) approx. 0.3-0.5). This effect is shown to be related to the orientation of the hydrogen planes with respect to the boundaries implied by the Mo/V interfaces. The hydrogen induced volume change, obtained by conventional theta-2 theta x-ray diffraction (XRD) and reciprocal space mapping, is found to be smaller for the (110) samples than for t...

  6. The ferromagnet spin-1/2 Ising superlattice in a transverse field

    International Nuclear Information System (INIS)

    Bouziane, T.; Saber, M.; Belaaraj, A.; Ainane, A.

    1998-09-01

    The phase transitions of a ferromagnet spin-1/2 Ising superlattice consisting of two different materials in a transverse field is examined with the use of effective field theory that accounts for the self-spin function correlation. The critical temperature of the system is studied as a function of the thickness of the constituents in a unit cell and of exchange interactions in each material. A critical interface exchange interaction above which the interface magnetism appears is found. The effects of a uniform transverse field and the interface exchange interaction on the parameters of the system are also investigated. (author)

  7. Theory of photoresistors on the base of trapezoidal δ-doped superlattices

    International Nuclear Information System (INIS)

    Osipov, V.V.; Selyakov, A.Yu.; Foygel, M.

    1999-01-01

    A theory of far infrared photoresistors that is based on trapezoidal δ-doped superlattices (TSL) has been developed. It is shown that photoconductivity of TSL in monopolar despite the interband absorption and generation of electron - hole pairs. In this way, photoelectric gain and responsivity of TSL photoresistor can reach gigantic values. It is established that tunneling-radiative lifetime of electron-hole pairs determines the kinetics of photoconductivity decay. It is shown that a surprising effect takes place: voltage responsivity does not depend virtually on lifetime of nonequilibrium carriers and on the doping level of TSL photoresistor and can reach gigantic values [ru

  8. In-plain electric properties of [CaMnO3/REMO3] (RE=Bi, La M=Fe, Fe0.8Mn0.2) superlattices grown by pulsed laser deposition method

    NARCIS (Netherlands)

    Iwata, N.; Watabe, Y.; Oikawa, T.; Takase, K.; Huijben, Mark; Inaba, T.; Oshima, K.; Rijnders, Augustinus J.H.M.; Yamamoto, H.

    2014-01-01

    The [CaMnO3 (CMO)/REMO3] (RE = Bi, La M = Fe, Fe0.8Mn0.2) superlattices show semiconducting behavior with transition temperatures (TEg) of 71, 127, and 151 K in the [CMO/BiFe0.8Mn0.2O3], [CMO/BiFeO3], and [CMO/LaFeO3] superlattices. The formation of a magnetic polaron is expected in the CMO layer of

  9. Influence of the interface corrugation on the subband dispersions and the optical properties of (113)-oriented GaAs/AlAs superlattices

    DEFF Research Database (Denmark)

    Langbein, Wolfgang Werner; Lüerssen, D.; Kalt, H.

    1996-01-01

    We report on the influence of the interface corrugation in (113)-grown GaAs/AlAs superlattices on their band-edge optical properties both in theory and experiment. We calculate the subband dispersions and the optical anisotropies in a multiband k . p formalism. The dominating contribution...... to the optical anisotropies is found to be due to the intrinsic properties of the valence-band structure. The corrugation modifies the density of states only slightly, giving no evidence of a quantum-win behavior. By comparing the calculation with the experimental optical anisotropy, we can estimate...... of the localized type-I states at the band-edge show an enhanced optical anisotropy in comparison to the luminescence of the extended states, revealing the anisotropic nature of their localization sites. In type-II samples, deeply localized, isolated type-I states (Gamma quantum boxes) dominate the luminescence...

  10. Misfit Strain in Superlattices Controlling the Electron-Lattice Interaction via Micro strain in Active Layers

    International Nuclear Information System (INIS)

    Poccia, N.; Ricci, A.; Bianconi, N.

    2010-01-01

    High-temperature superconductivity (HTS) emerges in quite different electronic materials: cuprates, diborides, and iron-pnictide superconductors. Looking for unity in the diversity we find in all these materials a common lattice architecture: they are practical realizations of heterostructures at atomic limit made of superlattices of metallic active layers intercalated by spacers as predicted in 1993 by one of us. The multilayer architecture is the key feature for the presence of electronic topological transitions where the Fermi surface of one of the subbands changes dimensionality. The superlattice misfit strain η between the active and spacer layers is shown to be a key variable to drive the system to the highest critical temperature Tc that occurs at a particular point of the 3D phase diagram Tc(θ, η) where d is the charge transfer or doping. The plots of Tc as a function of misfit strain at constant charge transfer in cuprates show a first-order quantum critical phase transition where an itinerant striped magnetic phase competes with superconductivity in the proximity of a structural phase transition, that is, associated with an electronic topological transition. The shape resonances in these multi gap superconductors is associated with the maximum Tc.

  11. Electronic and transport properties of noncollinear magnetic monatomic Mn chains: Fano resonances in the superlattice of noncollinear magnetic barriers and magnetic anisotropic bands

    International Nuclear Information System (INIS)

    Dai, C.J.; Yan, X.H.; Xiao, Y.; Guo, Y.D.

    2015-01-01

    By means of the density functional theory combined with non-equilibrium Green's function method, ballistic transport properties of one-dimensional noncollinear magnetic monatomic chains were investigated using the single-atomic Mn chains as a model system. Fano resonances are found to exist in the monatomic Mn chains with spin-spiral structure. Furthermore, in the monatomic Mn chains with magnetic soliton lattice, Fano resonances are enhanced and cause the conductance splitting in the transmission spectra. The Fano resonances in the noncollinear magnetic single-atomic Mn chains are arising from the coupling of the localized d-states and the extended states of the quantum channels. By constructing a theoretical model and calculating its conductance, it is found that the phenomena of Fano resonances and the accompanying conductance splitting exist universally in the superlattice of one-dimensional noncollinear magnetic barriers, due to the interference of the incident waves and reflected waves by the interfaces between the neighboring barriers. Moreover, the band structures of the ferromagnetic and spin-spiral monatomic Mn chains exhibit a strong dependence on the spatial arrangement of the magnetic moments of Mn atoms when spin–orbit coupling is considered. - Highlights: • Transport properties of noncollinear magnetic monatomic Mn chains are studied. • Fano resonances are found in the noncollinear magnetic monatomic Mn chains. • Magnetic soliton lattice leads to conductance splitting in the transmission curve. • Fano resonances exist in the superlattice of noncollinear magnetic barriers. • Effect of SOC on the band structure of FM and spin-spiral Mn chains are studied

  12. Modeling of the Interminiband Absorption Coefficient in InGaN Quantum Dot Superlattices

    Directory of Open Access Journals (Sweden)

    Giovanni Giannoccaro

    2016-01-01

    Full Text Available In this paper, a model to estimate minibands and theinterminiband absorption coefficient for a wurtzite (WZ indium gallium nitride (InGaN self-assembled quantum dot superlattice (QDSL is developed. It considers a simplified cuboid shape for quantum dots (QDs. The semi-analytical investigation starts from evaluation through the three-dimensional (3D finite element method (FEM simulations of crystal mechanical deformation derived from heterostructure lattice mismatch under spontaneous and piezoelectric polarization effects. From these results, mean values in QDs and barrier regions of charge carriers’ electric potentials and effective masses for the conduction band (CB and three valence sub-bands for each direction are evaluated. For the minibands’ investigation, the single-particle time-independent Schrödinger equation in effective mass approximation is decoupled in three directions and resolved using the one-dimensional (1D Kronig–Penney model. The built-in electric field is also considered along the polar axis direction, obtaining Wannier–Stark ladders. Then, theinterminiband absorption coefficient in thermal equilibrium for transverse electric (TE and magnetic (TM incident light polarization is calculated using Fermi’s golden rule implementation based on a numerical integration into the first Brillouin zone. For more detailed results, an absorption coefficient component related to superlattice free excitons is also introduced. Finally, some simulation results, observations and comments are given.

  13. Electronic structure modeling of InAs/GaSb superlattices with hybrid density functional theory

    Energy Technology Data Exchange (ETDEWEB)

    Garwood, Tristan [Univ. of New Mexico, Albuquerque, NM (United States). Center for High Technology Materials; Modine, Normand A. [Sandia National Laboratories (SNL-NM), Albuquerque, NM (United States); Krishna, S. [Univ. of New Mexico, Albuquerque, NM (United States). Center for High Technology Materials

    2016-12-18

    The application of first-principles calculations holds promise for greatly improving our understanding of semiconductor superlattices. By developing a procedure to accurately predict band gaps using hybrid density functional theory, it lays the groundwork for future studies investigating more nuanced properties of these structures. Our approach allows a priori prediction of the properties of SLS structures using only the band gaps of the constituent materials. Furthermore, it should enable direct investigation of the effects of interface structure, e.g., intermixing or ordering at the interface, on SLS properties. In this paper, we present band gap data for various InAs/GaSb type-II superlattice structures calculated using the generalized Kohn-Sham formulation of density functional theory. A PBE0-type hybrid functional was used, and the portion of the exact exchange was tuned to fit the band gaps of the binary compounds InAs and GaSb with the best agreement to bulk experimental values obtained with 18% of the exact exchange. The heterostructures considered in this study are 6 monolayer (ML) InAs/6 ML GaSb, 8 ML InAs/8 ML GaSb and 10 ML InAs/10 ML GaSb with deviations from the experimental band gaps ranging from 3% to 11%.

  14. Infrared spectroscopy of strained BaTiO.sub.3./sub./SrTiO.sub.3./sub. superlattices on scandate substrates

    Czech Academy of Sciences Publication Activity Database

    Železný, Vladimír; Soukiassian, A.; Xi, X.X.; Schlom, G.

    2014-01-01

    Roč. 87, 10-11 (2014), s. 929-937 ISSN 0141-1594 Institutional support: RVO:68378271 Keywords : infrared spectroscopy * ferroelectric superlattices * perovskites Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 0.954, year: 2014

  15. The phase diagrams of the site-diluted spin-1/2 Ising superlattice

    International Nuclear Information System (INIS)

    Saber, A.; Essaoudi, I.; Ainane, A.; Dujardin, F.; Saber, M.; Stebe, B.

    1998-08-01

    Using the effective field theory with a probability distribution technique that accounts for the single-site spin correlations, the critical behavior of a diluted spin-1/2 Ising superlattice consisting of two different ferromagnet materials is examined. The critical temperature of the system is studied as a function of the thickness of the constituents in a unit cell, the concentration of magnetic atoms, and the exchange interactions in each material. It is shown that the properties of the diluted system are different from those of the corresponding pure system. (author)

  16. Hydrophilic Modification of Multi-Walled Carbon Nanotube for Building Photonic Crystals with Enhanced Color Visibility and Mechanical Strength

    Directory of Open Access Journals (Sweden)

    Feihu Li

    2016-04-01

    Full Text Available Low color visibility and poor mechanical strength of polystyrene (PS photonic crystal films have been the main shortcomings for the potential applications in paints or displays. This paper presents a simple method to fabricate PS/MWCNTs (multi-walled carbon nanotubes composite photonic crystal films with enhanced color visibility and mechanical strength. First, MWCNTs was modified through radical addition reaction by aniline 2,5-double sulfonic acid diazonium salt to generate hydrophilic surface and good water dispersity. Then the MWCNTs dispersion was blended with PS emulsion to form homogeneous PS/MWCNTs emulsion mixtures and fabricate composite films through thermal-assisted method. The obtained films exhibit high color visibility under natural light and improved mechanical strength owing to the light-adsorption property and crosslinking effect of MWCNTs. The utilization of MWCNTs in improving the properties of photonic crystals is significant for various applications, such as in paints and displays.

  17. Dynamical x-ray diffraction studies of interfacial strain in superlattices grown by molecular beam epitaxy

    International Nuclear Information System (INIS)

    Vandenberg, J.M.; Chu, S.N.G.; Hamm, R.A.; Panish, M.B.; Ritter, D.; Mancrander, A.T.

    1992-01-01

    This paper reports on dynamical X-ray diffraction studies that have been carried out for lattice-matched InGaAs/InP superlattices grown by modified molecular beam epitaxy (MBE) techniques. The (400) X-ray satellite pattern, which is predominantly affected by the strain modulation, was analyzed. The strain and thickness of the actual layers including the presence of strained interfacial regions were determined

  18. Structural, dynamic, and vibrational properties during heat transfer in Si/Ge superlattices: A Car-Parrinello molecular dynamics study

    Energy Technology Data Exchange (ETDEWEB)

    Ji, Pengfei; Zhang, Yuwen, E-mail: zhangyu@missouri.edu [Department of Mechanical and Aerospace Engineering, University of Missouri, Columbia, Missouri 65211 (United States); Yang, Mo [College of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093 (China)

    2013-12-21

    The structural, dynamic, and vibrational properties during heat transfer process in Si/Ge superlattices are studied by analyzing the trajectories generated by the ab initio Car-Parrinello molecular dynamics simulation. The radial distribution functions and mean square displacements are calculated and further discussions are made to explain and probe the structural changes relating to the heat transfer phenomenon. Furthermore, the vibrational density of states of the two layers (Si/Ge) are computed and plotted to analyze the contributions of phonons with different frequencies to the heat conduction. Coherent heat conduction of the low frequency phonons is found and their contributions to facilitate heat transfer are confirmed. The Car-Parrinello molecular dynamics simulation outputs in the work show reasonable thermophysical results of the thermal energy transport process and shed light on the potential applications of treating the heat transfer in the superlattices of semiconductor materials from a quantum mechanical molecular dynamics simulation perspective.

  19. Structural, dynamic, and vibrational properties during heat transfer in Si/Ge superlattices: A Car-Parrinello molecular dynamics study

    International Nuclear Information System (INIS)

    Ji, Pengfei; Zhang, Yuwen; Yang, Mo

    2013-01-01

    The structural, dynamic, and vibrational properties during heat transfer process in Si/Ge superlattices are studied by analyzing the trajectories generated by the ab initio Car-Parrinello molecular dynamics simulation. The radial distribution functions and mean square displacements are calculated and further discussions are made to explain and probe the structural changes relating to the heat transfer phenomenon. Furthermore, the vibrational density of states of the two layers (Si/Ge) are computed and plotted to analyze the contributions of phonons with different frequencies to the heat conduction. Coherent heat conduction of the low frequency phonons is found and their contributions to facilitate heat transfer are confirmed. The Car-Parrinello molecular dynamics simulation outputs in the work show reasonable thermophysical results of the thermal energy transport process and shed light on the potential applications of treating the heat transfer in the superlattices of semiconductor materials from a quantum mechanical molecular dynamics simulation perspective

  20. Structural, dynamic, and vibrational properties during heat transfer in Si/Ge superlattices: A Car-Parrinello molecular dynamics study

    Science.gov (United States)

    Ji, Pengfei; Zhang, Yuwen; Yang, Mo

    2013-12-01

    The structural, dynamic, and vibrational properties during heat transfer process in Si/Ge superlattices are studied by analyzing the trajectories generated by the ab initio Car-Parrinello molecular dynamics simulation. The radial distribution functions and mean square displacements are calculated and further discussions are made to explain and probe the structural changes relating to the heat transfer phenomenon. Furthermore, the vibrational density of states of the two layers (Si/Ge) are computed and plotted to analyze the contributions of phonons with different frequencies to the heat conduction. Coherent heat conduction of the low frequency phonons is found and their contributions to facilitate heat transfer are confirmed. The Car-Parrinello molecular dynamics simulation outputs in the work show reasonable thermophysical results of the thermal energy transport process and shed light on the potential applications of treating the heat transfer in the superlattices of semiconductor materials from a quantum mechanical molecular dynamics simulation perspective.