WorldWideScience

Sample records for superlattice reflections deformation

  1. Influence of tension-twisting deformations and defects on optical and electrical properties of B, N doped carbon nanotube superlattices

    Science.gov (United States)

    Guili, Liu; Yan, Jiang; Yuanyuan, Song; Shuang, Zhou; Tianshuang, Wang

    2016-06-01

    As the era of nanoelectronics is dawning, CNT (carbon nanotube), a one-dimensional nano material with outstanding properties and performances, has aroused wide attention. In order to study its optical and electrical properties, this paper has researched the influence of tension-twisting deformation, defects, and mixed type on the electronic structure and optical properties of the armchair carbon nanotube superlattices doped cyclic alternately with B and N by using the first-principle method. Our findings show that if tension-twisting deformation is conducted, then the geometric structure, bond length, binding energy, band gap and optical properties of B, N doped carbon nanotube superlattices with defects and mixed type will be influenced. As the degree of exerted tension-twisting deformation increases, B, N doped carbon nanotube superlattices become less stable, and B, N doped carbon nanotube superlattices with defects are more stable than that with exerted tension-twisting deformations. Proper tension-twisting deformation can adjust the energy gap of the system; defects can only reduce the energy gap, enhancing the system metallicity; while the mixed type of 5% tension, twisting angle of 15° and atomic defects will significantly increase the energy gap of the system. From the perspective of optical properties, doped carbon nanotubes may transform the system from metallicity into semi-conductivity. Project supported by the National Natural Science Foundation of China (No. 51371049) and the Natural Science Foundation of Liaoning Province (No. 20102173).

  2. Interface Properties of InAs/AlSb Superlattices Characterized by Grazing Incidence X-Ray Reflectivity

    Institute of Scientific and Technical Information of China (English)

    LI Zhi-Hua; GUO Li-Wei; WU Shu-Dong; WANG Wen-Xin; HUANG Qi; ZHOU Jun-Ming

    2005-01-01

    @@ Two kinds of superlattice interfaces of InAs/AlSb superlattices are realized in an optimized interface growth process, where one is AlAs-like and the other is InSb-like grown on a relaxed AlSb buffer layer. The superlattice properties such as interface roughness and layer thickness are studied by grazing incidence x-ray reflectivity. The reflectivity curves are simulated by standard software till the simulation curves match well with the experimental curves. The simulation indicates that AlAs-like interfaces are much rougher than InSb-like interfaces. Grazing incidence x-ray reflectivity is also discussed as a powerful tool to assessing the structure properties of superlattices.

  3. Survey of Reflection-Asymmetric Nuclear Deformations

    Science.gov (United States)

    Olsen, Erik; Cao, Yuchen; Nazarewicz, Witold; Schunck, Nicolas

    2016-09-01

    Due to spontaneous symmetry breaking it is possible for a nucleus to have a deformed shape in its ground state. It is theorized that atoms whose nuclei have reflection-asymmetric or pear-like deformations could have non-zero electric dipole moments (EDMs). Such a trait would be evidence of CP-violation, a feature that goes beyond the Standard Model of Physics. It is the purpose of this project to predict which nuclei exhibit a reflection-asymmetric deformation and which of those would be the best candidates for an EDM measuring experiment. Using nuclear Density Functional Theory along with the new computer code AxialHFB and massively parallel computing we calculated ground state nuclear properties for thousands of even-even nuclei across the nuclear chart: from light to superheavy and from stable to short-lived systems. Six different Energy Density Functionals (EDFs) were used to assess systematic errors in our calculations. These results are to be added to the website Massexplorer (http://massexplorer.frib.msu.edu/) which contains results from earlier mass table calculations and information on single quasiparticle energies.

  4. Deformable Mirrors Capture Exoplanet Data, Reflect Lasers

    Science.gov (United States)

    2014-01-01

    To image and characterize exoplanets, Goddard Space Flight Center turned to deformable mirrors (DMs). Berkeley, California-based Iris AO, Inc. worked with Goddard through the SBIR program to improve the company’s microelectromechanical DMs, which are now being evaluated and used for biological research, industrial applications, and could even be used by drug manufacturers.

  5. The reflection and interference of electrons at the interface of superlattice

    Institute of Scientific and Technical Information of China (English)

    CHENG; Xingkui

    2002-01-01

    [1]Mukherji, D., Nag, B. R., Band structure of semiconductor superlattice, Phys. Rev., 1975,B12: 4338-4345.[2]Bastard, G., Superlattice band structure in the envelop-function approximation, Phys. Rev., 1981,B24: 5693-5697.[3]Ninno, D., Wong, K. B., Geh, M. A. et al., Optical transitions at confined resonance in(001)GaAs-Ga1-xAlxAs superlattice, Phys. Rev., 1985, B32: 2700-2702.[4]Cho, H. S., Prucnal, P. R., New formalism of the Kronig-Penney model with application to superlattice, Phys.Rev., 1987, B36: 3237-3242.[5]Adachi, S., GaAs, AlAs, and AlxGa1-xAs: Material parameters for use in research and device application, J. Appl. Phys., 1985, 58(3): R1-R29.[6]Levine, B. F., Bethea,C.G., Shen,V.O. et al., Tunable long-wavelength detectors using graded barrier quantum wells grown by electron beam source molecular beam epitaxy, Appl. Phys. Lett., 1990, 57(4): 383-385.

  6. X-ray reflectivity and atomic force microscopy studies of MOCVD grown AlxGa1-xN/GaN superlattice structures*

    Institute of Scientific and Technical Information of China (English)

    Wang Yuanzhang; Li Jinchai; Li Shuping; Chen Hangyang; Liu Dayi; Kang Junyong

    2011-01-01

    The grazing incidence X-ray reflectivity (GIXR) technique and atomic force microscopy (AFM) were exploited to obtain an accurate evaluation of the surfaces and interfaces for metalorganic chemical vapor deposition grown AlxGa1-xN/GaN superlattice structures. The X-ray diffraction results have been combined with reflectivity data to evaluate the layer thickness and Al mole fraction in the AlGaN layer. The presence ora smooth interface is responsible for the observation of intensity oscillation in GIXR, which is well correlated to step flow observation in AFM images of the surface. The structure with a low Al mole fraction (x = 0.25) and thin well width has a rather smooth surface for the Rrms of AFM data value is 0.45 nm.

  7. Propagation, reflection, and transmission of SH-waves in slightly compressible, finitely deformed elastic media

    Institute of Scientific and Technical Information of China (English)

    M. CHATTERJEE; A. CHATTOPADHYAY

    2015-01-01

    The propagation, reflection, and transmission of SH waves in slightly com-pressible, finitely deformed elastic media are considered in this paper. The dispersion relation for SH-wave propagation in slightly compressible, finitely deformed layer over-lying a slightly compressible, finitely deformed half-space is derived. The present paper also deals with the reflection and refraction (transmission) phenomena due to the SH wave incident at the plane interface between two distinct slightly compressible, finitely deformed elastic media. The closed form expressions for the amplitude ratios of reflection and refraction coefficients of the reflected and refracted SH waves are obtained from suit-able boundary conditions. For the numerical discussions, we consider the Neo-Hookean form of a strain energy function. The phase speed curves, the variations of reflection, and transmission coefficients with the angle of incidence, and the plots of the slowness sections are presented by means of graphs.

  8. Polyadic Cantor superlattices with variable lacunarity.

    Science.gov (United States)

    Jaggard, D L; Jaggard, A D

    1997-02-01

    Reflection and transmission properties of polyadic fractal superlattices are formulated, solved analytically, and characterized for variations in stage of growth, fractal dimension, and lacunarity. This is the first time to our knowledge that the effect of lacunarity on wave interactions with such structures has been considered. The results are summarized by families of reflection data that we denote twist plots. A new doubly recursive computational technique efficiently provides the reflection and transmission coefficients for a large class of Cantor superlattices with numerous interfaces.

  9. Propagation characteristics of annular laser beams passing through the reflection Bragg grating with deformation

    Science.gov (United States)

    Yin, Suqin; Zhang, Bin; Dan, Youquan

    2011-06-01

    When high-power annular laser beams produced by the unstable resonator pass through the volume Bragg grating (VBG), absorption of light in the VBG will induce a temperature increment, resulting in changes in surface distortion. Considering that the surface distortion of the grating induces index and period differences, the scalar wave equations for the annular laser beams propagating in the VBG have been solved numerically and iteratively using finite-difference and sparse matrix methods. The variation in intensity distributions, the total power reflection coefficient, and the power in the bucket (PIB) for the annular laser beams passing through the reflection VBG with deformation have been analyzed quantitatively. It can be shown that the surface distortion of the VBG and the beam orders of the annular beams affect evidently the intensity distributions, the power reflection coefficient, and the PIB of the output beam. The peak intensity decreases as the deformation of the VBG increases. The total power reflection efficiency decreases significantly with the increase in deformations of the VBG. The PIB of the output beam decreases as the obscuration ratio β and the deformation of the VBG increase. For the given obscuration ratio β, the influence of deformation of reflection VBG on the PIB of the annular beams is more sensitive with increase in distortion of the VBG and decrease in beam order.

  10. Reflection-asymmetric nuclear deformations within the Density Functional Theory

    CERN Document Server

    Olsen, E; Nazarewicz, W; Stoitsov, M; 10.1088/1742-6596/402/1/012034

    2013-01-01

    Within the nuclear density functional theory (DFT) we study the effect of reflection-asymmetric shapes on ground-state binding energies and binding energy differences. To this end, we developed the new DFT solver AxialHFB that uses an approximate second-order gradient to solve the Hartree-Fock-Bogoliubov equations of superconducting DFT with the quasi-local Skyrme energy density functionals. Illustrative calculations are carried out for even-even isotopes of radium and thorium.

  11. Reflection-Style Optical Antenna Deformations in Free-Space Laser Communications

    Directory of Open Access Journals (Sweden)

    Wanqing Xie

    2011-01-01

    Full Text Available Diversified deformations on reflection-style optical antennas caused by processing technique and complex outer-space environment in free-space laser communications are modeled by wavelet analysis. Influence due to the deformations on received power is studied by distortion attenuation factor based on the model. Theoretical and numerical analysis shows that distortion attenuation factor increases with shift factor of the model, decreases with dilation factor of the model, and decreases with irregular fluctuations with wavelet coefficient of the model. To the special case that the deformation can be well approximated to a constant, distortion attenuation factor varies periodically with the linear increase of the deformation coefficient. A reference for the requirement on the precision of optical antenna is proposed, and a method to reduce the effect of deformation is recommended. It is hoped that the study can be used in the design of free space laser communication systems.

  12. Specular reflection based sensing surface deformation of gas tungsten arc weld pool

    Institute of Scientific and Technical Information of China (English)

    Zhang Shiliang; Gao Jinqiang; Wu Chuansong; Zhang Yuming

    2007-01-01

    A sensing system is developed to measure the weld pool boundary and pool surface deformation in gas tungsten arc welding. LaserStrobe technique is used to eliminate the strong arc light interference, and specular reflection from the pool surface is sensed to describe the relation between the deformed stripes and pool surface depression. Clear images of both the pool boundary and the deformed stripes edges are obtained during gas tungsten arc welding process, which lays foundation for real-time monitoring the pool surface depression and weld penetration.

  13. 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.

  14. Exchange bias in Fe/Cr double superlattices.

    Energy Technology Data Exchange (ETDEWEB)

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

    1999-11-30

    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.

  15. Off-axis phonon and photon propagation in porous silicon superlattices studied by Brillouin spectroscopy and optical reflectance

    Energy Technology Data Exchange (ETDEWEB)

    Parsons, L. C., E-mail: lcparsons@mun.ca; Andrews, G. T., E-mail: tandrews@mun.ca [Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John' s, Newfoundland A1B 3X7 (Canada)

    2014-07-21

    Brillouin light scattering experiments and optical reflectance measurements were performed on a pair of porous silicon-based optical Bragg mirrors which had constituent layer porosity ratios close to unity. For off-axis propagation, the phononic and photonic band structures of the samples were modeled as a series of intersecting linear dispersion curves. Zone-folding was observed for the longitudinal bulk acoustic phonon and the frequency of the probed zone-folded longitudinal phonon was shown to be dependent on the propagation direction as well as the folding order of the mode branch. There was no conclusive evidence of coupling between the transverse and the folded longitudinal modes. Two additional observed Brillouin peaks were attributed to the Rayleigh surface mode and a possible pseudo-surface mode. Both of these modes were dispersive, with the velocity increasing as the wavevector decreased.

  16. Formation of diapiric structure in the deformation zone, central Indian Ocean: A model from gravity and seismic reflection data

    Digital Repository Service at National Institute of Oceanography (India)

    Krishna, K.S.; Rao, D.G.; Neprochnov, Y.P.

    Analyses of bathymetry, gravity and seismic reflection data of the diffusive plate boundary in the central Indian Ocean reveal a new kind of deformed structure besides the well-reported structures of long-wavelength anticlinal basement rises...

  17. Superlattice Optical Bistability Research.

    Science.gov (United States)

    2014-09-26

    multilayer heterojunction and superlattice device applications. 2.0 Growth Studies The MBE growth of mercury compound is still relatively new and novel...These superlattices are grown by molecular beam epitaxy in a MBE system specifically designed to handle mercury . MBE is an ultrahigh vacuum evaporative...therefore the growth process is not as well understood as that of III-V semiconductor - compounds . In HgTe-CdTe superlattices the CdTe deposition is

  18. Resonant x-ray scattering in perovskite manganite superlattice. Observation of 'orbital superlattice'

    CERN Document Server

    Kiyama, T; Ohsumi, H; Murakami, Y; Wakabayashi, Y; Izumi, M; Kawasaki, M; Tokura, Y

    2003-01-01

    We report the results of resonant X-ray scattering (RXS) measurement of superlattices which consist of La sub 0 sub . sub 4 sub 5 Sr sub 0 sub . sub 5 sub 5 MnO sub 3 and La sub 0 sub . sub 6 sub 0 Sr sub 0 sub . sub 4 sub 0 MnO sub 3 multilayers. An interference technique made it possible to observe RXS reflections from ferro-type orbital ordering in the superlattices. RXS can reveal the local circumstances around specific atoms in materials regulated atomically. In this experiment, we observed that the superlattice is actually composed of two kinds of layers with different lattice distortion states, presenting 'orbital superlattices', in which layers with different orbital states are stacked alternately in an atomic scale. (author)

  19. Crustal Structure and Deformation of the Yakutat Microplate: New Insights From STEEP Marine Seismic Reflection Data

    Science.gov (United States)

    Lowe, L. A.; Gulick, S. P.; Christeson, G.; van Avendonk, H.; Reece, R.; Elmore, R.; Pavlis, T.

    2008-12-01

    In fall 2008, we will conduct an active source marine seismic experiment of the offshore Yakutat microplate in the northern Gulf of Alaska. The survey will be conducted aboard the academic research vessel, R/V Marcus Langseth, collecting deep-penetrating multi-channel seismic reflection survey using an 8-km, 640 channel hydrophone streamer and a 6600 cu. in., 36 airgun array. The survey is the concluding data acquisition phase for the ST. Elias Erosion and tectonics Project (STEEP), a multi-institution NSF-Continental Dynamics project investigating the interplay of climate and tectonics in the Chugach-St. Elias Mountains in southern Alaska. The experiment will also provide important site survey information for possible future Integrated Ocean Drilling Program investigations. Two profiles coincident with wide-angle refraction data (see Christeson, et al., this session) will image structural changes across the Dangerous River Zone from east to west and the Transition Fault from south to north. We will also image the western portion of the Transition Fault to determine the nature of faulting along this boundary including whether or not the Pacific Plate is underthrusting beneath the Yakutat microplate as part of this collision. Our westernmost profile will image the Kayak Island Zone, typically described as the northern extension of the Aleutian megathrust but which may be a forming suture acting as a deformation backstop for the converging Yakutat and North American plates. Profiles across the Pamplona Zone, the current Yakutat-North America deformation front, will further constrain relative timing of structural development and the depth of deformation on the broad folds and thrust faults that comprise the area. This new dataset will allow further insight into regional tectonics of the St. Elias region as well as provide more detail regarding the development of the south Alaskan margin during major Plio-Pleistocene glacial- interglacial periods.

  20. 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.

  1. Dynamic square superlattice of Faraday waves

    Science.gov (United States)

    Kahouadji, Lyes; Chergui, Jalel; Juric, Damir; Shin, Seungwon; Tuckerman, Laurette

    2014-11-01

    Faraday waves are computed in a 3D container using BLUE, a code based on a hybrid Front-Tracking/Level-set algorithm for Lagrangian tracking of arbitrarily deformable phase interfaces. A new dynamic superlattice pattern is described which consists of a set of square waves arranged in a two-by-two array. The corners of this array are connected by a bridge whose position oscillates in time between the two diagonals.

  2. A workflow for sub-/seismic structure and deformation quantification of 3-D reflection seismic data sets across different scales

    Energy Technology Data Exchange (ETDEWEB)

    Krawczyk, C.M.; Lohr, T.; Oncken, O. [GFZ Potsdam (Germany); Tanner, D.C. [Goettingen Univ. (Germany). GZG; Endres, H. [RWTH Aachen (Germany)]|[TEEC, Isernhagen (Germany); Trappe, H.; Kukla, P. [TEEC, Isernhagen (Germany)

    2007-09-13

    The evolution of a sedimentary basin is mostly affected by deformation. Large-scale, subsurface deformation is typically identified by seismic data, sub-seismic small-scale fractures by well data. Between these two methods, we lack a deeper understanding of how deformation scales. We analysed a 3-D reflection seismic data set in the North German Basin, in order to determine the magnitude and distribution of deformation and its accumulation in space and time. A five-step approach is introduced for quantitative deformation and fracture prediction. An increased resolution of subtle tectonic lineaments is achieved by coherency processing, allowing to unravel the kinematics in the North German Basin from structural interpretation. Extensional events during basin initiation and later inversion are evident. 3-D retrodeformation shows major-strain magnitudes between 0-20% up to 1.3 km away from a fault trace, and variable deviations of associated extensional fractures. Good correlation of FMI data, strain distribution from retro-deformation and from geostatistic tools (see also Trappe et al., this volume) allows the validation of the results and makes the prediction of small-scale faults/fractures possible. The temporal component will be gained in the future by analogue models. The suggested workflow is applicable to reflection seismic surveys and yields in great detail both the tectonic history of a region as well as predictions for hydrocarbon plays or deep groundwater or geothermal reservoirs. (orig.)

  3. Soft-sediment deformations (convolute lamination and load structures) in turbidites as indicators of flow reflections against bounding slopes

    Science.gov (United States)

    Tinterri, Roberto; Muzzi Magalhaes, Pierre; Tagliaferri, Alessio; Cunha, Rogerio S.; Laporta, Michele

    2015-04-01

    turbidites containing these deformative structures show that they are genetically linked to contained-reflected beds in structurally-confined basins, suggesting a trigger mechanism associated with the cyclic-wave loading produced by flow impacts or reflected bores and internal waves related to ponded turbidity currents. The data that can demonstrate this hypothesis come from the foredeep turbidites of the Marnoso-arenacea Formation (northern Italy) and Annot Sandstones (southwestern France), where a basin scale high-resolution stratigraphic framework with bed-by-bed correlations is now available. These data show that the lateral and vertical distribution of convolute laminae and load structures is not random but has an evident depositional logic related to reflection processes against bounding slopes. Therefore, the main objectives of this work are: 1) to show that convolute laminae and load structures are strictly associated with other sedimentary structures that are unequivocally related to reflection and rebound processes of turbidity currents against morphological obstacles; 2) to show that their lateral and vertical distribution increases concomitantly with the number of contained-reflected beds in the proximity of structurally-controlled morphological highs; 3) to show that the increase in contained-reflected beds with convolute laminae is strictly related to the increase in the synsedimentary-structural uplifts producing more pronounced morphologic highs; 4) to discuss the processes that link soft-sediment deformations with cyclic-wave loading related to internal waves and bores produced by reflection processes.

  4. More on analyzing the reflection of a laser beam by a deformed highly reflective volume Bragg grating using iteration of the beam propagation method.

    Science.gov (United States)

    Shu, Hong; Mokhov, Sergiy; Zeldovich, Boris Ya; Bass, Michael

    2009-01-01

    A further extension of the iteration method for beam propagation calculation is presented that can be applied for volume Bragg gratings (VBGs) with extremely large grating strength. A reformulation of the beam propagation formulation is presented for analyzing the reflection of a laser beam by a deformed VBG. These methods will be shown to be very accurate and efficient. A VBG with generic z-dependent distortion has been analyzed using these methods.

  5. The mechanisms and temperature dependence of superlattice stacking fault formation in the single-crystal superalloy PWA 1480

    Science.gov (United States)

    Milligan, Walter W.; Antolovich, Stephen D.

    1991-01-01

    The mechanism of the formation of superlattice staking faults in the single-crystal nickel-base superalloy PWA 1480 was investigated by observing deformation microstructures in the superalloy single crystals in the temperature range 20-1100 C. Results showed that, in addition to superlattice stacking faults observed after slow strain rate deformation at temperatures from 700 to 950 C, a high-density of superlattice staking faults formed after deformation at 200 C and below. The mechanisms of fault formation, which are different in the high- and the low-temperature regimes, are discussed.

  6. 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....

  7. 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...

  8. A seismic reflection and GLORIA study of compressional deformation in the Gorringe Bank region, eastern North Atlantic

    Science.gov (United States)

    Hayward, N.; Watts, A. B.; Westbrook, G. K.; Collier, J. S.

    1999-09-01

    Seismic reflection and GLORIA side-scan sonar data obtained on RRS Charles Darwin cruise CD64 reveal new information on the styles of deformation in the Gorringe Bank region, at the eastern end of the Azores-Gibraltar plate boundary. Previous studies suggest that Gorringe Bank was formed by the overthrusting of a portion of the African plate upon the Eurasian plate. The new seismic data show, however, that the most intensely deformed region is located south of Gorringe Bank, on the northern flanks of a NW-SE-trending submarine ridge which includes the Ampere and Coral Patch seamounts. The deformation is expressed as long-wavelength (up to 60 km), large-amplitude (up to 800 m) folds in the sediments and underlying acoustic basement, which in places are associated with one or more reverse faults, and as a fabric of short-wavelength folds (up to 3 km) with a NE trend. In contrast, the same sedimentary units when traced beneath the flanking plains are undeformed, except for some faults with a small throw (~30 m), some of which offset the seafloor. GLORIA data show that recent deformation is broadly distributed over the region. Structural trends rotate from 45 deg in the west to 70 deg in the east of the region, nearly perpendicular to the NW-verging plate motion vectors as determined from plate kinematic models. Flexure modelling suggests that a portion of Gorringe Bank has loaded 152 Ma oceanic lithosphere and that a maximum of 50 km of shortening has occurred at Gorringe Bank since the mid-Miocene. Our observations support a model in which there is no single plate boundary in the region, rather that the deformation is distributed over a 200-330 km wide zone.

  9. Do crustal deformations observed by GPS in Tierra del Fuego (Argentina reflect glacial-isostatic adjustment?

    Directory of Open Access Journals (Sweden)

    L. Mendoza

    2010-09-01

    Full Text Available Vertical site velocities determined by geodetic GPS observations in the Lago Fagnano area, Tierra del Fuego main island, are interpreted with respect to their potential relation with the glacial-isostatic crustal response to ice mass changes. The spatial pattern of the uplift rates, in combination with the horizontal crustal deformation pattern, point towards a fault-tectonic rather than glacial-isostatic origin of the determined vertical crustal deformations. This implies rather small GIA effects pointing towards relatively small Holocene ice-mass changes in Tierra del Fuego. However, these findings are considered to be preliminary. They should be confirmed by additional observations covering an extended area with GPS sites.

  10. Magnetic Graphene Nanohole Superlattices

    CERN Document Server

    Yu, Decai; Liu, Miao; Liu, Wei; Liu, Feng

    2008-01-01

    We investigate the magnetic properties of nano-holes (NHs) patterned in graphene using first principles calculations. We show that superlattices consisting of a periodic array of NHs form a new family of 2D crystalline "bulk" magnets whose collective magnetic behavior is governed by inter-NH spin-spin interaction. They exhibit long-range magnetic order well above room temperature. Furthermore, magnetic semiconductors can be made by doping magnetic NHs into semiconducting NH superlattices. Our findings offer a new material system for fundamental studies of spin-spin interaction and magnetic ordering in low dimensions, and open up the exciting opportunities of making engineered magnetic materials for storage media and spintronics applications.

  11. 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...

  12. Highly Reflecting, Broadband Deformable Membrane Mirror for Wavefront Control Applications Project

    Data.gov (United States)

    National Aeronautics and Space Administration — This Phase I STTR project will develop a highly reflecting, broadband, radiation resistant, low-stress and lightweight, membrane integrated into an electrostatically...

  13. Crustal deformation styles along the reprocessed deep seismic reflection transect of the Central Iberian Zone (Iberian Peninsula)

    Science.gov (United States)

    Ehsan, Siddique Akhtar; Carbonell, Ramon; Ayarza, Puy; Martí, David; Pérez-Estaún, Andrés; Martínez-Poyatos, David Jesús; Simancas, Jose Fernando; Azor, Antonio; Mansilla, Luis

    2014-05-01

    The multichannel normal incidence (230 km long) deep seismic reflection profile ALCUDIA was acquired in summer 2007. This transect samples an intracontinental Variscan orogenic crust going across, from north to south, the major crustal domain (the Central Iberian Zone) and its suture zone with the Ossa-Morena Zone (the Central Unit) both build up most of the southwestern part of the Iberian Peninsula basement. This high resolution (60-90 fold) profile images about 70 km depth (20 s TWTT) of the continental lithosphere. A new data processing flow provides better structural constraints on the shallow and deep structures resulting in an image that reveals indentation features which strongly suggest horizontal tectonics. The ALCUDIA seismic image shows an upper crust c. 13 km thick decoupled from the comparatively reflective lower crust. The shallow reflectivity of the upper crust can be correlated with surface geological features mapped in the field whereas the deep reflectivity represents inferred imbricate thrust systems and listric extensional faults. The reflectivity of the mid-lower crust is continuous, high amplitude, and horizontal to arcuate though evidences of deformation are present as ductile boudinage structures, thrusting and an upper mantle wedge, suggesting a transpressional flower structure. The image reveals a laminated c. 1.5 km thick, subhorizontal to flat Moho indicating an average crustal thickness of 31-33 km. The Moho shows laterally variable signature, being highly reflective beneath the Central Iberian Zone, but discontinuous and diffuse below the Ossa-Morena Zone. The gravity response suggests relatively high density bodies in the mid-lower crust of the southern half of the transect. The seismic results suggest two major horizontal limits, a horizontal discontinuity at c. 13-15 km (corresponding to the brittle-ductile transition) and the Moho boundary both suggested to act as decoupling surfaces.

  14. Interlayer diffusion studies of a Laves phase exchange spring superlattice.

    Science.gov (United States)

    Wang, C; Kohn, A; Wang, S G; Ward, R C C

    2011-03-23

    Rare earth Laves phase (RFe(2)) superlattice structures grown at different temperatures are studied using x-ray reflectivity (XRR), x-ray diffraction, and transmission electron microscopy. The optimized molecular beam epitaxy growth condition is matched with the XRR simulation, showing minimum diffusion/roughness at the interfaces. Electron microscopy characterization reveals that the epitaxial growth develops from initial 3D islands to a high quality superlattice structure. Under this optimum growth condition, chemical analysis by electron energy loss spectroscopy with high spatial resolution is used to study the interface. The analysis shows that the interface roughness is between 0.6 and 0.8 nm and there is no significant interlayer diffusion. The locally sharp interface found in this work explains the success of simple structural models in predicting the magnetic reversal behavior of Laves exchange spring superlattices.

  15. Jaynes Cummings Photonic Superlattices

    CERN Document Server

    Longhi, Stefano

    2011-01-01

    A classical realization of the Jaynes-Cummings (JC) model, describing the interaction of a two-level atom with a quantized cavity mode, is proposed based on light transport in engineered waveguide superlattices. The optical setting enables to visualize in Fock space dynamical regimes not yet accessible in quantum systems, providing new physical insights into the deep strong coupling regime of the JC model. In particular, bouncing of photon number wave packets in Hilbert space and revivals of populations are explained as generalized Bloch oscillations in an inhomogeneous tight-binding lattice.

  16. 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...

  17. Aging in Magnetic Superlattices

    Science.gov (United States)

    Mukherjee, Tathagata; Pleimling, Michel; Binek, Christian

    2010-03-01

    Aging phenomena can be observed in non-equilibrium systems with slow relaxation dynamics. Magnetic specimens with well defined interactions and dimensions can serve as model systems for universal aspects of aging. Magnetic thin films provide access to a wide range of microscopic parameters. Superlattice structures allow tuning the intra and inter-plane exchange and enable geometrical confinement of the spin fluctuations. We use Co/Cr thin film superlattices to study magnetic aging. The static and dynamic magnetic properties are affected via the Co and Cr film thicknesses. The Curie temperature of the Co films is reduced from the bulk value by geometrical confinement. Cr provides antiferromagnetic coupling between the Co films. In-plane magnetic set fields of some 10-100 mT are applied and the sample is exposed to the latter for various waiting times. After removing the field, relaxation of the magnetization is recorded via longitudinal Kerr-magnetometry and SQUID. The relaxation data are analyzed by scaling plots revealing universal aspects of aging. Financial support by NRI, and NSF through EPSCoR, Career DMR-0547887, DMR-0904999, and MRSEC.

  18. Acoustoelectric effect in semiconductor superlattice

    Science.gov (United States)

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

    1993-10-01

    Acoustoelectric effect in semiconductor superlattice has been studied for acoustic wave whose wavelength lambda = 2pi/q is smaller than the mean free path of the electrons l (where ql approaches 1). Unlike the homogeneous bulk material where Weinreich relation is independent of the wave number q in the superlattice we observe a dependence on q i.e. spatial dispersion. In the presence of applied constant field E a threshold value was obtained where the acoustoelectric current changes direction.

  19. Supersolid Phase in One-Dimensional Hard-Core Boson Hubbard Model with a Superlattice Potential

    Institute of Scientific and Technical Information of China (English)

    GUO Huai-Ming; LIANG Ying

    2008-01-01

    The ground state of the one-dimensional hard-core boson Hubbard model with a superlattice potential is studied by quantum Monte Carlo methods. We demonstrate that besides the CDW phase and the Mort insulator phase, the supersolid phase emerges due to the presence of the superlattice potential, which reflects the competition with the hopping term. We also study the densities of sublattices and have a clear idea about the distribution of the bosons on the lattice.

  20. Study on band gap structure of Fibonacci quantum superlattices by using the transfer matrix method

    Science.gov (United States)

    Ferrando, V.; Castro-Palacio, J. C.; Marí, B.; Monsoriu, J. A.

    2014-02-01

    The scattering properties of particles in a one-dimensional Fibonacci sequence based potential have been analyzed by means of the Transfer Matrix Method. The electronic band gaps are examined comparatively with those obtained using the corresponding periodic potentials. The reflection coefficient shows self-similar properties for the Fibonacci superlattices. Moreover, by using the generalized Bragg's condition, the band gaps positions are derived from the golden mean involved in the design of the superlattice structure.

  1. Phoxonic Hybrid Superlattice.

    Science.gov (United States)

    Alonso-Redondo, Elena; Huesmann, Hannah; El Boudouti, El-Houssaine; Tremel, Wolfgang; Djafari-Rouhani, Bahram; Butt, Hans-Juergen; Fytas, George

    2015-06-17

    We studied experimentally and theoretically the direction-dependent elastic and electromagnetic wave propagation in a supported film of hybrid PMMA (poly[methyl-methacrylate])-TiO2 superlattice (SL). In the direction normal to the layers, this one-dimensional periodic structure opens propagation band gaps for both hypersonic (GHz) phonons and near-UV photons. The high mismatch of elastic and optical impedance results in a large dual phoxonic band gap. The presence of defects inherent to the spin-coating fabrication technique is sensitively manifested in the band gap region. Utilizing Brillouin light scattering, phonon propagation along the layers was observed to be distinctly different from propagation normal to them and can, under certain conditions (SL thickness and substrate elasticity), reveal the nanomechanical properties of the constituent layers. Besides the first realization of unidirectional phoxonic behavior, hybrid (soft-hard) periodic materials are a promising simple platform for opto-acoustic interactions and applications such as filters and Bragg mirrors.

  2. Superconducting superlattices 2: Native and artificial

    Energy Technology Data Exchange (ETDEWEB)

    Bozovic, I.; Pavuna, D. [eds.

    1998-12-31

    This volume is composed of 26 papers presented at the symposium. Topics covered include the following: high-{Tc} superlattices: intrinsic and artificial; low-{Tc} superlattices and multilayers; and theory.

  3. Evolving random fractal Cantor superlattices for the infrared using a genetic algorithm.

    Science.gov (United States)

    Bossard, Jeremy A; Lin, Lan; Werner, Douglas H

    2016-01-01

    Ordered and chaotic superlattices have been identified in Nature that give rise to a variety of colours reflected by the skin of various organisms. In particular, organisms such as silvery fish possess superlattices that reflect a broad range of light from the visible to the UV. Such superlattices have previously been identified as 'chaotic', but we propose that apparent 'chaotic' natural structures, which have been previously modelled as completely random structures, should have an underlying fractal geometry. Fractal geometry, often described as the geometry of Nature, can be used to mimic structures found in Nature, but deterministic fractals produce structures that are too 'perfect' to appear natural. Introducing variability into fractals produces structures that appear more natural. We suggest that the 'chaotic' (purely random) superlattices identified in Nature are more accurately modelled by multi-generator fractals. Furthermore, we introduce fractal random Cantor bars as a candidate for generating both ordered and 'chaotic' superlattices, such as the ones found in silvery fish. A genetic algorithm is used to evolve optimal fractal random Cantor bars with multiple generators targeting several desired optical functions in the mid-infrared and the near-infrared. We present optimized superlattices demonstrating broadband reflection as well as single and multiple pass bands in the near-infrared regime.

  4. Layered magnets: polarized neutron reflection studies

    Energy Technology Data Exchange (ETDEWEB)

    Zabel, H.; Schreyer, A. [Ruhr-Univ. Bochum, Lehrstuhl fuer Experimentalphysik/Festkoerperphysik, Bochum (Germany)

    1996-11-01

    Neutron reflectivity measurements from extended surfaces, thin films and superlattices provide information on the chemical profile parallel to the film normal, including film thicknesses, average composition and interfacial roughness parameters. Reflectivity measurements with polarized neutrons are particularly powerful for analyzing the magnetic density profiles in thin films and superlattices in addition to chemical profiles. The basic theory of polarized neutron reflectivity is provided, followed by some examples and more recent applications concerning polarized neutron reflectivity studies from exchange coupled Fe/Cr superlattices. (author) 5 figs., 13 refs.

  5. Formation of diapiric structure in the deformation zone, central Indian Ocean: A model from gravity and seismic reflection data

    Indian Academy of Sciences (India)

    K S Krishna; D Gopala Rao; Yu P Neprochnov

    2002-03-01

    Analyses of bathymetry, gravity and seismic reflection data of the diffusive plate boundary in the central Indian Ocean reveal a new kind of deformed structure besides the well-reported structures of long-wavelength anticlinal basement rises and high-angle reverse faults. The structure (basement trough) has a length of about 150 km and deepens by up to 1 km from its regional trend (northward dipping). The basement trough includes a rise at its center with a height of about 1.5 km. The rise is about 10 km wide with rounded upper surface and bounded by vertical faults. A broad free-air gravity low of about 20 mGal and a local high of 8 mGal in its center are associated with the identified basement trough and rise structure respectively. Seismic results reveal that the horizontal crustal compression prevailing in the diffusive plate boundary might have formed the basement trough possibly in early Pliocene time. Differential loading stresses have been generated from unequal crust/sediment thickness on lower crustal and upper mantle rocks. A thin semi-ductile serpentinite layer existing near the base of the crust that is interpreted to have been formed at mid-ocean ridge and become part of the lithosphere, may have responded to the downward loading stresses generated by the sediments and crustal rocks to inject the serpentinites into the overlying strata to form a classic diapiric structure.

  6. Thermodynamics of Co/Cr superlattices

    Science.gov (United States)

    Mukherjee, T.; Sahoo, S.; Skomski, R.; Sellmyer, D. J.; Binek, Ch.

    2008-03-01

    Progress in ultra thin film growth has resulted in many novel surface and interface induced properties of artificial heterostuctures. Here, we study magnetic superlattices of ultrathin Co and Cr films grown by Molecular Beam Epitaxy methodology at a base pressure below 1x10-10 mbar. Our approach is based on controlling two distinct magnetic degrees of freedom. First, the critical temperature, Tc, of individual Co films is tailored via geometrical confinement of the correlation length perpendicular to the film. Various thickness dependent values, Tc(d), between zero and the bulk Curie temperature of 1388 K are realized. Second, the Tc-tailored Co films are antiferromagnetically coupled through Cr interlayer films. The oscillating coupling strength is tailored via the Cr interlayer thickness. The resulting thermodynamic properties of such Co/Cr superlattices are studied with the help of SQUID magnetometry. Particular emphasis is laid on tailoring magnetic entropy changes in the vicinity of room temperature. X-ray diffraction and X-ray reflectivity are used to correlate structural data with the magnetic properties.

  7. Intermolecular Interactions in Crystalline Theobromine as Reflected in Electron Deformation Density and (13)C NMR Chemical Shift Tensors.

    Science.gov (United States)

    Bouzková, Kateřina; Babinský, Martin; Novosadová, Lucie; Marek, Radek

    2013-06-11

    An understanding of the role of intermolecular interactions in crystal formation is essential to control the generation of diverse crystalline forms which is an important concern for pharmaceutical industry. Very recently, we reported a new approach to interpret the relationships between intermolecular hydrogen bonding, redistribution of electron density in the system, and NMR chemical shifts (Babinský et al. J. Phys. Chem. A, 2013, 117, 497). Here, we employ this approach to characterize a full set of crystal interactions in a sample of anhydrous theobromine as reflected in (13)C NMR chemical shift tensors (CSTs). The important intermolecular contacts are identified by comparing the DFT-calculated NMR CSTs for an isolated theobromine molecule and for clusters composed of several molecules as selected from the available X-ray diffraction data. Furthermore, electron deformation density (EDD) and shielding deformation density (SDD) in the proximity of the nuclei involved in the proposed interactions are calculated and visualized. In addition to the recently reported observations for hydrogen bonding, we focus here particularly on the stacking interactions. Although the principal relations between the EDD and CST for hydrogen bonding (HB) and stacking interactions are similar, the real-space consequences are rather different. Whereas the C-H···X hydrogen bonding influences predominantly and significantly the in-plane principal component of the (13)C CST perpendicular to the HB path and the C═O···H hydrogen bonding modulates both in-plane components of the carbonyl (13)C CST, the stacking modulates the out-of-plane electron density resulting in weak deshielding (2-8 ppm) of both in-plane principal components of the CST and weak shielding (∼ 5 ppm) of the out-of-plane component. The hydrogen-bonding and stacking interactions may add to or subtract from one another to produce total values observed experimentally. On the example of theobromine, we demonstrate

  8. Multiferroicity in Perovskite Manganite Superlattice

    Science.gov (United States)

    Tao, Yong-Mei; Jiang, Xue-Fan; Liu, Jun-Ming

    2016-08-01

    Multiferroic properties of short period perovskite type manganite superlattice ((R1MnO3)n/(R2MnO3)n (n=1,2,3)) are considered within the framework of classical Heisenberg model using Monte Carlo simulation. Our result revealed the interesting behaviors in Mn spins structure in superlattice. Apart from simple plane spin cycloid structure which is shown in all manganites including bulk, film, and superlattice here in low temperature, a non-coplanar spiral spin structure is exhibited in a certain temperature range when n equals 1, 2 or 3. Specific heat, spin-helicity vector, spin correlation function, spin-helicity correlation function, and spin configuration are calculated to confirm this non-coplanar spiral spin structure. These results are associated with the competition among exchange interaction, magnetic anisotropy, and Dzyaloshinskii-Moriya interaction. Supported by the National Natural Science Foundation of China (NSFC) under Grant No. 11447136

  9. 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.

  10. Superlattice Thermoelectric Materials and Devices

    Science.gov (United States)

    Venkatasubramanian, Rama

    2002-03-01

    We have recently demonstrated a significant enhancement in thermoelectric figure-of-merit (ZT) at 300K, of about 2.4 in p-type Bi2Te3/Sb2Te3 superlattices, using the concept of phonon-blocking electron-transmitting superlattice structures [1]. The phonon blocking arises from a complex localization-like behavior for phonons in nano-structured superlattices and the electron transmission is facilitated by optimal choice of band-offsets in these semiconductor hetero-structures. We will also discuss the ZT 1.2 results in n-type Bi2Te3/Bi2Te3-xSex superlattices and our initial understanding on the reasons behind the less-than-dramatic performance of these materials compared to the p-type superlattices. Due to the high ZT of the material, devices potentially offer high coefficient of performance (COP) in solid-state refrigeration. The thin-film devices, resulting from rather simple microelectronic processing, allow high cooling power densities to be achieved for a variety of high-power electronic applications. We have obtained 32K and 40K sub-ambient cooling at 298K and 353K, respectively, in these superlattice micro-thermoelements with potential localized active-cooling power densities approaching 700 W/cm2. In addition to high-performance (in terms of COP) and power densities, these thin-film microdevices are also extremely fast-acting, within 10 microsec and about a factor of 23,000 better than bulk thermoelectric technology. Thus, these are of significance for preventing thermal run-away in high-power electronics. We will present results to demonstrate this concept with infrared imaging of cooling/heating with superlattice micro-devices. We will also discuss outstanding issues such as heat removal from the heat sink towards the full exploitation of this technology. In addition, we will compare the state-of-the-art with other thin-film superlattice materials and device concepts. [1] R. Venkatasubramanian, E. Siivola, T. Colpitts, and B.C. O’Quinn, Thin

  11. 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

  12. Complex band structure and superlattice electronic states

    Science.gov (United States)

    Schulman, J. N.; McGill, T. C.

    1981-04-01

    The complex band structures of the bulk materials which constitute the alternating layer (001) semiconductor-semiconductor superlattice are investigated. The complex bands near the center of the Brillouin zone in the [001] direction are studied in detail. The decay lengths of superlattice states whose energies lie in the bulk band gaps of one of the semiconductors are determined from the dispersion curves of these bands for imaginary k-->. This method is applied using a tight-binding band-structure calculation to two superlattices: the AlAs-GaAs superlattice and the CdTe-HgTe superlattice. The decay lengths of AlAs-GaAs superlattice conduction-band minimum states are found to be substantially shorter than those for the CdTe-HgTe superlattice. These differences in the decay of the states in the two superlattices result in differences in the variation of the conduction-band effective masses with the thickness of the AlAs and CdTe layers. The conduction-band effective masses increase more rapidly with AlAs thickness in the AlAs-GaAs superlattice than with CdTe thickness in the CdTe-HgTe superlattice.

  13. Phonon-induced polariton superlattices

    DEFF Research Database (Denmark)

    de Lima, Jr., M. M.; Poel, Mike van der; Santos, P. V.;

    2006-01-01

    We show that the coherent interaction between microcavity polaritons and externally stimulated acoustic phonons forms a tunable polariton superlattice with a folded energy dispersion determined by the phonon population and wavelength. Under high phonon concentration, the strong confinement of the...... of the optical and excitonic polariton components in the phonon potential creates weakly coupled polariton wires with a virtually flat energy dispersion....

  14. Thermodynamics and Magnetocaloric properties of Fe/Cr Superlattices

    Science.gov (United States)

    Mukherjee, T.; Michalski, S.; Skomski, R.; Sellmyer, D. J.; Binek, Ch.

    2011-03-01

    We explore MC properties of tailored Fe/Cr superlattices involving simple 3d metals. Our multilayers are fabricated by pulsed laser deposition with emphasis on maximizing magnetic entropy changes near room temperature. We use nanostructuring to tailor magnetic interaction and exploit geometrical confinement in order to fit the FM to paramagnetic transition temperature of the FM constituent films. In concert this leads to an optimized global metamagnetic transition maximizing the isothermal entropy change. Thermodynamic and MC properties of such Fe/Cr superlattices are studied with the help of SQUID magnetometry. Entropy changes are deduced via the Maxwell relation in single phase regions and via the Clausis-Clapeyron relations at first order metamagnetic transitions, X-ray diffraction and X-ray reflectivity are used to correlate structural data with the magnetic properties. Financial support by NRI, and NSF through EPSCoR, Career DMR-0547887, and MRSEC Grant No. 0820521.

  15. Analytical Study of Electromagnetic Wave in Superlattice

    Institute of Scientific and Technical Information of China (English)

    LIN Chang; ZHANG Xiu-Lian

    2004-01-01

    The theoretical description of soliton solutions and exact analytical solutions in the sine-Gordon equation is extended to superlattice physics. A family of interesting exact solutions and a new exact analytical solution have been obtained for the electromagnetic wave propagating through a superlattice. In more general cases, the vector potential along the propagating direction obeys the sine-Gordon equation. Some mathematical results of theoretical investigation are given for different cases in superlattices.

  16. Self-Organized Growth of Alloy Superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Chason, E.; Floro, J.A.; Follstaedt, D.M.; Lagally, M.G.; Liu, F.; Tersoff, J.; Venezuela, P.

    1998-10-19

    We predict theoretically and demonstrate experimentally the spontaneous formation of a superlattice during crystal growth. When a strained alloy grows by "step flow", the steps at the surface form periodic bunches. The resulting modulated strain biases the incorporation of the respective alloy components at different steps in the bunch, leading to the formation of a superlattice. X-ray diffraction and electron microscopy for SiGe grown on Si give clear evidence for such spontaneous superlattice formation.

  17. Spin-dependent optical superlattice

    Science.gov (United States)

    Yang, Bing; Dai, Han-Ning; Sun, Hui; Reingruber, Andreas; Yuan, Zhen-Sheng; Pan, Jian-Wei

    2017-07-01

    We propose and implement a lattice scheme for coherently manipulating atomic spins. Using a vector light shift and a superlattice structure, we demonstrate experimentally its capability on addressing spins in double wells and square plaquettes with subwavelength resolution. The quantum coherence of spin manipulations is verified through measuring atom tunneling and spin exchange dynamics. Our experiment presents a building block for engineering many-body quantum states in optical lattices for realizing quantum simulation and computation tasks.

  18. Structure and magnetic properties of thin films and superlattices

    CERN Document Server

    Bentall, M J

    2002-01-01

    Thin layers of rare earth elements and Laves phase superlattices were grown using molecular beam epitaxy. Their structure and magnetic properties have been probed using x-ray and neutron scattering, magnetisation measurements and high resolution electron microscopy. When holmium is grown on yttrium, the x-ray scattering from layers with a thickness below T sub c ' 115 A is characteristic of a pseudomorphic layer with the same in-plane lattice parameter as the yttrium substrate to within 0.05%. For layers above T sub c ' there is a sharp reduction in misfit strain which is probably due to the creation of edge dislocations. When gadolinium is grown on yttrium, no sharp change of strain of the thin layer was observed up to a thickness of 2920 A. This is characteristic of a pseudomorphic layer, and a failure to nucleate dislocations. For the Laves phase superlattices, a study of the x-ray scattering near several Bragg reflections revealed the presence of numerous superlattice peaks, showing that the samples exhib...

  19. Interwell excitons in GaAs superlattices

    DEFF Research Database (Denmark)

    Birkedal, Dan; Sayed, Karim El; Sanders, G.;

    1997-01-01

    The formation of spatially indirect excitons in superlattices with narrow minibands is investigated experimentally. The interwell exciton is similar to the first Wannier-Stark localized exciton of an electrically biased superlattice. However, in the present case the localization is mediated by th...

  20. MgO/Cu2O Superlattices: Growth of Epitaxial Two-Dimensional Nanostructures

    Science.gov (United States)

    Yang, M. J.; Wadekar, P. V.; Hsieh, W. C.; Huang, H. C.; Lin, C. W.; Chou, J. W.; Liao, C. H.; Chang, C. F.; Seo, H. W.; You, S. T.; Tu, L. W.; Lo, I. K.; Ho, N. J.; Yeh, S. W.; Liao, H. H.; Chen, Q. Y.; Chu, W. K.

    2016-12-01

    Alternated stacking of dissimilar layers can produce novel superlattice materials with multiple functionalities. The majority of such work reported in literature on epitaxial superlattices has been on alternating layers with the same space group (SG) and crystal structure (CS), whereas superlattices with the same CS but different SG have not been studied as much. We have grown superlattices with two well-known oxide materials, viz. cuprite (Cu2O, CS = cubic and SG = Pn bar{3} m) and magnesium oxide (MgO, CS = cubic, SG = Fm bar{3} m). An MgO buffer layer grown near 650°C at the film-substrate interface was found to be essential to achieving reasonable long-range atomic order. Grazing-angle x-ray diffraction, x-ray reflectivity, and electron diffraction analyses as well as transmission electron microscopy were used to investigate the interface abruptness, smoothness, and general crystallinity of the individual layers. Interdiffusion between MgO and Cu2O near interfacial regions places a limit of 250°C on the growth temperature for fabrication of superlattices with reasonably sharp interfaces.

  1. Revisiting HOPG superlattices: Structure and conductance properties

    Science.gov (United States)

    Patil, Sumati; Kolekar, Sadhu; Deshpande, Aparna

    2017-04-01

    Superlattices observed on highly oriented pyrolytic graphite (HOPG) have been studied extensively by scanning tunnelling microscopy (STM). The interest in the study of graphite superlattices has seen a resurgence since the discovery of graphene. Single layer graphene, bilayer graphene, and few layer graphene can now be grown on different substrates. The adherence of graphene to various substrates often leads to a periodic out-of-plane modulation and superlattices due to lattice mismatch. In this paper, we report STM imaging and scanning tunnelling spectroscopy (STS) of different kinds of superlattices on HOPG characterized by a variation in lattice periodicities. Our study also shows evidence of the displacement of the topmost HOPG layer by scanning different areas of the same superlattice. A correlation between the lattice periodicity with its conductance properties is derived. The results of this work are important for understanding the origin of the superlattice structure on HOPG. Investigation of such superlattices may open up possible ways to modify two dimensional electron systems to create materials with tailored electronic properties.

  2. Exact Surface States in Photonic Superlattices

    CERN Document Server

    Xie, Qiongtao

    2012-01-01

    We develop an analytical method to derive exact surface states in photonic superlattices. In a kind of infinite bichromatic superlattices satisfying some certain conditions, we analytically obtain their in-gap states, which are superpositions of finite numbers of unstable Bloch waves. By using the unstable in-gap states, we construct exactly several stable surface states in various photonic superlattices. We analytically explore the parametric dependence of these exact surface states. Our analysis provides an exact demonstration for the existence of surface states and would be also helpful to understand surface states in other lattice systems.

  3. 剪切形变对硼氮掺杂碳纳米管超晶格电子结构和光学性能的影响∗%Influences of shear deformation on electronic structure and optical prop erties of B, N dop ed carb on nanotub e sup erlattices

    Institute of Scientific and Technical Information of China (English)

    姜艳; 刘贵立

    2015-01-01

    with B and N under different shear deformations are investigated by the first-principles method through using the CASTEP code in MS 6.0. It is found that the structures of carbon nanotube superlattices can be dramatically changed by shear deformation. When the shear deformation is less than 9%, the optimization geometry structures of carbon nanotube superlattices are still similar to tubular structures, when the shear deformation is greater than 12%, the geometry structures of these systems have large distortions. The results about the binding energy show that the shear deformation changes the stability of the armchair doped carbon nanotube superlattice. The larger the shear deformation, the lower the stability of the doped carbon nanotube superlattices is. The analysis of charge population show that the covalent bond and ionic bond coexist in the armchair carbon nanotube superlattices doped cyclically alternately with B and N. The band gap of the carbon nanotube superlattice is affected by N, B dopants, as a result, the carbon nanotube superlattice changes from a metal to a semiconductor. Compared with the (5, 5) nanotube superlattices, the band gaps of the (7, 7), (9, 9) doped carbon nanotube superlattices increase. With increasing the shear deformation, the band gap of the doped carbon nanotube superlattices decreases gradually, when the shear deformation is greater than 12%, the band gap changes into 0 eV, the carbon nanotube superlattice changes back into a metal from a semiconductor. The analysis of density of states obtains the same conclusions as the energy band analysis. In optical properties, compared with the armchair carbon nanotube superlattices doped cyclically alternately with B and N without shear deformation, those systems under shear deformation have the peaks of the absorption coefficient and the reflectivity that are all reduced, and are all red-shifted.

  4. Onshore-offshore seismic reflection profiling across the southern margin of the Sea of Japan: back-arc opening, shortening and active strike-slip deformation

    Science.gov (United States)

    Sato, Hiroshi; Ishiyama, Tatsuya; Kato, Naoko; Toda, Shigeru; Kawasaki, Shinji; Fujiwara, Akira; Tanaka, Yasuhisa; Abe, Susumu

    2017-04-01

    M7-class crustal earthquakes of overlying plate in subduction system have tendency to increase before megathrust earthquake events. Due to stress buildup by the upcoming Nankai Trough megathrust earthquake, SW Japan has being seismically active for last 20 years. In terms of the mitigation of earthquake and tsunami hazards, to construct seismogenic source fault models is first step for evaluating the strong ground motions and height of tsunamis. Since 2013, we performed intense seismic profiling in and around the southern part of the Sea of Japan. In 2016, a 180-km-long onshore -offshore seismic survey was carried out across the volcanic arc and back-arc basins (from Kurayoshi to the Yamato basin). Onshore section, CMP seismic reflection data were collected using four vibroseis trucks and fixed 1150 channel recorders. Offshore part we acquired the seismic reflection data using 1950 cu inch air-guns towing a 4-km-long streamer cable. We performed CMP reflection and refraction tomography analysis. Obtained seismic section portrays compressively deformed rifted continental crust and undeformed oceanic back-arc basin, reflecting the rheological features. These basic structures were formed during the opening of the Sea of Japan in early Miocene. The sub-horizontal Pliocene sediments unconformably cover the folded Miocene sediments. The opening and clock-wise rotation of SW Japan has been terminated at 15 Ma and contacted to the young Shikoku basin along the Nankai trough. Northward motion of Philippine Sea plate (PHS) and the high thermal regime in the Shikoku basin produced the strong resistance along the Nankai trough. The main shortening deformation observed in the seismic section has been formed this tectonic event. After the initiation of the subduction along the Nankai trough, the rate of shortening deformation was decreased and the folded strata were covered by sub-horizontal Pliocene sediments. The thrusting trending parallel to the arc has been continued from

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

    Energy Technology Data Exchange (ETDEWEB)

    Meng, Fanchao, E-mail: fanchao.meng@mail.mcgill.ca [Department of Mining and Materials Engineering, McGill University, Montréal, QC H3A 0C5 (Canada); Zhang, Shiqi [School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, AZ 85281 (United States); Lee, In-Ho [Korea Research Institute of Standards and Science, 1 Doryong-Dong, Yuseong-Gu, Daejeon 305-600 (Korea, Republic of); Jun, Sukky [Department of Mechanical Engineering, University of Wyoming, Laramie, WY 82071 (United States); Ciobanu, Cristian V., E-mail: cciobanu@mines.edu [Department of Mechanical Engineering, Colorado School of Mines, Golden, CO 80401 (United States)

    2016-07-01

    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.

  6. Nanomechanical characterization of rod-like superlattice assembled from tobacco mosaic viruses

    Science.gov (United States)

    Wang, Haoran; Wang, Xinnan; Li, Tao; Lee, Byeongdu

    2013-01-01

    Tobacco mosaic virus (TMV) and TMV-derived materials have demonstrated their great potential in biomedical applications, where the mechanical properties are determining factors for their proper functionalities and structural integrity. Recently, it has been found that a superlattice structure can be formed by two-dimensional hexagonal packing TMV self-assembly in Barium ions solution. In parallel to the exploration of possible applications of TMV superlattice, the mechanical properties were characterized by the atomic force microscopy based nanoindentation. The elastic modulus of 2.14 GPa was obtained by application of the extended Johnson-Kendall-Roberts (JKR) model with the force vs sample deformation data. The adhesion force was taken into consideration, and an easy-to-implement approach of using the extended JKR model was proposed by processing both the theoretical model and the experimental data. Finite element analysis was conducted to evaluate the reinforcing effect of the like-charge forces between the TMVs and the mechanical properties of the TMV superlattice. Using the Halpin-Tsai model, the transverse elastic modulus of the superlattice sample varied within 2.00-4.38 GPa, depending on the indentation locations. Attraction-repulsion equilibrium was found to maintain the packing of TMVs. This provides useful information to address the sources of the attraction and repulsion forces to control the TMV assembly.

  7. Magnetic structure of holmium-yttrium superlattices

    DEFF Research Database (Denmark)

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

    1993-01-01

    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......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...... 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...

  8. Interwell excitons in GaAs superlattices

    DEFF Research Database (Denmark)

    Birkedal, Dan; Sayed, Karim El; Sanders, G.;

    1996-01-01

    The formation of spatially indirect excitons in superlattices with narrow minibands is theoretically and experimentally investigated. We identify the experimental conditions for the observation of interwell excitons and find a distinct excitonic state energetically located between the Is exciton ...

  9. Analytical Study of Electromagnetic Wave in Superlattice

    Institute of Scientific and Technical Information of China (English)

    LINChang; ZHANGXiu-Lian

    2004-01-01

    The theoretical description of soliton solutions and exact analytical solutions in the sine-Gordon equation is extended to superlattice physics. A family of interesting exact solutions and a new exact analytical solution have been obtained for the electromagnetic wave propagating through a superlattice. In more general cases, the vector potential along the propagating direction obeys the sine-Gordon equation. Some mathematical results of theoretical investigation are given for different cases in supedattices.

  10. HgTe-CdTe SUPERLATTICES

    OpenAIRE

    Smith, D; Mcgill, T.

    1984-01-01

    We report on a theoretical study of the electronic properties of HgTe-CdTe superlattices. The band gap as a function of layer thickness, effective masses normal to the layer plane and tunneling length are compared to the corresponding (Hg, Cd)Te alloys. We find that the superlattice possesses a number of properties that may make it superior to the corresponding alloy as an infrared material.

  11. Deposition of TiN/CrN hard superlattices by reactive d.c. magnetron sputtering

    Indian Academy of Sciences (India)

    Harish C Barshilia; K S Rajam

    2003-02-01

    Multilayer superlattice coatings of TiN/CrN were deposited on silicon substrates using a reactive d.c. magnetron sputtering process. Superlattice period, also known as modulation wavelength (), was controlled by controlling the dwell time of the substrate underneath Ti and Cr targets. X-ray diffraction (XRD), nanoindentation and atomic force microscopy (AFM) were used to characterize the films. The XRD data showed 1st and 2nd order satellite reflections along the principal reflection for films having 132 Å $\\geq \\Lambda \\geq$ 84 Å, thus confirming the formation of superlattice. The multilayer coatings exhibited hardness () as high as 3200 kg/mm2, which is 2 times the rule-of-mixtures value (i.e. $H_{TiN}$ = 2200 kg/mm2 and $H_{CrN}$ = 1000 kg/mm2). Detailed investigations on the effects of various process parameters indicated that hardness of the superlattice coatings was affected not only by modulation wavelength but also by nitrogen partial pressure and ion bombardment during deposition.

  12. THE POST-SEDIMENTARY CHARACTER OF DEVELOPMENT OF INTRAPLATE DISLOCATIONS AS A REFLECTION OF IMPULSIVENESS OF DEFORMATION PROCESSES

    Directory of Open Access Journals (Sweden)

    V. I. Popkov

    2015-09-01

    Full Text Available The available data on well-studied areas of the Turan platform (as an example are reviewed and analyzed to reveal the role of con-sedimentation and post-sedimentation tectonic movements in formation of dislocations of the sedimentary cover. At the background of the long-term (tens and hundreds of million years quiet evolution of the territory under study, short-term intervals are distinguished, which duration amounts to the first millions of years (typically manifested in one or two stratigraphic layers; in such time intervals, tectonic movements were dramatically boosted and accompanied by land uplifting, sea regression, erosion of sediments accumulated earlier and manifestation of deformation processes.The paleotectonic reconstructions show that during such ‘revolutionary’ stages, large tectonic elements occurred along with local uplifts that added to their complexity. In the region under study, the Pre-Jurassic, Pre-Cretaceous (Late Okoma, Pre-Danish and the Pre-Middle Miocene gaps in sedimentation are studied in detail. It is shown that only during the above four periods of sedimentation gaps and accompanying erosion-denudation processes, the regional structures gained from 50 to 80% of their current amplitudes at the bottom of the cover, and the Pre-Danish and Pre-Middle Miocene washout periods were most important.Local uplift also developed impulsively and primarily due to the post-sedimentation movements. Cross-sections of anticlines studied in detail (Figures 1 to 3 are discussed as examples that clearly show the increase of erosional shearing of the sediments accumulated earlier towards domes of uplifts without any con-sedimentation decrease of their thicknesses. During these periods of the geologic history, regardless of their short duration, folded dislocation gained up to 65–90% of their current amplitudes. The periods of activation were separated by long relatively quiescent tectonic periods with the gradually slowing down

  13. Strong reduction of the lattice thermal conductivity in superlattices and quantum dot superlattices

    Science.gov (United States)

    Fomin, V. M.; Nika, D. L.; Cocemasov, A. I.; Isacova, C. I.; Schmidt, O. G.

    2012-06-01

    Thermal transport is theoretically investigated in the planar Si/Ge superlattices and Si/Ge quantum dot superlattices. The phonon states in the considered nanostructures are obtained using the Face-centered Cubic Cell model of lattice dynamics. A significant reduction of the lattice thermal conductivity is revealed in both considered structures in a wide range of temperatures from 100 K to 400 K. This effect is explained by the removal of the high-energy and high-velocity phonon modes from the heat flux due to their localization in superlattice segments and the phonon scattering on the interfaces. The obtained results show prospects of the planar superlattices and quantum-dot superlattices for thermoelectric and thermo-insulating applications.

  14. Ultrasound focusing images in superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Narita, Michiko; Tanaka, Yukihiro; Tamura, Shin-ichiro [Department of Applied Physics, Hokkaido University, Sapporo (Japan)

    2002-03-04

    We study theoretically ultrasound focusing in periodic multilayered structures, or superlattices, by solving the wave equation with the Green function method and calculating the transmitted ultrasound amplitude images of both the longitudinal and transverse modes. The constituent layers assumed are elastically isotropic but the periodically stacked structure is anisotropic. Thus anisotropy of ultrasound propagation is predicted even at low frequencies and it is enhanced significantly at higher frequencies due to the zone-folding effect of acoustic dispersion relations. An additional effect studied is the interference of ultrasound (known as the internal diffraction), which can be recognized when the propagation distance is comparable to the ultrasound wavelength. Numerical examples are developed for millimetre-scale Al/polymer multilayers used recently for imaging experiment with surface acoustic waves. (author)

  15. PHASE TRANSITION PROPERTIES OF A TWO COMPONENT FINITE MAGNETIC SUPERLATTICE

    Institute of Scientific and Technical Information of China (English)

    WANG XIAO-GUANG; LIU NING-NING; PAN SHAO-HUA; YANG GUO-ZHEN

    2000-01-01

    We study an (l, n) finite superlattice, which consists of two alternative magnetic materials(components) of l and n atomic layers, respectively. Based on the Ising model, we examine the phase transition properties of the magnetic superlattice. By transfer matrix method we derive the equation for Curie temperature of the superlattice. Numerical results are obtained for the dependence of Curie temperature on the thickness and exchange constants of the superlattice.

  16. 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.

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

    Science.gov (United States)

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

    2015-08-01

    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.

  18. Anisotropy in layered half-metallic Heusler alloy superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Azadani, Javad G.; Munira, Kamaram; Sivakumar, Chockalingam; Butler, William H. [Center for Materials for Information Technology, University of Alabama, Tuscaloosa, Alabama 35487 (United States); Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama 35487 (United States); Romero, Jonathon [Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama 35487 (United States); Ma, Jianhua; Ghosh, Avik W. [Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, Virginia 22904 (United States)

    2016-01-28

    We show that when two Heusler alloys are layered in the [001], [110], or [111] directions for various thicknesses to form a superlattice, the Slater-Pauling rule may still be satisfied and the resulting superlattice is often half-metallic with gaps comparable to or larger than those of its constituents. In addition, uniaxial magnetocrystalline anisotropy is induced because of the differences in the electronic structure of the two Heuslers in the superlattice. Various full-full, full-half, and half-half Heusler superlattices are studied, and potential half-metallic superlattices with perpendicular magnetocrystalline anisotropy are identified.

  19. Magnetic properties of rare earth superlattices

    CERN Document Server

    Wilkins, C J T

    2001-01-01

    Single-crystal Tm/Y and Tm/Lu superlattices have been grown using molecular beam epitaxy and their chemical structures have been determined using X-ray diffraction. Magnetisation measurements have revealed a more complicated phase diagram than that of pure Tm. Application of a field along the c-direction gave rise to an extra transition, and transitions were detected for the superlattices when the field was applied along the b-axis. In neutron diffraction studies, c-axis longitudinally modulated magnetic structures were found for both Tm/Y and Tm/Lu, which propagate coherently through the non-magnetic layers. In the case of Tm/Lu superlattices, there is evidence for ordering of the basal plane components.

  20. Electrical transport engineering of semiconductor superlattice structures

    Science.gov (United States)

    Shokri, Aliasghar

    2014-04-01

    We investigate the influence of doping concentration on band structures of electrons and electrical transmission in a typical aperiodic semiconductor superlattice consisting of quantum well and barrier layers, theoretically. For this purpose, we assume that each unit cell of the superlattice contains alternately two types of material GaAs (as a well) and GaAlAs (as a barrier) with six sublayers of two materials. Our calculations are based on the generalized Kronig-Penny (KP) model and the transfer matrix method within the framework of the parabolic conductance band effective mass approximation in the coherent regime. This model reduces the numerical calculation time and enables us to use the transfer matrix method to investigate transport in the superlattices. We show that by varying the doping concentration and geometrical parameters, one can easily block the transmission of the electrons. The numerical results may be useful in designing of nanoenergy filter devices.

  1. Electrical transport engineering of semiconductor superlattice structures

    Energy Technology Data Exchange (ETDEWEB)

    Shokri, Aliasghar, E-mail: aashokri@tpnu.ac.ir

    2014-04-01

    We investigate the influence of doping concentration on band structures of electrons and electrical transmission in a typical aperiodic semiconductor superlattice consisting of quantum well and barrier layers, theoretically. For this purpose, we assume that each unit cell of the superlattice contains alternately two types of material GaAs (as a well) and GaAlAs (as a barrier) with six sublayers of two materials. Our calculations are based on the generalized Kronig–Penny (KP) model and the transfer matrix method within the framework of the parabolic conductance band effective mass approximation in the coherent regime. This model reduces the numerical calculation time and enables us to use the transfer matrix method to investigate transport in the superlattices. We show that by varying the doping concentration and geometrical parameters, one can easily block the transmission of the electrons. The numerical results may be useful in designing of nanoenergy filter devices.

  2. Ballistic miniband conduction in a graphene superlattice

    Science.gov (United States)

    Lee, Menyoung; Wallbank, John R.; Gallagher, Patrick; Watanabe, Kenji; Taniguchi, Takashi; Fal'ko, Vladimir I.; Goldhaber-Gordon, David

    2016-09-01

    Rational design of long-period artificial lattices yields effects unavailable in simple solids. The moiré pattern in highly aligned graphene/hexagonal boron nitride (h-BN) heterostructures is a lateral superlattice with high electron mobility and an unusual electronic dispersion whose miniband edges and saddle points can be reached by electrostatic gating. We investigated the dynamics of electrons in moiré minibands by measuring ballistic transport between adjacent local contacts in a magnetic field, known as the transverse electron focusing effect. At low temperatures, we observed caustics of skipping orbits extending over hundreds of superlattice periods, reversals of the cyclotron revolution for successive minibands, and breakdown of cyclotron motion near van Hove singularities. At high temperatures, electron-electron collisions suppress focusing. Probing such miniband conduction properties is a necessity for engineering novel transport behaviors in superlattice devices.

  3. Energy Band Calculations for Maximally Even Superlattices

    Science.gov (United States)

    Krantz, Richard; Byrd, Jason

    2007-03-01

    Superlattices are multiple-well, semiconductor heterostructures that can be described by one-dimensional potential wells separated by potential barriers. We refer to a distribution of wells and barriers based on the theory of maximally even sets as a maximally even superlattice. The prototypical example of a maximally even set is the distribution of white and black keys on a piano keyboard. Black keys may represent wells and the white keys represent barriers. As the number of wells and barriers increase, efficient and stable methods of calculation are necessary to study these structures. We have implemented a finite-element method using the discrete variable representation (FE-DVR) to calculate E versus k for these superlattices. Use of the FE-DVR method greatly reduces the amount of calculation necessary for the eigenvalue problem.

  4. Resonance frequency in ferromagnetic superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Qiu Rongke; Huang Andong [School of Science, Shenyang University of Technology, Shenyang 110870 (China); Li Da; Zhang Zhidong, E-mail: rkqiu@163.com [Shenyang National Laboratory for Materials Science, Institute of Metal Research and International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016 (China)

    2011-10-19

    The resonance frequency in two-layer and three-layer ferromagnetic superlattices is studied, using the Callen's Green function method, the Tyablikov decoupling approximation and the Anderson-Callen decoupling approximation. The effects of interlayer exchange coupling, anisotropy, external magnetic field and temperature on the resonance frequency are investigated. It is found that the resonance frequencies increase with increasing external magnetic field. In a parameter region of the asymmetric system, each sublayer corresponds to its own resonance frequency. The anisotropy of a sublayer affects only the resonance frequency corresponding to this sublayer. The stronger the anisotropy, the higher is the resonance frequency. The interlayer exchange coupling affects only the resonance frequencies belonging to the sublayers connected by it. The stronger the interlayer exchange coupling, the higher are the resonance frequencies. All the resonance frequencies decrease as the reduced temperature increases. The results direct the method to enhance and adjust the resonance frequency of magnetic multilayered materials with a wide band.

  5. 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.

    2017-04-01

    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.

  6. 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.

  7. Theory of Semiconducting Superlattices and Microstructures

    Science.gov (United States)

    1992-03-01

    Core excitons ir. superlattices We have developed the first theory of Hjalmarsor.- Frenke ’ core excitons in superlattices, and applied it to strained...technique has been described are accelerated. A kinetic tempcrature TK is defined as by Kirkpatrick et al.31 and uses thr Monte Carlo algo- the average...classical kinetic energy of the atoms, rithm of Metropolis et al.32 Monte Carlo steps are taken 3/2kTK=(l/N)4rn’mlv,, where i=1,2, . . . ,n is the

  8. FABRICATION OF PHOTONIC CRYSTAL WITH SUPERLATTICES

    Institute of Scientific and Technical Information of China (English)

    SUN Cheng; Chen Haihua; Zhang Jizhong; Wei Hongmei; Gu Zhongze

    2006-01-01

    A novel technique was used to fabricate three-dimensional photonic crystals with superlattices. The super structure was fabricated by assembling monodispersed microspheres in the grooves of the scales of morpho butterfly, which makes the photonic crystal being composed of two kinds of different photonic structures (natural groove structure of butterfly wing and artificial microspherical colloids arrangement). The superstructural photonic crystal exhibits some unique optical properties different from both the butterfly wing and the colloidal crystal. The approach exhibited here provides a new way for fabricate photonic crystals with superlattices.

  9. 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.

  10. 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.

  11. A study on the synthesis and microstructure of WC-TiN superlattice coating

    Energy Technology Data Exchange (ETDEWEB)

    Yoon, J.S.; Myung, H.S.; Han, J.G. [Sung Kyun Kwan Univ., Suwon (Korea). Plasma Appl. Mater. Lab.; Musil, J. [Dept. of Physics, Univ. of West Bohemia (Czech Republic)

    2000-09-01

    WC-TiN superhard coatings are formulated to form a nanoscaled superlattice by separate arc reactive evaporation of Ti and WC. The microstructure of WC-TiN films was identified to be a superlattice of TiN and {beta}-WC{sub 1-x} phases with modulation period ({lambda}) of 5-13 nm and the lattice planes were continuous through the TiN and {beta}-WC{sub 1-x} layers. The residual stress of WC-TiN films was measured to be 7.9 GPa. This high stress was reduced to 2.2 GPa by introducing Ti or Ti-WC interlayers. Ti-WC interlayer also increased the film adhesion strength. In spite of almost the same residual stress of 2.2-2.3 GPa, Ti-WC/WC-TiN film showed a higher adhesion strength of 48.5 N than that of Ti/WC-TiN film. These results are attributed to the low residual stress and higher stiffness of the Ti-WC interlayer than the soft Ti interlayer. The microhardness of Ti-WC/WC-TiN film on cemented carbide was measured to be 40 GPa and the maximum hardness was obtained as the period ({lambda}) was approximately 7 nm. This value is approximately 1.5 times higher than that of the TiN single layer film. Other WC-TiN superlattice coatings with Ti and WC interlayers showed a hardness range of 38-40 GPa. The ratio H{sup 3}/E{sup 2} (plastic deformation resistance) of WC-TiN superlattice films with various interlayers was calculated to be in a range from 0.18 to 0.33. This paper reports the preparation of WC-TiN superlattice coatings on WC-Co and Si substrates using a multi-cathode arc ion-plating system. The microstructures and mechanical properties of WC-TiN superlattice films were investigated, too. (orig.)

  12. Interface bands in carbon nanotube superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Jaskolski, W.; Pelc, M. [Instytut Fizyki UMK, Grudziadzka 5, 87-100 Torun (Poland); Santos, H.; Chico, L. [Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas, Cantoblanco, 28049 Madrid (Spain); Ayuela, A. [Centro de Fisica de Materiales CSIC-UPV/EHU, Departamento de Fisica de Materiales (Facultad de Quimicas), and Donostia International Physics Center (DIPC), 20080 Donostia (Spain)

    2010-02-15

    We study the electronic band structure of several carbon nanotube superlattices built of two kinds of intermolecular junctions: (12, 0)/(6, 6) and (8, 0)/(14, 0). In particular, we focus on the energy bands originating from interface states. We find that in case of the metallic (12, 0)/(6, 6) superlattices, the interface bands change periodically their character from bonding- to antibonding-like vs. increasing length of the (6, 6) tube. We show that these changes are related to the decay of the charge density Friedel oscillations in the metallic (6, 6) tube. However, when we explore other chiralities without rotational symmetry, no changes in bondingantibonding character are observed for semiconductor superlattices, as exemplified in the case of (8, 0)/(14, 0) superlattices. Our results indicate that unless metallic tubes are employed in the junctions, the bonding-antibonding crossings are not present (copyright 2010 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  13. Synthesis of Bulk Nanostructured DO22 Superlattice of Ni3(Mo, Nb with High Strength, High Ductility, and High Thermal Stability

    Directory of Open Access Journals (Sweden)

    H. M. Tawancy

    2012-01-01

    Full Text Available We show that a bulk nanostructured material combining high strength, high ductility, and high thermal stability can be synthesized in a Ni-Mo-Nb alloy with composition approaching Ni3(Mo, Nb. By means of a simple aging treatment at 700°C, the grains of the parent face-centered cubic phase are made to transform into nanosized ordered crystals with DO22 superlattice maintaining a size of 10–20 nm after up to 100 hours of aging and corresponding room-temperature yield strength of 820 MPa and tensile ductility of 35%. Deformation of the superlattice is found to predominantly occur by twinning on {111} planes of the parent phase. It is concluded that, although the respective slip systems are suppressed, most of the twinning systems are preserved in the DO22 superlattice enhancing the ductility.

  14. Tailored inter and intra layer exchange coupled superlattices for optimized magnetocaloric effect

    Science.gov (United States)

    Mukherjee, Tathagata; Michalski, S.; Skomski, R.; Sellmyer, D. J.; Binek, Ch.

    2012-02-01

    We explore Magnetocaloric (MC) properties of Fe/Cr superlattices with tailored inter- and intra-layer interaction using simple 3d metals. Our multilayers are fabricated by pulsed-laser deposition with emphasis on maximizing magnetic entropy changes near room temperature. NanostructuringootnotetextT. Mukherjee, S. Sahoo, R. Skomski, D. J. Sellmyer, and Ch. Binek, Phys. Rev. B 79, 144406 (2009). allows tailoring ferromagnetic and antiferromagnetic coupling. This in concert with finite size scaling of the ferromagnetic Fe films has the potential to lead to optimized MC materials. Thermodynamic and MC properties of such Fe/Cr superlattices are studied with the help of SQUID magnetometry. Entropy changes are deduced via the Maxwell relation in single phase regions, X-ray diffraction and X-ray reflectivity are used to correlate structural data with the magnetic properties.

  15. Tuneable perpendicular magnetic anisotropy in single crystal [Co/Ni](111) superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Gottwald, M; Girod, S; Andrieu, S; Mangin, S, E-mail: gottwald@lpm.u-nancy.fr [Institut Jean Lamour, CNRS - Nancy Universite, BP 239, F-54506 Vandoeuvre (France)

    2010-06-15

    This paper is dedicated to the preparation of thin film with a strong perpendicular to the film plane magnetic anisotropy, behaviour of great interest for spintronics. Single-crystalline [Co/Ni] (111) superlattices have been grown by molecular beam epitaxy. The epitaxial growth of Co and Ni was controlled by using reflection high energy diffraction (RHEED), allowing us to get an accurate control of the thicknesses. The superlattices magnetic properties were studied using magnetometry. All of them exhibit strong perpendicular to the plane magnetic anisotropy. The maximum of magneto-crystalline anisotropy is obtained for one cobalt mo nolayer. A simple model which takes into account surface and volume anisotropy explains the evolution of perpendicular anisotropy in these layers.

  16. 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.

  17. Ultrafast structural dynamics of perovskite superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Woerner, M.; Korff Schmising, C. von; Zhavoronkov, N.; Elsaesser, T. [Max-Born-Institut fuer Nichtlineare Optik und Kurzzeitspektroskopie, Berlin (Germany); Bargheer, M. [Universitaet Potsdam, Institut fuer Physik und Astronomie, Potsdam (Germany); Vrejoiu, I.; Hesse, D.; Alexe, M. [Max-Planck-Institut fuer Mikrostrukturphysik, Halle (Germany)

    2009-07-15

    Femtosecond X-ray diffraction provides direct insight into the ultrafast reversible lattice dynamics of materials with a perovskite structure. Superlattice (SL) structures consisting of a sequence of nanometer-thick layer pairs allow for optically inducing a tailored stress profile that drives the lattice motions and for limiting the influence of strain propagation on the observed dynamics. We demonstrate this concept in a series of diffraction experiments with femtosecond time resolution, giving detailed information on the ultrafast lattice dynamics of ferroelectric and ferromagnetic superlattices. Anharmonically coupled lattice motions in a SrRuO{sub 3}/PbZr{sub 0.2}Ti{sub 0.8}O{sub 3} (SRO/PZT) SL lead to a switch-off of the electric polarizations on a time scale of the order of 1 ps. Ultrafast magnetostriction of photoexcited SRO layers is demonstrated in a SRO/SrTiO{sub 3} (STO) SL. (orig.)

  18. Tunneling of electrons through semiconductor superlattices

    Indian Academy of Sciences (India)

    C L Roy

    2002-11-01

    The purpose of the present paper is to report a study of tunneling of electrons through semiconductor superlattices (SSL); specially, we have analysed diverse features of transmission coefficient of SSL. The SSL we have considered is Ga0.7Al0.3As–GaAs which has been drawing considerable attention during the recent past on account of some typical features of its band structure. We have indicated how our results would help fabrication of ultra high speed devices.

  19. Highly variable coastal deformation in the 2016 MW7.8 Kaikōura earthquake reflects rupture complexity along a transpressional plate boundary

    Science.gov (United States)

    Clark, K. J.; Nissen, E. K.; Howarth, J. D.; Hamling, I. J.; Mountjoy, J. J.; Ries, W. F.; Jones, K.; Goldstien, S.; Cochran, U. A.; Villamor, P.; Hreinsdóttir, S.; Litchfield, N. J.; Mueller, C.; Berryman, K. R.; Strong, D. T.

    2017-09-01

    Coseismic coastal deformation is often used to understand slip on offshore faults in large earthquakes but in the 2016 MW 7.8 Kaikōura earthquake multiple faults ruptured across and sub-parallel to the coastline. Along ∼110 km of coastline, a rich dataset of coastal deformation comprising airborne lidar differencing, field surveying and satellite geodesy reveals highly variable vertical displacements, ranging from -2.5 to 6.5 m. These inform a refined slip model for the Kaikōura earthquake which incorporates changes to the slip on offshore faults and inclusion of an offshore reverse crustal fault that accounts for broad, low-amplitude uplift centered on Kaikōura Peninsula. The exceptional detail afforded by differential lidar and the high variability in coastal deformation combine to form the highest-resolution and most complex record of coseismic coastal deformation yet documented. This should prompt reassessment of coastal paleoseismic records that may not have considered multi-fault ruptures and high complexity deformation fields.

  20. Properties of ultra-thin vanadium layers in V/Ru superlattices

    Science.gov (United States)

    Liscio, F.; Maret, M.; Meneghini, C.; Hazemann, J. L.; Albrecht, M.

    2007-12-01

    The properties of ultra-thin vanadium layers in V/Ru(0001) superlattices grown by molecular beam epitaxy were studied. The atomic structure of V was investigated by various methods including reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD) and polarized x-ray-absorption fine structure (PXAFS). It appears that, for up to three atomic layers, V adopts a slightly distorted hexagonal-close-packed (hcp) structure induced by pseudomorphic growth on Ru(0001). By increasing the V thickness to four atomic layers, this structure almost completely relaxes towards the body-centered-cubic (bcc) bulk structure. This sharp structural transition is also manifested in the electronic properties. A reduced superconducting transition temperature between 0.6 and 1.05 K was found in the bcc V/hcp Ru superlattice, while superconductivity is quenched in the superlattice with hexagonal V. This behavior might be linked to the existence of a ferromagnetic ground state of the metastable V induced by the hybridization of the d-bands at the hcp V/Ru interface, as predicted from first-principles density-functional theory.

  1. Properties of ultra-thin vanadium layers in V/Ru superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Liscio, F [Science et Ingenierie des Materiaux et Procedes, INPGrenoble, CNRS-UJF, BP75, 38402 Saint Martin d' Heres (France); Maret, M [Science et Ingenierie des Materiaux et Procedes, INPGrenoble, CNRS-UJF, BP75, 38402 Saint Martin d' Heres (France); Meneghini, C [Dipartimento Di Fisica, Universita di Roma Tre, 00146-Rome (Italy); Hazemann, J L [Institut Neel, MCMF, CNRS, BP 166, 38042 Grenoble (France); Albrecht, M [Department of Physics, University of Konstanz, D-78457 Konstanz (Germany)

    2007-12-05

    The properties of ultra-thin vanadium layers in V/Ru(0001) superlattices grown by molecular beam epitaxy were studied. The atomic structure of V was investigated by various methods including reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD) and polarized x-ray-absorption fine structure (PXAFS). It appears that, for up to three atomic layers, V adopts a slightly distorted hexagonal-close-packed (hcp) structure induced by pseudomorphic growth on Ru(0001). By increasing the V thickness to four atomic layers, this structure almost completely relaxes towards the body-centered-cubic (bcc) bulk structure. This sharp structural transition is also manifested in the electronic properties. A reduced superconducting transition temperature between 0.6 and 1.05 K was found in the bcc V/hcp Ru superlattice, while superconductivity is quenched in the superlattice with hexagonal V. This behavior might be linked to the existence of a ferromagnetic ground state of the metastable V induced by the hybridization of the d-bands at the hcp V/Ru interface, as predicted from first-principles density-functional theory.

  2. 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...

  3. 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...

  4. 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...

  5. Plasmon nanoparticle superlattices as optical-frequency magnetic metamaterials.

    Science.gov (United States)

    Alaeian, Hadiseh; Dionne, Jennifer A

    2012-07-02

    Nanocrystal superlattices have emerged as a new platform for bottom-up metamaterial design, but their optical properties are largely unknown. Here, we investigate their emergent optical properties using a generalized semi-analytic, full-field solver based on rigorous coupled wave analysis. Attention is given to superlattices composed of noble metal and dielectric nanoparticles in unary and binary arrays. By varying the nanoparticle size, shape, separation, and lattice geometry, we demonstrate the broad tunability of superlattice optical properties. Superlattices composed of spherical or octahedral nanoparticles in cubic and AB(2) arrays exhibit magnetic permeabilities tunable between 0.2 and 1.7, despite having non-magnetic constituents. The retrieved optical parameters are nearly polarization and angle-independent over a broad range of incident angles. Accordingly, nanocrystal superlattices behave as isotropic bulk metamaterials. Their tunable permittivities, permeabilities, and emergent magnetism may enable new, bottom-up metamaterials and negative index materials at visible frequencies.

  6. Transverse magnetic mode along THz waveguides with biased superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Aceituno, P. [Dpto. Fisica Basica, Universidad de La Laguna, La Laguna, 38206 Tenerife (Spain)], E-mail: paceitun@ull.es; Hernandez-Cabrera, A. [Dpto. Fisica Basica, Universidad de La Laguna, La Laguna, 38206 Tenerife (Spain); Vasko, F.T. [Institute of Semiconductor Physics, NAS Ukraine, Pr. Nauki 41, Kiev 03028 (Ukraine)

    2008-05-15

    We study the propagation of transverse magnetic modes arising from a waveguide consisting on a GaAs-based superlattice located at vacuum-dielectric interface. The transverse mode is generated by the ultrafast intersubband response of the superlattice subjected to a high-frequency electric field. The superlattice is also subjected to a homogeneous bias potential to get a biased superlattice with equipopulated levels. The heterostructure is analyzed through the tight-binding approximation, and considering the level broadening caused by different scattering processes (homogeneous and inhomogeneous broadening mechanisms). We pay special attention to the dispersion relations of the complex dielectric permittivity because of real and imaginary parts of this function play a key role in wide miniband superlattices.

  7. Theory of silicon superlattices - Electronic structure and enhanced mobility

    Science.gov (United States)

    Moriarty, J. A.; Krishnamurthy, S.

    1983-01-01

    A realistic tight-binding band-structure model of silicon superlattices is formulated and used to study systems of potential applied interest, including periodic layered Si-Si(1-x)Ge(x) heterostructures. The results suggest a possible new mechanism for achieving enhanced transverse carrier mobility in such structures: reduced transverse conductivity effective masses associated with the superlattice band structure. For electrons in 100-line-oriented superlattices, a reduced conductivity mass arises intrinsically from the lower symmetry of the superlattice and its unique effect on the indirect bulk silicon band gap. An order of magnitude estimate of the range of mobility enhancement expected from this mechanism appears to be consistent with preliminary experimental results on Si-Si(1-x)Ge(x) superlattices.

  8. Superwide-angle acoustic propagations above the critical angles of the Snell law in liquid—solid superlattice

    Science.gov (United States)

    Zhang, Sai; Zhang, Yu; Gao, Xiao-Wei

    2014-12-01

    In this paper, superwide-angle acoustic propagations above the critical angles of the Snell law in liquid—solid superlattice are investigated. Incident waves above the critical angles of the Snell law usually inevitably induce total reflection. However, incident waves with big oblique angles through the liquid—solid superlattice will produce a superwide angle transmission in a certain frequency range so that total reflection does not occur. Together with the simulation by finite element analysis, theoretical analysis by using transfer matrix method suggests the Bragg scattering of the Lamb waves as the physical mechanism of acoustic wave super-propagation far beyond the critical angle. Incident angle, filling fraction, and material thickness have significant influences on propagation. Superwide-angle propagation phenomenon may have potential applications in nondestructive evaluation of layered structures and controlling of energy flux.

  9. Nonlinear THz response of metallic armchair graphene nanoribbon superlattices

    Science.gov (United States)

    Wang, Yichao; Andersen, David R.

    2016-11-01

    We study the third order THz nonlinear response of metallic armchair graphene nanoribbon superlattices in the presence of an elliptically-polarized excitation field using the time dependent perturbation theory. For a one-dimensional Kronig-Penney potential of infinite length, the nonlinear response can be described perturbatively by a low energy \\mathbf{k}\\centerdot \\mathbf{p} N-photon coupling model. Remarkably, as shown by Burset et al the energy dispersion of the metallic band in the direction parallel to the superlattice wavevector is independent of the applied superlattice potential while the energy dispersion in the direction perpendicular to the superlattice wavevector depends strongly on the superlattice parameters. As a result, we predict novel behavior for the nonlinear response of single layer metallic acGNR superlattices to an applied elliptically-polarized electric field. Our work shows that the superlattice potential, periodicity, Fermi level, excitation field polarization state, and temperature all play a significant role in the resulting THz nonlinear conductances.

  10. Transfer Matrix for Fibonacci Dielectric Superlattice

    Institute of Scientific and Technical Information of China (English)

    蔡祥宝

    2001-01-01

    The transfer matrices, which transfer the amplitudes of the electric fields of second- and third-harmonic waves from one side of the interface to the other, are defined for layers joined coherently, and the total transfer matrices for several sequential interfaces can be simply obtained by multiplication of the matrices. Using the transfer matrix method, the interacting processes of second- and third-harmonic waves in a one-dimensional finite Fibonacci dielectric superlattice are investigated. Applying the numerical procedure described in this letter, the dependence of the second- and third-harmonic fields on sample thickness is obtained. The numerical results agree with the quasi-phase-matching theory.

  11. Einstein's Photoemission from Quantum Confined Superlattices.

    Science.gov (United States)

    Debbarma, S; Ghatak, K P

    2016-01-01

    This paper is dedicated to the 83th Birthday of Late Professor B. R. Nag, D.Sc., formerly Head of the Departments of Radio Physics and Electronics and Electronic Science of the University of Calcutta, a firm believer of the concept of theoretical minimum of Landau and an internationally well known semiconductor physicist, to whom the second author remains ever grateful as a student and research worker from 1974-2004. In this paper, an attempt is made to study, the Einstein's photoemission (EP) from III-V, II-VI, IV-VI, HgTe/CdTe and strained layer quantum well heavily doped superlattices (QWHDSLs) with graded interfaces in the presence of quantizing magnetic field on the basis of newly formulated electron dispersion relations within the frame work of k · p formalism. The EP from III-V, II-VI, IV-VI, HgTe/CdTe and strained layer quantum wells of heavily doped effective mass superlattices respectively has been presented under magnetic quantization. Besides the said emissions, from the quantum dots of the aforementioned heavily doped SLs have further investigated for the purpose of comparison and complete investigation in the context of EP from quantum confined superlattices. Using appropriate SLs, it appears that the EP increases with increasing surface electron concentration and decreasing film thickness in spiky manners, which are the characteristic features of such quantized hetero structures. Under magnetic quantization, the EP oscillates with inverse quantizing magnetic field due to Shuvnikov-de Haas effect. The EP increases with increasing photo energy in a step-like manner and the numerical values of EP with all the physical variables are totally band structure dependent for all the cases. The most striking features are that the presence of poles in the dispersion relation of the materials in the absence of band tails create the complex energy spectra in the corresponding HD constituent materials of such quantum confined superlattices and effective electron

  12. Magnetic Bloch oscillations in nanowire superlattice rings.

    Science.gov (United States)

    Citrin, D S

    2004-05-14

    The recent growth of semiconductor nanowire superlattices encourages hope that Bloch-like oscillations in such structures formed into rings may soon be observed in the presence of a time-dependent magnetic flux threading the ring. These magnetic Bloch oscillations are a consequence of Faraday's law; the time-dependent flux produces an electromotive force around the ring, thus leading to the Bloch-like oscillations. In the spectroscopic domain, generalized Wannier-Stark states are found that are manifestations of the emf-induced localization of the states.

  13. Microcavities with distributed Bragg reflectors based on ZnSe/MgS superlattice grown by MOVPE

    Science.gov (United States)

    Tawara, T.; Yoshida, H.; Yogo, T.; Tanaka, S.; Suemune, I.

    2000-12-01

    Monolithic II-VI semiconductor microcavities for the blue-green region grown by metal-organic vapor-phase epitaxy have been demonstrated. ZnSe/MgS-superlattice (ZnSe/MgS-SL) layers were used for the distributed Bragg reflectors (DBRs). The DBR with only 5 periods showed the high reflectivity of 92% at the wavelength of 510 nm due to the large difference of refractive indices between ZnSe and MgS layers. In a monolithic II-VI microcavity structure based on these DBRs, a clear cavity resonance mode was observed in the blue-green region for the first time.

  14. Developing high-performance III-V superlattice IRFPAs for defense: challenges and solutions

    Science.gov (United States)

    Zheng, Lucy; Tidrow, Meimei; Aitcheson, Leslie; O'Connor, Jerry; Brown, Steven

    2010-04-01

    The antimonide superlattice infrared detector technology program was established to explore new infrared detector materials and technology. The ultimate goal is to enhance the infrared sensor system capability and meet challenging requirements for many applications. Certain applications require large-format focal plane arrays (FPAs) for a wide field of view. These FPAs must be able to detect infrared signatures at long wavelengths, at low infrared background radiation, and with minimal spatial cross talk. Other applications require medium-format pixel, co-registered, dual-band capability with minimal spectral cross talk. Under the technology program, three leading research groups have focused on device architecture design, high-quality material growth and characterization, detector and detector array processing, hybridization, testing, and modeling. Tremendous progress has been made in the past few years. This is reflected in orders-of-magnitude reduction in detector dark-current density and substantial increase in quantum efficiency, as well as the demonstration of good-quality long-wavelength infrared FPAs. Many technical challenges must be overcome to realize the theoretical promise of superlattice infrared materials. These include further reduction in dark current density, growth of optically thick materials for high quantum efficiency, and elimination of FPA processing-related performance degradation. In addition, challenges in long-term research and development cost, superlattice material availability, FPA chip assembly availability, and industry sustainability are also to be met. A new program was established in 2009 with a scope that is different from the existing technology program. Called Fabrication of Superlattice Infrared FPA (FastFPA), this 4-year program sets its goal to establish U.S. industry capability of producing high-quality superlattice wafers and fabricating advanced FPAs. It uses horizontal integration strategy by leveraging existing III

  15. 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.

  16. Influence of Deviation on Optical Transmission through Aperiodic Superlattices

    Institute of Scientific and Technical Information of China (English)

    YIN Hai-Long; YANG Xiang-Bo; LAN Sheng; HU Wei

    2007-01-01

    We propose a deviation model and study the influences of the relative error and sensitivity of a machine on the transmission coefficients (TCs) of Fibonacci superlattices. It is found that for a system with fewer layers, the influence of deviation can be ignored. When superlattices become more complicated, they may be fabricated by a machine with suitable relative error and possess the designed value of TC. However, when the number of system layers exceeds some critical value, superlattices should be manufactured only by precise machines. The influence of the sensitivity is also discussed.

  17. 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.

  18. ZnSe/ZnSeTe Superlattice Nanotips

    Science.gov (United States)

    2010-01-01

    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. PMID:20672085

  19. Raman fingerprint of aligned graphene/h-BN superlattices.

    Science.gov (United States)

    Eckmann, Axel; Park, Jaesung; Yang, Huafeng; Elias, Daniel; Mayorov, Alexander S; Yu, Geliang; Jalil, Rashid; Novoselov, Kostya S; Gorbachev, Roman V; Lazzeri, Michele; Geim, Andre K; Casiraghi, Cinzia

    2013-11-13

    Graphene placed on hexagonal-boron nitride (h-BN) experiences a superlattice (Moiré) potential, which leads to a strong reconstruction of graphene's electronic spectrum with new Dirac points emerging at sub-eV energies. Here we study the effect of such superlattices on graphene's Raman spectrum. In particular, the 2D Raman peak is found to be exquisitely sensitive to the misalignment between graphene and h-BN lattices, probably due to the presence of a strain distribution with the same periodicity of the Moiré potential. This feature can be used to identify graphene superlattices with a misalignment angle smaller than 2°.

  20. Sculptured 3D twister superlattices embedded with tunable vortex spirals.

    Science.gov (United States)

    Xavier, Jolly; Vyas, Sunil; Senthilkumaran, Paramasivam; Denz, Cornelia; Joseph, Joby

    2011-09-01

    We present diverse reconfigurable complex 3D twister vortex superlattice structures in a large area embedded with tunable vortex spirals as well as dark rings, threaded by vortex helices. We demonstrate these tunable complex chiral vortex superlattices by the superposition of relatively phase engineered plane waves. The generated complex 3D twister lattice vortex structures are computationally as well as experimentally analyzed using various tools to verify the presence of phase singularities. Our observation indicates the application-specific flexibility of our approach to tailor the transverse superlattice spatial irradiance profile of these longitudinally whirling vortex-cluster units and dark rings.

  1. Aging in Co/Cr Superlattices

    Science.gov (United States)

    Mukherjee, T.; Pleimling, M.; Binek, Ch.

    2009-03-01

    Aging phenomena are observed in various systems brought into non-equilibrium and subsequently showing slow relaxation dynamics. Magnetic specimens with well defined interactions and dimensions can serve as model systems for universal aspects of aging. Magnetic thin films provide access to a wide range of microscopic parameters. Superlattice structures allow tuning the intra and inter-plane exchange and enable geometrical confinement of the spin fluctuations. We use Co/Cr thin film superlattices to study magnetic aging. The static and dynamic properties are affected via the Co and Cr film thicknesses. TC of the Co films is reduced from the bulk value by geometrical confinement. Non-ergodic behavior sets in at a tunable temperature T^* in a range of some 100K above zero. Cr provides antiferromagnetic coupling between the Co films. Non-equilibrium spin states are set via low field cooling in 5mT in-plane magnetic field to below T^*. Next various in-plane magnetic set fields of some 10-100 mT are applied and the sample is exposed to the latter for various waiting times tw, respectively. After removing the field, relaxation of the magnetization is recorded via longitudinal Kerr-magnetometry. The relaxation data are analyzed by scaling plots revealing universal aspects of aging. Financial support by Teledyne-Isco, NRI, and NSF through EPSCoR, Career DMR-0547887, and MRSEC.

  2. Coercivity enhancement in (Co/CoO)n superlattices

    Science.gov (United States)

    Polisetty, Srinivas; Binek, Christian

    2009-03-01

    The temperature dependence of the coercivity is studied in (Co/CoO)n periodic multilayer thin film superstructures below and above the exchange bias blocking temperature. The ferromagnetic Co thin films are grown with the help of MBE at a base pressure of 10E-10 m.bar whereas antiferromagnetic CoO thin films are grown from in-situ oxidized Co. The thicknesses of these films are monitored by reflection high energy electron diffraction (RHEED). A mean-field theory^1 is outlined which provides an analytic and intuitive expression for the enhancement of the coercivity of the ferromagnet which experiences the exchange coupling with a neighboring antiferromagnet. An experimental approach is developed allowing to determine the interface susceptibility of an individual antiferromagnetic pinning layer by systematic change in the thickness of the antiferromagnet thin films in various sets of superlattice samples measured at different temperatures, respectively. The experiment enables us to separate out the intrinsic coercivity from the contribution induced by exchange coupling at the interface. It is the goal of our study to evidence or disprove if it is simply this susceptibility of the reversible interface magnetization creating the spin drag effect and by that the coercivity enhancement. Financial support by NSF through CAREER DMR-0547887, NRI and Nebraska MRSEC. ^1G. Scholten, K. D. Usadel, and U. Nowak, Phys. Rev B. 71, 064413 (2005).

  3. 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.

  4. 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...

  5. Plasmonic Enhanced Type-II Superlattice Focal Plane Arrays Project

    Data.gov (United States)

    National Aeronautics and Space Administration — SVT Associates proposes an novel type II superlattice structure to extend the cutoff wavelength and CBIRD SL photo diode structure with unipolar barriers to suppress...

  6. The soliton properties of dipole domains in superlattices

    Institute of Scientific and Technical Information of China (English)

    张启义; 田强

    2002-01-01

    The formation and propagation of dipole domains in superlattices are studied both by the modified discrete driftmodel and by the nonlinear Schrodinger equation. The spatiotemporal distribution of the electric field and electrondensity are presented. The numerical results are compared with the soliton solutions of the nonlinear Schrodingerequation and analysed. It is shown that the numerical solutions agree with the soliton solutions of the nonlinearSchrodinger equation. The dipole electric-field domains in semiconductor superlattices have the properties of solitons.

  7. Superlattice Intermediate Band Solar Cell on Gallium Arsenide

    Science.gov (United States)

    2015-02-09

    AFRL-RV-PS- AFRL-RV-PS- TR-2015-0048 TR-2015-0048 SUPERLATTICE INTERMEDIATE BAND SOLAR CELL ON GALLIUM ARSENIDE Alexandre Freundlich...SUBTITLE 5a. CONTRACT NUMBER FA9453-13-1-0232 Superlattice Intermediate Band Solar Cell on Gallium Arsenide 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER...band solar cell incorporating low dimensional structures made with dilute nitrogen alloys of III-V semiconductors is investigated theoretically and

  8. Experimental evidence of delocalized states in random dimer superlattices

    OpenAIRE

    Bellani, V.; Díez, E.; Hey, R.; Toni, L.; Tarricone, L.; Parravicini, G.B.; Domínguez-Adame Acosta, Francisco; Gómez-Alcalá, R.

    1999-01-01

    We study the electronic properties of GaAs-AlGaAs superlattices with intentional correlated disorder by means of photoluminescence and vertical dc resistance. The results are compared to those obtained in ordered and uncorrelated disordered superlattices. We report the first experimental evidence that spatial correlations inhibit localization of states in disordered low-dimensional systems, as our previous theoretical calculations suggested, in contrast to the earlier belief that all eigensta...

  9. 强光辐照下白宝石高反镜尺寸对热变形的影响%Effect of dimensions on thermal deformation of high reflectance sapphire mirrors under high power laser irradiation

    Institute of Scientific and Technical Information of China (English)

    梅艳莹; 杨涛

    2014-01-01

    为了进一步减小白宝石( Al2 O3)高反镜在强光辐照下的热变形,提高光束质量,研究了白宝石高反镜厚度、直径尺寸对热变形的影响。采用以极坐标表示的热传导方程和热变形公式来描述白宝石高反镜的温度场分布和位移场分布;在有限元分析软件中建立数值计算模型,并计算了不同厚度、直径尺寸下的温度场和位移场,得到了热变形随厚度尺寸和直径尺寸变化的规律。结果表明,影响白宝石高反镜反射面峰谷值变化的主要因素是温度,而尺寸变化对温度和刚度均有影响;选择合适的高反镜直径和厚度尺寸,可以有效降低镜面温升,同时得到合适的轴向结构刚度,从而减小反射镜镜面热变形。该研究结果对强光辐照下白宝石高反镜尺寸设计和选择具有一定的参考价值。%In order to reduce thermal deformation of high reflectance sapphire mirrors under high power laser irradiation and improve the beam quality , effect of the diameter and thickness on thermal deformation of the high reflectance mirror was studied .The heat conduction equation expressed in polar coordinate and the thermal deformation formula were used to describe the distribution of the temperature field and the displacement field of the sapphire mirror .Then, the numerical calculation model was built with the finite element analysis software and the temperature field and displacement field in different thickness and diameter was calculated .The thermal deformation with respect to the thickness and diameter was obtained .The results indicate that the main factors affecting the PV value of the sapphire mirror surface is the temperature variation and the change of the size has effect on both temperature and stiffness .Appropriate size and thickness effectively reduce the rise of mirror ’ s temperature and keep suitable axial structural stiffness , and thus the thermal deformation of the

  10. Heteroepitaxy of Ge-Si{sub 1{minus}x}Ge{sub x} superlattices on Si (100) substrates by GeH{sub 4}-Si MBE

    Energy Technology Data Exchange (ETDEWEB)

    Orlov, L.K.; Tolomasov, V.A.; Potapov, A.V.; Drozdov, Yu.N. [Russian Academy of Sciences, Nizhny Novgorod (Russian Federation). Inst. for Physics of Microstructures; Vdovin, V.I. [Inst. for Rare Metals Giredmet, Moscow (Russian Federation)

    1996-12-31

    The authors applied GeH{sub 4}-SI MBE for growing Ge-Si{sub 1{minus}x}Ge{sub x} superlattices on Si(100). They investigated the distribution and the structure of defects inside heteroepitaxial Si{sub 1{minus}x}Ge{sub x} layers grown on Si(100). It was shown that the system has unique peculiarities of a dislocation structure formation. They found out that the plastic deformation on a layer-substrate heteroboundary eliminates strong elastic deformation inside the grown layer.

  11. Coherent quantum transport features in carbon superlattice structures

    Science.gov (United States)

    McIntosh, R.; Henley, S. J.; Silva, S. R. P.; Bhattacharyya, S.

    2016-10-01

    Whilst resonant transmission is well understood and can be fully harnessed for crystalline superlattices, a complete picture has not yet emerged for disordered superlattices. It has proven difficult to tune resonant transmission in disordered diamond-like carbon (DLC) superlattices as conventional models are not equipped to incorporate significant structural disorder. In this work, we present concurrent experimental and theoretical analysis which addresses resonant transmission in DLC superlattices. Devices were fabricated by growing alternate layers of DLC with different percentages of sp3 hybridized carbon.Coherent quantum transport effects were demonstrated in these structurally disordered DLC superlattices through distinct current modulation with negative differential resistance (NDR) in the current-voltage (I-V) measurements. A model was developed using tight-binding calculations assuming a random variation of the hopping integral to simulate structural (bond-length) disorder. Calculations of the I-V characteristics compliment the interpretation of the measurements and illustrate that while DLC superlattice structures are unlike their classical counterparts, the near-field structural order will help with the confinement of quantised states. The present model provides an empirical guide for tailoring the properties of future devices, giving rise to much hope that carbon electronics operating at high frequencies over large areas can now be developed.

  12. Carbon-coated nanoparticle superlattices for energy applications

    Science.gov (United States)

    Li, Jun; Yiliguma, Affa; Wang, Yifei; Zheng, Gengfeng

    2016-07-01

    Nanoparticle (NP) superlattices represent a unique material architecture for energy conversion and storage. Recent reports on carbon-coated NP superlattices have shown exciting electrochemical properties attributed to their rationally designed compositions and structures, fast electron transport, short diffusion length, and abundant reactive sites via enhanced coupling between close-packed NPs, which are distinctive from their isolated or disordered NP or bulk counterparts. In this minireview, we summarize the recent developments of highly-ordered and interconnected carbon-coated NP superlattices featuring high surface area, tailorable and uniform doping, high conductivity, and structure stability. We then introduce the precisely-engineered NP superlattices by tuning/studying specific aspects, including intermetallic structures, long-range ordering control, and carbon coating methods. In addition, these carbon-coated NP superlattices exhibit promising characteristics in energy-oriented applications, in particular, in the fields of lithium-ion batteries, fuel cells, and electrocatalysis. Finally, the challenges and perspectives are discussed to further explore the carbon-coated NP superlattices for optimized electrochemical performances.

  13. Fluorescence x-ray standing wave study on (AlAs)(GaAs) superlattices

    CERN Document Server

    Lessmann, A; Munkholm, A; Schuster, M; Riechert, H; Materlik, G

    1999-01-01

    X-ray standing waves (XSW) were used to investigate the structure of molecular beam epitaxy (MBE) grown (AlAs) sub 3 (GaAs) sub 7 short-period superlattices (SPSL). The modulation of the Al K, As L, and Ga L x-ray fluorescence induced by XSW was measured at the zero-order superlattice (SL) satellite (AlAs)(GaAs)(004,0) and the GaAs(004) substrate Bragg reflection. From the shape of the fluorescence yield modulations and the diffraction pattern, a model of the interfaces is derived by comparing the experimental data with dynamical calculations of the x-ray wave field distribution and reflectivity. A straightforward analysis of the fluorescence measurements at the SL satellite shows that in AlAs layers a high crystalline order is established, whereas in GaAs layers a fraction of the Ga and As atoms is not on ideal lattice sites, but is displaced towards the substrate. The data can be explained by a model in which, at each AlAs/GaAs interface of the GaAs layers, two Ga atom planes are displaced by 0.035 nm and 0...

  14. Fluorescence x-ray standing wave study on (AlAs)(GaAs) superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Lessmann, A.; Brennan, S.; Munkholm, A. [Stanford Synchrotron Radiation Laboratory SSRL/SLAC, Menlo Park, CA (United States); Schuster, M.; Riechert, H. [Siemens AG, Corporate Technology, Munich (Germany); Materlik, G. [Hamburger Synchrotronstrahlungslabor HASYLAB/DESY, Hamburg (Germany)

    1999-05-21

    X-ray standing waves (XSW) were used to investigate the structure of molecular beam epitaxy (MBE) grown (AlAs){sub 3}(GaAs){sub 7} short-period superlattices (SPSL). The modulation of the Al K, As L, and Ga L x-ray fluorescence induced by XSW was measured at the zero-order superlattice (SL) satellite (AlAs)(GaAs)(004,0) and the GaAs(004) substrate Bragg reflection. From the shape of the fluorescence yield modulations and the diffraction pattern, a model of the interfaces is derived by comparing the experimental data with dynamical calculations of the x-ray wave field distribution and reflectivity. A straightforward analysis of the fluorescence measurements at the SL satellite shows that in AlAs layers a high crystalline order is established, whereas in GaAs layers a fraction of the Ga and As atoms is not on ideal lattice sites, but is displaced towards the substrate. The data can be explained by a model in which, at each AlAs/GaAs interface of the GaAs layers, two Ga atom planes are displaced by 0.035 nm and 0.008 nm and one As atom plane by 0.023 nm. The displacements within the GaAs layers exhibit a mirror symmetry with respect to the centre of each layer. (author)

  15. Ion beam studies in strained layer superlattices

    CERN Document Server

    Pathak, A P; Bhattacharya, D P; Dev, B N; Ghosh, S; Goswami, D K; Lakshmi-Bala, S; Nageswara-Rao, S V S; Satyam, P V; Siddiqui, A M; Srivastava, S K; Turos, A

    2002-01-01

    The potential device application of semiconductor heterostructures and strained layer superlattices has been highlighted. Metal organic chemical vapour deposition grown In sub 0 sub . sub 5 sub 3 Ga sub 0 sub . sub 4 sub 7 As/InP lattice-matched structure has been irradiated by 130 MeV Ag sup 1 sup 3 sup + and studied by RBS/Channelling using 3.5 MeV He sup 2 sup + ions. Ion irradiation seems to have induced a finite tensile strain in the InGaAs layer, indicating thereby that ion beam mixing occurs at this energy. Other complementary techniques like high resolution XRD and STM are needed to conclude the structural modifications in the sample.

  16. Nonreciprocal Multiferroic Superlattices with Broken Parity Symmetry

    Science.gov (United States)

    Tang, Zhenghua; Zhang, Weiyi

    Multiferroic materials are characterized by the coexistence of ferroelectric and ferromagnetic (or antiferromagnetic) orders, the coupling to lattice vibration can be invoked either through piezoelectric or piezomagnetic effects. In this paper, the polaritonic band structures of multiferroic superlattices composed of oppositely polarized domains are investigated using the generalized transfer matrix method. For the primitive cell with broken parity symmetry, the polaritonic band structure is asymmetrical with respect to the forward and backward propagation directions (nonreciprocality). In particular, the band extreme points move away from the Brillouin zone center. This asymmetry in band-gap positions and widths can be used to design compact one-way optical isolators, while the extremely slow light velocities near the asymmetrical upper edges of lower bands includes the essential ingredients for designing slow light devices.

  17. Magnetocaloric properties of Co/Cr superlattices

    Science.gov (United States)

    Mukherjee, Tathagata; Skomski, Ralph; Sellmyer, David; Binek, Christian

    2010-03-01

    Nanostructured materials aiming on refrigeration applications are experimentally realized by molecular beam epitaxial (MBE) growth of Co/Cr superlattices using mean-field theoretical concepts as guiding principles.footnotetextT. Mukherjee, S. Sahoo, R. Skomski, D. J. Sellmyer, and Ch. Binek, Phys. Rev. B 79, 144406-1-9 (2009). Magnetocaloric properties are deduced from measurements of the temperature and field dependence of the magnetization of our samples. More generally, the potential of artificial antiferromagnets for near room-temperature refrigeration is explored. The effects of intra-plane and inter-plane exchange interactions on the magnetic phase diagram in Ising-type model systems are revisited in mean-field considerations with special emphasis on tailoring magnetocaloric properties. The experimental results are discussed in light of our theoretical findings, and extrapolations for future improved nanostructures are provided. Financial support by NRI, and NSF through EPSCoR, Career DMR-0547887, and MRSEC.

  18. Magnetic Field in Superlattices Semiconductors of Crystals

    Directory of Open Access Journals (Sweden)

    Luciano Nascimento

    2015-05-01

    Full Text Available In this work we present a study on the super-semiconductor networks, using the Kronig-Penney model for the effective mass approximation, and then the calculations for the application of the magnetic field perpendicular and parallel to the layers of super lattices crystals. The magnetic field applied parallel to the layers, was used to adjust the resonance of a higher energy subband of a well by thermal excitation with a lower energy subband of the adjacent well, increasing energy levels in its tunneling rate. We use the formalism of Schrödinger equation of quantum mechanics. Introducing the calculations in a systematic way in superlattices for each semiconductor quantum well to assess their energy spectrum systematically studied.

  19. 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.   In our FY12 IRAD “Strained Layer Superlattice Infrared Detector Array...

  20. Crossover from Incoherent to Coherent Phonon Scattering in Epitaxial Oxide Superlattices

    Science.gov (United States)

    2013-12-08

    of the superlattices. Figure 3a shows a high-resolution, short-angular-range θ–2θ X - ray diffraction (XRD) scan of a (STO)6/(CTO)6 superlattice...function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides 1. REPORT DATE (DD-MM-YYYY) 4. TITLE...synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two

  1. Quantum Dot Superlattice Enabled Rational Design in Optoelectronics and Hydrogen Generation

    Science.gov (United States)

    2014-11-25

    Final 3. DATES COVERED (From - To) 22-April-2013 to 21-April-2014 4. TITLE AND SUBTITLE Quantum Dot Superlattice Enabled Rational Design...15. SUBJECT TERMS Quantum Dots , Optoelectronic Applications, Charge Transfer, Superlattices, Density Functional Theory, Coupling...FA2386-13-1-4074 “ Quantum Dot Superlattice Enabled Rational Design in Optoelectronics and Hydrogen Generation” April 21, 2014 PI and Co-PI

  2. Metallic interface in non-SrTiO{sub 3} based titanate superlattice

    Energy Technology Data Exchange (ETDEWEB)

    Liu, Xiaoran, E-mail: xxl030@uark.edu; Cao, Yanwei; Kareev, M.; Middey, S.; Chakhalian, J. [Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701 (United States); Choudhury, D. [Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701 (United States); Department of Physics, Indian Institute of Technology, Kharagpur 721302 (India)

    2015-11-09

    We report on the fabrication of all perovskite Mott insulator/band insulator YTiO{sub 3}/CaTiO{sub 3} superlattices by pulsed laser deposition. The combination of in situ reflection high energy electron diffraction, X-ray diffraction, and X-ray reflectivity confirms the high quality of the films grown in a layer-by-layer mode. Electrical transport measurements reveal that a non-SrTiO{sub 3} based two-dimensional electron gas system has formed at the YTiO{sub 3}/CaTiO{sub 3} interface. These studies offer another route in the pursuit of complex oxide two-dimensional electron gas systems, which allows to obtain greater insights into the exotic many-body phenomena at such interfaces.

  3. Effects of the local field and inherent deformation in reflectance anisotropy spectra of A{sup III}B{sup V} semiconductor surfaces

    Energy Technology Data Exchange (ETDEWEB)

    Berkovits, V.L.; Gordeeva, A.B.; Kosobukin, V.A. [A.F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg 194021 (Russian Federation)

    2010-08-15

    Reflectance anisotropy (RA) spectra of naturally oxidized GaAs(001) and InAs(001) surfaces reveal features that differ qualitatively from those caused by reconstruction of clean surfaces or surface electric fields of the crystals. We show that the observed RA spectra of the semiconductor/oxide systems are simultaneously influenced by interface and near-surface anisotropies whose physical origins are different. The interface anisotropy is associated with anisotropic polarizability of the valence bonds belonging to a monatomic layer of excess As atoms occurring between the crystal and oxide. The near-surface anisotropy is due to uniaxial strain of a near-surface region of the crystals. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

  4. Quasi-Dirac points in one-dimensional graphene superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Chen, C.H.; Tseng, P.; Hsueh, W.J., E-mail: hsuehwj@ntu.edu.tw

    2016-08-26

    Quasi-Dirac points (QDPs) with energy different from the traditional Dirac points (TDPs) have been found for the first time in one-dimensional graphene superlattices. The angular-averaged conductance reaches a minimum value at the QDPs, at which the Fano factor approaches 1/3. Surprisingly, the minimum conductance at these QDPs may be lower than that at the TDPs under certain conditions. This is remarkable as the minimum conductance attainable in graphene superlattices was believed to appear at TDPs. - Highlights: • Quasi-Dirac points (QDPs) are found for the first time in one-dimensional graphene superlattices. • The QDP is different from the traditional Dirac points (TDPs) in graphene superlattices. • The angular-averaged conductance reaches a minimum value at the QDPs, at which the Fano factor approaches 1/3. • The minimum conductance at these QDPs may be lower than that at the TDPs under certain conditions. • The minimum conductance attainable in graphene superlattices was believed to appear at TDPs.

  5. Study of Ni80Fe20/Fe50Mn50 superlattice microstructures by transmission electron microscopy and x—ray diffraction

    Institute of Scientific and Technical Information of China (English)

    MingXu; TaoYang; GuangmingLuo; ZhengqiLu; CuixiuLiu; NingYang; Zhenh

    2001-01-01

    The detailed microstructures of Ni80Fe20/Fe50Mn50 superlattices have been characterized using both x-ray diffraction techniques and transmission electron microscopy.The obrivous layered structure,typical column structure and twins which exist in Ni80Fe20/Fe50Mn50 superlattices were observed through performing transmission microscopy.By combining the technique of lowangle x-ray reflectivity(specular and off-specular scans)with the anomalous scattering effect and high-angle x-ray diffraction(using conventional x-ray),wequantitatively analysed the microstructural variation as a function of annealing temperature.It is found that the lateral correlation length,the(111)peak intensity of the superlattices and the average multilayer coherence length all increase with a rise in annealing temperature annealing can decrease the rootmean-square roughness at the interfaces of Ni80Fe20/Fe50Mn50 superlattices.the obtained microstructural knowledge will be helpful in understanding the magnetic properties of the ni80Fe20/Fe50Mn50 exchange bias system.

  6. Molecular dynamics simulation of thermal conductivities of superlattice nanowires

    Institute of Scientific and Technical Information of China (English)

    杨决宽; 陈云飞; 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.

  7. Rabi Oscillations in Realistic Superlattice with Finite Bloch Bands

    Institute of Scientific and Technical Information of China (English)

    FAN Wen-Bin; ZHANG Ping; LUO Ying; ZHAO Xian-Geng

    2001-01-01

    We investigate the dynamical processes taking place in nanodevices by high-frequency dc-ac fields. We found that Rabi oscillations between minibands are clearly identified under theoretical resonant conditions derived by an ideal two-band superlattice model, the resonant conditions have broadened, and the amount of broadening is about four times of the Rabi oscillation frequency. We also want to elucidate the role of different mechanisms that could lead to loss of quantum coherence. Our results show how the dephasing effects of disorder of interface roughness and doping fluctuation that after some periods destroy coherent oscillations, such as Rabi oscillations,can be reduced dramatically if we apply a bias static electric field to the superlattice system. The doping fluctuation dephasing effect is much stronger than that of interface roughness in the coherent process of realistic superlattices.

  8. Defect enhanced spin and valley polarizations in silicene superlattices

    Science.gov (United States)

    Li, Wen; Lu, Wei-Tao; Li, Yun-Fang; Han, Hai-Hua

    2017-04-01

    We studied the effect of a defect of superlattice on the spin and valley dependent transport properties in silicene, where there is an abnormal barrier in height. It is found that the transmission resonance is greatly suppressed, because the symmetry of superlattice structure is destroyed by the defect. The spin-up and spin-down electrons near the K and K ‧ valleys are dominated by different effective superlattices and defects. Therefore, the conductances are strongly dependent on the spin and valley of electron. By adjusting the defect strength properly, the spin and valley polarizations could be dramatically enhanced in a wide energy region. Furthermore, the result suggests an application of the structure as a defect-controlled switch.

  9. The magnetic structure of holmium-erbium superlattices

    Energy Technology Data Exchange (ETDEWEB)

    McMorrow, D.F. [Risoe National Lab., Roskilde (Denmark); Simpson, J.A.; Cowley, R.A.; Jehan, D.A.; Ward, R.C.C.; Wells, M.R. [Oxford Physics, Clarendon Lab. (United Kingdom); Thurston, T.R.; Gibbs, D. [Brookhaven National Lab., Upton, NY (United States)

    1994-06-01

    The effect of completing crystal-field anisotropies on magnetic order has been investigated in a series of Ho/Er superlattices using neutron and resonant x-ray magnetic diffraction techniques. The neutron diffraction reveals that for temperatures in the interval T{sub N}(Er) {le} T {le} T{sub N}(Ho) the Ho basal-plane order propagates coherently through the paramagnetic Er, and that below T{sub N}(Er) the longitudinal component of the Er moments fails to order across the Ho block. The magnetic superlattice peaks observed in the x-ray scattering display an anomalous energy dependence: a sharp resonance is found at L{sub III}(Ho), with no resonance visible at L{sub III}(Er). These results are discussed with reference to models of exchange in metallic superlattices.

  10. Electronic states of InSe/GaSe superlattice

    Science.gov (United States)

    Erkoç, Ş.; Allahverdi, K.; Ibrahim, Z.

    1994-06-01

    Analysis of recent publications revealed an increasing interest in epitaxial growth of InSe/GaSe superlattice. Within the effective mass theory we carried out self-consistent calculations of the confined and itinerant electronic states, potential profile and charge density distribution of InSe/GaSe superlattice, where the InSe layers are the well and the GaSe layers the barrier. Calculations were performed for three types of doping: uniform, modulated in the well, and modulated in the barrier. It has been found that the Coulomb interaction in the well and barrier forces the formation of localized states in the barrier region. The possibility of an insulator-metal transition in InSe/GaSe superlattice is predicted for modulation doping in the barrier and for a doping level n = 10 19cm-3. A decrease of the barrier height has been found for modulation doping in the well.

  11. Contracture deformity

    Science.gov (United States)

    Deformity - contracture ... Contracture can be caused by any of the following: Brain and nervous system disorders, such as cerebral ... Follow your health care provider's instructions for treating contracture at home. Treatments may include: Doing exercises and ...

  12. Electric and magnetic superlattices in trilayer graphene

    Science.gov (United States)

    Uddin, Salah; Chan, K. S.

    2016-01-01

    The properties of one dimensional Kronig-Penney type of periodic electric and vector potential on ABC-trilayer graphene superlattices are investigated. The energy spectra obtained with periodic vector potentials shows the emergence of extra Dirac points in the energy spectrum with finite energies. For identical barrier and well widths, the original as well as the extra Dirac points are located in the ky = 0 plane. An asymmetry between the barrier and well widths causes a shift in the extra Dirac points away from the ky = 0 plane. Extra Dirac points having same electron hole crossing energy as that of the original Dirac point as well as finite energy Dirac points are generated in the energy spectrum when periodic electric potential is applied to the system. By applying electric and vector potential together, the symmetry of the energy spectrum about the Fermi level is broken. A tunable band gap is induced in the energy spectrum by applying both electric and vector potential simultaneously with different barrier and well widths.

  13. Negative capacitance in multidomain ferroelectric superlattices

    Science.gov (United States)

    Zubko, Pavlo; Wojdeł, Jacek C.; Hadjimichael, Marios; Fernandez-Pena, Stéphanie; Sené, Anaïs; Luk'Yanchuk, Igor; Triscone, Jean-Marc; Íñiguez, Jorge

    2016-06-01

    The stability of spontaneous electrical polarization in ferroelectrics is fundamental to many of their current applications, which range from the simple electric cigarette lighter to non-volatile random access memories. Research on nanoscale ferroelectrics reveals that their behaviour is profoundly different from that in bulk ferroelectrics, which could lead to new phenomena with potential for future devices. As ferroelectrics become thinner, maintaining a stable polarization becomes increasingly challenging. On the other hand, intentionally destabilizing this polarization can cause the effective electric permittivity of a ferroelectric to become negative, enabling it to behave as a negative capacitance when integrated in a heterostructure. Negative capacitance has been proposed as a way of overcoming fundamental limitations on the power consumption of field-effect transistors. However, experimental demonstrations of this phenomenon remain contentious. The prevalent interpretations based on homogeneous polarization models are difficult to reconcile with the expected strong tendency for domain formation, but the effect of domains on negative capacitance has received little attention. Here we report negative capacitance in a model system of multidomain ferroelectric-dielectric superlattices across a wide range of temperatures, in both the ferroelectric and paraelectric phases. Using a phenomenological model, we show that domain-wall motion not only gives rise to negative permittivity, but can also enhance, rather than limit, its temperature range. Our first-principles-based atomistic simulations provide detailed microscopic insight into the origin of this phenomenon, identifying the dominant contribution of near-interface layers and paving the way for its future exploitation.

  14. Intrinsic noncollinear magnetization in Fe/Cr superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Yartseva, N.S., E-mail: yartseva@imp.uran.ru [Institute of Metal Physics, UD of RAS, Ekaterinburg 620990 (Russian Federation); Yartsev, S.V. [ZAO NPO “Spektr”, 14 Berezovskiy 623700 (Russian Federation); Demangeat, C. [UFR de Physique et d’Ingéniérie, Université de Strasbourg, 3 rue de l’Université, 67000 Strasbourg (France)

    2014-12-15

    Magnetic moments distribution in Fe{sub 3}Cr{sub n} superlattice series with fixed middle Fe monolayer and number of Cr monolayers (MLs) n from 1 to 45 is computed in the framework of collinear and noncollinear Periodic Anderson model. The superlattices are composed of layers in (0 0 1) and (1 1 0) plane with ideal interface. The total energy shows that noncollinear orientation of the magnetic moments remains the ground state for all superlattices with Cr thickness above 5 MLs. Distribution of the magnetic moments for Fe/Cr(0 0 1) superlattices depends on parity of the Cr MLs. For odd numbers Cr magnetic moments are canted and symmetrically distributed between the neighboring Fe slabs. The values of Cr moments are enhanced at the interface and weakened to the bulk in the middle. For even numbers of Cr MLs quasi-helicoidal magnetic moments distribution consisting of two interleaved spirals is found. The moments are screwing sequentially from Fe/Cr interface to perpendicular orientation, keeping the angles and moments for some successive MLs, and then continue screwing towards the next interface. In Fe/Cr(1 1 0) superlattices the magnetic moments of two nonequivalent atoms in the monolayer are canted to each other near Fe/Cr interface and then swing the direction on perpendicular to the fixed Fe moments. - Highlights: • Frustration destroys the collinear magnetization in Fe/Cr superlattices. • Spin spiral were investigated within basic noncollinear Periodic Anderson Model. • Total energy of the spin spiral is generally more stable than collinear magnetization. • Neither step nor alloying at the Fe/Cr interface is necessary for Spin Spiral stability. • The two interleaved spirals calculated are similar to Fishman’s helical state.

  15. Raman-induced Spin-Orbit Coupling in Optical Superlattices

    Science.gov (United States)

    Li, Junru; Huang, Wujie; Shteynas, Boris; Burchesky, Sean; Top, Furkan; Jamison, Alan; Ketterle, Wolfgang

    2016-05-01

    We demonstrate a new scheme for spin-orbit coupling (SOC) of ultracold atoms. Instead of internal (hyperfine) states, two lowest bands in an optical superlattice were used as pseudospins. A Raman process was implemented to provide coupling between pseudospin and momentum. With single internal state and far-detuned beams used, our new scheme will allow convenient generalisation to a wide range of atoms. Pseudospin interaction is tuneable by controlling the superlattice, allowing us to study many-body phenomena in SOC systems such as the stripe phase.

  16. 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.

  17. The solition properties of dipole domains in superlattices

    Institute of Scientific and Technical Information of China (English)

    张启义; 田强

    2002-01-01

    The formation and propagation of dipole domains in superlattices are studied both by the modified discrete drift model and by the nonlinear schroedinger equation,the spatiotemporal distribution of the electric field and electron density are presented.The numerical results are compared with the soliton solutions of the nonlinear Schroedinger equation and analysed.It is shown that the numerical solutions agree with the soliton solutions of the nonlinear Schroedinger equation.The dipole electric-field domains in semiconductor superlattices have the properties of solitons.

  18. Spin-dependent terahertz oscillator based on hybrid graphene superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Díaz, E.; Miralles, K.; Domínguez-Adame, F. [GISC, Departamento Física de Materiales, Universidad Complutense, E-28040 Madrid (Spain); Gaul, C., E-mail: cgaul@pks.mpg.de [Max Planck Institute for the Physics of Complex Systems, 01187 Dresden (Germany)

    2014-09-08

    We theoretically study the occurrence of Bloch oscillations in biased hybrid graphene systems with spin-dependent superlattices. The spin-dependent potential is realized by a set of ferromagnetic insulator strips deposited on top of a gapped graphene nanoribbon, which induce a proximity exchange splitting of the electronic states in the graphene monolayer. We numerically solve the Dirac equation and study Bloch oscillations in the lowest conduction band of the spin-dependent superlattice. While the Bloch frequency is the same for both spins, we find the Bloch amplitude to be spin dependent. This difference results in a spin-polarized ac electric current in the THz range.

  19. Photon BLOCH oscillations in porous silicon optical superlattices.

    Science.gov (United States)

    Agarwal, V; del Río, J A; Malpuech, G; Zamfirescu, M; Kavokin, A; Coquillat, D; Scalbert, D; Vladimirova, M; Gil, B

    2004-03-01

    We report the first observation of oscillations of the electromagnetic field in an optical superlattice based on porous silicon. These oscillations are an optical equivalent of well-known electronic Bloch oscillations in crystals. Elementary cells of our structure are composed by microcavities whose coupling gives rise to the extended collective modes forming optical minigaps and minibands. By varying thicknesses of the cavities along the structure axis, we have created an effective electric field for photons. A very high quality factor of the confined optical state of the Wannier-Stark ladder may allow lasing in porous silicon-based superlattices.

  20. Binding Graphene Sheets Together Using Silicon: Graphene/Silicon Superlattice

    Directory of Open Access Journals (Sweden)

    Zhang Yong

    2010-01-01

    Full Text Available Abstract We propose a superlattice consisting of graphene and monolayer thick Si sheets and investigate it using a first-principles density functional theory. The Si layer is found to not only strengthen the interlayer binding between the graphene sheets compared to that in graphite, but also inject electrons into graphene, yet without altering the most unique property of graphene: the Dirac fermion-like electronic structure. The superlattice approach represents a new direction for exploring basic science and applications of graphene-based materials.

  1. Nanoscale Deformable Optics

    Science.gov (United States)

    Strauss, Karl F.; Sheldon, Douglas J.

    2011-01-01

    Several missions and instruments in the conceptual design phase rely on the technique of interferometry to create detectable fringe patterns. The intimate emplacement of reflective material upon electron device cells based upon chalcogenide material technology permits high-speed, predictable deformation of the reflective surface to a subnanometer or finer resolution with a very high degree of accuracy. In this innovation, a layer of reflective material is deposited upon a wafer containing (perhaps in the millions) chalcogenic memory cells with the reflective material becoming the front surface of a mirror and the chalcogenic material becoming a means of selectively deforming the mirror by the application of heat to the chalcogenic material. By doing so, the mirror surface can deform anywhere from nil to nanometers in spots the size of a modern day memory cell, thereby permitting realtime tuning of mirror focus and reflectivity to mitigate aberrations caused elsewhere in the optical system. Modern foundry methods permit the design and manufacture of individual memory cells having an area of or equal to the Feature (F) size of the design (assume 65 nm). Fabrication rules and restraints generally require the instantiation of one memory cell to another no closer than 1.5 F, or, for this innovation, 90 nm from its neighbor in any direction. Chalcogenide is a semiconducting glass compound consisting of a combination of chalcogen ions, the ratios of which vary according to properties desired. It has been shown that the application of heat to cells of chalcogenic material cause a large alteration in resistance to the range of 4 orders of magnitude. It is this effect upon which chalcogenidebased commercial memories rely. Upon removal of the heat source, the chalcogenide rapidly cools and remains frozen in the excited state. It has also been shown that the chalcogenide expands in volume because of the applied heat, meaning that the coefficient of expansion of chalcogenic

  2. Active Beam Shaping System and Method Using Sequential Deformable Mirrors

    Science.gov (United States)

    Norman, Colin A. (Inventor); Pueyo, Laurent A. (Inventor)

    2015-01-01

    An active optical beam shaping system includes a first deformable mirror arranged to at least partially intercept an entrance beam of light and to provide a first reflected beam of light, a second deformable mirror arranged to at least partially intercept the first reflected beam of light from the first deformable mirror and to provide a second reflected beam of light, and a signal processing and control system configured to communicate with the first and second deformable mirrors. The first deformable mirror, the second deformable mirror and the signal processing and control system together provide a large amplitude light modulation range to provide an actively shaped optical beam.

  3. Type II superlattice technology for LWIR detectors

    Science.gov (United States)

    Klipstein, P. C.; Avnon, E.; Azulai, D.; Benny, Y.; Fraenkel, R.; Glozman, A.; Hojman, E.; Klin, O.; Krasovitsky, L.; Langof, L.; Lukomsky, I.; Nitzani, M.; Shtrichman, I.; Rappaport, N.; Snapi, N.; Weiss, E.; Tuito, A.

    2016-05-01

    SCD has developed a range of advanced infrared detectors based on III-V semiconductor heterostructures grown on GaSb. The XBn/XBp family of barrier detectors enables diffusion limited dark currents, comparable with MCT Rule-07, and high quantum efficiencies. This work describes some of the technical challenges that were overcome, and the ultimate performance that was finally achieved, for SCD's new 15 μm pitch "Pelican-D LW" type II superlattice (T2SL) XBp array detector. This detector is the first of SCD's line of high performance two dimensional arrays working in the LWIR spectral range, and was designed with a ~9.3 micron cut-off wavelength and a format of 640 x 512 pixels. It contains InAs/GaSb and InAs/AlSb T2SLs, engineered using k • p modeling of the energy bands and photo-response. The wafers are grown by molecular beam epitaxy and are fabricated into Focal Plane Array (FPA) detectors using standard FPA processes, including wet and dry etching, indium bump hybridization, under-fill, and back-side polishing. The FPA has a quantum efficiency of nearly 50%, and operates at 77 K and F/2.7 with background limited performance. The pixel operability of the FPA is above 99% and it exhibits a stable residual non uniformity (RNU) of better than 0.04% of the dynamic range. The FPA uses a new digital read-out integrated circuit (ROIC), and the complete detector closely follows the interfaces of SCD's MWIR Pelican-D detector. The Pelican- D LW detector is now in the final stages of qualification and transfer to production, with first prototypes already integrated into new electro-optical systems.

  4. Decoupling the refractive index from the electrical properties of transparent conducting oxides via periodic superlattices

    Science.gov (United States)

    Caffrey, David; Norton, Emma; Coileáin, Cormac Ó.; Smith, Christopher M.; Bulfin, Brendan; Farrell, Leo; Shvets, Igor V.; Fleischer, Karsten

    2016-09-01

    We demonstrate an alternative approach to tuning the refractive index of materials. Current methodologies for tuning the refractive index of a material often result in undesirable changes to the structural or optoelectronic properties. By artificially layering a transparent conducting oxide with a lower refractive index material the overall film retains a desirable conductivity and mobility while acting optically as an effective medium with a modified refractive index. Calculations indicate that, with our refractive index change of 0.2, a significant reduction of reflective losses could be obtained by the utilisation of these structures in optoelectronic devices. Beyond this, periodic superlattice structures present a solution to decouple physical properties where the underlying electronic interaction is governed by different length scales.

  5. Deformation microstructures

    DEFF Research Database (Denmark)

    Hansen, N.; Huang, X.; Hughes, D.A.

    2004-01-01

    Microstructural characterization and modeling has shown that a variety of metals deformed by different thermomechanical processes follows a general path of grain subdivision, by dislocation boundaries and high angle boundaries. This subdivision has been observed to very small structural scales...... of the order of 10 nm, produced by deformation under large sliding loads. Limits to the evolution of microstructural parameters during monotonic loading have been investigated based on a characterization by transmission electron microscopy. Such limits have been observed at an equivalent strain of about 10...

  6. Superlattice conductivity sign change induced by intense electromagnetic radiation

    Science.gov (United States)

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

    2016-07-01

    The current density in a superlattice exposed to a quantizing electric field and the terahertz field has been calculated. The calculations have been carried out taking into account inelastic scattering of charge carriers by phonons. The possibility of an absolute negative conductivity, i.e., the emergence of electric current opposing the direction of the quantizing electric field, has been demonstrated.

  7. Strong impact of impurity bands on domain formation in superlattices

    DEFF Research Database (Denmark)

    Wacker, Andreas; Jauho, Antti-Pekka

    1998-01-01

    The formation of electric field domains in doped semiconductor superlattices is described within a microscopic model. Due to the presence of impurity bands in low-doped samples the current-voltage characteristic is essentially different compared to medium-doped samples. (C) 1998 Published by Else...

  8. Coherent magnetic structures in terbium/holmium superlattices

    DEFF Research Database (Denmark)

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

    1997-01-01

    Neutron-scattering techniques have been used to investigate the magnetic properties of three Tb/Ho superlattices grown by molecular-beam epitaxy. It is revealed that for temperatures in the range T = 10 to T-N(Ho)approximate to 130 K, there is a basal-plane ferromagnetic alignment of Tb moments...

  9. The structural and magnetic properties of holmium/scandium superlattices

    DEFF Research Database (Denmark)

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

    1997-01-01

    The properties of Ho/Sc superlattices grown by molecular beam epitaxy (MBE) have been investigated using X-ray and neutron diffraction techniques. Structural studies reveal the novel existence of more than one a lattice parameter. Examining the magnetic properties, it is found that the Ho 4f...

  10. 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...

  11. Bypassing of a barrier by dissociated and superlattice dislocations

    DEFF Research Database (Denmark)

    Bhushan, Karihaloo

    1975-01-01

    Very simple procedures are used to calculate the upper and lower bounds for the applied stress required for the leading extended (superlattice) dislocation in a group of n coplanar screw dislocations of like sign with Burgers vector b to bypass a noncoplanar perfect screw dislocation with Burgers...... vector mb (m...

  12. Type-II superlattice photodiodes: an alternative for VLWIR detection

    Science.gov (United States)

    Brown, Gail J.; Houston, Shanee; Szmulowicz, Frank; Mahalingam, Krishnamur; Haugan, Heather; Wei, Yajun; Gin, Aaron; Razeghi, Manijeh

    2003-09-01

    In the very long wavelength infrared (VLWIR) band, λ>14 microns, the detector materials are currently limited to extrinsic semiconductors. These extrinsic materials can be either heavily doped bulk semiconductor, like silicon or germanium, or a doped quantum well heterostructure. An alternative choice that provides the opportunity for higher temperature operation for VLWIR sensing is an intrinsic material based on a type-II InAs/Ga(In)Sb superlattice. There are many possible designs for these superlattices which will produce the same narrow band gap by adjusting individual layer thicknesses, indium content or substrate orientation. The infrared properties of various compositions and designs of these type-II superlattices have been studied. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 15 microns. An overview of the status of this material system will be presented. In addition, the latest experimental results for superlattice photodiodes with cut-off wavelengths as long as 30 microns will be covered.

  13. Heterojunction and superlattice detectors for infrared to ultraviolet

    Science.gov (United States)

    Perera, A. G. U.

    2016-07-01

    The interest in Infrared and Ultraviolet detectors has increased immensely due to the emergence of important applications over a wide range of activities. Detectors based on free carrier absorption known as Hetero-junction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors and variations of these heterojunction structures to be used as intervalence band detectors for a wide wavelength region are presented. Although this internal photoemission concept is valid for all semiconductor materials systems, using a well-studied III-V system of GaAs/AlxGa1-x As to cover a wide wavelength range from UV to far-infrared (THz) is an important development in detector technology. Using the intervalence band (heavy hole, light hole and split off) transitions for high operating temperature detection of mid Infrared radiation is also discussed. A promising new way to extend the detection wavelength threshold beyond the standard threshold connected with the energy gap in a GaAs/AlxGa1-x As system is also presented. Superlattice detector technology, which is another promising detector architecture, can be optimized using both Type I and Type II heterostructures. Here the focus will be on Type II Strained Layer (T2SL) Superlattice detectors. T2SL Superlattices based on InAs/(In,GA)Sb have made significant improvements demonstrating focal plane arrays operating around 80 K and with multiple band detection capability. A novel spectroscopic method to evaluate the band offsets of both heterojunction and superlattice detectors is also discussed.

  14. 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...

  15. Haglund's Deformity

    Science.gov (United States)

    ... to follow the surgeon’s instructions for postsurgical care. Prevention To help prevent a recurrence of Haglund’s deformity: wear appropriate shoes; avoid shoes with a rigid heel back use arch supports or orthotic devices perform stretching exercises to prevent the Achilles tendon from tightening ...

  16. Multistability, chaos, and random signal generation in semiconductor superlattices

    Science.gov (United States)

    Ying, Lei; Huang, Danhong; Lai, Ying-Cheng

    2016-06-01

    Historically, semiconductor superlattices, artificial periodic structures of different semiconductor materials, were invented with the purpose of engineering or manipulating the electronic properties of semiconductor devices. A key application lies in generating radiation sources, amplifiers, and detectors in the "unusual" spectral range of subterahertz and terahertz (0.1-10 THz), which cannot be readily realized using conventional radiation sources, the so-called THz gap. Efforts in the past three decades have demonstrated various nonlinear dynamical behaviors including chaos, suggesting the potential to exploit chaos in semiconductor superlattices as random signal sources (e.g., random number generators) in the THz frequency range. We consider a realistic model of hot electrons in semiconductor superlattice, taking into account the induced space charge field. Through a systematic exploration of the phase space we find that, when the system is subject to an external electrical driving of a single frequency, chaos is typically associated with the occurrence of multistability. That is, for a given parameter setting, while there are initial conditions that lead to chaotic trajectories, simultaneously there are other initial conditions that lead to regular motions. Transition to multistability, i.e., the emergence of multistability with chaos as a system parameter passes through a critical point, is found and argued to be abrupt. Multistability thus presents an obstacle to utilizing the superlattice system as a reliable and robust random signal source. However, we demonstrate that, when an additional driving field of incommensurate frequency is applied, multistability can be eliminated, with chaos representing the only possible asymptotic behavior of the system. In such a case, a random initial condition will lead to a trajectory landing in a chaotic attractor with probability 1, making quasiperiodically driven semiconductor superlattices potentially as a reliable

  17. The correlation between the temperature dependence of the CRSS and the formation of superlattice-intrinsic stacking faults in the nickel-base superalloy PWA 1480. [critical resolved shear stress

    Science.gov (United States)

    Milligan, Walter W.; Antolovich, Stephen D.

    1989-01-01

    The PWA 1480 nickel-base superalloy is known to exhibit a unique minimum in the critical resolved shear stress (CRSS) at about 400 C. This paper reports an observation of a deformation mechanism whose temperature dependence correlates exactly with the reduction in the CRSS. It was found that, after monotonic or cyclic deformation of PWA 1480 at 20 C, the deformation substructures typically contain high density of superlattice-intrinsic stacking faults (S-ISFs) within the gamma-prime precipitates. As the temperature of deformation is increased, the density of S-ISFs is reduced, until finally no faults are observed after deformation in the range from 400 to 705 C. The reduction in the fault density corresponds exactly to the reduction in the CRSS, and the temperature at which the fault density is zero corresponds with the minimum in the CRRS. Two possible mechanisms related to the presence of the S-ISFs in the alloy are considered.

  18. Piezoelectrics by Design: A Route through Short-period Perovskite Superlattices

    CERN Document Server

    Das, Hena; Saha-Dasgupta, T

    2010-01-01

    Using first-principles density functional theory, we study piezoelectricity in short-period superlattices made with combination of ferroelectric and paraelectric components and exhibiting polar discontinuities. We show that piezoelectric response of such a superlattice can be tuned both in terms of sign and magnitude with a choice of its components. As these superlattices with nonswitchable polarization do not undergo ferroelectric transitions, we predict them to exhibit a robust piezoelectric response with weaker temperature dependence compared to their bulk counterparts.

  19. Picosecond luminescence approach to vertical transport in GaAs/GaAlAs superlattices

    Science.gov (United States)

    Deveaud, B.; Chomette, A.; Lambert, B.; Regreny, A.; Romestain, R.; Edel, P.

    1986-03-01

    Picosecond luminescence of GaAs/GaAlAs superlattices has been measured at 5 K. Asymetrical structures where one larger well is introduced at 9000 Å from the surface are studied. It is then possible to estimate the mean transfer time of photoexcited carriers through 9000 Å of superlattice. This time is found to be about 4 nsec in a 40/40 Å superlattice and 800 psec in a 30/30 Å one. This evidences the rather high mobility of small period superlattices in the growth direction.

  20. A GaAssolarAlAs superlattice autocorrelator for picosecond THz radiation pulses

    Science.gov (United States)

    Winnerl, S.; Pesahl, S.; Schomburg, E.; Grenzer, J.; Renk, K. F.; Pellemans, H. P. M.; van der Meer, A. F. G.; Pavel'ev, D. G.; Koschurinov, Yu.; Ignatov, A. A.; Melzer, B.; Ustinov, V.; Ivanov, S.; Kop'ev, P. S.

    1999-01-01

    We report on a GaAs/AlAs, wide-miniband, superlattice autocorrelator for picosecond THz radiation pulses (operated at room temperature); the autocorrelator is based on the THz radiation-induced reduction of current through the superlattice. THz radiation (frequency 7.2 THz) from the FELIX (free-electron laser for infrared experiments) was coupled into the superlattice with an antenna system. We measured the current reduction for two time-delayed pulses and found that the signal decreased when the time delay was smaller than the pulse duration. With this superlattice autocorrelator we were able to resolve laser pulses that had a duration of a few picoseconds.

  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. 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.

  3. 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.

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

    Science.gov (United States)

    Malyshev, K. V.

    2015-01-01

    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 F011(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.

  5. High Coefficient of Performance HgCdTe And Metallic Superlattice-Based Thermoelectric Coolers Project

    Data.gov (United States)

    National Aeronautics and Space Administration — We propose the development of nanoscale superlattices (SLs) as the active elements of high efficiency thermoelectric coolers. Recent models predict that the...

  6. Perceptual transparency from image deformation.

    Science.gov (United States)

    Kawabe, Takahiro; Maruya, Kazushi; Nishida, Shin'ya

    2015-08-18

    Human vision has a remarkable ability to perceive two layers at the same retinal locations, a transparent layer in front of a background surface. Critical image cues to perceptual transparency, studied extensively in the past, are changes in luminance or color that could be caused by light absorptions and reflections by the front layer, but such image changes may not be clearly visible when the front layer consists of a pure transparent material such as water. Our daily experiences with transparent materials of this kind suggest that an alternative potential cue of visual transparency is image deformations of a background pattern caused by light refraction. Although previous studies have indicated that these image deformations, at least static ones, play little role in perceptual transparency, here we show that dynamic image deformations of the background pattern, which could be produced by light refraction on a moving liquid's surface, can produce a vivid impression of a transparent liquid layer without the aid of any other visual cues as to the presence of a transparent layer. Furthermore, a transparent liquid layer perceptually emerges even from a randomly generated dynamic image deformation as long as it is similar to real liquid deformations in its spatiotemporal frequency profile. Our findings indicate that the brain can perceptually infer the presence of "invisible" transparent liquids by analyzing the spatiotemporal structure of dynamic image deformation, for which it uses a relatively simple computation that does not require high-level knowledge about the detailed physics of liquid deformation.

  7. 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.

  8. Isolated structures in two-dimensional optical superlattice

    Science.gov (United States)

    Zou, Xin-Hao; Yang, Bao-Guo; Xu, Xia; Tang, Peng-Ju; Zhou, Xiao-Ji

    2017-10-01

    Overlaying commensurate optical lattices with various configurations called superlattices can lead to exotic lattice topologies and, in turn, a discovery of novel physics. In this study, by overlapping the maxima of lattices, a new isolated structure is created, while the interference of minima can generate various "sublattice" patterns. Three different kinds of primitive lattices are used to demonstrate isolated square, triangular, and hexagonal "sublattice" structures in a two-dimensional optical superlattice, the patterns of which can be manipulated dynamically by tuning the polarization, frequency, and intensity of laser beams. In addition, we propose the method of altering the relative phase to adjust the tunneling amplitudes in "sublattices". Our configurations provide unique opportunities to study particle entanglement in "lattices" formed by intersecting wells and to implement special quantum logic gates in exotic lattice geometries.

  9. Measuring spin correlations in optical lattices using superlattice potentials

    DEFF Research Database (Denmark)

    Pedersen, Kim Georg Lind; Andersen, Brian Møller; Bruun, Georg Morten;

    2011-01-01

    We suggest two experimental methods for probing both short- and long-range spin correlations of atoms in optical lattices using superlattice potentials. The first method involves an adiabatic doubling of the periodicity of the underlying lattice to probe neighboring singlet (triplet) correlations...... for fermions (bosons) by the occupation of the resulting vibrational ground state. The second method utilizes a time-dependent superlattice potential to generate spin-dependent transport by any number of prescribed lattice sites, and probes correlations by the resulting number of doubly occupied sites....... For experimentally relevant parameters, we demonstrate how both methods yield large signatures of antiferromagnetic correlations of strongly repulsive fermionic atoms in a single shot of the experiment. Lastly, we show how this method may also be applied to probe d-wave pairing, a possible ground-state candidate...

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

    Energy Technology Data Exchange (ETDEWEB)

    Lima, Jonas R. F., E-mail: jonas.iasd@gmail.com [Instituto de Ciencia de Materiales de Madrid (CSIC) - Cantoblanco, Madrid 28049 (Spain)

    2015-02-28

    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{sub g} can be tuned in the range 0 ≤ E{sub 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.

  11. Nonlinear thermoelectric efficiency of superlattice-structured nanowires

    Science.gov (United States)

    Karbaschi, Hossein; Lovén, John; Courteaut, Klara; Wacker, Andreas; Leijnse, Martin

    2016-09-01

    We theoretically investigate nonlinear ballistic thermoelectric transport in a superlattice-structured nanowire. By a special choice of nonuniform widths of the superlattice barriers—analogous to antireflection coating in optical systems—it is possible to achieve a transmission which comes close to a square profile as a function of energy. We calculate the low-temperature output power and power-conversion efficiency of a thermoelectric generator based on such a structure and show that the efficiency remains high also when operating at a significant power. To provide guidelines for experiments, we study how the results depend on the nanowire radius, the number of barriers, and on random imperfections in barrier width and separation. Our results indicate that high efficiencies can indeed be achieved with today's capabilities in epitaxial nanowire growth.

  12. Coupled bloch-phonon oscillations in semiconductor superlattices

    Science.gov (United States)

    Dekorsy; Bartels; Kurz; Kohler; Hey; Ploog

    2000-07-31

    We investigate coherent Bloch oscillations in GaAs/AlxGa1-xAs superlattices with electronic miniband widths larger than the optical phonon energy. In these superlattices the Bloch frequency can be tuned into resonance with the optical phonon. Close to resonance a direct coupling of Bloch oscillations to LO phonons is observed which gives rise to the coherent excitation of LO phonons. The density necessary for driving coherent LO phonons via Bloch oscillations is about 2 orders of magnitude smaller than the density necessary to drive coherent LO phonons in bulk GaAs. The experimental observations are confirmed by the theoretical description of this phenomenon [A.W. Ghosh et al., Phys. Rev. Lett. 85, 1084 (2000)].

  13. Resonant tunnelling in a Fibonacci bilayer graphene superlattice

    Energy Technology Data Exchange (ETDEWEB)

    Mukhopadhyay, S.; Sinha, C. [Department of Theoretical Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, West Bengal (India); Biswas, R. [Department of Physics, PK College, Contai, Purba Medinipur, West Bengal (India)

    2010-02-15

    The transmission coefficients (TCs) and angularly averaged conductance for quasi-particle transport are studied for a bilayer graphene superlattice arranged according to the Fibonacci sequence. The transmission is found to be symmetric around the superlattice growth direction and highly sensitive to the direction of the quasi-particle incidence. The transmission spectra are fragmented and appear in groups due to the quasi-periodicity of the system. The average conductance shows interesting structures sharply dependent on the height of the potential barriers between two graphene strips. The low-energy conductance due to Klein transmission is substantially modified by the inclusion of quasi-periodicity in the system. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

  14. Phonons in Ge/Si superlattices with Ge quantum dots

    CERN Document Server

    Milekhin, A G; Pchelyakov, O P; Schulze, S; Zahn, D R T

    2001-01-01

    Ge/Si superlattices with Ge quantum dots obtained by means of molecular-beam epitaxy were investigated by means of light Raman scattering under resonance conditions. These structures are shown to have oscillation properties of both two-dimensional and zero-dimensional objects. Within spectrum low-frequency range one observes twisted acoustic phonons (up to 15 order) typical for planar superlattices. Lines of acoustic phonons are overlapped with a wide band of continuous emission. Analysis of frequencies of Ge and Ge-Si optical phonons shows that Ge quantum dots are pseudoamorphous ones and mixing of Ge and Si atoms is a negligible one. One detected low-frequency shift of longitudinal optical phonons at laser excitation energy increase (2.54-2.71 eV)

  15. Isolated Structures in Two-Dimensional Optical Superlattice

    CERN Document Server

    Zou, Xinhao; Xu, Xia; Tang, Pengju; Zhou, Xiaoji

    2016-01-01

    Overlaying commensurate optical lattices with various configurations called superlattices can lead to exotic lattice topologies and, in turn, a discovery of novel physics. In this study, by overlapping the maxima of lattices, a new isolated structure is created, while the interference of minima can generate various "sublattice" patterns. Three different kinds of primitive lattices are used to demonstrate isolated square, triangular, and hexagonal "sublattice" structures in a two-dimensional optical superlattice, the patterns of which can be manipulated dynamically by tuning the polarization, frequency, and intensity of laser beams. In addition, we propose the method of altering the relative phase to adjust the tunneling amplitudes in "sublattices." Our configurations provide unique opportunities to study particle entanglement in "lattices" formed by intersecting wells and to implement special quantum logic gates in exotic lattice geometries.

  16. Heterogenous Material Integration and Band Engineering With Type II Superlattice

    Science.gov (United States)

    2015-10-26

    of chemical vapor deposited graphene transferred to SiO2 . Appl. Phys. Lett. 99, 122108 (2011). 113 Ferrari, A. C. Raman spectroscopy of graphene ...extrinsic performance limits of graphene devices on SiO2 . Nat. Nanotechnol. 3, 206-209 (2008). 130 Fang, T., Konar, A., Xing, H. & Jena, D. Mobility...AlSb strained layer superlattices. 15. SUBJECT TERMS crystal growth, characterization, semiconductor fabrication, infrared detectors, graphene

  17. Transparent conducting oxides: a δ-doped superlattice approach.

    Science.gov (United States)

    Cooper, Valentino R; Seo, Sung S Ambrose; Lee, Suyoun; Kim, Jun Sung; Choi, Woo Seok; Okamoto, Satoshi; Lee, Ho Nyung

    2014-08-11

    Metallic states appearing at interfaces between dissimilar insulating oxides exhibit intriguing phenomena such as superconductivity and magnetism. Despite tremendous progress in understanding their origins, very little is known about how to control the conduction pathways and the distribution of charge carriers. Using optical spectroscopic measurements and density-functional theory (DFT) simulations, we examine the effect of SrTiO3 (STO) spacer layer thickness on the optical transparency and carrier distribution in La δ-doped STO superlattices. We experimentally observe that these metallic superlattices remain highly transparent to visible light; a direct consequence of the appropriately large gap between the O 2p and Ti 3d states. In superlattices with relatively thin STO layers, we predict that three-dimensional conduction would occur due to appreciable overlap of quantum mechanical wavefunctions between neighboring δ-doped layers. These results highlight the potential for using oxide heterostructures in optoelectronic devices by providing a unique route for creating novel transparent conducting oxides.

  18. 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.

  19. 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.

  20. Theory of THz harmonic generation in semiconductor superlattices (Conference Presentation)

    Science.gov (United States)

    Pereira, Mauro F.; Winge, David O.; Wacker, Andreas

    2016-10-01

    Superlattices are artificial structures with a wide range of applications and open possibilities for controlling and study transport and optical [M.F. Pereira Jr., Phys. Rev. B 52, (1995)] properties of semiconductors. In this work, we start from the full Nonequilibrium Greens Functions approach [A. Wacker et a, IEEE Journal of Sel. Top. in Quantum Electron.,19 1200611, (2013),T. Schmielau and M.F. Pereira, Appl. Phys. Lett. 95 231111, (2009)] to obtain Voltage-Current curves and compare them with experiments. By adjusting the numerical solutions of the corresponding Dyson equations to a simple model, analytical solutions are given for the nonlinear response of a biased superlattice under sub-THz radiation. The frequency multiplication process leading to multiple harmonicgeneration is described. This hybrid approach leads to predictive simulations and may have important application for a new generation of devices where the superlattices are used as both sources and detectors and may be particular useful for high resolution transient spectroscopy [A.A. Yablokov et at, IEEE Transactions on THz Science and Technology 5, 845 (2015)].

  1. 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.

  2. Bose-Hubbard model on a checkerboard superlattice

    Science.gov (United States)

    Iskin, Menderes

    2011-05-01

    We study the ground-state phases of the Bose-Hubbard model on a checkerboard superlattice in two dimensions, including the superfluid phase and the Mott and charge-density-wave insulators. First, we discuss the single-particle Hofstadter problem, and show that the presence of a checkerboard superlattice gives rise to a magnetic flux-independent energy gap in the excitation spectrum. Then, we consider the many-particle problem, and derive an analytical mean-field expression for the superfluid-Mott and superfluid-charge-density-wave insulator phase transition boundaries. Finally, since the phase diagram of the Bose-Hubbard model on a checkerboard superlattice is in many ways similar to that of the extended Bose-Hubbard model, we comment on the effects of magnetic field on the latter model, and derive an analytical mean-field expression for the superfluid-insulator phase transition boundaries as well. This work is supported by Marie Curie International Reintegration Grant (FP7-PEOPLE-IRG-2010-268239).

  3. Spectral properties of Fibonacci superlattices formed using armchair graphene nanoribbons

    Science.gov (United States)

    Korol, A. M.; Litvynchuk, S. I.; Bagliuk, S. V.; Lazarenko, M. V.

    2016-03-01

    We discuss and analyze the dependence spectra of the transmission coefficient T on the quasiparticle energy E of one variety of graphene-based Fibonacci superlattices (SL). The SL is built from armchair graphene nanoribbons (GNR), and the quasi-periodicity is produced by metal-like (MGNR) and semiconductor (SCGNR) ribbons, placed along the lattice growth axis in accordance with the Fibonacci sequence, which are used as individual SL elements. It is shown that the difference in the values of quantized transverse quasi-momentum of electrons in MGNR and SCGNR is enough to form an effective quasi-periodic modulation in the examined structure (no additional factors required), and the optimal nanoribbon width range for this purpose is determined. We also analyzed the dependence of the spectral properties of the test structure on the geometric parameters of the superlattice, and the external electrostatic potential. We paid particular attention to the fact that each Fibonacci generation had a Dirac superlattice band gap. The results of the study can be useful in the determination of optimal parameters for graphene-based nanoelectronic devices.

  4. 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.

  5. Reversible solvent vapor-mediated phase changes in nanocrystal superlattices.

    Science.gov (United States)

    Goodfellow, Brian W; Korgel, Brian A

    2011-04-26

    Colloidal nanocrystals are being explored for use in a variety of applications, from solar cells to transistors to medical diagnostics and therapy. Ordered assemblies of nanocrystals, or superlattices, are one particularly interesting class of these materials, in which the nanocrystals serve as modular building blocks to construct nanostructures by self-assembly with spatial and temporal complexity and unique properties. From a fundamental perspective, the nanocrystals are simple molecular models that can be manipulated and studied to test statistical mechanical and thermodynamic models of crystallization and disorder. An article by Bian et al. in this issue of ACS Nano reports surprising new phase behavior in semiconductor nanocrystal superlattices: reversible transitions between non-close-packed body-centered cubic (bcc) and body-centered tetragonal (bct) structures, and close-packed face-centered cubic (fcc) structures, observed by real-time in situ grazing incidence small-angle X-ray scattering (GISAXS) measurements, upon solvent vapor exposure and increased interparticle separation. These studies offer new insight and raise new questions about superlattice structure and the forces that control self-assembly. Accompanying computer simulations show that ligand-ligand interactions are important. Furthermore, it appears that ligand-coated nanocrystals have more in common with soft microphase-separated materials, like diblock copolymers and surfactant assemblies, than previously realized.

  6. Shape-anisotropy driven symmetry transformations in nanocrystal superlattice polymorphs.

    Science.gov (United States)

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

    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.

  7. Laser molecular-beam epitaxy and second-order optical nonlinearity of BaTiO3/SrTiO3 superlattices

    Institute of Scientific and Technical Information of China (English)

    2000-01-01

    A series of c-axis oriented BaTiO3/SrTiO3 superlattices with the atomic-scale precision were epitaxially grown on single-crystal SrTiO3 (100) substrates using laser molecular-beam epitaxy (LMBE). A periodic modulation of the intensity of reflection high-energy electron diffraction (RHEED) in BaTiO3 and SrTiO3 layers was observed and attributed to the lattice-misfit-induced periodic variation of the terrace density in film surface. The relationship between the second-order nonlinear optical susceptibilities and the superlattice structure was systematically studied. The experimental and theoretical fitting results indicate that the second-order nonlinear optical susceptibilities of BaTiO3/SrTiO3 superlattices were greatly enhanced with the maximum value being more than one order of magnitude larger than that of bulk BaTiO3 crystal. The mechanism of the enhancement of the second-order optical nonlinearity was discussed by taking into account the stress-induced lattice distortion and polarization enhancement.

  8. Reflecting reflection in supervision

    DEFF Research Database (Denmark)

    Lystbæk, Christian Tang

    Reflection has moved from the margins to the mainstream in supervision. Notions of reflection have become well established since the late 1980s. These notions have provided useful framing devices to help conceptualize some important processes in guidance and counseling. However, some applications...

  9. Spin Filtering in a Nanowire Superlattice by Dresselhause Spin-Orbit Coupling

    Institute of Scientific and Technical Information of China (English)

    Samad Javidan

    2011-01-01

    @@ An InAs/GaSb nanowire Superlattice using GaAs for the impure layers is proposed.Dresselhaus spin-orbit coupling eliminates spin degeneracy, induces one miniband in the superlattices to split into two minibands and leads to complete spin polarization and excellent filtering by optimizing the well and barrier widths and GaAs layer distances.

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

    Energy Technology Data Exchange (ETDEWEB)

    Bouchard, A.M.

    1994-07-27

    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.

  11. Enhanced tunnel transport in disordered carbon superlattice structures incorporated with nitrogen

    Science.gov (United States)

    Katkov, Mikhail V.; Bhattacharyya, Somnath

    2012-06-01

    The possibility for enhanced tunnel transport through the incorporation of nitrogen in a quasi-one dimensional superlattice structure of amorphous carbon (a -C) made of sp2-C and sp3-C rich phases is shown by using a tight-binding model. The proposed superstructure can be described by a set of disordered graphite-like carbon clusters (acting as quantum wells) separated by a thin layer of diamond-like carbon (barriers) where the variation of the width and depth of the carbon clusters significantly control the electron transmission peaks. A large structural disorder in the pure carbon system, introduced through the variation of the bond length and associated deformation potential for respective carbon phases, was found to suppress the sharp features of the transmission coefficients. A small percentage of nitrogen addition to the carbon clusters can produce a distinct transmission peak at the low energy; however, it can be practically destroyed due to increase of the level of disorder of carbon sites. Whereas pronounced resonance peaks, both for C and N sites can be achieved through controlling the arrangement of the nitrogen sites of increased concentration within the disordered sp2-C clusters. The interplay of disorder associated with N and C sites illustrated the tunable nature of resistance of the structures as well as their characteristic times.

  12. 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.

  13. Nonlinear optical response in Kronig-Penney type graphene superlattice in terahertz regime

    Science.gov (United States)

    Jiang, Lijuan; Yuan, Rui-Yang; Zhao, Xin; Lv, Jing; Yan, Hui

    2015-05-01

    The terahertz nonlinear optical response in Kronig-Penney (KP) type graphene superlattice is demonstrated. The single-, triple- and quintuple-frequencies of the fifth-order nonlinear responses are investigated for different frequencies and temperatures with the angle φ along the periodicity of the superlattice toward the external field tuning from 0 to π/2. The results show that the fifth-order nonlinear optical conductance of graphene superlattice is enhanced in the terahertz regime when φ = 0, i.e. an external field is applied along the periodicity of the superlattice. The fifth-order nonlinear optical conductances at φ = 0 for different frequencies and temperatures are calculated. The results show that the nonlinear optical conductance is enhanced in low frequency and low temperature. Our results suggest that KP type graphene superlattices are preferred structures for developing graphene-based nonlinear photonics and optoelectronics devices.

  14. ON THE CHARACTERIZATION OF METALLIC SUPERLATTICE STRUCTURES BY X—RAY DIFFRACTION

    Institute of Scientific and Technical Information of China (English)

    MINGXU; WenxueYU; 等

    1999-01-01

    To solve the problem on the microstructural characterization of metallic superlattices,taking the NiFe/Cu superlattices as example,we show that the sturctures of metallic superlattices can be characterized exactly by combining low-angle X-ray diffraction with high-angle X-ray diffraction.First,we determine exactly the total film thickness by a straightforward and precise method based on a modified Bragg law from the subsidiary maxima around the low-angle X-ray diffraction peak.Then.by combining with the simulation of high-angle X-ray diffraction.we obtain the sturctural parameters such as the superlattice period,the sublayer and buffer thickness,This characterization procedure is also applicable to other types of metallic superlattices.

  15. Interface properties of magnetic tunnel junction La0.7Sr0.3MnO3/SrTiO3 superlattices studied by standing-wave excited photoemission spectroscopy

    NARCIS (Netherlands)

    Gray, A.X.; Papp, C.; Balke, B.; Yang, S.-H.; Huijben, M.; Rotenberg, E.; Bostwick, A.; Ueda, S.; Yamashita, Y.; Kobayashi, K.; Gullikson, E.M.; Kortright, J.B.; Groot, de F.M.F.; Rijnders, G.; Blank, D.H.A.; Ramesh, R.; Fadley, C.S.

    2010-01-01

    The chemical and electronic-structure profiles of magnetic tunnel junction (MTJ) La0.7Sr0.3MnO3/SrTiO3 (LSMO/STO) superlattices have been quantitatively determined via soft and hard x-ray standing-wave excited photoemission, x-ray absorption and x-ray reflectivity, in conjunction with x-ray optical

  16. Coherent acoustic phonons in YBa{sub 2}Cu{sub 3}O{sub 7}/La{sub 1/3}Ca{sub 2/3}MnO{sub 3} superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Li, Wei [National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093 (China); Institute of Signal Processing and Transmission, Nanjing University of Posts and Telecommunications, Nanjing 210003, Jiangsu (China); He, Bin; Zhang, Chunfeng, E-mail: cfzhang@nju.edu.cn; Liu, Shenghua; Wang, Xiaoyong [National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093 (China); Liu, Xiaoran; Middey, S.; Chakhalian, J. [Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701 (United States); Xiao, Min, E-mail: mxiao@uark.edu [National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093 (China); Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701 (United States)

    2016-03-28

    We investigate photo-induced coherent acoustic phonons in complex oxide superlattices consisting of high-Tc superconductor YBa{sub 2}Cu{sub 3}O{sub 7−x} and ferromagnetic manganite La{sub 1/3}Ca{sub 2/3}MnO{sub 3} epitaxial layers with broadband pump-probe spectroscopy. Two oscillatory components have been observed in time-resolved differential reflectivity spectra. Based on the analysis, the slow oscillation mode with a frequency sensitive to the probe wavelength is ascribed to the stimulated Brillouin scattering due to the photon reflection by propagating train of coherent phonons. The fast oscillation mode with a probe-wavelength-insensitive frequency is attributed to the Bragg oscillations caused by specular phonon reflections at oxide interfaces or the electron-coupling induced modulation due to free carrier absorption in the metallic superlattices. Our findings suggest that oxide superlattice is an ideal system to tailor the coherent behaviors of acoustic phonons and to manipulate the thermal and acoustic properties.

  17. Dynamics of alkyl chains in monolayer protected metal clusters and their superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Mukhopadhyay, R [Solid State Physics Division, BARC, Mumbai 400085 (India); Mitra, S [Solid State Physics Division, BARC, Mumbai 400085 (India); Johnson, M [Institute Lau-Langevin, BP156, F-38042, Grenoble, Cedex 9 (France); Pradeep, T [Department of Chemistry and SAIF, IITm, Chennai 600 036 (India)

    2007-12-15

    Alkyl chains dynamics in monolayer protected metal cluster (MPC) systems of gold and silver have been studied by the quasielastic neutron scattering (QENS) technique. Isolated MPCs investigated are 6, 12 and 18 carbon n-alkyl chain thiolate protected 4 nm diameter gold clusters while the superlattices are their silver analogues. Evolution of dynamics with temperature is found to be very different in the isolated clusters and their superlattices. While continuous evolution of the dynamics of the monolayer was observed in isolated MPCs, it is abrupt in superlattice systems and occurs at a temperature consistent with the superlattice melting detected in calorimetry measurements. A model where the chain undergoes uniaxial rotational diffusion with additional body axis fluctuation was found to describe the data consistently. For the superlattice systems, the chains are found to be held by strong inter-chain interactions below the superlattice melting. The data from the planar silver thiolate systems show similar behavior like the superlattice systems, consistent with the calorimetric data.

  18. The Role of Ligand Packing Frustration in Body-Centered Cubic (bcc) Superlattices of Colloidal Nanocrystals.

    Science.gov (United States)

    Goodfellow, Brian W; Yu, Yixuan; Bosoy, Christian A; Smilgies, Detlef-M; Korgel, Brian A

    2015-07-02

    This paper addresses the assembly of body centered-cubic (bcc) superlattices of organic ligand-coated nanocrystals. First, examples of bcc superlattices of dodecanethiol-capped Au nanocrystals and oleic acid-capped PbS and PbSe nanocrystals are presented and examined by transmission electron microscopy (TEM) and grazing incidence small-angle X-ray scattering (GISAXS). These superlattices tend to orient on their densest (110) superlattice planes and exhibit a significant amount of {112} twinning. The same nanocrystals deposit as monolayers with hexagonal packing, and these thin films can coexist with thicker bcc superlattice layers, even though there is no hexagonal plane in a bcc lattice. Both the preference of bcc in bulk films over the denser face-centered cubic (fcc) superlattice structure and the transition to hexagonal monolayers can be rationalized in terms of packing frustration of the ligands. A model is presented to calculate the difference in entropy associated with capping ligand packing frustration in bcc and fcc superlattices.

  19. Progress in MBE grown type-II superlattice photodiodes

    Science.gov (United States)

    Hill, Cory J.; Li, Jian V.; Mumolo, Jason M.; Gunapala, Sarath D.

    2006-01-01

    We report on the status of GaSb/InAs type-II superlattice diodes grown and fabricated at the Jet Propulsion Laboratory designed for infrared absorption in the 8-12(mu)m range. Recent devices have produced detectivities as high as 8x10 to the tenth power Jones with a differential resistance-area product greater than 6 Ohmcm(sup 2) at 80K with a long wavelength cutoff of approximately 12(mu)m. The measured quantum efficiency of these front-side illuminated devices is close to 30% in the 10-11(mu)m range without antireflection coatings.

  20. Zener tunneling of light waves in an optical superlattice.

    Science.gov (United States)

    Ghulinyan, Mher; Oton, Claudio J; Gaburro, Zeno; Pavesi, Lorenzo; Toninelli, Costanza; Wiersma, Diederik S

    2005-04-01

    We report on the observation of Zener tunneling of light waves in spectral and time-resolved transmission measurements, performed on an optical superlattice made of porous silicon. The structure was designed to have two photonic minibands, spaced by a narrow frequency gap. A gradient in the refractive index was introduced to create two optical Wannier-Stark ladders and, at a critical value of the optical gradient, tunneling between energy bands was observed in the form of an enhanced transmission peak and a characteristic time dependence of the transmission.

  1. Theory of the Fermi-level energy in semiconductor superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Luscombe, J.H. (Central Research Laboratories, Texas Instruments Incorporated, Dallas, Texas (USA)); Aggarwal, R. (Central Research Laboratories, Texas Instruments Incorporated, Dallas, Texas (USA) Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts (USA)); Reed, M.A. (Central Research Laboratories, Texas Instruments Incorporated, Dallas, Texas (USA) Department of Electrical Engineering, Yale University, New Haven, Connecticut (USA)); Frensley, W.R. (Central Research Laboratories, Texas Instruments Incorporated, Dallas, Texas (USA) Department of Electrical Engineering, University of Texas at Dallas, Richardson, Texas (USA)); Luban, M. (Iowa Univ., Iowa City, IA (USA). Dept. of Physics and Astronomy Ames Lab., IA (USA))

    1991-09-15

    A theoretical study of the properties of the Fermi level in semiconductor superlattices (SL's) is made which is based upon the carrier occupation of the minibands in thermal equilibrium. We find, for a fixed carrier density and temperature, that the SL Fermi level can differ significantly from that obtained using commonly employed three-dimensional approximations, depending upon the relative spacings and widths of the minibands, with the SL Fermi level being higher than the corresponding bulk value. We find that the SL Fermi level is a sensitive function of the relative widths of the quantum wells and barriers.

  2. The DUV Stability of Superlattice-Doped CMOS Detector Arrays

    Science.gov (United States)

    Hoenk, M. E.; Carver, A. G.; Jones, T.; Dickie, M.; Cheng, P.; Greer, H. F.; Nikzad, S.; Sgro, J.; Tsur, S.

    2013-01-01

    JPL and Alacron have recently developed a high performance, DUV camera with a superlattice doped CMOS imaging detector. Supperlattice doped detectors achieve nearly 100% internal quantum efficiency in the deep and far ultraviolet, and a single layer, Al2O3 antireflection coating enables 64% external quantum efficiency at 263nm. In lifetime tests performed at Applied Materials using 263 nm pulsed, solid state and 193 nm pulsed excimer laser, the quantum efficiency and dark current of the JPL/Alacron camera remained stable to better than 1% precision during long-term exposure to several billion laser pulses, with no measurable degradation, no blooming and no image memory at 1000 fps.

  3. Electronic Bloch oscillation in bilayer graphene gradient superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Cheng, Hemeng; Li, Changan; Song, Yun [Department of Physics, Beijing Normal University, Beijing 100875 (China); Ma, Tianxing, E-mail: txma@bnu.edu.cn [Department of Physics, Beijing Normal University, Beijing 100875 (China); Beijing Computational Science Research Center, Beijing 100084 (China); Wang, Li-Gang, E-mail: sxwlg@yahoo.com [Department of Physics, Zhejiang University, Hangzhou 310027 (China); Lin, Hai-Qing [Beijing Computational Science Research Center, Beijing 100084 (China)

    2014-08-18

    We investigate the electronic Bloch oscillation in bilayer graphene gradient superlattices using transfer matrix method. By introducing two kinds of gradient potentials of square barriers along electrons propagation direction, we find that Bloch oscillations up to terahertz can occur. Wannier-Stark ladders, as the counterpart of Bloch oscillation, are obtained as a series of equidistant transmission peaks, and the localization of the electronic wave function is also signature of Bloch oscillation. Furthermore, the period of Bloch oscillation decreases linearly with increasing gradient of barrier potentials.

  4. Surface photovoltage spectroscopy of quantum wells and superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Bachrach-Ashkenasy, N.; Kronik, L.; Shapira, Y. [Department of Physical Electronics, Faculty of Engineering, Tel-Aviv University, Ramat-Aviv 69978 (Israel); Rosenwaks, Y.; Hanna, M.C. [National Renewable Energy Laboratory, Golden, Colorado 80401 (United States); Leibovitch, M.; Ram, P. [Physics Department, Brooklyn College of the City University of New York, Brooklyn, New York 11210 (United States)

    1996-02-01

    Surface photovoltage spectroscopy (SPS) has been employed to monitor optical transitions in quantum well and superlattice structures at room temperature. Excellent agreement is found between theoretical predictions of heavy hole and electron energy level positions and the observed transitions. The results show that using this technique, the complete band diagram of the quantum structure may be constructed. SPS emerges as a powerful tool capable of monitoring optical transitions above the lowest one in a simple to interpret, contactless, and nondestructive way. {copyright} {ital 1996 American Institute of Physics.}

  5. Reflective Teaching

    Science.gov (United States)

    Farrell, Thomas S. C.

    2013-01-01

    Thomas Farrell's "Reflective Teaching" outlines four principles that take teachers from just doing reflection to making it a way of being. Using the four principles, Reflective Practice Is Evidence Based, Reflective Practice Involves Dialogue, Reflective Practice Links Beliefs and Practices, and Reflective Practice Is a Way of Life,…

  6. Elastic superlattices with simultaneously negative effective mass density and shear modulus

    Science.gov (United States)

    Solís-Mora, I. S.; Palomino-Ovando, M. A.; Pérez-Rodríguez, F.

    2013-03-01

    We investigate the vibrational properties of superlattices with layers of rubber and polyurethane foam, which can be either conventional or auxetic. Phononic dispersion calculations show a second pass band for transverse modes inside the lowest band gap of the longitudinal modes. In such a band, the superlattices behave as a double-negative elastic metamaterial since the effective dynamic mass density and shear modulus are both negative. The pass band is associated to a Fabry-Perot resonance band which turns out to be very narrow as a consequence of the high contrast between the acoustic impedances of the superlattice components.

  7. Physical properties of ferroelectric superlattice A3/B3 system in electric field

    Institute of Scientific and Technical Information of China (English)

    Jiang Wei; Lo Veng-Cheong; Bai Bao-Dong

    2005-01-01

    Based on the differential operator technique, a transverse Ising model (TIM) in the effective-field theory is developed to study the physical properties of a ferroelectric superlattice A3/B3 system. The effects of an external electric field on the polarization, susceptibility and pyroelectric coefficient of the ferroelectric superlattice A3/B3 system are discussed in detail. The susceptibility of the ferroelectric superlattice A3/B3 system decreases with the increase of the electric field, implying that the polarization is weak.

  8. The intensive terahertz electroluminescence induced by Bloch oscillations in SiC natural superlattices.

    Science.gov (United States)

    Sankin, Vladimir; Andrianov, Alexandr; Petrov, Alexey; Zakhar'in, Alexey; Lepneva, Ala; Shkrebiy, Pavel

    2012-10-09

    : We report on efficient terahertz (THz) emission from high-electric-field-biased SiC structures with a natural superlattice at liquid helium temperatures. The emission spectrum demonstrates a single line, the maximum of which shifts linearly with increases in bias field. We attribute this emission to steady-state Bloch oscillations of electrons in the SiC natural superlattice. The properties of the THz emission agree fairly with the parameters of the Bloch oscillator regime, which have been proven by high-field electron transport studies of SiC structures with natural superlattices.

  9. 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...... 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...

  10. Effect of the degree of disorder on electronic and optical properties in random superlattices

    Science.gov (United States)

    Wang, E. G.; Su, W. P.; Ting, C. S.

    1994-01-01

    A three-dimensional tight-binding calculation is developed and used to study disorder effects in a realistic random superlattice. With increasing disorder, a tendency of possible indirect-direct band-gap transition is suggested. Direct evidence of mobility edges between localized and extended states in three-dimensional random systems is given. As system disorder increases, the optical absorption intensities increase dramatically from five to forty-five times stronger than the ordered (GaAs)(sub 1)/(AlAs)(sub 1) superlattice. It is believed that the degree of disorder significantly affects electronic and optical properties of GaAs/AlAs random superlattices.

  11. Optical Properties of Self-Organized PbS Quantum Dot Superlattices

    Institute of Scientific and Technical Information of China (English)

    YE Chang-Hui; YAO Lian-Zeng; MU Ji-Mei; SHI Gang; ZHANG Li-De

    2000-01-01

    Self-organization of PbS into quantum dot superlattices has been demonstrated for the first time, and hexaplanar colloidal crystals 1 - 10 μm in size made from PbS quantum dots 3 - 6 nm in diameter are revealed in transmissionelectron microscope micrographs, and the inner structures of the superlattices can be seen by a high resolution transmission electron microscopy. The optical absorption and photoluminescence spectra have been recorded. The ordering of the superlattices is crucial for the understanding of the fundamental properties of quantum-dot arrays, as well as for their optimal utilization in optical and electronic applications.

  12. Demonstration of long minority carrier lifetimes in very narrow bandgap ternary InAs/GaInSb superlattices

    Science.gov (United States)

    Haugan, H. J.; Brown, G. J.; Olson, B. V.; Kadlec, E. A.; Kim, J. K.; Shaner, E. A.

    2015-09-01

    Minority carrier lifetimes in very long wavelength infrared (VLWIR) InAs/GaInSb superlattices (SLs) are reported using time-resolved microwave reflectance measurements. A strain-balanced ternary SL absorber layer of 47.0 Å InAs/21.5 Å Ga0.75In0.25Sb, corresponding to a bandgap of ˜50 meV, is found to have a minority carrier lifetime of 140 ± 20 ns at ˜18 K. This lifetime is extraordinarily long, when compared to lifetime values previously reported for other VLWIR SL detector materials. This enhancement is attributed to the strain-engineered ternary design, which offers a variety of epitaxial advantages and ultimately leads to a reduction of defect-mediated recombination centers.

  13. Low dark current N structure superlattice MWIR photodetectors

    Science.gov (United States)

    Salihoglu, Omer; Muti, Abdullah; Turan, Rasit; Ergun, Yuksel; Aydinli, Atilla

    2014-06-01

    Commercially available read out integrated circuits (ROICs) require the FPA to have high dynamic resistance area product at zero bias (R0A) which is directly related to dark current of the detector. Dark current arises from bulk and surface contributions. Recent band structure engineering studies significantly suppressed the bulk contribution of the type-II superlattice infrared photodetectors (N structure, M structure, W structure). In this letter, we will present improved dark current results for unipolar barrier complex supercell superlattice system which is called as "N structure". The unique electronic band structure of the N structure increases electron-hole overlap under bias, significantly. N structure aims to improve absorption by manipulating electron and hole wavefunctions that are spatially separated in T2SLs, increasing the absorption while decreasing the dark current. In order to engineer the wavefunctions, we introduce a thin AlSb layer between InAs and GaSb layers in the growth direction which also acts as a unipolar electron barrier. Despite the difficulty of perfect lattice matching of InAs and AlSb, such a design is expected to reduce dark current. Experiments were carried out on Single pixel with mesa sizes of 100 × 100 - 700 × 700 μm photodiodes. Temperature dependent dark current with corresponding R0A resistance values are reported.

  14. Weak Topological Insulators in PbTe/SnTe superlattice

    Science.gov (United States)

    Yang, Gang; Liu, Junwei; Fu, Liang; Duan, Wenhui; Liu, Chaoxing

    2014-03-01

    It is desirable to realize topological phases in artificial structures by engineering electronic band structures. In this paper, we investigate (PbTe)m(SnTe)2n-m superlattices along the [001] direction and find a robust weak topological insulator phase for a large variety of layer numbers m and 2 n - m . We confirm this topologically non-trivial phase by calculating Z2 topological invariants and topological surface states based on the first-principles calculations. We show that the folding of Brillouin zone due to the superlattice structure plays an essential role in inducing topologically non-trivial phases in this system. This mechanism can be generalized to other systems in which band inversion occurs at multiple momenta, and gives us a brand-new way to engineer topological materials in artificial structures. We acknowledge support from the Ministry of Science and Technology of China and the National Natural Science Foundation of China. LF is supported by the DOE Office of Basic Energy Sciences.

  15. Beating the amorphous limit in thermal conductivity by superlattices design.

    Science.gov (United States)

    Mizuno, Hideyuki; Mossa, Stefano; Barrat, Jean-Louis

    2015-09-16

    The value measured in the amorphous structure with the same chemical composition is often considered as a lower bound for the thermal conductivity of any material: the heat carriers are strongly scattered by disorder, and their lifetimes reach the minimum time scale of thermal vibrations. An appropriate design at the nano-scale, however, may allow one to reduce the thermal conductivity even below the amorphous limit. In the present contribution, using molecular-dynamics simulation and the Green-Kubo formulation, we study systematically the thermal conductivity of layered phononic materials (superlattices), by tuning different parameters that can characterize such structures. We have discovered that the key to reach a lower-than-amorphous thermal conductivity is to block almost completely the propagation of the heat carriers, the superlattice phonons. We demonstrate that a large mass difference in the two intercalated layers, or weakened interactions across the interface between layers result in materials with very low thermal conductivity, below the values of the corresponding amorphous counterparts.

  16. Type-ii binary superlattices for infrared detector

    Energy Technology Data Exchange (ETDEWEB)

    Razeghi, M.; Mohseni, H. [Northwestern Univ., Evanston (United States); Brown, G. J. [WPAFB, Colombus (United States)

    2001-12-01

    III-V quantum wells and superlattices based on InAs/GaSb/AlSb, and related compounds have attracted many attentions due to their unique band alignments and physical properties. Recently, novel electronic and optoelectronic heterostructures have been proposed from this material system for hundred gigahertz logic circuits, terahertz transistors. RTDs, infrared lasers, and infrared detectors. In this paper we will describe the ongoing research at the Center for Quantum Devices to develop the theory, modeling, growth, characterization, and device fabrication techniques for this material system. We have demonstarted the first uncooled infrared detectors from type-II superlattices. The measured detectivity is more than 1 x 10{sup 8} cmHz{sup 1/2}/W at 10.6 {mu}m at room temperature which is higher than the commercially available uncooled photon detectors at similar wavelength. In paralle, we have demonstraed the first high-performance p-i-n type-II photodiode in the very long wavelength infrared (VLWIR) range operating at T=80K. The devices with cutoff wavelength of 16 mm showed a responsivity of 3.5 A/W at 80 K leading to a detectivity of {approx}1.51x10{sup 10} cmHz{sup 1/2}/W. Similar devices with cutoff wavelengths up to 25 {mu}m was demonstrated at 80 K. To enhance this technology further, we plan to move from quantum wells to quantum wire and quantum dots.

  17. Microemulsion-based synthesis of copper nanodisk superlattices

    Science.gov (United States)

    Sun, Lei; Zhao, Yanbao; Guo, Wenjing; Tao, Xiaojun; Zhang, Zhijun

    2011-06-01

    Nanocrystal superlattices (NCSs) comprised of self-assembled copper nanodisks were successfully synthesized in quaternary W/O microemulsions containing Span 80-Tween 80, liquid paraffin and n-butanol. Morphologies, structure and thermal properties of the Cu nanocrystals were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectra, thermogravimetry (TG) and differential thermogravimetry (DTG). The reaction conditions which effect the growth of the Cu nanodisks were explored, and a mechanism for the formation of the Cu NCSs is proposed. XRD and TEM studies show that the as-synthesized Cu nanodisks exhibit a cubic crystal structure, and FT-IR and TG analysis show that the surfaces of the Cu nanodisks are covered with surfactants, which assist in the formation of the superlattice and prevent the oxidation of the Cu nanocrystals. Variation of the reaction parameters such as mass ratio of the surfactants and the presence of oleic acid is found to have a significant effect on the formation of the Cu nanodisks.

  18. Commensurability oscillations in a two-dimensional lateral superlattice

    Science.gov (United States)

    Davies, John; Long, Andrew; Grant, David; Chowdhury, Suja

    2000-03-01

    We have calculated and measured conduction in a two-dimensional electron gas subject to a weak two-dimensional periodic potential and a normal magnetic field. Simulations with a potential Vx \\cos(2π x/a) + Vy \\cos(2π y/a) show the usual commensurability oscillations in ρ_xx(B) with Vx alone. The introduction of Vy suppresses these oscillations, rather than introducing the additional oscillations in ρ_yy(B) expected from previous perturbation theories. We explain this in terms of drift of the guiding center of cyclotron motion along contours of an effective potential: open orbits of the guiding center contribute to conduction but closed orbits do not. All orbits are closed in a symmetric superlattice with |V_x| = |V_y| and commensurability oscillations are therefore quenched. Experiments on etched superlattices confirm this picture. Conventional lattice-matched samples give a symmetric potential and weak oscillations; the symmetry is broken by the piezoelectric effect in stressed samples, leading to strong oscillations. Periodic modulation of the magnetic field can be treated in the same way, which explains previous experimental results.

  19. Magnetic domain wall energy in Ni/Co superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Toyoki, Kentaro; Nishimura, Takashi; Harimoto, Shotaro; Shiratsuchi, Yu, E-mail: shiratsuchi@mat.eng.osaka-u.ac.jp; Nakatani, Ryoichi

    2014-12-15

    The magnetic domain wall energy density σ{sub W} of a Ni/Co superlattice possessing perpendicular magnetic anisotropy was determined using the magnetic domain theory derived by Kooy and Enz (1960). To determine σ{sub W}, we obtained the saturation magnetization, magnetic domain period, and perpendicular magnetic anisotropy energy by individual measurements. Using the magnetic domain period and the ferromagnetic layer thickness, we first determined the dipolar length. The estimated dipolar length was about 15–25 nm, which is in good agreement with the change in the magnetization curve with the ferromagnetic layer thickness. By using the dipolar length and saturation magnetization, the σ{sub W} was calculated to be 4–7 erg/cm{sup 2}. - Highlights: • Magnetic domain wall energy of a Ni/Co superlattice was determined experimentally. • The magnetic domain wall energy was estimated to be 4–8 erg/cm{sup 2}. • Using estimated value, the magnetization curves were reproduced well. • The estimated value is reasonable compared with the other ferromagnetic materials.

  20. Artificially engineered Heusler ferrimagnetic superlattice exhibiting perpendicular magnetic anisotropy

    Science.gov (United States)

    Ma, Q. L.; Zhang, X. M.; Miyazaki, T.; Mizukami, S.

    2015-01-01

    To extend density limits in magnetic recording industry, two separate strategies were developed to build the storage bit in last decade, introduction of perpendicular magnetic anisotropy (PMA) and adoption of ferrimagnetism/antiferromagnetism. Meanwhile, these properties significantly improve device performance, such as reducing spin-transfer torque energy consumption and decreasing signal-amplitude-loss. However, materials combining PMA and antiferromagnetism rather than transition-metal/rare-earth system were rarely developed. Here, we develop a new type of ferrimagnetic superlattice exhibiting PMA based on abundant Heusler alloy families. The superlattice is formed by [MnGa/Co2FeAl] unit with their magnetizations antiparallel aligned. The effective anisotropy (Kueff) over 6 Merg/cm3 is obtained, and the SL can be easily built on various substrates with flexible lattice constants. The coercive force, saturation magnetization and Kueff of SLs are highly controllable by varying the thickness of MnGa and Co2FeAl layers. The SLs will supply a new choice for magnetic recording and spintronics memory application such as magnetic random access memory.

  1. Fine structure of the exciton electroabsorption in semiconductor superlattices

    Science.gov (United States)

    Monozon, B. S.; Schmelcher, P.

    2017-02-01

    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.

  2. 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.

  3. Dimensional control of cobalt spin state in oxide superlattices

    Science.gov (United States)

    Jeong, Da Woon; Choi, W. S.; Okamoto, S.; Sohn, C. H.; Park, H. J.; Kim, J.-Y.; Lee, H. N.; Kim, K. W.; Moon, S. J.; Noh, T. W.

    2013-03-01

    Perovskite cobalt oxide is a very intriguing system with various spin states owing to the delicate balance between crystal field splitting and Hund exchange energy. In this talk, we show that its spin state can be altered through dimensional control, enabled by digital synthesis of perovskite cobalt oxide superlattices. We employed a few unit cells of LaCoO3 as an active magnetic layer, separated by LaAlO3 spacer layer. High quality [(LaCoO3) n (LaAlO3) n ]8 (n = 2, 6, and 10) superlattices were fabricated using pulsed laser epitaxy. Spectroscopic tools including x-ray absorption spectroscopy and optical spectroscopy revealed clear evolution of the electronic structure and resultant spin state by changing dimensionality. Specifically, the spin state changed from a high to a low spin state with a larger optical band gap, as the dimension reduced from 3D to 2D. Dynamic mean field calculation supported the critical role of dimensionality on the spin state and electronic structure of LaCoO3.

  4. Artificially engineered Heusler ferrimagnetic superlattice exhibiting perpendicular magnetic anisotropy.

    Science.gov (United States)

    Ma, Q L; Zhang, X M; Miyazaki, T; Mizukami, S

    2015-01-19

    To extend density limits in magnetic recording industry, two separate strategies were developed to build the storage bit in last decade, introduction of perpendicular magnetic anisotropy (PMA) and adoption of ferrimagnetism/antiferromagnetism. Meanwhile, these properties significantly improve device performance, such as reducing spin-transfer torque energy consumption and decreasing signal-amplitude-loss. However, materials combining PMA and antiferromagnetism rather than transition-metal/rare-earth system were rarely developed. Here, we develop a new type of ferrimagnetic superlattice exhibiting PMA based on abundant Heusler alloy families. The superlattice is formed by [MnGa/Co2FeAl] unit with their magnetizations antiparallel aligned. The effective anisotropy (K(u)(eff)) over 6 Merg/cm(3) is obtained, and the SL can be easily built on various substrates with flexible lattice constants. The coercive force, saturation magnetization and K(u)(eff) of SLs are highly controllable by varying the thickness of MnGa and Co2FeAl layers. The SLs will supply a new choice for magnetic recording and spintronics memory application such as magnetic random access memory.

  5. High quantum efficiency Type-II superlattice N-structure photodetectors with thin intrinsic layers

    Science.gov (United States)

    Ergun, Yuksel; Hostut, Mustafa; Tansel, Tunay; Muti, Abdullah; Kilic, Abidin; Turan, Rasit; Aydinli, Atilla

    2013-06-01

    We report on the development of InAs/AlSb/GaSb based N-structure superlattice pin photodiode. In this new design, AlSb layer in between InAs and GaSb layers acts as an electron barrier that pushes electron and hole wave functions towards the GaSb/InAs interface to perform strong overlap under reverse bias. Experimental results show that, with only 20 periods of intrinsic layers, dark current density and dynamic resistance at -50 mV bias are measured as 6x10-3 A/cm2 and 148 Ωcm2 at 77K, respectively. Under zero bias, high spectral response of 1.2A/W is obtained at 5 μm with 50% cut-off wavelengths (λc) of 6 μm. With this new design, devices with only 146 nm thick i-regions exhibit a quantum efficiency of 42% at 3 μm with front-side illimunation and no anti-reflection coatings.

  6. Sterol superlattice affects antioxidant potency and can be used to assess adverse effects of antioxidants.

    Science.gov (United States)

    Olsher, Michelle; Chong, Parkson Lee-Gau

    2008-11-01

    We have developed a fluorescence method to examine how membrane sterol lateral organization affects the potency of antioxidants, and used this information to evaluate possible adverse effects of lipid-soluble antioxidants seen in recent clinical studies. In the presence of an antioxidant, the lag time (tau) produced during free radical-induced sterol oxidation in lipid vesicles reflects the potency of the antioxidant. The ascorbic acid-induced tau value varies with sterol mol% in a biphasic manner, showing a minimum at the critical sterol mole fraction for maximal superlattice formation (C r), in ascorbic acid concentrations antioxidant potency. In contrast, the biphasic change in tau at C r was observed only at doses of ascorbyl palmitate antioxidant than its water-soluble counterpart as judged by the tau value, it can easily perturb sterol lateral organization by insertion into membrane bilayers, which could impose detrimental effects on cells. The threshold antioxidant concentration (C th) to abolish biphasic change in tau at C r may vary with antioxidant and could be used to assess potential adverse effects of other lipid-soluble antioxidants.

  7. Quantum efficiency investigations of type-II InAs/GaSb midwave infrared superlattice photodetectors

    Energy Technology Data Exchange (ETDEWEB)

    Giard, E., E-mail: edouard.giard@onera.fr; Ribet-Mohamed, I.; Jaeck, J.; Viale, T.; Haïdar, R. [ONERA, DOTA, Chemin de la Hunière, 91761 Palaiseau Cedex (France); Taalat, R.; Delmas, M.; Rodriguez, J.-B.; Christol, P. [Institut d' Electronique du Sud, UMR-CNRS 5214, Université Montpellier 2, Place Eugène Bataillon, 34095 Montpellier Cedex 5 (France); Steveler, E.; Bardou, N. [Laboratoire de Photonique et de Nanostructures (LPN-CNRS), Route de Nozay, 91460 Marcoussis (France); Boulard, F. [CEA, LETI, MINATEC Campus, 17 Avenue des martyrs, 38054 Grenoble (France)

    2014-07-28

    We present in this paper a comparison between different type-II InAs/GaSb superlattice (T2SL) photodiodes and focal plane array (FPA) in the mid-wavelength infrared domain to understand which phenomenon drives the performances of the T2SL structure in terms of quantum efficiency (QE). Our measurements on test photodiodes suggest low minority carrier diffusion length in the “InAs-rich” design, which penalizes carriers' collection in this structure for low bias voltage and front side illumination. This analysis is completed by a comparison of the experimental data with a fully analytic model, which allows to infer a hole diffusion length shorter than 100 nm. In addition, measurements on a FPA with backside illumination are finally presented. Results show an average QE in the 3–4.7 μm window equal to 42% for U{sub bias} = −0.1 V, 77 K operating temperature and no anti-reflection coating. These measurements, completed by modulation transfer function and noise measurements, reveal that the InAs-rich design, despite a low hole diffusion length, is promising for high performance infrared imaging applications.

  8. Optical Observation of Plasmonic Nonlocal Effects in a 2D Superlattice of Ultrasmall Gold Nanoparticles.

    Science.gov (United States)

    Shen, Hao; Chen, Li; Ferrari, Lorenzo; Lin, Meng-Hsien; Mortensen, N Asger; Gwo, Shangjr; Liu, Zhaowei

    2017-03-02

    The advances in recent nanofabrication techniques have facilitated explorations of metal structures into nanometer scales, where the traditional local-response Drude model with hard-wall boundary conditions fails to accurately describe their optical responses. The emerging nonlocal effects in single ultrasmall silver nanoparticles have been experimentally observed in single-particle spectroscopy enabled by the unprecedented high spatial resolution of electron energy loss spectroscopy (EELS). However, the unambiguous optical observation of such new effects in gold nanoparticles has yet not been reported, due to the extremely weak scattering and the obscuring fingerprint of strong interband transitions. Here we present a nanosystem, a superlattice monolayer formed by sub-10 nm gold nanoparticles. Plasmon resonances are spectrally well-separated from interband transitions, while exhibiting clearly distinguishable blueshifts compared to predictions by the classical local-response model. Our far-field spectroscopy was performed by a standard optical transmission and reflection setup, and the results agreed excellently with the hydrodynamic nonlocal model, opening a simple and widely accessible way for addressing quantum effects in nanoplasmonic systems.

  9. 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.

  10. Simple theoretical analysis of the Einstein’s photoemission from quantum confined superlattices

    Science.gov (United States)

    Pahari, S.; Bhattacharya, S.; Roy, S.; Saha, A.; De, D.; Ghatak, K. P.

    2009-11-01

    In this paper, we study the Einstein's photoemission from III-V, II-VI, IV-VI and HgTe/CdTe quantum well superlattices (QWSLs) with graded interfaces and quantum well effective mass superlattices in the presence of a quantizing magnetic field on the basis of newly formulated dispersion relations in the respective cases. Besides, the same has been studied from the afore-mentioned quantum dot superlattices and it appears that the photoemission oscillates with increasing carrier degeneracy and quantizing magnetic field in different manners. In addition, the photoemission oscillates with film thickness and increasing photon energy in quantum steps together with the fact that the solution of the Boltzmann transport equation will introduce new physical ideas and new experimental findings under different external conditions. The influence of band structure is apparent from all the figures and we have suggested three applications of the analyses of this paper in the fields of superlattices and microstructures.

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

    DEFF Research Database (Denmark)

    Gorczyca, Iza; Suski, Tadek; Staszczak, Grzegorz;

    2013-01-01

    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......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...

  12. High-Detectivity Type-II Superlattice Detectors for 6-14 um Infrared Applications Project

    Data.gov (United States)

    National Aeronautics and Space Administration — SVT Associates proposes an novel type II superlattice structure to extend the cutoff wavelength and CBIRD SL photo diode structure with unipolar barriers to suppress...

  13. High Electric Field Quantum Transport: Submillimeter Wave AC Stark Localization in Vertical and Lateral Superlattices.

    Science.gov (United States)

    2007-11-02

    superlattices. These experiments have opened the arena of photon assisted transport to semiconductor devices and paved the way for future teraherz: electronics based on quantum transport in semiconductor nanostructures.

  14. Engineering the electronic structure of graphene superlattices via Fermi velocity modulation

    Science.gov (United States)

    Lima, Jonas R. F.

    2017-01-01

    Graphene superlattices have attracted much research interest in the last years, since it is possible to manipulate the electronic properties of graphene in these structures. It has been verified that extra Dirac points appear in the electronic structure of the system. The electronic structure in the vicinity of these points has been studied for a gapless and gapped graphene superlattice and for a graphene superlattice with a spatially modulated energy gap. In each case a different behavior was obtained. In this work we show that via Fermi velocity engineering it is possible to tune the electronic properties of a graphene superlattice to match all the previous cases studied. We also obtained new features of the system never observed before, reveling that the electronic structure of graphene is very sensitive to the modulation of the Fermi velocity. The results obtained here are relevant for the development of novel graphene-based electronic devices.

  15. Huge spin-transfer torque in a magnetic tunnel junction by a superlattice barrier

    Science.gov (United States)

    Chen, C. H.; Tseng, P.; Ko, C. W.; Hsueh, W. J.

    2017-09-01

    Huge spin-transfer torque (STT) in a magnetic tunnel junction (MTJ) achieved by superlattice barrier composed of alternate layers of a nonmagnetic metal and an insulator is proposed. The magnitude of the STT depends on the number of cells in the superlattice barrier and the nonmagnetic metal layer's thickness. The result shows that the STT of the novel superlattice-barrier MTJ can reach values up to four orders of magnitude greater than those of traditional single-barrier stacks based on three cells superlattice by designing the nonmagnetic metal layer's thickness. In addition, the spin-transfer torque of the proposed MTJ can also be thousands of magnitude greater than those of traditional double-barrier MTJs.

  16. Harmonic hexagonal superlattice pattern in a dielectric barrier discharge at atmospheric pressure

    Institute of Scientific and Technical Information of China (English)

    DONG LiFang; XIE WeiXia; ZHAO HaiTao; FAN WeiLi

    2009-01-01

    We report a harmonic hexagonal superlattice pattern in a dielectric barrier discharge in air/argon mixture at atmospheric pressure.The bifurcation scenario of harmonic hexagonal superlattice pattern with the applied voltage increasing is given.The phase diagram of the pattern types as a function of the applied voltage and the air-concentration is obtained.The hysteresis of pattern transitions at the upward and downward stage of the applied voltage is observed.The correlation measurements indicate that harmonic hexagonal superlattice pattern is an interleaving of two different transient sublattices.The spatial power spectrum demonstrates that harmonic hexagonal superlattice pattern has two separate wave vectors.Both small wave vector qh and big wave vector Kh,belong to the harmonic mode,and they obey a triad resonant interaction q1h + q2h,=Kh.

  17. Influence of Fermi velocity engineering on electronic and optical properties of graphene superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Aram, Tahereh Nemati [Research Institute for Applied Physics and Astronomy, University of Tabriz, Tabriz (Iran, Islamic Republic of); Université Grenoble Alpes, Institut Neel, 38042 Grenoble (France); Asgari, Asghar, E-mail: asgari@tabrizu.ac.ir [Research Institute for Applied Physics and Astronomy, University of Tabriz, Tabriz (Iran, Islamic Republic of); School of Electrical, Electronic and Computer Engineering, The University of Western Australia, Crawley, WA 6009 (Australia)

    2015-06-05

    In this paper, using Kronig–Penney model, the electronic states in graphene-based superlattices with various substrates and considering exact electron Fermi velocity values are investigated. The analysis of our results clearly indicates that the difference between Fermi velocity values of gaped and gapless graphene regions determines the patency rate of band gap. Also, using transfer matrix method (TMM) the absorbance spectrum of mentioned structures is calculated. The more important result is that the absorbance of these structures is significantly near zero. - Highlights: • The electronic states in graphene superlattices with various substrates are investigated. • The exact electron Fermi velocity values are considered. • Using TMM the absorbance spectrum of two graphene-based superlattices is calculated. • The widest (narrowest) energy band gap belong to quartz–SiC (quartz–h-BN) superlattice.

  18. Thermal conductivity measurement of InGaAs/InGaAsP superlattice thin films

    Institute of Scientific and Technical Information of China (English)

    CHEN Zhen; YANG Juekuan; ZHUANG Ping; CHEN Minhua; ZHU Jian; CHEN Yunfei

    2006-01-01

    The thermal conductivities of InGaAs/InGaAsP superlattices with different period lengths were measured from 100 to 320 K using 3ω method.In this temperature range, the thermal conductivities were found to decrease with an increase in temperature. For the period length-dependant thermal conductivity, the minimum value does exist at a certain period length, which demonstrates that at a short period length, superlattice thermal conductivity increases with a decrease in the period length. When the period is longer than a certain period length, the interface thermal resistance dominates in phonon transport. The experimental and theoretical results confirmed the previous predictions from the lattice dynamics analysis, i.e. with the increase in period length, the dominant mechanisms of phonon transport in superlattices will shift from wave mode to particle mode. This is crucial for the cutoff of the phonons and lays a sound foundation for the design of superlattice structures.

  19. 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....... It is found that a good description of the dependence of T-N upon concentration is given by a virtual-crystal model where the peak in the conduction-electron susceptibility varies linearly between that of the pure constituents. In the superlattices, the moments also form a basal-plane helix at T...

  20. q-Deformed Relativistic Fermion Scattering

    Directory of Open Access Journals (Sweden)

    Hadi Sobhani

    2017-01-01

    Full Text Available In this article, after introducing a kind of q-deformation in quantum mechanics, first, q-deformed form of Dirac equation in relativistic quantum mechanics is derived. Then, three important scattering problems in physics are studied. All results have satisfied what we had expected before. Furthermore, effects of all parameters in the problems on the reflection and transmission coefficients are calculated and shown graphically.

  1. Dislocations: 75 years of Deformation Mechanisms

    Science.gov (United States)

    Schneider, Judy

    2009-01-01

    The selection of papers presented in this section reflect on themes to be explored at the "Dislocations: 75 years of Deformation Mechanisms" Symposium to be held at the Annual 2009 TMS meeting. The symposium was sponsored by the Mechanical Behavior of Materials Committee to give tribute to the evolution of a concept that has formed the basis of our mechanistic understanding of how crystalline solids plastically deform and how they fail.

  2. Structure property relationships of nitride superlattice hard coatings prepared by pulsed laser deposition

    Science.gov (United States)

    Patel, Nitin

    Today, more than 40% of all cutting tools used in machining applications are covered with coatings. Coatings improve wear resistance, increase tool life, enable use at higher speed, and broaden the application range. Superlattices, where thin layers (typically deposited in an alternating fashion, are widely used commercially. Importantly, the hardness value of a superlattice (e.g. TiN/AlN) can significantly exceed the rule of mixture value. Superlattice coatings built from crystallographically dissimilar materials are not widely studied but hold promise for improvements in performance by allowing for both hardness and toughness to be simultaneously optimized. This is what this thesis is concerned with: a structure-property comparison of isostructural superlattices with corresponding non-isostructural superlattices. In order to grow both isostructural and non-isostructural superlattices from the same set of materials, it is necessary to grow monolithic films in different phases. Towards this end, the synthesis of different phases of AlN, (Ti,Al)N, TaN, and TiN was investigated. Films were grown by pulsed laser deposition in two different chambers that had different base pressures to study the effect of background gases on the phases and orientations of the films. Growth of AlN and (Ti,Al)N films is strongly affected in a chamber that had a base pressure of 10-6 Torr, but the films adopt their stable nitride structures in a chamber with the lower base pressure of 10-8 Torr. TaN adopts either the cubic rock salt structure or its stable hexagonal structure, depending on the growth temperature, while TiN grows as rock salt in all conditions. Single crystal epitaxial superlattices were then grown with different compositions, periodicities, and crystallographic orientations to compare the effect of chemistry, nanostructure, and crystallographic texture on hardness. Finally, the structure-property relationships of non-isostructural (cubic/hexagonal) superlattices are

  3. Transport in weakly-coupled superlattices: A quantitative approach for photon-assisted tunneling

    DEFF Research Database (Denmark)

    Wacker, Andreas; Jauho, Antti-Pekka

    1997-01-01

    Photon-assisted tunneling is studied in weakly-coupled semiconductor superlattices under THz irradiation. Using a microscopic transport model we find excellent quantitative agreement with experimental data for two different samples without using any fitting parameters.......Photon-assisted tunneling is studied in weakly-coupled semiconductor superlattices under THz irradiation. Using a microscopic transport model we find excellent quantitative agreement with experimental data for two different samples without using any fitting parameters....

  4. Electron transport across a quantum wire embedding a saw-tooth superlattice

    Institute of Scientific and Technical Information of China (English)

    Chen Yuan-Ping; Yan Xiao-Hong; Lu Mao-Wang; Deng Yu-Xiang

    2004-01-01

    By developing the recursive Green function method, the transport properties through a quantum wire embedding a finite-length saw-tooth superlattice are studied in the presence of magnetic field. The effects of magnetic modulation and the geometric structures of the superlattice on transmission coefficient are discussed. It is shown that resonant electron gas. The transmission spectrum can be tailored to match requirements through adjusting the size of saw-tooth quantum dot and field strength.

  5. Photonic band structure of one-dimensional aperiodic superlattices composed of negative refraction metamaterials

    Science.gov (United States)

    Tyc, Michał H.; Salejda, Włodzimierz; Klauzer-Kruszyna, Agnieszka; Tarnowski, Karol

    2007-05-01

    The dispersion relation for polarized light transmitting through a one-dimensional superlattice composed of aperiodically arranged layers made of ordinary dielectric and negative refraction metamaterials is calculated with finite element method. Generalized Fibonacci, generalized Thue-Morse, double-periodic and Rudin-Shapiro superlattices are investigated, using their periodic approximants. Strong dispersion of metamaterials is taken into account. Group velocities and effective refraction indices in the structures are calculated. The self-similar structure of the transmission spectra is observed.

  6. Tunable Negative Differential Resistance in Planer Graphene Superlattice Resonant Tunneling Diode

    OpenAIRE

    Sattari-Esfahlan, S. M.; Fouladi-Oskuei, J.; S. Shojaei

    2017-01-01

    In this paper, we report on the controllable negative differential resistance (NDR) in a proposed planar graphene superlattice structure. High value of peak to valley ratio (PVR) is predicted. This is significant because of appearance of NDR with high PVR at low biases. Our finding is important since beside the other potential applications of the graphene, proposes implementation of the graphene based superlattice in electronic devices such as resonant tunneling diode and filters.

  7. 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)

  8. Self-Organization of PbS into Quantum Dots Superlattices

    Institute of Scientific and Technical Information of China (English)

    2000-01-01

    Self-organization of PbS into quantum dots superlattices is demonstrated for the first time, and hexaplanar colloidal crystals 1-10m m in size made from PbS quantum dots 4nm in diameter are shown in Transmission Electron Microscope (TEM) micrograph, and the inner structures of the superlattices can be seen from the High Resolution Transmission Electron Microscope (HRTEM).

  9. Controlling Chaos Probability of a Bose-Einstein Condensate in a Weak Optical Superlattice

    Institute of Scientific and Technical Information of China (English)

    XU Jun; LUO Xiao-Bing

    2009-01-01

    @@ The spatial chaos probability of a Bose-Einstein condensate perturbed by a weak optical superlattice is studied. It is demonstrated that the spatial chaotic solution appears with a certain probability in a given parameter region under a random boundary condition. The effects of the lattice depths and wave vectors on the chaos probability are illustrated, and different regions associated with different chaos probabilities are found. This suggests a feasible scheme for suppressing and strengthening chaos by adjusting the optical superlattice experimentally.

  10. Minority Carrier Lifetime in Beryllium-Doped InAs/InAsSb Strained Layer Superlattices

    Science.gov (United States)

    2014-06-03

    SECURITY CLASSIFICATION OF: Minority carrier lifetimes in undoped and Beryllium -doped Type-2 Ga-free, InAs/InAsSb strained layer superlattices (SLS) with...is unlimited. Minority Carrier Lifetime in Beryllium -Doped InAs/InAsSb Strained Layer Superlattices The views, opinions and/or findings contained in...Brook University W-5510 Melville Library West Sayville, NY 11796 -3362 1 ABSTRACT Minority Carrier Lifetime in Beryllium -Doped InAs/InAsSb Strained

  11. Manganites in Perovskite Superlattices: Structural and Electronic Properties

    KAUST Repository

    Jilili, Jiwuer

    2016-07-13

    Perovskite oxides have the general chemical formula ABO3, where A is a rare-earth or alkali-metal cation and B is a transition metal cation. Perovskite oxides can be formed with a variety of constituent elements and exhibit a wide range of properties ranging from insulators, metals to even superconductors. With the development of growth and characterization techniques, more information on their physical and chemical properties has been revealed, which diversified their technological applications. Perovskite manganites are widely investigated compounds due to the discovery of the colossal magnetoresistance effect in 1994. They have a broad range of structural, electronic, magnetic properties and potential device applications in sensors and spintronics. There is not only the technological importance but also the need to understand the fundamental mechanisms of the unusual magnetic and transport properties that drive enormous attention. Manganites combined with other perovskite oxides are gaining interest due to novel properties especially at the interface, such as interfacial ferromagnetism, exchange bias, interfacial conductivity. Doped manganites exhibit diverse electrical properties as compared to the parent compounds. For instance, hole doped La0.7Sr0.3MnO3 is a ferromagnetic metal, whereas LaMnO3 is an antiferromagnetic insulator. Since manganites are strongly correlated systems, heterojunctions composed of manganites and other perovskite oxides are sunject to complex coupling of the spin, orbit, charge, and lattice degrees of freedom and exhibit unique electronic, magnetic, and transport properties. Electronic reconstructions, O defects, doping, intersite disorder, magnetic proximity, magnetic exchange, and polar catastrophe are some effects to explain these interfacial phenomena. In our work we use first-principles calculations to study the structural, electronic, and magnetic properties of manganite based superlattices. Firstly, we investigate the electronic

  12. Design of MWIR Type-II Superlattices for Infrared Photon Detectors

    Science.gov (United States)

    Grein, Christoph

    The Type II InAs/GaInSb and InAs/InAsSb superlattices are material systems for implementation as photodetector absorbers in infrared imaging applications. In addition to cutoff wavelengths spanning the infrared spectrum, they offer degrees of freedom in their materials design (e.g. layer thicknesses, alloy compositions, number of layers in one superlattice period) that permit the optimization of an infrared photon detector's figures of merit such as detectivity through the tuning of material properties like generation/recombination lifetimes and optical absorption. We describe efforts to obtain accurate electronic band structures of superlattice semiconductors with infrared energy gaps, and employing them to evaluate nonradiative minority carrier lifetimes. Simple device models are utilized to suggest potential performance enhancements that arise from employing superlattices as infrared absorber. We also discuss current efforts to simulate the molecular beam epitaxial growth of InAs/InAsSb superlattices to predict dominant native point defects and other growth nonidealities. Design of MWIR Type-II Superlattices for Infrared Photon Detectors.

  13. High-temperature crystallization of nanocrystals into three-dimensional superlattices

    Science.gov (United States)

    Wu, Liheng; Willis, Joshua J.; McKay, Ian Salmon; Diroll, Benjamin T.; Qin, Jian; Cargnello, Matteo; Tassone, Christopher J.

    2017-08-01

    Crystallization of colloidal nanocrystals into superlattices represents a practical bottom-up process with which to create ordered metamaterials with emergent functionalities. With precise control over the size, shape and composition of individual nanocrystals, various single- and multi-component nanocrystal superlattices have been produced, the lattice structures and chemical compositions of which can be accurately engineered. Nanocrystal superlattices are typically prepared by carefully controlling the assembly process through solvent evaporation or destabilization or through DNA-guided crystallization. Slow solvent evaporation or cooling of nanocrystal solutions (over hours or days) is the key element for successful crystallization processes. Here we report the rapid growth (seconds) of micrometre-sized, face-centred-cubic, three-dimensional nanocrystal superlattices during colloidal synthesis at high temperatures (more than 230 degrees Celsius). Using in situ small-angle X-ray scattering, we observe continuous growth of individual nanocrystals within the lattices, which results in simultaneous lattice expansion and fine nanocrystal size control due to the superlattice templates. Thermodynamic models demonstrate that balanced attractive and repulsive interparticle interactions dictated by the ligand coverage on nanocrystal surfaces and nanocrystal core size are responsible for the crystallization process. The interparticle interactions can also be controlled to form different superlattice structures, such as hexagonal close-packed lattices. The rational assembly of various nanocrystal systems into novel materials is thus facilitated for both fundamental research and for practical applications in the fields of magnetics, electronics and catalysis.

  14. Molecular dynamics study of the lattice thermal conductivity of Kr/Ar superlattice nanowires

    Energy Technology Data Exchange (ETDEWEB)

    Chen Yunfei; Li Deyu; Yang Juekuan; Wu Yonghua; Lukes, J.R.; Majumdar, Arun

    2004-06-15

    The nonequilibrium molecular dynamics (NEMD) method has been used to calculate the lattice thermal conductivities of Ar and Kr/Ar nanostructures in order to study the effects of interface scattering, boundary scattering, and elastic strain on lattice thermal conductivity. Results show that interface scattering poses significant resistance to phonon transport in superlattices and superlattice nanowires. The thermal conductivity of the Kr/Ar superlattice nanowire is only about ((1)/(3)) of that for pure Ar nanowires with the same cross-sectional area and total length due to the additional interfacial thermal resistance. It is found that nanowire boundary scattering provides significant resistance to phonon transport. As the cross-sectional area increases, the nanowire boundary scattering decreases, which leads to increased nanowire thermal conductivity. The ratio of the interfacial thermal resistance to the total effective thermal resistance increases from 30% for the superlattice nanowire to 42% for the superlattice film. Period length is another important factor affecting the effective thermal conductivity of the nanostructures. Increasing the period length will lead to increased acoustic mismatch between the adjacent layers, and hence increased interfacial thermal resistance. However, if the total length of the superlattice nanowire is fixed, reducing the period length will lead to decreased effective thermal conductivity due to the increased number of interfaces. Finally, it is found that the interfacial thermal resistance decreases as the reference temperature increases, which might be due to the inelastic interface scattering.

  15. 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.

  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. Two-dimensional Confinement of Heavy Fermions in Artificial Superlattices

    Science.gov (United States)

    Shishido, Hiroaki

    2011-03-01

    Low dimensionality and strong electron-electron Coulomb interactions are both key parameters for novel quantum states of condensed matter. A metallic system with the strongest electron correlations is reported in rare-earth and actinide compounds with f electrons, known as heavy-fermion compounds, where the effective mass of the conduction electrons are strikingly enhanced by the electron correlations up to some hundreds times the free electron mass. To date the electronic structure of all heavy-fermion compounds is essentially three-dimensional. We realized experimentally a two-dimensional heavy fermion system, adjusting the dimensionality in a controllable fashion. We grew artificial superlattices of CeIn 3 (m)/ LaIn 3 (n), in which m -layers of heavy-fermion antiferromagnet CeIn 3 and n -layers of a non-magnetic isostructual compound LaIn 3 are stacked alternately, by a molecular beam epitaxy. By reducing the thickness of the CeIn 3 layers, the magnetic order was suppressed and the effective electron mass was further enhanced. The Néel temperature becomes zero at around m = 2 , concomitant with striking deviations from the standard Fermi liquid low-temperature electronic properties. Standard Fermi liquid behaviors are, however, recovered under high magnetic field. These behaviors imply new ``dimensional tuning'' towards a quantum critical point. We also succeeded to fabricate artificial superlattices of a heavy fermion superconductor CeCoIn 5 and non-magnetic divalent Yb-compound YbCoIn 5 . Superconductivity survives even in CeCoIn 5 (3)/ YbCoIn 5 (5) films, while the thickness of CeCoIn 5 layer, 2.3 nm, is comparable to the c -axis coherence length ξc ~ 2 nm. This work has been done in collaboration with Y. Mizukami, S. Yasumoto, M. Shimozawa, H. Kontani, T. Shibauchi, T. Terashima and Y. Matsuda.superconductivity is realized in the artificial superlattices. This work has been done in collaboration with Y. Mizukami, S. Yasumoto, M. Shimozawa, H. Kontani, T

  18. Structural and electronic properties of germanene/MoS2 monolayer and silicene/MoS2 monolayer superlattices

    OpenAIRE

    Li, Xiaodan; Wu, Shunqing; Zhou, Sen; Zhu, Zizhong

    2014-01-01

    Superlattice provides a new approach to enrich the class of materials with novel properties. Here, we report the structural and electronic properties of superlattices made with alternate stacking of two-dimensional hexagonal germanene (or silicene) and a MoS2 monolayer using the first principles approach. The results are compared with those of graphene/MoS2 superlattice. The distortions of the geometry of germanene, silicene, and MoS2 layers due to the formation of the superlattices are all r...

  19. Monitoring and analysis of dam deformations Sherman Island hydroelectric development

    Energy Technology Data Exchange (ETDEWEB)

    Rohde, M.W.; Asce, A.M. [Coler & Colantonio, Inc., Norwell, MA (United States); Zuccolotto, J.M. [Niagara Mohawak Power Corporation, Syracuse, NY (United States)

    1995-12-31

    A modern integrated deformation monitoring program established for a reconstructed buttress dam is presented. A brief overview of the monitoring program will be provided along with discussions on equipment calibration aspects, how to assess the quality of the observation data, methods used to verify stability of the deformation network and to establish a trend of the deformations, how to develop deformation models which accurately reflect the spatial and temporal motions of the structure, including an investigation into the use of the Global Positioning System (GPS) for monitoring the deformations.

  20. Effective negative refractive index in ferromagnet-semiconductor superlattices.

    Science.gov (United States)

    Tarkanyan, Roland H; Niarchos, Dimitris G

    2006-06-12

    Problem of anomalous refraction of electromagnetic waves is analyzed in a superlattice which consists of alternating layers of ferromagnetic insulator and nonmagnetic semiconductor. Effective permittivity and permeability tensors are derived in the presence of an external magnetic field parallel to the plane of the layers. It is shown that in the case of the Voigt configuration, the structure behaves as a left-handed medium with respect to TE-type polarized wave, in the low-frequency region of propagation. The relative orientation of the Poynting vector and the refractive wave vector is examined in different frequency ranges. It is shown that the frequency region of existence for the backward mode can be changed using external magnetic field as tuning parameter.

  1. Superlattices assembled through shape-induced directional binding.

    Science.gov (United States)

    Lu, Fang; Yager, Kevin G; Zhang, Yugang; Xin, Huolin; Gang, Oleg

    2015-01-01

    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.

  2. Dynamical Axion Field in a Magnetic Topological Insulator Superlattice

    Science.gov (United States)

    Wang, Jing; Lian, Biao; Zhang, Shou-Cheng

    We propose that the dynamical axion field can be realized in a magnetic topological insulator superlattice or a topological paramagnetic insulator. The magnetic fluctuations of these systems produce a pseudoscalar field which has an axionic coupling to the electromagnetic field, and thus it gives a condensed-matter realization of the axion electrodynamics. Compared to the previously proposed dynamical axion materials where a long range antiferromagnetic order is required, the systems proposed here have the advantage that only an uniform magnetization or a paramagnetic state is needed for the dynamic axion. We further propose several experiments to detect such a dynamical axion field. This work is supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Contract No. DE-AC02-76SF00515.

  3. Kinetics of electron transfer from photoexcited superlattice electrodes

    Energy Technology Data Exchange (ETDEWEB)

    Nozik, A.J.; Turner, J.A.; Peterson, M.W.

    1988-05-05

    A kinetic model has been developed that quantitatively describes electron transfer from photoexcited superlattice electrodes into liquid solutions. The model permits electron transfer from all quantum levels as well as from surface states; it also takes into account recombination in the bulk, space charge layer, and surfaces states, and band-edge movement. The model calculations define the values of the rate constants for heterogeneous electron transfer and hot electron thermalization among the various energy levels in the supperlattice quantum wells that are necessary to achieve hot electron transfer from excited quantum states. The question of whether hot electron transfer is manifested by a dependence of the photocurrent action spectra on acceptor redox potential is examined in detail.

  4. Plasmon modes of a massive Dirac plasma, and their superlattices

    Science.gov (United States)

    Sachdeva, Rashi; Thakur, Anmol; Vignale, Giovanni; Agarwal, Amit

    2015-05-01

    We explore the collective density oscillations of a collection of charged massive Dirac particles, in one, two, and three dimensions, and their one-dimensional (1D) superlattice. We calculate the long-wavelength limit of the dynamical polarization function analytically, and use the random phase approximation to obtain the plasmon dispersion. The density dependence of the long-wavelength plasmon frequency in massive Dirac systems is found to be different compared to systems with parabolic and gapless Dirac dispersion. We also calculate the long-wavelength plasmon dispersion of a 1D metamaterial made from 1D and 2D massive Dirac plasma. Our analytical results will be useful for exploring the use of massive Dirac materials as electrostatically tunable plasmonic metamaterials and can be experimentally verified by infrared spectroscopy, as in the case of graphene [L. Ju et al., Nat. Nanotechnol. 6, 630 (2011), 10.1038/nnano.2011.146].

  5. Electronic and magnetic properties of zincblende half-metal superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Fong, C Y; Qian, M C; Pask, J; Yang, L H; Dag, S

    2003-11-05

    Zincblende half-metallic compounds such as CrAs, with large magnetic moments and high Curie temperatures, are promising materials for spintronic applications. They explore layered materials, consisting of alternating layers of zincblende half-metals, by first principles calculations, and find that superlattices of (CrAs){sub 1}(MnAs){sub 1} and (CrAs){sub 2}(MnAs){sub 2} are half-metallic with magnetic moments of 7.0{mu}{sub B} and 14.0{mu}{sub B} per unit cell, respectively. They discuss the nature of the bonding and half-metallicity in these materials and, based on the understanding acquired, develop a simple expression for the magnetic moment in such materials. They explore the range of lattice constants over which half-metallicity is manifested, and suggest corresponding substrates for growth in thin film form.

  6. Photon transport enhanced by transverse Anderson localization in disordered superlattices

    CERN Document Server

    Hsieh, Pin-Chun; McMillan, James; Tsai, Min-An; Lu, Ming; Panoiu, Nicolae; Wong, Chee Wei

    2014-01-01

    One of the daunting challenges in optical physics is to accurately control the flow of light at the subwavelength scale, by patterning the optical medium one can design anisotropic media. The light transport can also be significantly affected by Anderson localization, namely the wave localization in a disordered medium, a ubiquitous phenomenon in wave physics. Here we report the photon transport and collimation enhanced by transverse Anderson localization in chip-scale dispersion engineered anisotropic media. We demonstrate a new type of anisotropic photonic structure in which diffraction is nearly completely arrested by cascaded resonant tunneling through transverse guided resonances. By perturbing the shape of more than 4,000 scatterers in these superlattices we add structural disordered in a controlled manner and uncover the mechanism of disorder-induced transverse localization at the chip-scale. Arrested spatial divergence is captured in the power-law scaling, along with exponential asymmetric mode profil...

  7. Optical Studies on Antimonide Superlattice Infrared Detector Material

    Science.gov (United States)

    Hoglund, Linda; Soibel, Alexander; Hill, Cory J.; Ting, David Z.; Khoshakhlagh, Arezou; Liao, Anna; Keo, Sam; Lee, Michael C.; Nguyen, Jean; Mumolo, Jason M.; hide

    2010-01-01

    In this study the material quality and optical properties of type II InAs/GaSb superlattices are investigated using transmission and photoluminescence (PL) spectroscopy. The influence of the material quality on the intensity of the luminescence and on the electrical properties of the detectors is studied and a good correlation between the photodetector current-voltage (IV) characteristics and the PL intensity is observed. Studies of the temperature dependence of the PL reveal that Shockley-Read-Hall processes are limiting the minority carrier lifetime in both the mid-IR wavelength and the long-IR wavelength detector material studied. These results demonstrate that PL spectroscopy is a valuable tool for optimization of infrared detectors.

  8. Tunable anisotropic superfluidity in an optical kagome superlattice

    Science.gov (United States)

    Zhang, Xue-Feng; Wang, Tao; Eggert, Sebastian; Pelster, Axel

    2015-07-01

    We study the phase diagram of the Bose-Hubbard model on the kagome lattice with a broken sublattice symmetry. Such a superlattice structure can naturally be created and tuned by changing the potential offset of one sublattice in the optical generation of the frustrated lattice. The superstructure gives rise to a rich quantum phase diagram, which is analyzed by combining quantum Monte Carlo simulations with the generalized effective potential Landau theory. Mott phases with noninteger filling and a characteristic order along stripes are found, which show a transition to a superfluid phase with an anisotropic superfluid density. Surprisingly, the direction of the superfluid anisotropy can be tuned by changing the particle number, the hopping strength, or the interaction. Finally, we discuss characteristic signatures of anisotropic phases in time-of-flight absorption measurements.

  9. Laser induced structural transformation in chalcogenide based superlattices

    Science.gov (United States)

    Zallo, Eugenio; Wang, Ruining; Bragaglia, Valeria; Calarco, Raffaella

    2016-05-01

    Superlattices made of alternating layers of nominal GeTe and Sb2Te3 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.

  10. Transport properties of graphene under periodic and quasiperiodic magnetic superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Lu, Wei-Tao, E-mail: luweitao@lyu.edu.cn [School of Science, Linyi University, 276005 Linyi (China); Institute of Condensed Matter Physics, Linyi University, 276005 Linyi (China); Wang, Shun-Jin [Department of Physics, Sichuan University, 610064 Chengdu (China); Wang, Yong-Long; Jiang, Hua [School of Science, Linyi University, 276005 Linyi (China); Institute of Condensed Matter Physics, Linyi University, 276005 Linyi (China); Li, Wen [School of Science, Linyi University, 276005 Linyi (China)

    2013-08-15

    We study the transmission of Dirac electrons through the one-dimensional periodic, Fibonacci, and Thue–Morse magnetic superlattices (MS), which can be realized by two different magnetic blocks arranged in certain sequences in graphene. The numerical results show that the transmission as a function of incident energy presents regular resonance splitting effect in periodic MS due to the split energy spectrum. For the quasiperiodic MS with more layers, they exhibit rich transmission patterns. In particular, the transmission in Fibonacci MS presents scaling property and fragmented behavior with self-similarity, while the transmission in Thue–Morse MS presents more perfect resonant peaks which are related to the completely transparent states. Furthermore, these interesting properties are robust against the profile of MS, but dependent on the magnetic structure parameters and the transverse wave vector.

  11. Influence of impurity on electronic properties of carbon nanotube superlattices

    Directory of Open Access Journals (Sweden)

    AA Shokri

    2013-09-01

    Full Text Available   In this paper, electronic properties of single-wall armchair and zigzag carbon nanotubes (CNTs superlattices, n(12,0/m(6,6 and n(12,0/m(11,0 are investigated. For this reason, the topological defects of pentagon–heptagon pairs at interfaces of carbon hexagonal network appear. These defects break the symmetry of the system, and then change the electrical properties. The calculations include two parts: investigation of the structures in the absence and presence of the impurity effect, which are calculated by the nearest-neighbor tight binding model . Out numerical results can be useful in designing nanoelectronic devices based on carbon nanotubes.

  12. An organic donor/acceptor lateral superlattice at the nanoscale.

    Science.gov (United States)

    Otero, Roberto; Ecija, David; Fernandez, Gustavo; Gallego, José María; Sanchez, Luis; Martín, Nazario; Miranda, Rodolfo

    2007-09-01

    A precise control of the nanometer-scale morphology in systems containing mixtures of donor/acceptor molecules is a key factor to improve the efficiency of organic photovoltaic devices. Here we report on a scanning tunneling microscopy study of the first stages of growth of 2-[9-(1,3-dithiol-2-ylidene)anthracen-10(9H)-ylidene]-1,3-dithiole, as electron donor, and phenyl-C61-butyric acid methyl ester, as electron acceptor, on a Au(111) substrate under ultrahigh vacuum conditions. Due to differences in bonding strength with the substrate and different interactions with the Au(111) herringbone surface reconstruction, mixed thin films spontaneously segregate into a lateral superlattice of interdigitated nanoscale stripes with a characteristic width of about 10-20 nm, a morphology that has been predicted to optimize the efficiency of organic solar cells.

  13. Spin-polarized transport in graphene nanoribbon superlattices

    Institute of Scientific and Technical Information of China (English)

    Yu Xin-Xin; Xie Yue-E; OuYang Tao; Chen Yuan-Ping

    2012-01-01

    By the Green's function method,we investigate spin transport properties of a zigzag graphene nanoribbon superlattice (ZGNS) under a ferromagnetic insulator and edge effect.The exchange splitting induced by the ferromagnetic insulator eliminates the spin degeneracy,which leads to spin-polarized transport in structure.Spin-dependent minibands and minigaps are exhibited in the conductance profile near the Fermi energy.The location and width of the miniband are associated with the geometry of the ZGNS.In the optimal structure,the spin-up and spin-down minibands can be separated completely near the Fermi energy.Therefore,a wide,perfect spin polarization with clear stepwise pattern is observed,i.e.,the perfect spin-polarized transport can be tuned from spin up to spin down by varying the electron energy.

  14. Titanium-based silicide quantum dot superlattices for thermoelectrics applications.

    Science.gov (United States)

    Savelli, Guillaume; Stein, Sergio Silveira; Bernard-Granger, Guillaume; Faucherand, Pascal; Montès, Laurent; Dilhaire, Stefan; Pernot, Gilles

    2015-07-10

    Ti-based silicide quantum dot superlattices (QDSLs) are grown by reduced-pressure chemical vapor deposition. They are made of titanium-based silicide nanodots scattered in an n-doped SiGe matrix. This is the first time that such nanostructured materials have been grown in both monocrystalline and polycrystalline QDSLs. We studied their crystallographic structures and chemical properties, as well as the size and the density of the quantum dots. The thermoelectric properties of the QDSLs are measured and compared to equivalent SiGe thin films to evaluate the influence of the nanodots. Our studies revealed an increase in their thermoelectric properties-specifically, up to a trifold increase in the power factor, with a decrease in the thermal conductivity-making them very good candidates for further thermoelectric applications in cooling or energy-harvesting fields.

  15. Designing magnetic superlattices that are composed of single domain nanomagnets

    Directory of Open Access Journals (Sweden)

    Derek M. Forrester

    2014-07-01

    Full Text Available Background: The complex nature of the magnetic interactions between any number of nanosized elements of a magnetic superlattice can be described by the generic behavior that is presented here. The hysteresis characteristics of interacting elliptical nanomagnets are described by a quasi-static method that identifies the critical boundaries between magnetic phases. A full dynamical analysis is conducted in complement to this and the deviations from the quasi-static analysis are highlighted. Each phase is defined by the configuration of the magnetic moments of the chain of single domain nanomagnets and correspondingly the existence of parallel, anti-parallel and canting average magnetization states.Results: We give examples of the phase diagrams in terms of anisotropy and coupling strength for two, three and four magnetic layers. Each phase diagrams character is defined by the shape of the magnetic hysteresis profile for a system in an applied magnetic field. We present the analytical solutions that enable one to define the “phase” boundaries between the emergence of spin-flop, anti-parallel and parallel configurations. The shape of the hysteresis profile is a function of the coupling strength between the nanomagnets and examples are given of how it dictates a systems magnetic response. Many different paths between metastable states can exist and this can lead to instabilities and fluctuations in the magnetization.Conclusion: With these phase diagrams one can find the most stable magnetic configurations against perturbations so as to create magnetic devices. On the other hand, one may require a magnetic system that can easily be switched between phases, and so one can use the information herein to design superlattices of the required shape and character by choosing parameters close to the phase boundaries. This work will be useful when designing future spintronic devices, especially those manipulating the properties of CoFeB compounds.

  16. Deformations of crystal frameworks

    CERN Document Server

    Borcea, Ciprian S

    2011-01-01

    We apply our deformation theory of periodic bar-and-joint frameworks to tetrahedral crystal structures. The deformation space is investigated in detail for frameworks modelled on quartz, cristobalite and tridymite.

  17. Deformed General Relativity

    CERN Document Server

    Bojowald, Martin

    2013-01-01

    Deformed special relativity is embedded in deformed general relativity using the methods of canonical relativity and loop quantum gravity. Phase-space dependent deformations of symmetry algebras then appear, which in some regimes can be rewritten as non-linear Poincare algebras with momentum-dependent deformations of commutators between boosts and time translations. In contrast to deformed special relativity, the deformations are derived for generators with an unambiguous physical role, following from the relationship between canonical constraints of gravity with stress-energy components. The original deformation does not appear in momentum space and does not give rise to non-locality issues or problems with macroscopic objects. Contact with deformed special relativity may help to test loop quantum gravity or restrict its quantization ambiguities.

  18. Investigation of Anisotropic Thermal Conductivity of GaAs/AlAs Superlattices

    Science.gov (United States)

    Li, Ran

    The thermal conductivities of superlattices are essential to improve the properties of thermoelectrics and optoelectronics; however, limited results in relation to both the in-plane and cross-plane thermal conductivities have been reported. A convenient, effective, and accurate experimental method is required to improve the current research on the thermal properties of superlattices. We conducted an experimental research study on two GaAs/AlAs superlattice samples with a total superlattice layer thickness of 2 microm using a combination of the 2-omega and 3-omega techniques. The samples have period thicknesses of 4 nm and 10 nm, respectively. To explore the thermal conductivities of the substrate and insulation layer of the superlattice samples indirectly, a controlled sample with the same structure, but without a superlattice layer, is used. We obtained the thermal conductivities of the GaAs substrate and insulation layer (SiO2 thin film) using the 3-omega technique and FEM simulation model. We also explored the deviation of the experimental results of the 2-omega technique from the Fourier's Law through the controlled sample. These parameters obtained from the controlled sample are used in the data analysis in the following superlattice research. In the superlattice study, we combine the 3-omega and 2-omega techniques to characterize the anisotropic thermal conductivity of GaAs/AlAs superlattice from the same wafer. The in-plane thermal conductivity, cross-plane thermal conductivity, and anisotropy are obtained from the same wafer by comparing the experimental results with the FEM simulated results. This combination works fine in general and demonstrates a significant reduction in thermal conductivity compared to that of equivalent bulk materials. Superlattices with different period thicknesses but the same total superlattice thickness present a significant difference in both the in-plane and cross-plane thermal conductivities of the superlattices. However, we

  19. P-type AlAs/[GaAs/AlAs] Semiconductor/Superlattice DBR Grown by MBE

    Institute of Scientific and Technical Information of China (English)

    2001-01-01

    A p-type AlAs(70.2 nm)/16.5 period [GaAs(3 nm)/AlAs(0.7 nm)] semiconductor/superlatice distributed Bragg reflector (DBR) has been grown on n+-GaAs(100) substrate by V80H molecular beam epitaxy system. Experimental reflection spectrum shows that its central wavelength is 820 nm, with the peak reflectivity for 10-pair DBR of as high as 96 %, and the reflection bandwidth of as wide as 90 nm. We formed a 20×20 μm2 square mesa to measure the series resistance using wet chemical etching. From the measurement result, the series resistance of about 50 Ω is obtained at a moderate doping (3×1018 cm-3). Finally, the dependence of the resistance of the DBR on the temperature is analyzed. From the experimental result, it is found that the mechanism of the low series resistance of this kind of DBR may increase the tunneling current in the semiconductor/superlattice mirror structure, which will result in a decrease in series resistance.

  20. Two types of q-deformed Wigner algebra

    Energy Technology Data Exchange (ETDEWEB)

    Chung, Won Sang [Department of Physics and Research Institute of Natural Science, College of Natural Science, Gyeongsang National University, Jinju (Korea, Republic of)

    2014-07-01

    In this paper we introduce two types of q-deformed Wigner algebras. One is q-deformation of Wigner algebra with q-reflection symmetry and another is q-deformation of Wigner algebra with reflection symmetry. For two types of q-deformed Wigner algebras, we investigate the representation and the eigenvalue of the position operator. Like q-calculus, we introduce the (q; ν)- numbers, (q; ν)-derivatives and (q; ν)-Hermite polynomials for two algebras. For the deformation parameter q = 1 - ε with small ε, we discuss the thermodynamics of the particle obeying the q-deformed Wigner algebra. (Copyright copyright 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  1. Reflective Writing

    DEFF Research Database (Denmark)

    Ahrenkiel Jørgensen, Andriette

    2016-01-01

    Høeg etetera. The dialogues work as a tool of reflection in terms of providing opportunity to examine his own beliefs, to explore the possible reasons for engaging in a particular activity. On the basis of Sven-Ingvar Andersson’s book a teaching program at the Aarhus School of Architecture provides...... a contribution to the discussions about the role of reflection in design work and in learning situations at large. By engaging with the dialogic reflection, which is one of the four essential types of reflection, (the three others being descriptive writing, descriptive reflection and critical reflection...

  2. Deformable Nanolaminate Optics

    Energy Technology Data Exchange (ETDEWEB)

    Olivier, S S; Papavasiliou, A P; Barbee, T W; Miles, R R; Walton, C C; Cohn, M B; Chang, K

    2006-05-12

    We are developing a new class of deformable optic based on electrostatic actuation of nanolaminate foils. These foils are engineered at the atomic level to provide optimal opto-mechanical properties, including surface quality, strength and stiffness, for a wide range of deformable optics. We are combining these foils, developed at Lawrence Livermore National Laboratory (LLNL), with commercial metal processing techniques to produce prototype deformable optics with aperture sizes up to 10 cm and actuator spacing from 1 mm to 1 cm and with a range of surface deformation designed to be as much as 10 microns. The existing capability for producing nanolaminate foils at LLNL, coupled with the commercial metal processing techniques being used, enable the potential production of these deformable optics with aperture sizes of over 1 m, and much larger deformable optics could potentially be produced by tiling multiple deformable segments. In addition, based on the fabrication processes being used, deformable nanolaminate optics could potentially be produced with areal densities of less than 1 kg per square m for applications in which lightweight deformable optics are desirable, and deformable nanolaminate optics could potentially be fabricated with intrinsically curved surfaces, including aspheric shapes. We will describe the basic principles of these devices, and we will present details of the design, fabrication and characterization of the prototype deformable nanolaminate optics that have been developed to date. We will also discuss the possibilities for future work on scaling these devices to larger sizes and developing both devices with lower areal densities and devices with curved surfaces.

  3. Dispersion relation for localized magnetic polaritons propagating at the junction of two ferromagnetic/ non-magnetic superlattices

    Indian Academy of Sciences (India)

    R T Tagiyeva

    2004-09-01

    Localized magnetic polaritons are investigated in the systems consisting of two magnetic superlattices, coupled by a ferromagnetic contact layer. The general dispersion relation for localized magnetic polaritons are derived in the framework of the electromagnetic wave theory in the Voigt geometry by the `transfer' matrix method. The numerical calculations were carried out for different parameters of the superlattices and contact layer and then discussed.

  4. Giant piezoelectric response in piezoelectric/dielectric superlattices due to flexoelectric effect

    Science.gov (United States)

    Liu, Chang; Wu, Huaping; Wang, Jie

    2016-11-01

    Flexoelectricity describes the linear response of electrical polarization to a strain gradient, which can be used to enhance the piezoelectric effect of piezoelectric material or realize the piezoelectric effect in nonpiezoelectric materials. Here, we demonstrate from thermodynamics theory that a giant piezoelectric effect exists in piezoelectric/dielectric superlattices due to flexoelectric effect. The apparent piezoelectric coefficient is calculated from the closed-form of analytical expression of the polarization distribution in the piezoelectric/dielectric superlattice subjected to a normal stress, in which the flexoelectric effect is included. It is found that there exists a strong nonlinear coupling between the flexoelectric and piezoelectric effects, which significantly enhances the apparent piezoelectric coefficient in the piezoelectric/dielectric superlattice. For a specific thickness ratio of the piezoelectric and dielectric layers, the enhanced apparent piezoelectric coefficient in the superlattice is ten times larger than that of its pure piezoelectric counterpart. The present work suggests an effective way to obtain giant apparent piezoelectric effect in piezoelectric/dielectric superlattices through flexoelectric effect.

  5. Temperature-Dependent X-ray Diffraction Measurements of Infrared Superlattices Grown by MBE

    Directory of Open Access Journals (Sweden)

    Charles J. Reyner

    2016-11-01

    Full Text Available Strained-layer superlattices (SLSs are an active research topic in the molecular beam epitaxy (MBE and infrared focal plane array communities. These structures undergo a >500 K temperature change between deposition and operation. As a result, the lattice constants of the substrate and superlattice are expected to change by approximately 0.3%, and at approximately the same rate. However, we present the first temperature-dependent X-ray diffraction (XRD measurements of SLS material on GaSb and show that the superlattice does not contract in the same manner as the substrate. In both InAs/InAs0.65Sb0.35 and In0.8Ga0.2As/InAs0.65Sb0.35 SLS structures, the apparent out-of-plane strain states of the superlattices switch from tensile at deposition to compressive at operation. These changes have ramifications for material characterization, defect generation, carrier lifetime, and overall device performance of superlattices grown by MBE.

  6. Spin-dependent Seebeck effects in a graphene superlattice p–n junction with different shapes

    Science.gov (United States)

    Zhou, Benhu; Zhou, Benliang; Yao, Yagang; Zhou, Guanghui; Hu, Ming

    2017-10-01

    We theoretically calculate the spin-dependent transmission probability and spin Seebeck coefficient for a zigzag-edge graphene nanoribbon p–n junction with periodically attached stubs under a perpendicular magnetic field and a ferromagnetic insulator. By using the nonequilibrium Green’s function method combining with the tight-binding Hamiltonian, it is demonstrated that the spin-dependent transmission probability and spin Seebeck coefficient for two types of superlattices can be modulated by the potential drop, the magnetization strength, the number of periods of the superlattice, the strength of the perpendicular magnetic field, and the Anderson disorder strength. Interestingly, a metal to semiconductor transition occurs as the number of the superlattice for a crossed superlattice p–n junction increases, and its spin Seebeck coefficient is much larger than that for the T-shaped one around the zero Fermi energy. Furthermore, the spin Seebeck coefficient for crossed systems can be much pronounced and their maximum absolute value can reach 528 μV K-1 by choosing optimized parameters. Besides, the spin Seebeck coefficient for crossed p–n junction is strongly enhanced around the zero Fermi energy for a weak magnetic field. Our results provide theoretical references for modulating the thermoelectric properties of a graphene superlattice p–n junction by tuning its geometric structure and physical parameters.

  7. Aberration Corrected Scanning Transmission Electron Microscopy of (Ca , Sr)Fe2O5 Brownmillerite superlattices

    Science.gov (United States)

    Mukherjee, Debangshu; Stone, Greg; Moon, Eun Ju; Young, Joshua; Gopalan, Venkatraman; Rondinelli, James; May, Steven; Alem, Nasim

    The brownmillerite phase A2B2O5 consists of ordered oxygen vacancies in alternate perovskite layers forming chiral tetrahedral chains. The handedness of these tetrahedral chains control the polarization of the structure. The current study focuses on 1-1 brownmillerite superlattices grown on a SrTiO3 substrates using molecular beam epitaxy. The B-site in this structure is iron throughout the superlattice film, while the A-site alternates between calcium and strontium in the superlattice layers. In this study, we use atomic resolution aberration corrected scanning transmission electron microscopy (STEM) to investigate the structure and chemistry of the film-substrate interface as well as the chemical structure of the superlattice. Atom positions are determined to measure displacement vectors of A-site cations in the superlattice structure. D.M., G.A.S., V.G. and N.A. were supported by the National Science Foundation under Grant No. DMR-1420620. E.J.M. and S.J.M. were supported by the National Science Foundation under Grant No. DMR-1151649.

  8. Energy landscape of self-assembled superlattices of PbSe nanocrystals.

    Science.gov (United States)

    Quan, Zewei; Wu, Di; Zhu, Jinlong; Evers, Wiel H; Boncella, James M; Siebbeles, Laurens D A; Wang, Zhongwu; Navrotsky, Alexandra; Xu, Hongwu

    2014-06-24

    Self-assembly of nanocrystals (NCs) into superlattices is an intriguing multiscale phenomenon that may lead to materials with novel collective properties, in addition to the unique properties of individual NCs compared with their bulk counterparts. By using different dispersion solvents, we synthesized three types of PbSe NC superlattices--body-centered cubic (bcc), body-centered tetragonal (bct), and face-centered cubic (fcc)--as confirmed by synchrotron small-angle X-ray scattering. Solution calorimetric measurements in hexane show that the enthalpy of formation of the superlattice from dispersed NCs is on the order of -2 kJ/mol. The calorimetric measurements reveal that the bcc superlattice is the energetically most stable polymorph, with the bct being 0.32 and the fcc 0.55 kJ/mol higher in enthalpy. This stability sequence is consistent with the decreased packing efficiency of PbSe NCs from bcc (17.2%) to bct (16.0%) and to fcc (15.2%). The small enthalpy differences among the three polymorphs confirm a closely spaced energy landscape and explain the ease of formation of different NC superlattices at slightly different synthesis conditions.

  9. Optical constants of GaAs-AlGaAs superlattices and multiple quantum wells

    Science.gov (United States)

    Kahen, K. B.; Leburton, J. P.

    1986-01-01

    The optical properties of GaAs-Al sub x Ga sub 1-xAs superlattices are calculated as a function of the frequency and superlattice structure. The comutations are performed using a partition method which combines the vectors k.p method with the pseudopotential technique. The influence of the super-structure on the electronic properties of the systems is accounted for by appropriate quantization conditions. The anisotropy and structure dependence of the dielectric constant result mainly from the contribution of the gamma region while the contributions of the other regions of the Brillouin zone are rather insensitive to the superlattice structure. The superlattice index of refraction values are shown to attain maxima at the various quantized transition energies, where for certain structures, the difference between the refractive indices of the superlattices and its corresponding Al sub x Ga sub 1-xAs alloy can be as large as 2%. In general results are in good agreement with the experimental data.

  10. Far-infrared and dc-Magnetotransport of CaMnO3 CaRuO3 Superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Yordanov, P. [Max-Planck-Institut fur Feskorperforschung, Stuttgart, Germany; Boris, A. V. [Max-Planck-Institut fur Feskorperforschung, Stuttgart, Germany; Freeland, J. W. [Argonne National Laboratory (ANL); Kavich, J. J. [University of Illinois, Chicago; Chakhalian, J. [University of Arkansas; Lee, Ho Nyung [ORNL; Keimer, Bernhard [Max-Planck-Institut fur Feskorperforschung, Stuttgart, Germany

    2012-01-01

    We report temperature- and magnetic-field-dependent measurements of the dc resistivity and the far-infrared reflectivity (FIR) (photon energies {h_bar}{omega} = 50-700 cm{sup -1}) of superlattices comprising ten consecutive unit cells of the antiferromagnetic insulator CaMnO{sub 3}, and four to ten unit cells of the correlated paramagnetic metal CaRuO{sub 3}. Below the Neel temperature of CaMnO{sub 3}, the dc resistivity exhibits a logarithmic divergence upon cooling, which is associated with a large negative, isotropic magnetoresistance. The {omega} {yields} 0 extrapolation of the resistivity extracted from the FIR reflectivity, on the other hand, shows a much weaker temperature and field dependence. We attribute this behavior to scattering of itinerant charge carriers in CaRuO{sub 3} from sparse, spatially isolated magnetic defects at the CaMnO{sub 3}-CaRuO{sub 3} interfaces. This field-tunable 'transport bottleneck' effect may prove useful for functional metal-oxide devices.

  11. 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.

  12. Controllable spin and valley polarized current through a superlattice of normal/ferromagnetic/normal silicene junction

    Science.gov (United States)

    Rashidian, Z.; Hajati, Y.; Rezaeipour, S.; Baher, S.

    2017-02-01

    The spin and valley transports in a superlattice of normal/ferromagnetic/normal silicene junction are studied theoretically. Transport properties in particular valley-resolved conductance, spin and valley polarization have been computed by the Landauer Buttiker formula. We achieve fully valley and spin polarized current in the superlattice N/F/N structure. Our findings also imply that by increasing the number of ferromagnetic barriers, the onset of fully spin and valley polarized current always occur for lower values of staggered potential(Δz/E) and length of the ferromagnetic region (Kf L) in the silicene supelattice structure as compared with N/F/N silicene junction. Fully spin and valley polarizations make silicene superlattice a suitable candidate for spin-valleytronics applications.

  13. The Tip-Induced Twisted Bilayer Graphene Superlattice on HOPG: Capillary Attraction Effect

    CERN Document Server

    Yin, Long Jing; Feng, Ke Ke; Dou, Rui-Fen; Nie, Jia-Cai

    2014-01-01

    We use the tip of the scanning tunneling microscope (STM) to manipulate single weakly bound nanometer-sized sheets on the the highly oriented pyrolytic graphite (HOPG) surface through artifically increasing the tip and sample interaction in humid environment. By this means it is possible to tear apart a graphite sheet againt a step and fold this part onto the HOPG surface and thus generate the gaphene superlattices with hexagonal symmetry. The tip and sample surface interactions, including the van der Waals force, eletrostatic force and capillary attraction force originating from the Laplace pressure due to the formation of a highly curved fluid meniscus connecting the tip and sample, are discussed in details to understand the fromation mechnism of graphen superlattice induced by the STM tip. Especially, the capillary force is the key role in manipulating the graphite surface sheet in the hunmidity condition. Our approach may provides a simple and feasible route to prepare the controllable superlattices and g...

  14. Noise-enhanced spontaneous chaos in semiconductor superlattices at room temperature

    Science.gov (United States)

    Alvaro, M.; Carretero, M.; Bonilla, L. L.

    2014-08-01

    Physical systems exhibiting fast spontaneous chaotic oscillations are used to generate high-quality true random sequences in random number generators. The concept of using fast practical entropy sources to produce true random sequences is crucial to make storage and transfer of data more secure at very high speeds. While the first high-speed devices were chaotic semiconductor lasers, the discovery of spontaneous chaos in semiconductor superlattices at room temperature provides a valuable nanotechnology alternative. Spontaneous chaos was observed in 1996 experiments at temperatures below liquid nitrogen. Here we show spontaneous chaos at room temperature appears in idealized superlattices for voltage ranges where sharp transitions between different oscillation modes occur. Internal and external noises broaden these voltage ranges and enhance the sensitivity to initial conditions in the superlattice snail-shaped chaotic attractor thereby rendering spontaneous chaos more robust.

  15. Optically and Electrically Tunable Dirac Points and Zitterbewegung in Graphene-Based Photonic Superlattices

    CERN Document Server

    Deng, Hanying; Malomed, Boris A; Chen, Xianfeng; Panoiu, Nicolae C

    2015-01-01

    We demonstrate that graphene-based photonic superlattices provide a versatile platform for electrical and all-optical control of photonic beams with deep-subwavelength accuracy. Specifically, by inserting graphene sheets into periodic metallo-dielectric structures one can design optical superlattices that posses photonic Dirac points (DPs) at frequencies at which the spatial average of the permittivity of the superlattice, $\\bar{ \\varepsilon}$, vanishes. Similar to the well-known zero-$\\bar{n}$ bandgaps, we show that these zero-$\\bar{\\varepsilon}$ DPs are highly robust against structural disorder. We also show that, by tuning the graphene permittivity via the optical Kerr effect or electrical doping, one can induce a spectral variation of the DP exceeding \\SI{30}{\

  16. Bulk elastic waves with unidirectional backscattering-immune topological states in a time-dependent superlattice

    Energy Technology Data Exchange (ETDEWEB)

    Swinteck, N., E-mail: swinteck@email.arizona.edu; Matsuo, S.; Runge, K.; Lucas, P.; Deymier, P. A. [Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721 (United States); Vasseur, J. O. [Institut d' Electronique, de Micro-électronique et de Nanotechnologie, UMR CNRS 8520, Cité Scientifique, 59652 Villeneuve d' Ascq Cedex (France)

    2015-08-14

    Recent progress in electronic and electromagnetic topological insulators has led to the demonstration of one way propagation of electron and photon edge states and the possibility of immunity to backscattering by edge defects. Unfortunately, such topologically protected propagation of waves in the bulk of a material has not been observed. We show, in the case of sound/elastic waves, that bulk waves with unidirectional backscattering-immune topological states can be observed in a time-dependent elastic superlattice. The superlattice is realized via spatial and temporal modulation of the stiffness of an elastic material. Bulk elastic waves in this superlattice are supported by a manifold in momentum space with the topology of a single twist Möbius strip. Our results demonstrate the possibility of attaining one way transport and immunity to scattering of bulk elastic waves.

  17. Phonon-pumped terahertz gain in n-type GaAs/AlGaAs superlattices

    Science.gov (United States)

    Sun, Gregory; Soref, Richard A.

    2001-05-01

    Local population inversion and far-IR gain are proposed and theoretically analyzed for an unbiased n-doped GaAs/Al0.15Ga0.85As superlattice pumped solely by phonons. The lasing transition occurs at the Brillouin zone boundary of the superlattice wave vector kz between the two conduction minibands CB1 and CB2 of the opposite curvature in kz space. The proposed waveguided structure is contacted above and below by heat sinks at 300 K and 77 K, respectively. Atop the superlattice, a heat buffer layer confines longitudinal optical phonons for enhanced optical-phonon pumping of CB1 electrons. A gain of 345 cm-1 at 4.5 THz is predicted for a doping density of 2.8×1016cm-3.

  18. Lattice Thermal Conductivity of Superlattices from an Adiabatic Bond Charge Model

    Science.gov (United States)

    Ward, Alistair; Broido, David

    2007-03-01

    The adiabatic bond charge model (ABCM) has successfully rendered phonon dispersions of a host of bulk semiconductors [1,2] and has also been used to calculate the phonon dispersions in quantum well superlattices [3]. We have developed an ABCM for superlattices and combined it with a symmetry-based representation of the anharmonic interatomic forces to calculate the lattice thermal conductivity of short-period superlattices, using an iterative solution to the Boltzmann-Peierls equation [4]. We compare our ABCM results with those obtained from some commonly used models for the interatomic forces in semiconductors to assess the importance of accurate descriptions of the phonon dispersions in thermal conductivity calculations. [1] W. Weber, Physical Review B 15, 4789 (1977). [2] K. C. Rustagi and W. Weber, Solid State Communications 18, 673 (1976). [3] S. K. Yip and Y. C. Chang, Physical Review B 30 7037 (1984). [4] D. A. Broido, A. Ward, and N. Mingo, Physical Review B 72, 014308 (2005).

  19. Synthesis and electrical properties of In2O3(ZnO)m superlattice nanobelt

    Institute of Scientific and Technical Information of China (English)

    唐欣月; 高红; 武立立; 温静; 潘思明; 刘欣; 张喜田

    2015-01-01

    One-dimensional (1D) In2O3(ZnO)m superlattice nanobelts are synthesized by chemical vapor deposition method. The formation of In2O3(ZnO)m superlattice is verified by the high-resolution transmission electron microscopy images. The typical zigzag boundaries could be clearly observed. An additional peak at 614 cm−1 is found in the Raman spec-trum, which may correspond to the superlattice structure. The study about the electrical transport properties reveals that the In2O3(ZnO)m nanobelts exhibit peculiar nonlinear I–V characteristics even under the Ohmic contact measurement con-dition, which are different from the Ohmic behaviors of the In-doped ZnO nanobelts. The photoelectrical measurements show the differences in photocurrent property between them, and their transport mechanisms are also discussed.

  20. 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.

  1. Wannier-Stark localization and terahertz electroluminescence of natural SiC superlattice

    Energy Technology Data Exchange (ETDEWEB)

    Sankin, V. I.; Andrianov, A. V.; Petrov, A. G.; Zakhar' in, A. O. [A.F. Ioffe Physical Technical Institute, 194021 St. Petersburg (Russian Federation)

    2013-12-04

    We report on efficient terahertz electroluminescence in the region of 1.5-2 THz from high electric field biased 6H-SiC n{sup +}−n{sup −}−n{sup +} structures with a natural superlattice at 7 K. The properties of the terahertz emission allow it to be attributed to spontaneous radiation resulting from electron Bloch oscillations in SiC natural superlattice. The use of the unique object, namely, natural superlattice of SiC allowed us to demonstrate a whole series of remarkable effects of Wannier-Stark localization and to get the intensive terahertz emission under steady-state electrical excitation of Bloch oscillations.

  2. Spin selector based on periodic diluted-magnetic-semiconductor/nonmagnetic-barrier superlattices

    Directory of Open Access Journals (Sweden)

    Ping-Fan Yang

    2015-07-01

    Full Text Available We propose a spin selector based on periodic diluted-magnetic-semiconductor/nonmagnetic-barrier (DMS/NB superlattices subjected to an external magnetic field. We find that the periodic DMS/NB superlattices can achieve 100% spin filtering over a dramatically broader range of incident energies than the diluted-magnetic-semiconductor/semiconductor (DMS/S case studied previously. And the positions and widths of spin-filtering bands can be manipulated effectively by adjusting the geometric parameters of the system or the strength of external magnetic field. Such a compelling filtering feature stems from the introduction of nonmagnetic barrier and the spin-dependent giant Zeeman effect induced by the external magnetic field. We also find that the external electric field can exert a significant influence on the spin-polarized transport through the DMS/NB superlattices.

  3. Spin selector based on periodic diluted-magnetic-semiconductor/nonmagnetic-barrier superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Yang, Ping-Fan; Guo, Yong, E-mail: guoy66@tsinghua.edu.cn [Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084 (China); Collaborative Innovation Center of Quantum Matter, Beijing (China); Zhu, Rui [Department of Physics, South China University of Technology, Guangzhou 510641 (China)

    2015-07-15

    We propose a spin selector based on periodic diluted-magnetic-semiconductor/nonmagnetic-barrier (DMS/NB) superlattices subjected to an external magnetic field. We find that the periodic DMS/NB superlattices can achieve 100% spin filtering over a dramatically broader range of incident energies than the diluted-magnetic-semiconductor/semiconductor (DMS/S) case studied previously. And the positions and widths of spin-filtering bands can be manipulated effectively by adjusting the geometric parameters of the system or the strength of external magnetic field. Such a compelling filtering feature stems from the introduction of nonmagnetic barrier and the spin-dependent giant Zeeman effect induced by the external magnetic field. We also find that the external electric field can exert a significant influence on the spin-polarized transport through the DMS/NB superlattices.

  4. Enhancing chaotic behavior at room temperature in GaAs/(Al,Ga)As superlattices

    Science.gov (United States)

    Ruiz-Garcia, M.; Essen, J.; Carretero, M.; Bonilla, L. L.; Birnir, B.

    2017-02-01

    Previous theoretical and experimental work has put forward 50-period semiconductor superlattices as fast, true random number generators at room temperature. Their randomness stems from feedback between nonlinear electronic dynamics and stochastic processes that are intrinsic to quantum transitions. This paper theoretically demonstrates that shorter superlattices with higher potential barriers contain fully chaotic dynamics over several intervals of the applied bias voltage compared to the 50-period device which presented a much weaker chaotic behavior. The chaos arises from deterministic dynamics, hence it persists even in the absence of additional stochastic processes. Moreover, the frequency of the chaotic current oscillations is higher for shorter superlattices. These features should allow for faster and more robust generation of true random numbers.

  5. Effect of roughness on perpendicular magnetic anisotropy in (Co90Fe10/Pt)n superlattices

    Science.gov (United States)

    Qiu, Jinjun; Meng, Zhaoliang; Yang, Yi; Ying, Ji Feng; Yap, Qi Jia; Han, Guchang

    2016-05-01

    Superlattice [Co90Fe10(0.21)/Pt(0.23)]n (unit in nm) with the repeat cycles n ranging from 3 to 30 were studied. Both effective anisotropy (Keff) and PMA constant (KU) reached a maximum at n=8. When the 3 nm Pt underlayer was deposited at low energy condition, the Keff and KU of (CoFe/Pt)8 are 4.0 and 6.1 Merg/cc, respectively. On the other hand, the Keff and KU increased to 6.8 and 9.7 Merg/cc, respectively, when the Pt underlayer deposited at high energy condition. As the n increases, the surface roughness monotonously increases and d111 inside the superlattice layers increase and relax from bottom to top part. The interface roughness and relaxation in superlattice reduce the PMA considerably.

  6. Effect of roughness on perpendicular magnetic anisotropy in (Co90Fe10/Ptn superlattices

    Directory of Open Access Journals (Sweden)

    Jinjun Qiu

    2016-05-01

    Full Text Available Superlattice [Co90Fe10(0.21/Pt(0.23]n (unit in nm with the repeat cycles n ranging from 3 to 30 were studied. Both effective anisotropy (Keff and PMA constant (KU reached a maximum at n=8. When the 3 nm Pt underlayer was deposited at low energy condition, the Keff and KU of (CoFe/Pt8 are 4.0 and 6.1 Merg/cc, respectively. On the other hand, the Keff and KU increased to 6.8 and 9.7 Merg/cc, respectively, when the Pt underlayer deposited at high energy condition. As the n increases, the surface roughness monotonously increases and d111 inside the superlattice layers increase and relax from bottom to top part. The interface roughness and relaxation in superlattice reduce the PMA considerably.

  7. -Deformed nonlinear maps

    Indian Academy of Sciences (India)

    Ramaswamy Jaganathan; Sudeshna Sinha

    2005-03-01

    Motivated by studies on -deformed physical systems related to quantum group structures, and by the elements of Tsallis statistical mechanics, the concept of -deformed nonlinear maps is introduced. As a specific example, a -deformation procedure is applied to the logistic map. Compared to the canonical logistic map, the resulting family of -logistic maps is shown to have a wider spectrum of interesting behaviours, including the co-existence of attractors – a phenomenon rare in one-dimensional maps.

  8. Alar Rim Deformities.

    Science.gov (United States)

    Totonchi, Ali; Guyuron, Bahman

    2016-01-01

    The alar rim plays an important role in nasal harmony. Alar rim flaws are common following the initial rhinoplasty. Classification of the deformities helps with diagnosis and successful surgical correction. Diagnosis of the deformity requires careful observation of the computerized or life-sized photographs. Techniques for treatment of these deformities can easily be learned with attention to detail. Copyright © 2016 Elsevier Inc. All rights reserved.

  9. Modeling functional piezoelectricity in perovskite superlattices with competing instabilities

    Science.gov (United States)

    Swartz, Charles; Wu, Xifan

    2012-02-01

    Multi-component Perovskite Superlattices (SLs) of the form ABO3, provide a very promising avenue for the design of materials with multifunctional properties. Furthermore the interfaces of such multi-component SLs are home to competing anti-ferrodistortive and ferroelectric instabilities which can produce unexpected functionalities. However, at present first principles calculations exceeding more than 10 units cells, are particularly costly as they scale with the valence electrons as N^3. We present a first-principles modeling technique that allows us to accurately model the piezoelectric strains of paraelectric/ferroelectric SLs, BaTiO3/CaTiO3 and PbTiO3/SrTiO3, under a fixed displacement field. The model is based on a maximally localized wannier center layer polarization technique, as well as a truncated cluster expansion, that makes use of the fact that such PE/FE SLs have been shown to have highly localized ionic and electronic interface effects. The prediction of the piezoelectricity for a SL of an arbitrary stacking sequence will be demonstrated. We also use our model to conduct a systemic study of the interface effects on piezoelectric response in the above SLs paying special attention to a strong non-linear effect observed in Bulk SrTiO3.

  10. ``N'' structure for type-II superlattice photodetectors

    Science.gov (United States)

    Salihoglu, Omer; Muti, Abdullah; Kutluer, Kutlu; Tansel, Tunay; Turan, Rasit; Ergun, Yuksel; Aydinli, Atilla

    2012-08-01

    In the quest to raise the operating temperature and improve the detectivity of type II superlattice (T2SL) photodetectors, we introduce a design approach that we call the "N structure." N structure aims to improve absorption by manipulating electron and hole wavefunctions that are spatially separated in T2SLs, increasing the absorption while decreasing the dark current. In order to engineer the wavefunctions, we introduce a thin AlSb layer between InAs and GaSb layers in the growth direction which also acts as a unipolar electron barrier. Unlike the symmetrical insertion of AlSb into GaSb layers, N design aims to exploit the shifting of the electron and hole wavefunctions under reverse bias. With cutoff wavelength of 4.3 μm at 77 K, temperature dependent dark current and detectivity measurements show that the dark current density is 3.6 × 10-9 A/cm2, under zero bias. Photodetector reaches background limited infrared photodetection (BLIP) condition at 125 K with the BLIP detectivity (D*BLIP) of 2.6 × 1010 Jones under 300 K background and -0.3 V bias voltage.

  11. Artificial charge-modulationin atomic-scale perovskite titanate superlattices.

    Science.gov (United States)

    Ohtomo, A; Muller, D A; Grazul, J L; Hwang, H Y

    2002-09-26

    The nature and length scales of charge screening in complex oxides are fundamental to a wide range of systems, spanning ceramic voltage-dependent resistors (varistors), oxide tunnel junctions and charge ordering in mixed-valence compounds. There are wide variations in the degree of charge disproportionation, length scale, and orientation in the mixed-valence compounds: these have been the subject of intense theoretical study, but little is known about the microscopic electronic structure. Here we have fabricated an idealized structure to examine these issues by growing atomically abrupt layers of LaTi(3+)O(3) embedded in SrTi(4+)O(3). Using an atomic-scale electron beam, we have observed the spatial distribution of the extra electron on the titanium sites. This distribution results in metallic conductivity, even though the superlattice structure is based on two insulators. Despite the chemical abruptness of the interfaces, we find that a minimum thickness of five LaTiO(3) layers is required for the centre titanium site to recover bulk-like electronic properties. This represents a framework within which the short-length-scale electronic response can be probed and incorporated in thin-film oxide heterostructures.

  12. Quasi-ternary nanoparticle superlattices through nanoparticle design

    Energy Technology Data Exchange (ETDEWEB)

    Kortright, Jeffrey; Shevchenko, Elena V.; Kortright, Jeffrey B.; Talapin, Dmitri V.; Aloni, Shaul; Alivisatos, A. Paul

    2007-06-19

    Individual nanoscale building blocks exhibit a wide range of size-dependent properties, since their size can be tuned over known characteristic length scales of bulk materials. In the last several years, the possibility of combining different materials in the form of two and three component nanoparticles (NPs) has been extensively explored. Also multi-component materials can be obtained via self-assembly of NPs from their binary colloidal mixtures. These new nanocrystal solids may possess tunable collective properties that originate from interactions between size and composition controlled building blocks. Exchange coupling between neighboring NPs of magnetically soft and hard materials enhances the magnetic energy product of the nanocomposite material. Randomly mixed solids of small and large semiconducting CdSe NPs revealed enhancement of photoluminescence intensity of large semiconductor particles accompanied by quenching of photoluminescence of the small particles because of long-range resonant transfer of electronic excitations from the more electronically confined small particles to higher excited states of the large particles. Recently, it was demonstrated that binary semiconducting composite materials can show strongly enhanced electronic properties with about 100-fold higher conductance as compared to the sum of individual conductances of single-component films. Creation of highly periodic superlattices is expected not just provide the control of the homogeneity of the sample but also affect their properties. It was shown that silver nanocrystals organized into periodic cubic structures vibrated coherently [20] and demonstrated a change in electronic transport properties.

  13. Thiol passivation of MWIR type II superlattice photodetectors

    Science.gov (United States)

    Salihoglu, O.; Muti, A.; Aydinli, A.

    2013-06-01

    Poor passivation on photodetectors can result in catastrophic failure of the device. Abrupt termination of mesa side walls during pixel definition generates dangling bonds that lead to inversion layers and surface traps leading to surface leakage currents that short circuit diode action. Good passivation, therefore, is critical in the fabrication of high performance devices. Silicondioxide has been the main stay of passivation for commercial photodetectors, deposited at high temperatures and high RF powers using plasma deposition techniques. In photodetectors based on III-V compounds, sulphur passivation has been shown to replace oxygen and saturate the dangling bonds. Despite its effectiveness, it degrades over time. More effort is required to create passivation layers which eliminate surface leakage current. In this work, we propose the use of sulphur based octadecanethiol (ODT), CH3(CH2)17SH, as a passivation layer for the InAs/GaSb superlattice photodetectors that acts as a self assembled monolayer (SAM). ODT SAMs consist of a chain of 18 carbon atoms with a sulphur atom at its head. ODT Thiol coating is a simple process that consist of dipping the sample into the solution for a prescribed time. Excellent electrical performance of diodes tested confirm the effectiveness of the sulphur head stabilized by the intermolecular interaction due to van der Walls forces between the long chains of ODT SAM which results in highly stable ultrathin hydrocarbon layers without long term degradation.

  14. One-way electromagnetic waveguide using multiferroic Fibonacci superlattices

    Science.gov (United States)

    Tang, Zhenghua; Lei, Dajun; Huang, Jianquan; Jin, Gui; Qiu, Feng; Yan, Wenyan

    2015-12-01

    The multiferroic Fibonacci superlattices (MFSs) are composed of single-phase multiferroic domains with polarization and magnetization according to the rule of Fibonacci sequence. We propose to construct a one-way electromagnetic waveguide by the MFSs. The forbidden band structures of the MFSs for the forward and backward electromagnetic waves are not completely overlapped, and an obvious translation between them occurs around the fixed point ω bar = 1 with broken time-reversal and space inversion symmetries (TRSIS), which indicates the existence of one-way electromagnetic modes in the MFSs. Transmission spectrum is utilized to present this property and to indicate further one-way electromagnetic modes lying within the polaritonic band gap. The maximum forbidden bandwidth (divided by midgap frequency) of 5.4% for the backward electromagnetic wave (BEW) is found, in which the forward electromagnetic wave (FEW) can pass. The functions of one-way propagation modes and polaritonic band gap integrated into the MFSs can miniaturize the one-way photonic devices. The properties can also be applied to construct compact microwave isolators.

  15. Topological hierarchy matters — topological matters with superlattices of defects

    Science.gov (United States)

    He, Jing; Kou, Su-Peng

    2016-11-01

    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. Project supported by the National Basic Research Program of China (Grant Nos. 2011CB921803 and 2012CB921704), the National Natural Science Foundation of China (Grant Nos. 11174035, 11474025, 11404090, and 11674026), the Natural Science Foundation of Hebei Province, China (Grant No. A2015205189), the Hebei Education Department Natural Science Foundation, China (Grant No. QN2014022), and the Specialized Research Fund for the Doctoral Program of Higher Education, China.

  16. Ge/SiGe superlattices for nanostructured thermoelectric modules

    Energy Technology Data Exchange (ETDEWEB)

    Chrastina, D., E-mail: daniel@chrastina.net [L-NESS Politecnico di Milano, Polo di Como, via Anzani 42, 22100 Como (Italy); Cecchi, S. [L-NESS Politecnico di Milano, Polo di Como, via Anzani 42, 22100 Como (Italy); Hague, J.P. [Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA (United Kingdom); Frigerio, J. [L-NESS Politecnico di Milano, Polo di Como, via Anzani 42, 22100 Como (Italy); Samarelli, A.; Ferre–Llin, L.; Paul, D.J. [School of Engineering, University of Glasgow, Oakfield Avenue, Glasgow, G12 8LT (United Kingdom); Müller, E. [Electron Microscopy ETH Zurich (EMEZ), ETH-Zürich, CH-8093 (Switzerland); Etzelstorfer, T.; Stangl, J. [Institut für Halbleiter und Festkörperphysik, Universität Linz, A-4040 Linz (Austria); Isella, G. [L-NESS Politecnico di Milano, Polo di Como, via Anzani 42, 22100 Como (Italy)

    2013-09-30

    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.

  17. Study of Crystals Semiconductors in Superlattices via Quantum Mechanics

    Directory of Open Access Journals (Sweden)

    *1A. L. C. L. Jamshidi

    2013-12-01

    Full Text Available This work analyzes, from the effects related to the processes of transportation of carrier and the changes in the electronic structure of semiconductors materials due to the presence of defects and disorders in the crystalline net. These defects are located in specific areas of the material and either interact or remain inert. In general, they are described by local wave functions. The study of superlattices of semiconductor crystal considers important parameters such as disorder effects in crystals and the alternate periodic growth of the layer of two semiconductors with different gaps and minigaps energies. The quantum mechanical calculations are applied for determining the physical properties of the semiconductors crystals. This study encompasses the effects of defects and the crystalline disorders evaluation by quantum mechanics. Further, it is discuss the presence of defects in the periodic, quasiperiodic and disordered arrangements. The theoretical approach use to understand the mechanism and the results of experimental techniques in which are characterized the current and optic transportation of a semiconductor crystal.

  18. Angle-dependent bandgap engineering in gated graphene superlattices

    Energy Technology Data Exchange (ETDEWEB)

    García-Cervantes, H.; Sotolongo-Costa, O. [Centro de Investigación en Ciencias, IICBA, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, 62209 Cuernavaca, Morelos, México (Mexico); Gaggero-Sager, L. M. [CIICAp, IICBA, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, 62209 Cuernavaca, Morelos, México (Mexico); Naumis, G. G. [Instituto Física, Depto. de Física-Química, Universidad Nacional Autónoma de México (UNAM). Apdo. Postal 20-364, 01000, México D.F., México (Mexico); Rodríguez-Vargas, I., E-mail: isaac@fisica.uaz.edu.mx [Centro de Investigación en Ciencias, IICBA, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, 62209 Cuernavaca, Morelos, México (Mexico); Unidad Académica de Física, Universidad Autónoma de Zacatecas, Calzada Solidaridad Esquina Con Paseo La Bufa S/N, 98060 Zacatecas, Zac., México (Mexico)

    2016-03-15

    Graphene Superlattices (GSs) have attracted a lot of attention due to its peculiar properties as well as its possible technological implications. Among these characteristics we can mention: the extra Dirac points in the dispersion relation and the highly anisotropic propagation of the charge carriers. However, despite the intense research that is carried out in GSs, so far there is no report about the angular dependence of the Transmission Gap (TG) in GSs. Here, we report the dependence of TG as a function of the angle of the incident Dirac electrons in a rather simple Electrostatic GS (EGS). Our results show that the angular dependence of the TG is intricate, since for moderated angles the dependence is parabolic, while for large angles an exponential dependence is registered. We also find that the TG can be modulated from meV to eV, by changing the structural parameters of the GS. These characteristics open the possibility for an angle-dependent bandgap engineering in graphene.

  19. Hetero-engineering infrared detectors with type-II superlattices

    Science.gov (United States)

    Tian, Z.-B.; DeCuir, E. A.; Gautam, N.; Krishna, S.; Wijewarnasuriya, P. S.; Pattison, J. W.; Dhar, N.; Welser, R. E.; Sood, A. K.

    2013-09-01

    InAs/GaSb type-II superlattices (T2-SLs) are of great interest as they provide a lot of band engineering flexibility. A wide variety of unipolar barrier structures have been investigated with this material system. In this report, we will present our recent work on the development of low noise long-wave infrared (LWIR) InAs/GaSb T2-SLs photodetectors. By adopting a so-called pBiBn design, the dark current of LWIR photodetectors is greatly suppressed. The LWIR pBiBn device has demonstrated a dark current density as low as 1.42×10-5 A/cm2 at -60 mV, and R0A of 5365 Ωcm2 at 76 K. A peak detectivity at 7.8 μm of 7.7×1011 cmHz1/2W-1 is obtained at 76 K. Further effort to reduce the operating bias is also reported. By refining the energy-band alignment, a 2-μm-thick LWIR pBiBn device has demonstrated a single pass (no AR coating) quantum efficiency of 20% at 10 μm under zero-bias at 77 K. We have recently extended our efforts to further reduce the dark current by using an interband cascade (IC) photodetector structure. Some further details about the device operation and results will be discussed.

  20. Dimensionality Control of d-orbital Occupation in Oxide Superlattices

    Science.gov (United States)

    Jeong, Da Woon; Choi, Woo Seok; Okamoto, Satoshi; Kim, Jae-Young; Kim, Kyung Wan; Moon, Soon Jae; Cho, Deok-Yong; Lee, Ho Nyung; Noh, Tae Won

    2014-08-01

    Manipulating the orbital state in a strongly correlated electron system is of fundamental and technological importance for exploring and developing novel electronic phases. Here, we report an unambiguous demonstration of orbital occupancy control between t2g and eg multiplets in quasi-two-dimensional transition metal oxide superlattices (SLs) composed of a Mott insulator LaCoO3 and a band insulator LaAlO3. As the LaCoO3 sublayer thickness approaches its fundamental limit (i.e. one unit-cell-thick), the electronic state of the SLs changed from a Mott insulator, in which both t2g and eg orbitals are partially filled, to a band insulator by completely filling (emptying) the t2g (eg) orbitals. We found the reduction of dimensionality has a profound effect on the electronic structure evolution, which is, whereas, insensitive to the epitaxial strain. The remarkable orbital controllability shown here offers a promising pathway for novel applications such as catalysis and photovoltaics, where the energy of d level is an essential parameter.

  1. Development of Type-II superlattice VLWIR detectors in JAXA

    Science.gov (United States)

    Sakai, Michito; Murooka, Jumpei; Kumeta, Ayaka; Kimura, Toshiyoshi; Inada, Hiroshi; Iguchi, Yasuhiro; Hiroe, Yuta; Kimata, Masafumi

    2017-02-01

    One of JAXA's future missions, using an imaging Fourier Transform Spectrometer (FTS), requires the focal plane array (FPA) that has high sensitivity up to the very long-wavelength infrared (VLWIR) region. Since a Type-II superlattice (T2SL) is the only known infrared material to exhibit performance that is theoretically predicted to be higher than that of HgCdTe additionally the cutoff wavelength can be tailored in the wavelength region of 3-30 μm, we started the research and development of the T2SL detector in 2009. In order to confirm our final goal, which is to realize the FPA with a cutoff wavelength of 15 μm, we first fabricated the 320 × 256 (QVGA format) InAs/GaInSb T2SL FPA with a cutoff wavelength of 15 μm, and the large-format 640 × 512 (VGA format) T2SL FPA is followed because the other missions, using an infrared imager, require the large-format FPA. The noise-equivalent delta temperature measured with F1.4 optics was 0.15 K for QVGA format T2SL FPA at 77 K. It was 0.35 K for VGA format T2SL FPA at 77 K, but there is non-uniformity, and further improvements are necessary to achieve high performance FPAs.

  2. Fluctuations as stochastic deformation

    Science.gov (United States)

    Kazinski, P. O.

    2008-04-01

    A notion of stochastic deformation is introduced and the corresponding algebraic deformation procedure is developed. This procedure is analogous to the deformation of an algebra of observables like deformation quantization, but for an imaginary deformation parameter (the Planck constant). This method is demonstrated on diverse relativistic and nonrelativistic models with finite and infinite degrees of freedom. It is shown that under stochastic deformation the model of a nonrelativistic particle interacting with the electromagnetic field on a curved background passes into the stochastic model described by the Fokker-Planck equation with the diffusion tensor being the inverse metric tensor. The first stochastic correction to the Newton equations for this system is found. The Klein-Kramers equation is also derived as the stochastic deformation of a certain classical model. Relativistic generalizations of the Fokker-Planck and Klein-Kramers equations are obtained by applying the procedure of stochastic deformation to appropriate relativistic classical models. The analog of the Fokker-Planck equation associated with the stochastic Lorentz-Dirac equation is derived too. The stochastic deformation of the models of a free scalar field and an electromagnetic field is investigated. It turns out that in the latter case the obtained stochastic model describes a fluctuating electromagnetic field in a transparent medium.

  3. Deformed discrete symmetries

    Science.gov (United States)

    Arzano, Michele; Kowalski-Glikman, Jerzy

    2016-09-01

    We construct discrete symmetry transformations for deformed relativistic kinematics based on group valued momenta. We focus on the specific example of κ-deformations of the Poincaré algebra with associated momenta living on (a sub-manifold of) de Sitter space. Our approach relies on the description of quantum states constructed from deformed kinematics and the observable charges associated with them. The results we present provide the first step towards the analysis of experimental bounds on the deformation parameter κ to be derived via precision measurements of discrete symmetries and CPT.

  4. Direct probing of vertical electron movement in superlattices by sub-picosecond luminescence

    Science.gov (United States)

    Deveaud, B.; Chomette, A.; Clérot, F.; Lambert, B.; Auvray, P.; Gauneau, M.; Regreny, A.

    Vertical transport in GaAs/AlGaAs superlattices is probed in structures with graded composition. Such structures allow boi to impose a quasi-electric field to the carriers and to evidence the carrier movement by the temporal changes in the luminescence lineshape. The fit of this lineshape by a drift-diffusion model gives the transport properties of electrons. High mobility of the electrons is evidenced for the shortest period superlattices, in agreement with previous optical measurements. Smaller mobilities are observed when the miniband width becomes smaller.

  5. Fabrication of Si/SiO2 Superlattice Microwire Array Solar Cells Using Microsphere Lithography

    Directory of Open Access Journals (Sweden)

    Shigeru Yamada

    2016-01-01

    Full Text Available A fabrication process for silicon/silicon dioxide (Si/SiO2 superlattice microwire array solar cells was developed. The Si/SiO2 superlattice microwire array was fabricated using a microsphere lithography process with polystyrene particles. The solar cell shows a photovoltaic effect and an open-circuit voltage of 128 mV was obtained. The limiting factors of the solar cell performance were investigated from the careful observations of the solar cell structures. We also investigated the influence of the microwire array structure on light trapping in the solar cells.

  6. Molecular dynamics simulation of thermal conductivity of GaN/AlN quantum dot superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Kawamura, Takahiro [Graduate School of Engineering, Kyushu University, 6-1, Kasuga-Koen, Kasuga, Fukuoka 816-8580 (Japan); Kangawa, Yoshihiro; Kakimoto, Koichi [Research Institute for Applied Mechanics, Kyushu University, 6-1, Kasuga-Koen, Kasuga, Fukuoka 816-8580 (Japan)

    2007-06-15

    We calculated thermal conductivity of GaN/AlN quantum dot superlattices by molecular dynamics simulation. The results of investigation of the effect of quantum dots on thermal conductivity as a function of superlattice period are presented in this paper. An empirical potential function of Stillinger-Weber potential was used for simulations. Thermal conductivity was obtained by Green-Kubo's equation. The results show that the values of thermal conductivity parallel to the wetting layers decreased due to the effect of quantum dots. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  7. Second-Harmonic and Third-Harmonic Generations in the Thue-Morse Dielectric Superlattice

    Institute of Scientific and Technical Information of China (English)

    蔡祥宝

    2002-01-01

    Theoretical work on the optical properties of the one-dimensional dielectric superlattice is extended. 3Byv means of a transfer matrix method, the second-harmonic and third-harmonic generations in a one-dimensional tinite Thue Morse dielectric superlattice are analysed. The electric field amplitude variables of the second-harmonic and third-harmonic can be expressed by the formula of matrices. Taking advantage of numerical procedure, we discuss the dependence of the second-harmonic and third-harmonic on the fundamental wavelength and the field amplitude variables of the fundamental wave. High conversion efficiency of the third-harmonic can be obtained at some special fundamental wavelength.

  8. 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.

  9. 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...

  10. Effect of exchange interaction in ferromagnetic superlattices: A Monte Carlo study

    Science.gov (United States)

    Masrour, R.; Jabar, A.

    2016-10-01

    The Monte Carlo simulation is used to investigate the magnetic properties of ferromagnetic superlattices through the Ising model. The reduced critical temperatures of the ferromagnetic superlattices are studied each as a function of layer thickness for different values of exchange interaction. The exchange interaction in each layer within the interface and the crystal field in the unit cell are studied. The magnetic coercive fields and magnetization remnants are obtained for different values of exchange interaction, different values of temperature and crystal field with fixed values of physical parameters.

  11. 2D wave-front shaping in optical superlattices using nonlinear volume holography.

    Science.gov (United States)

    Yang, Bo; Hong, Xu-Hao; Lu, Rong-Er; Yue, Yang-Yang; Zhang, Chao; Qin, Yi-Qiang; Zhu, Yong-Yuan

    2016-07-01

    Nonlinear volume holography is employed to realize arbitrary wave-front shaping during nonlinear processes with properly designed 2D optical superlattices. The concept of a nonlinear polarization wave in nonlinear volume holography is investigated. The holographic imaging of irregular patterns was performed using 2D LiTaO3 crystals with fundamental wave propagating along the spontaneous polarization direction, and the results agree well with the theoretical predictions. This Letter not only extends the application area of optical superlattices, but also offers an efficient method for wave-front shaping technology.

  12. Strain-free polarization superlattice in silicon carbide: a theoretical investigation.

    Science.gov (United States)

    Deák, Peter; Buruzs, Adam; Gali, Adam; Frauenheim, Thomas

    2006-06-16

    A strain-free superlattice of inversion domains along the hexagonal axis of SiC is investigated by theoretical calculations. The induced polarization causes a zigzag shape in the band edges, leading to spatial separation of photoexcited carriers and to an effective band gap narrowing tunable over a wide range by the geometry and on a smaller scale by the intensity of the excitation. Calculations on the SiC surface indicate that preparation of such a superlattice might be possible in atomic layer epitaxy with properly chosen sources and temperatures.

  13. Acoustic analogue of electronic BLOCH oscillations and resonant Zener tunneling in ultrasonic superlattices.

    Science.gov (United States)

    Sanchis-Alepuz, Helios; Kosevich, Yuriy A; Sánchez-Dehesa, José

    2007-03-30

    We demonstrate the existence of Bloch oscillations of acoustic fields in sound propagation through a superlattice of water cavities and layers of methyl methacrylate. To obtain the acoustic equivalent of a Wannier-Stark ladder, we employ a set of cavities with different thicknesses. Bloch oscillations are observed as time-resolved oscillations of transmission in a direct analogy to electronic Bloch oscillations in biased semiconductor superlattices. Moreover, for a particular gradient of cavity thicknesses, an overlap of two acoustic minibands occurs, which results in resonant Zener-like transmission enhancement.

  14. Semiconductor-Superlattice Parametric Oscillator as a Subterahertz and Possible Terahertz Radiation Source

    Directory of Open Access Journals (Sweden)

    Karl F. Renk

    2007-01-01

    Full Text Available We describe the operation of a semiconductor-superlattice parametric oscillator (SPO at a subterahertz frequency (near 300 GHz. The oscillator is driven by a microwave source (frequency near 100 GHz. We also present an analysis indicating that operation at frequencies above 1 THz should be possible. The SPO is based on the ability of conduction electrons in a superlattice to perform Bloch oscillations. Broadband tunability as well as the monochromacy of a driving microwave field are transferred to the SPO.

  15. Synchrotron X-Ray Study on Structures of Ni80Fe20/Cu Superlattices

    Institute of Scientific and Technical Information of China (English)

    XU Ming; LUO Guang-Ming; CHAI Chun-Lin; YANG Tao; MAI Zhen-Hong; LAI Wu-Yan; WU Zhong-Hua; WANG De-Wu

    2001-01-01

    We have shown that, in contrast to the results in the literature, the Bragg peak intensity of Ni80Fe20/Cu superlattices is enhanced at the incident x-ray energy slightly higher than the absorption edge of the heavier element (Cu). The atomic density at Ni80Fe20/Cu interface was analysed by the diffraction anomalous fine structure technology with the incident angle of x-ray fixed at the first Bragg peak. Our results demonstrate the epitaxy growth of Ni80Fe20/Cu superlattices. Upon annealing, the epitaxity of Ni80Fe20/Cu multilayers becomes poor but the local crystallinity in each layer is improved.

  16. The chemical and magnetic structures of holmium-yttrium and holmium-lutetium superlattices

    DEFF Research Database (Denmark)

    McMorrow, D.F.; Jehan, D.A.; Swaddling, P.P.;

    1993-01-01

    We present the results of a study of the chemical and magnetic structures of Ho/Y and Ho/Lu superlattices, all grown by molecular beam epitaxy. By combining the results of high-resolution X-ray diffraction with detailed modelling we show that the superlattices have high crystallographic integrity......: the average structural coherence length in the growth direction is approximately 2000 angstrom, while the interfaces between the two elements are well defined, extending over approximately four lattice planes. The magnetic structures were determined using neutron scattering techniques. In the case of the Ho...

  17. 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)

  18. Design and Manufacture of GeSi/Si Superlattice Nanocrystalline Photodetector

    Institute of Scientific and Technical Information of China (English)

    2006-01-01

    According to Maxwell's theory, the optical transmission characteristics in GeSi/Si superlattice nanocrystalline layer have been analyzed and calculated. The calculated result shows that when the total thickness L is 340nm, the single mode lightwave can be transmitted only at periodic number M≥15.5. In addition, at the direction of transmission, when the transmission distance is larger than 500μm, the lightwave intensity is decreased greatly. Based on the above parameters, the design and manufacture of GeSi/Si superlattice nanocrystalline photodetector are carried out.

  19. Resonant tunnelling and intersubband absorption in AlN - GaN superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Baumann, E.; Giorgetta, F.R.; Hofstetter, D. [University of Neuchatel, 1 A.-L. Breguet, Neuchatel, 2000 (Switzerland); Wu, H.; Schaff, W.J.; Eastman, L.F. [Cornell University, Ithaca, NY 14850 (United States); Kirste, L. [Fraunhofer-Institute of Applied Solid State Physics, Tullastrasse 72, Freiburg, 79108 (Germany)

    2005-02-01

    We report on intersubband absorption and photovoltage measurements on regular GaN/AlN-based superlattice structures at 1.55 {mu}m. For high barriers, the photovoltage peaks at a higher energy than the absorbance spectrum due to the decrease of the tunnelling probability. The observed photovoltage is thus the macroscopic manifestation that the 2-dimensional electron gas at the top of the superlattice gets depleted by a vertical transport of electrons. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  20. Thermal conductivity of GaAs/AlAs superlattices:The Umklapp process

    Institute of Scientific and Technical Information of China (English)

    Yang Yu-Rong; Yan Xiao-Hong; Cao Jue-Xian; Xiao Yang; Mao Yu-Liang; Xiang Jun; Yu Hai-Lin

    2004-01-01

    The thermal conductivity of GaAs/AlAs superlattices limited by the three-phonon Umklapp process and boundary scattering has been studied theoretically based on the model of lattice dynamics with force constant matrix. It was found that the Umklapp relaxation rate approximates BTω2/n with a fitting parameter B. The thermal conductivity increases with the increase of temperature at low temperatures, and would show a peak behaviour at about 60K before falling off at high temperatures. In addition, the thermal conductivity increases with the increase of period thickness of the superlattices.

  1. Structural and electronic properties of germanene/MoS2 monolayer and silicene/MoS2 monolayer superlattices

    Science.gov (United States)

    Li, Xiaodan; Wu, Shunqing; Zhou, Sen; Zhu, Zizhong

    2014-03-01

    Superlattice provides a new approach to enrich the class of materials with novel properties. Here, we report the structural and electronic properties of superlattices made with alternate stacking of two-dimensional hexagonal germanene (or silicene) and a MoS2 monolayer using the first principles approach. The results are compared with those of graphene/MoS2 superlattice. The distortions of the geometry of germanene, silicene, and MoS2 layers due to the formation of the superlattices are all relatively small, resulting from the relatively weak interactions between the stacking layers. Our results show that both the germanene/MoS2 and silicene/MoS2 superlattices are manifestly metallic, with the linear bands around the Dirac points of the pristine germanene and silicene seem to be preserved. However, small band gaps are opened up at the Dirac points for both the superlattices due to the symmetry breaking in the germanene and silicene layers caused by the introduction of the MoS2 sheets. Moreover, charge transfer happened mainly within the germanene (or silicene) and the MoS2 layers (intra-layer transfer), as well as some part of the intermediate regions between the germanene (or silicene) and the MoS2 layers (inter-layer transfer), suggesting more than just the van der Waals interactions between the stacking sheets in the superlattices.

  2. Intracrystalline deformation of calcite

    NARCIS (Netherlands)

    de Bresser, Hans

    1991-01-01

    It is well established from observations on natural calcite tectonites that intracrystalline plastic mechanisms are important during the deformation of calcite rocks in nature. In this thesis, new data are presented on fundamental aspects of deformation behaviour of calcite under conditions where 'd

  3. Resurgent deformation quantisation

    Energy Technology Data Exchange (ETDEWEB)

    Garay, Mauricio, E-mail: garay91@gmail.com [Institut für Mathematik, FB 08 Physik, Mathematik und Informatik, Johannes Gutenberg-Universität, 55099 Mainz (Germany); Goursac, Axel de, E-mail: Axelmg@melix.net [Chargé de Recherche au F.R.S.-FNRS, IRMP, Université Catholique de Louvain, Chemin du Cyclotron, 2, B-1348 Louvain-la-Neuve (Belgium); Straten, Duco van, E-mail: straten@mathematik.uni-mainz.de [Institut für Mathematik, FB 08 Physik, Mathematik und Informatik, Johannes Gutenberg-Universität, 55099 Mainz (Germany)

    2014-03-15

    We construct a version of the complex Heisenberg algebra based on the idea of endless analytic continuation. The algebra would be large enough to capture quantum effects that escape ordinary formal deformation quantisation. -- Highlights: •We construct resurgent deformation quantisation. •We give integral formulæ. •We compute examples which show that hypergeometric functions appear naturally in quantum computations.

  4. Deformations of Superconformal Theories

    CERN Document Server

    Cordova, Clay; Intriligator, Kenneth

    2016-01-01

    We classify possible supersymmetry-preserving relevant, marginal, and irrelevant deformations of unitary superconformal theories in $d \\geq 3$ dimensions. Our method only relies on symmetries and unitarity. Hence, the results are model independent and do not require a Lagrangian description. Two unifying themes emerge: first, many theories admit deformations that reside in multiplets together with conserved currents. Such deformations can lead to modifications of the supersymmetry algebra by central and non-central charges. Second, many theories with a sufficient amount of supersymmetry do not admit relevant or marginal deformations, and some admit neither. The classification is complicated by the fact that short superconformal multiplets display a rich variety of sporadic phenomena, including supersymmetric deformations that reside in the middle of a multiplet. We illustrate our results with examples in diverse dimensions. In particular, we explain how the classification of irrelevant supersymmetric deformat...

  5. Massey products and deformations

    CERN Document Server

    Fuchs, D; Fuchs, Dmitry; Lang, Lynelle

    1996-01-01

    The classical deformation theory of Lie algebras involves different kinds of Massey products of cohomology classes. Even the condition of extendibility of an infinitesimal deformation to a formal one-parameter deformation of a Lie algebra involves Massey powers of two dimensional cohomology classes which are not powers in the usual definition of Massey products in the cohomology of a differential graded Lie algebra. In the case of deformations with other local bases, one deals with other, more specific Massey products. In the present work a construction of generalized Massey products is given, depending on an arbitrary graded commutative, associative algebra. In terms of these products, the above condition of extendibility is generalized to deformations with arbitrary local bases. Dually, a construction of generalized Massey products on the cohomology of a differential graded commutative associative algebra depends on a nilpotent graded Lie algebra. For example, the classical Massey products correspond to the...

  6. Deformation mechanisms in experimentally deformed Boom Clay

    Science.gov (United States)

    Desbois, Guillaume; Schuck, Bernhard; Urai, Janos

    2016-04-01

    Bulk mechanical and transport properties of reference claystones for deep disposal of radioactive waste have been investigated since many years but little is known about microscale deformation mechanisms because accessing the relevant microstructure in these soft, very fine-grained, low permeable and low porous materials remains difficult. Recent development of ion beam polishing methods to prepare high quality damage free surfaces for scanning electron microscope (SEM) is opening new fields of microstructural investigation in claystones towards a better understanding of the deformation behavior transitional between rocks and soils. We present results of Boom Clay deformed in a triaxial cell in a consolidated - undrained test at a confining pressure of 0.375 MPa (i.e. close to natural value), with σ1 perpendicular to the bedding. Experiments stopped at 20 % strain. As a first approximation, the plasticity of the sample can be described by a Mohr-Coulomb type failure envelope with a coefficient of cohesion C = 0.117 MPa and an internal friction angle ϕ = 18.7°. After deformation test, the bulk sample shows a shear zone at an angle of about 35° from the vertical with an offset of about 5 mm. We used the "Lamipeel" method that allows producing a permanent absolutely plane and large size etched micro relief-replica in order to localize and to document the shear zone at the scale of the deformed core. High-resolution imaging of microstructures was mostly done by using the BIB-SEM method on key-regions identified after the "Lamipeel" method. Detailed BIB-SEM investigations of shear zones show the following: the boundaries between the shear zone and the host rock are sharp, clay aggregates and clastic grains are strongly reoriented parallel to the shear direction, and the porosity is significantly reduced in the shear zone and the grain size is smaller in the shear zone than in the host rock but there is no evidence for broken grains. Comparison of microstructures

  7. Quantifying Reflection

    DEFF Research Database (Denmark)

    Alcock, Gordon Lindsay

    2013-01-01

    This paper documents 1st semester student reflections on “learning to learn” in a team-based PBL environment with quantitative and qualitative student reflective feedback on the learning gains of 60 Architectural Technology and Construction Management students at VIA University College, Denmark....... It contrasts the students’ self-assessment in a range of ‘product’ skills such as Revit, Structural Design, Mathematics of construction, Technical Installations; as well as ‘process’ competencies such as ‘Working in a team’, Sharing knowledge, Maintaining a portfolio and Reflecting ON learning and FOR learning......´ These are all based on Blooms taxonomy and levels of competence and form a major part of individual student and group learning portfolios. Key Words :Project-Based learning, Reflective Portfolios, Self assessment, Defining learning gains, Developing learning strategies , Reflections on and for learning...

  8. Deformation modeling and constitutive modeling for anisotropic superalloys

    Science.gov (United States)

    Milligan, Walter W.; Antolovich, Stephen D.

    1989-01-01

    A study of deformation mechanisms in the single crystal superalloy PWA 1480 was conducted. Monotonic and cyclic tests were conducted from 20 to 1093 C. Both (001) and near-(123) crystals were tested, at strain rates of 0.5 and 50 percent/minute. The deformation behavior could be grouped into two temperature regimes: low temperatures, below 760 C; and high temperatures, above 820 to 950 C depending on the strain rate. At low temperatures, the mechanical behavior was very anisotropic. An orientation dependent CRSS, a tension-compression asymmetry, and anisotropic strain hardening were all observed. The material was deformed by planar octahedral slip. The anisotropic properties were correlated with the ease of cube cross-slip, as well as the number of active slip systems. At high temperatures, the material was isotropic, and deformed by homogeneous gamma by-pass. It was found that the temperature dependence of the formation of superlattice-intrinsic stacking faults was responsible for the local minimum in the CRSS of this alloy at 400 C. It was proposed that the cube cross-slip process must be reversible. This was used to explain the reversible tension-compression asymmetry, and was used to study models of cross-slip. As a result, the cross-slip model proposed by Paidar, Pope and Vitek was found to be consistent with the proposed slip reversibility. The results were related to anisotropic viscoplastic constitutive models. The model proposed by Walter and Jordan was found to be capable of modeling all aspects of the material anisotropy. Temperature and strain rate boundaries for the model were proposed, and guidelines for numerical experiments were proposed.

  9. Game model of safety monitoring for arch dam deformation

    Institute of Scientific and Technical Information of China (English)

    2008-01-01

    Arch dam deformation is comprehensively affected by water pressure,temperature,dam’s structural behavior and material properties as well as other factors.Among them the water pressure and temperature are external factors(source factors) that cause dam deformation,and dam’s structural behavior and material properties are the internal factors of deformation(resistance factors).The dam deformation is the result of the mutual game playing between source factors and resistance factors.Therefore,resistance factors of structure and materials that reflect resistance character of arch dam structure are introduced into the traditional model,where structure factor is embodied by the flexibility coefficient of dam body and the maximum dam height,and material property is embodied by the elastic modulus of dam.On the basis of analyzing the correlation between dam deformation and resistance factors,the game model of safety monitoring for arch dam deformation is put forward.

  10. Structural and magnetic properties of an InGaAs/Fe3Si superlattice in cylindrical geometry

    Science.gov (United States)

    Deneke, Ch; Schumann, J.; Engelhard, R.; Thomas, J.; Müller, C.; Khatri, M. S.; Malachias, A.; Weisser, M.; Metzger, T. H.; Schmidt, O. G.

    2009-01-01

    The structure and magnetic properties of an InGaAs/Fe3Si superlattice in a cylindrical geometry are investigated by electron microscopy techniques, x-ray diffraction and magnetometry. To form a radial superlattice, a pseudomorphic InGaAs/Fe3Si bilayer has been released from its substrate self-forming into rolled-up microtubes. Oxide-free interfaces as well as areas of crystalline bonding are observed and an overall lattice mismatch between succeeding layers is determined. The cylindrical symmetry of the final radial superlattice shows a significant effect on the magnetization behavior of the rolled-up layers.

  11. Reflection ciphers

    DEFF Research Database (Denmark)

    Boura, Christina; Canteaut, Anne; Knudsen, Lars Ramkilde

    2017-01-01

    study the necessary properties for this coupling permutation. Special care has to be taken of some related-key distinguishers since, in the context of reflection ciphers, they may provide attacks in the single-key setting.We then derive some criteria for constructing secure reflection ciphers...... and analyze the security properties of different families of coupling permutations. Finally, we concentrate on the case of reflection block ciphers and, as an illustration, we provide concrete examples of key schedules corresponding to several coupling permutations, which lead to new variants of the block...

  12. Artificial semiconductor/insulator superlattice channel structure for high-performance oxide thin-film transistors.

    Science.gov (United States)

    Ahn, Cheol Hyoun; Senthil, Karuppanan; Cho, Hyung Koun; Lee, Sang Yeol

    2013-01-01

    High-performance thin-film transistors (TFTs) are the fundamental building blocks in realizing the potential applications of the next-generation displays. Atomically controlled superlattice structures are expected to induce advanced electric and optical performance due to two-dimensional electron gas system, resulting in high-electron mobility transistors. Here, we have utilized a semiconductor/insulator superlattice channel structure comprising of ZnO/Al2O3 layers to realize high-performance TFTs. The TFT with ZnO (5 nm)/Al2O3 (3.6 nm) superlattice channel structure exhibited high field effect mobility of 27.8 cm(2)/Vs, and threshold voltage shift of only < 0.5 V under positive/negative gate bias stress test during 2 hours. These properties showed extremely improved TFT performance, compared to ZnO TFTs. The enhanced field effect mobility and stability obtained for the superlattice TFT devices were explained on the basis of layer-by-layer growth mode, improved crystalline nature of the channel layers, and passivation effect of Al2O3 layers.

  13. The magnetic phase diagram and zero field structure of holmium-lutetium superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Swaddling, P.P. [Clarendon Laboratory, Oxford University, Oxford (United Kingdom); McMorrow, D.F. [Risoe National Laboratory, Roskilde (Denmark); Cowley, R.A. [Clarendon Laboratory, Oxford University, Oxford (United Kingdom); Simpson, J.A. [Clarendon Laboratory, Oxford University, Oxford (United Kingdom); Wells, M.R. [Clarendon Laboratory, Oxford University, Oxford (United Kingdom); Ward, R.C.C. [Clarendon Laboratory, Oxford University, Oxford (United Kingdom); Clausen, K.N. [Risoe National Laboratory, Roskilde (Denmark); Collins, M.F. [Dept of Physics and Astronomy, McMaster University, Hamilton (Canada); Buyers, W.J.L. [AECL, Chalk River, Ontario (Canada)

    1995-02-09

    Neutron diffraction has been used to study the magnetic structure of a series of Ho/Lu superlattices. In zero field a transition to a ferromagnetic phase of Ho is observed at low temperatures, and the stability of this phase has been investigated by applying a magnetic field in the basal plane. ((orig.)).

  14. Photoluminescence and pressure effects in short period InN/nGaN superlattices

    DEFF Research Database (Denmark)

    Staszczak, G.; Gorczyca, I.; Suski, T.

    2013-01-01

    Measurements of photoluminescence and its dependence on hydrostatic pressure are performed on a set of InN/nGaN superlattices with one InN monolayer and with different numbers of GaN monolayers. The emission energies, EPL, measured at ambient pressure, are close to the value of the band gap, Eg...

  15. FIR Induced Intrinsic Exciton Transitions in GaAs/AlGaAs Superlattices

    DEFF Research Database (Denmark)

    Dremin, A. A.; Timofeev, V. B.; Birkedal, Dan;

    1997-01-01

    Intrinsic transitions of confined excitons in GaAs/AlGaAs superlattices with different barrier widths have been studied with the use of resonant far-infrared absorption under variation of magnetic field perpendicular and tilted with respect to the growth directions. Few resonances have been...

  16. 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.

  17. Studies of Spectroscopic Ellipsometry in Cd1-xMnx Te/CdTe Superlattices

    Institute of Scientific and Technical Information of China (English)

    CHEN Chen-Jia; WANG Xue-Zhong; Vittorio BELLANI; Angiolino STELLA

    2006-01-01

    Cd1-xMnxTe/CdTe superlattices and thin films were grown by molecular beam epitaxy on GaAs (001) substrates. Spectroscopic ellipsometry measurements were performed on Cd1-xMnxTe/CdTe superlattices with compositions x = 0.4, 0.8, and Cd1-xMnxTe thin films with x = 0.2, 0.4, 0.6 at room temperature in the photon energy range 1.4-5eV. In superlattices the pseudodielectric functions measured by ellipsometry show specific features related to the exciton transition between quantized interbands. The exciton transitions related to the heavy holes of 11H, 22H, and 33H are observed and identified. In thin films spectroscopic ellipsometry allows the clear identification of the energy gap E0. Additionally, critical point transitions are observable in both the spectra of the superlattices and films. Photoreflectance spectra were also performed at room temperature in order to compare with our ellipsometry results. After taking into account the strain-induced and quantum confinement effects, the theoretical calculations are in good agreement with our experimental spectra. Ellipsometry appears to be a suited technique to monitor the MBE growth, ultimately also in situ, of diluted magnetic low-dimensional systems.

  18. Enhanced valley-resolved thermoelectric transport in a magnetic silicene superlattice

    Science.gov (United States)

    Niu, Zhi Ping; Zhang, Yong Mei; Dong, Shihao

    2015-07-01

    Electrons in two-dimensional crystals with a honeycomb lattice structure possess a valley degree of freedom in addition to charge and spin, which has revived the field of valleytronics. In this work we investigate the valley-resolved thermoelectric transport through a magnetic silicene superlattice. Since spin is coupled to the valley, this device allows a coexistence of the insulating transmission gap of one valley and the metallic resonant band of the other, resulting in a strong valley polarization Pv. Pv oscillates with the barrier strength V with its magnitude greatly enhanced by the superlattice structure. In addition, a controllable fully valley polarized transport and an on/off switching effect in the conductance spectra are obtained. Furthermore, the spin- and valley-dependent thermopowers can be controlled by V, the on-site potential difference between A and B sublattices and Fermi energy, and enhanced by the superlattice structure. Enhanced valley-resolved thermoelectric transport and its control by means of gate voltages make the magnetic silicene superlattice attractive in valleytronics applications.

  19. Simulational studies of epitaxial semiconductor superlattices: Quantum dynamical phenomena in ac and dc electric fields

    Energy Technology Data Exchange (ETDEWEB)

    Reynolds, Joseph [Iowa State Univ., Ames, IA (United States)

    1997-10-08

    Using high-accuracy numerical methods the author investigates the dynamics of independent electrons in both ideal and realistic superlattices subject to arbitrary ac and/or dc electric fields. For a variety of superlattice potentials, optically excited initial wave packets, and combinations of ac and dc electric fields, he numerically solves the time-dependent Schroedinger equation. In the case of ideal periodic superlattice potentials, he investigates a long list of dynamical phenomena involving multiple miniband transitions and time-dependent electric fields. These include acceleration effects associated with interminiband transitions in strong fields, Zener resonances between minibands, dynamic localization with ac fields, increased single-miniband transport with an auxiliary resonant ac field, and enhanced or suppressed interminiband probability exchange using an auxiliary ac field. For all of the cases studied, the resulting time-dependent wave function is analyzed by projecting the data onto convenient orthonormal bases. This allows a detailed comparison with approximately analytic treatments. In an effort to explain the rapid decay of experimentally measured Bloch oscillation (BO) signals the author incorporates a one-dimensional representation of interface roughness (IR) into their superlattice potential. He shows that as a result of IR, the electron dynamics can be characterized in terms of many discrete, incommensurate frequencies near the Block frequency. Chapters 2, 3, 4 and 5 have been removed from this report and will be processed separately.

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

    Energy Technology Data Exchange (ETDEWEB)

    Hu, Shenyang [Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Burkes, Douglas E. [Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Lavender, Curt A. [Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Senor, David J. [Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Setyawan, Wahyu [Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Xu, Zhijie [Pacific Northwest National Lab. (PNNL), Richland, WA (United States)

    2016-07-08

    Nano-gas bubble superlattices are often observed in irradiated UMo nuclear fuels. However, the for- mation 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 devel- oped. 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 $\\langle$110$\\rangle$ directions in the body-centered cubic U matrix causes the gas bubble alignment along $\\langle$110$\\rangle$ 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.

  1. Inelastic Quantum Transport in Superlattices: Success and Failure of the Boltzmann Equation

    DEFF Research Database (Denmark)

    Wacker, Andreas; Jauho, Antti-Pekka; Rott, Stephan;

    1999-01-01

    Electrical transport in semiconductor superlattices is studied within a fully self-consistent quantum transport model based on nonequilibrium Green functions, including phonon and impurity scattering. We compute both the drift-velocity-held relation and the momentum distribution function covering...

  2. Dynamic Behavior and Quasi-energy Spectrum of Multiband Superlattice Bloch Electrons in Quantum Kicked Potential

    Institute of Scientific and Technical Information of China (English)

    OUYANG BiYao; ZHAO XianGeng; CHEN ShiGang; LIU Jie

    2001-01-01

    In this paper, we study the dynamic behavior and quasi-energy spectrum of multiband superlattice Bloch electrons in quantum kicked potential. We show analytically and numerically the avoided crossing and band suppression about the quasi-energy spectrum, the dynamic nonlocalization, and the electron oscillation behavior between two bands.

  3. Electronic structure of atomically coherent square semiconductor superlattices with dimensionality below two

    NARCIS (Netherlands)

    Kalesaki, E.; Evers, W.H.; Vanmaekelbergh, D.; Delerue, C.

    2013-01-01

    The electronic structure of recently synthesized square superlattices with atomic coherence composed of PbSe, CdSe, or CdTe nanocrystals (NCs) attached along {100} facets is investigated using tight-binding calculations. In experimental realizations of these systems [W. H. Evers et al., Nano Lett. 1

  4. Photoluminescence and pressure effects in short period InN/nGaN superlattices

    DEFF Research Database (Denmark)

    Staszczak, G.; Gorczyca, I.; Suski, T.;

    2013-01-01

    Measurements of photoluminescence and its dependence on hydrostatic pressure are performed on a set of InN/nGaN superlattices with one InN monolayer and with different numbers of GaN monolayers. The emission energies, EPL, measured at ambient pressure, are close to the value of the band gap, Eg, ...

  5. Metal-insulator transitions in LaTiO3 / CaTiO3 superlattices

    Science.gov (United States)

    Seo, Sung Seok A.; Lee, Ho Nyung

    2010-03-01

    Strongly correlated electrons at an interface of complex oxide heterostructures often show interesting behaviors that require an introduction of new physical concepts. For example, the metallic transport behavior found in the superlattices of a Mott insulator LaTiO3 and a band insulator SrTiO3 (STO) has established the concept of interfacial electronic reconstruction. In this work, we have studied the transport property of a new type of Mott/band insulator LaTiO3/CaTiO3 (LTO/CTO) superlattices grown by pulsed laser deposition (PLD). In order to rule out concerns about the PLD plume-triggered oxygen vacancies generated in STO substrates, which might influence transport measurement, and to investigate the effect of epitaxial strain, we have used insulating NdGaO3 substrates. While both LTO and CTO single films are highly insulating, we have observed intriguing metal-insulator transitions (MIT) in the LTO/CTO superlattices depending on the global LTO/CTO thickness ratio and temperature. (Note that LTO/STO superlattices are metallic at all temperatures (2-300 K)). In this talk, we will discuss the origin of the MIT in the scheme of self compensation mechanism of d-electrons at the hetero-interface between LTO and CTO.

  6. Design of a terahertz CW photomixer based on PIN and superlattice PIN devices

    DEFF Research Database (Denmark)

    Krozer, Viktor; Eichhorn, Finn

    2006-01-01

    We present the design of a photomixer LO based on standard and superlattice PIN diodes, operating at 1 THz. The design is based on a direct integration of a double slot antenna with the PIN device and a suitable matching circuit. The antenna has been designed together with a dielectric lens using...

  7. Goos-Hänchen shifts in AA-stacked bilayer graphene superlattices

    Science.gov (United States)

    Zahidi, Youness; Redouani, Ilham; Jellal, Ahmed

    2016-07-01

    The quantum Goos-Hänchen shifts of the transmitted electron beam through an AA-stacked bilayer graphene superlattices are investigated. We found that the band structures of graphene superlattices can have more than one Dirac point, their locations do not depend on the number of barriers. It was revealed that any n-barrier structure is perfectly transparent at normal incidence around the Dirac points created in the superlattices. We showed that the Goos-Hänchen shifts display sharp peaks inside the transmission gap around two Dirac points (E =VB + τ, E =VW + τ), which are equal to those of transmission resonances. The obtained Goos-Hänchen shifts are exhibiting negative as well as positive behaviors and strongly depending on the location of Dirac points. It is observed that the maximum absolute values of the shifts increase as long as the number of barriers is increased. Our analysis is done by considering four cases: single, double barriers, superlattices without and with defect.

  8. 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 ...

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

    Science.gov (United States)

    Hu, Shenyang; Burkes, Douglas E.; Lavender, Curt A.; Senor, David J.; Setyawan, Wahyu; Xu, Zhijie

    2016-10-01

    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 directions in the body-centered cubic U matrix causes the gas bubble alignment along 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.

  10. Zigzag-Shaped Superlattices on the Basis of Graphene Nanoribbons: Structure and Electronic Properties

    Science.gov (United States)

    Saroka, V. A.; Batrakov, K. G.

    2016-09-01

    The paper focuses on superlattices consisting of two coplanar fragments of one-layer graphene nanoribbons that have different width and are connected at an angle. Classification of such superlattices was carried out; their electronic properties were studied using the tight-binding method. It was demonstrated that in superlattices consisting of two fragments of graphene nanoribbons with armchair edges connected at an angle of 60°, the band gap can be regulated by the number of dimeric carbon atom chains of one of the fragments. In that case one can observe a periodic dependence of the band gap on the number of chains with a characteristic period equal to three dimeric chains. The number of dimeric chains of the second superlattice fragment regulates the average band gap value near which the periodic oscillations occur, as well as the amplitude of those oscillations. Therefore, one can accomplish a sufficiently precise band gap tuning for such structures. Such tuning can find its wide application in the booming carbon nanoelectronics industry when creating generators, amplifiers and sensors in the nanochains.

  11. 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 an....... 74, 1831 (1995)], where a set of numerical simulations was presented....

  12. New research progress on the ultrastructure of tectonically deformed coals

    Institute of Scientific and Technical Information of China (English)

    Yiwen Ju; Xiaoshi Li

    2009-01-01

    The structure of tectonically deformed coals shows some characteristics and regulations on the ultra-scale when influenced by different factors such as temperature, pressure and directional stress. Deformations of the macromolecular structure may induce changes in the nano-scale pore structure, which are very important for gas adsorption and pervasion. The geological impact of different mechanisms of deformation on different types of ultrastructure of tectonically deformed coals is not only of significance as a topic for scientific research, but also in studying environments of metamorphism-deformation at differing metamorphic stages. This research is also very important in studying the accumulation and occurrence conditions of coalbed methane gas, mechanisms of coal and gas outbursts, predictions of coalbed methane resources and dangers of coal and gas outbursts. This paper analyzes current research, within China and other coun-tries, in the ultrastructure of tectonically deformed coals. The research includes (1) structural models of tectonically deformed coals; (2) the relationship between ultrastructure and stress; (3) the structure of tectonically deformed coals and its strain environment. These results are also based on our own experiments including vitirnite reflectance (R_(o,max)), X-ray diffraction (XRD) and nuclear magnetic res-onance (NMR (CP/MAS+TOSS)). We discuss the important effects of the structure of tectonically deformed coals and their physical properties, and then point out some problems concerning the research progress of tectonically deformed coals.

  13. The Spherical Deformation Model

    DEFF Research Database (Denmark)

    Hobolth, Asgar

    2003-01-01

    Miller et al. (1994) describe a model for representing spatial objects with no obvious landmarks. Each object is represented by a global translation and a normal deformation of a sphere. The normal deformation is defined via the orthonormal spherical-harmonic basis. In this paper we analyse...... the spherical deformation model in detail and describe how it may be used to summarize the shape of star-shaped three-dimensional objects with few parameters. It is of interest to make statistical inference about the three-dimensional shape parameters from continuous observations of the surface and from...

  14. Calcaneo-valgus deformity.

    Science.gov (United States)

    Evans, D

    1975-08-01

    A discussion of the essential deformity in calcaneo-valgus feet develops a theme originally put forward in 1961 on the relapsed club foot (Evans 1961). Whereas in the normal foot the medial and lateral columns are about equal in length, in talipes equino-varus the lateral column is longer and in calcaneo-valgus shorter than the medial column. The suggestion is that in the treatment of both deformities the length of the columns be made equal. A method is described of treating calcaneo-valgus deformity by inserting cortical bone grafts taken from the tibia to elongate the anterior end of the calcaneus.

  15. 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.

  16. Effect of crystalline/amorphous interfaces on thermal transport across confined thin films and superlattices

    Science.gov (United States)

    Giri, Ashutosh; Braun, Jeffrey L.; Hopkins, Patrick E.

    2016-06-01

    We report on the thermal boundary resistances across crystalline and amorphous confined thin films and the thermal conductivities of amorphous/crystalline superlattices for Si/Ge systems as determined via non-equilibrium molecular dynamics simulations. Thermal resistances across disordered Si or Ge thin films increase with increasing length of the interfacial thin films and in general demonstrate higher thermal boundary resistances in comparison to ordered films. However, for films ≲3 nm, the resistances are highly dependent on the spectral overlap of the density of states between the film and leads. Furthermore, the resistances at a single amorphous/crystalline interface in these structures are much lower than those at interfaces between the corresponding crystalline materials, suggesting that diffusive scattering at an interface could result in higher energy transmissions in these systems. We use these findings, together with the fact that high mass ratios between amorphous and crystalline materials can lead to higher thermal resistances across thin films, to design amorphous/crystalline superlattices with very low thermal conductivities. In this regard, we study the thermal conductivities of amorphous/crystalline superlattices and show that the thermal conductivities decrease monotonically with increasing interface densities above 0.1 nm-1. These thermal conductivities are lower than that of the homogeneous amorphous counterparts, which alludes to the fact that interfaces non-negligibly contribute to thermal resistance in these superlattices. Our results suggest that the thermal conductivity of superlattices can be reduced below the amorphous limit of its material constituent even when one of the materials remains crystalline.

  17. Interfacial ferromagnetism and exchange bias in CaRuO3/CaMnO3 superlattices.

    Science.gov (United States)

    He, C; Grutter, A J; Gu, M; Browning, N D; Takamura, Y; Kirby, B J; Borchers, J A; Kim, J W; Fitzsimmons, M R; Zhai, X; Mehta, V V; Wong, F J; Suzuki, Y

    2012-11-01

    We have found ferromagnetism in epitaxially grown superlattices of CaRuO(3)/CaMnO(3) that arises in one unit cell at the interface. Scanning transmission electron microscopy and electron energy loss spectroscopy indicate that the difference in magnitude of the Mn valence states between the center of the CaMnO(3) layer and the interface region is consistent with double exchange interaction among the Mn ions at the interface. Polarized neutron reflectivity and the CaMnO(3) thickness dependence of the exchange bias field together indicate that the interfacial ferromagnetism is only limited to one unit cell of CaMnO(3) at each interface. The interfacial moment alternates between the 1 μ(B)/interface Mn ion for even CaMnO(3) layers and the 0.5 μ(B)/interface Mn ion for odd CaMnO(3) layers. This modulation, combined with the exchange bias, suggests the presence of a modulating interlayer coupling between neighboring ferromagnetic interfaces via the antiferromagnetic CaMnO(3) layers.

  18. Extremely deformable structures

    CERN Document Server

    2015-01-01

    Recently, a new research stimulus has derived from the observation that soft structures, such as biological systems, but also rubber and gel, may work in a post critical regime, where elastic elements are subject to extreme deformations, though still exhibiting excellent mechanical performances. This is the realm of ‘extreme mechanics’, to which this book is addressed. The possibility of exploiting highly deformable structures opens new and unexpected technological possibilities. In particular, the challenge is the design of deformable and bi-stable mechanisms which can reach superior mechanical performances and can have a strong impact on several high-tech applications, including stretchable electronics, nanotube serpentines, deployable structures for aerospace engineering, cable deployment in the ocean, but also sensors and flexible actuators and vibration absorbers. Readers are introduced to a variety of interrelated topics involving the mechanics of extremely deformable structures, with emphasis on ...

  19. Cross-sectional scanning thermal microscopy of ErAs/GaAs superlattices grown by molecular beam epitaxy.

    Science.gov (United States)

    Park, K W; Krivoy, E M; Nair, H P; Bank, S R; Yu, E T

    2015-07-03

    Scanning thermal microscopy has been implemented in a cross-sectional geometry, and its application for quantitative, nanoscale analysis of thermal conductivity is demonstrated in studies of an ErAs/GaAs nanocomposite superlattice. Spurious measurement effects, attributable to local thermal transport through air, were observed near large step edges, but could be eliminated by thermocompression bonding to an additional structure. Using this approach, bonding of an ErAs/GaAs superlattice grown on GaAs to a silicon-on-insulator wafer enabled thermal signals to be obtained simultaneously from Si, SiO2, GaAs, and ErAs/GaAs superlattice. When combined with numerical modeling, the thermal conductivity of the ErAs/GaAs superlattice measured using this approach was 11 ± 4 W m(-1) K(-1).

  20. Dualband MW/LW Strained Layer Superlattice Focal Plane Arrays For Satellite-Based Wildfire Detection Project

    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...

  1. Highly ordered and highly aligned two-dimensional binary superlattice of a SWNT/cylindrical-micellar system.

    Science.gov (United States)

    Lim, Sung-Hwan; Jang, Hyung-Sik; Ha, Jae-Min; Kim, Tae-Hwan; Kwasniewski, Pawel; Narayanan, Theyencheri; Jin, Kyeong Sik; Choi, Sung-Min

    2014-11-10

    We report a highly ordered intercalated hexagonal binary superlattice of hydrophilically functionalized single-walled carbon nanotubes (p-SWNTs) and surfactant (C12 E5 ) cylindrical micelles. When p-SWNTs (with a diameter slightly larger than that of the C12 E5 cylinders) were added to the hexagonally packed C12 E5 cylindrical-micellar system, p-SWNTs positioned themselves in such a way that the free-volume entropies for both p-SWNTs and C12 E5 cylinders were maximized, thus resulting in the intercalated hexagonal binary superlattice. In this binary superlattice, a hexagonal array of p-SWNTs is embedded in a honeycomb lattice of C12 E5 cylinders. The intercalated hexagonal binary superlattice can be highly aligned in one direction by an oscillatory shear field and remains aligned after the shear is removed.

  2. Interaction of optical and interface phonons and their anisotropy in GaAs/AlAs superlattices: Experiment and calculations

    Energy Technology Data Exchange (ETDEWEB)

    Volodin, V. A., E-mail: volodin@isp.nsc.ru [Russian Academy of Sciences, Rzhanov Institute of Semiconductor Physics, Siberian Branch (Russian Federation); Sachkov, V. A. [Russian Academy of Sciences, Omsk Scientific Center, Siberian Branch (Russian Federation); Sinyukov, M. P. [Russian Academy of Sciences, Rzhanov Institute of Semiconductor Physics, Siberian Branch (Russian Federation)

    2015-05-15

    The angular anisotropy of interface phonons and their interaction with optical phonons in (001) GaAs/AlAs superlattices are calculated and experimentally studied. Experiments were performed by Raman light scattering in different scattering geometries for phonons with the wave vector directed normally to the superlattice and along its layers. Phonon frequencies were calculated by the extended Born method taking the Coulomb interaction into account in the rigid-ion approximation. Raman scattering spectra were calculated in the Volkenshtein bond-polarizability approximation. Calculations confirmed that the angular anisotropy of phonons observed in experiments appears due to interaction (mixing) of optical phonons, in which atoms are mainly displaced normally to superlattices, with interface phonons (TO-IF modes). In the scattering geometry, when the wave vector lies in the plane of superlattice layers, the mixed TO-IF modes are observed under nonresonance conditions. The Raman spectra for TO-IF modes depend on the mixing of atoms at heteroboundaries.

  3. Interaction of optical and interface phonons and their anisotropy in GaAs/AlAs superlattices: Experiment and calculations

    Science.gov (United States)

    Volodin, V. A.; Sachkov, V. A.; Sinyukov, M. P.

    2015-05-01

    The angular anisotropy of interface phonons and their interaction with optical phonons in (001) GaAs/AlAs superlattices are calculated and experimentally studied. Experiments were performed by Raman light scattering in different scattering geometries for phonons with the wave vector directed normally to the superlattice and along its layers. Phonon frequencies were calculated by the extended Born method taking the Coulomb interaction into account in the rigid-ion approximation. Raman scattering spectra were calculated in the Volkenshtein bond-polarizability approximation. Calculations confirmed that the angular anisotropy of phonons observed in experiments appears due to interaction (mixing) of optical phonons, in which atoms are mainly displaced normally to superlattices, with interface phonons (TO-IF modes). In the scattering geometry, when the wave vector lies in the plane of superlattice layers, the mixed TO-IF modes are observed under nonresonance conditions. The Raman spectra for TO-IF modes depend on the mixing of atoms at heteroboundaries.

  4. Deformations of singularities

    CERN Document Server

    Stevens, Jan

    2003-01-01

    These notes deal with deformation theory of complex analytic singularities and related objects. The first part treats general theory. The central notion is that of versal deformation in several variants. The theory is developed both in an abstract way and in a concrete way suitable for computations. The second part deals with more specific problems, specially on curves and surfaces. Smoothings of singularities are the main concern. Examples are spread throughout the text.

  5. Diffeomorphic Statistical Deformation Models

    DEFF Research Database (Denmark)

    Hansen, Michael Sass; Hansen, Mads/Fogtman; Larsen, Rasmus

    2007-01-01

    In this paper we present a new method for constructing diffeomorphic statistical deformation models in arbitrary dimensional images with a nonlinear generative model and a linear parameter space. Our deformation model is a modified version of the diffeomorphic model introduced by Cootes et al. Th...... with ground truth in form of manual expert annotations, and compared to Cootes's model. We anticipate applications in unconstrained diffeomorphic synthesis of images, e.g. for tracking, segmentation, registration or classification purposes....

  6. Deformation in nanocrystalline metals

    OpenAIRE

    Helena Van Swygenhoven; Julia R. Weertman

    2006-01-01

    It is now possible to synthesize polycrystalline metals made up of grains that average less than 100 nm in size. Such nanocrystalline metals contain a significant volume fraction of interfacial regions separated by nearly perfect crystals. The small sizes involved limit the conventional operation of dislocation sources and thus a fundamental question arises: how do these materials deform plastically? We review the current views on deformation mechanisms in nanocrystalline, face-centered cubic...

  7. The influence of AlN/GaN superlattice intermediate layer on the properties of GaN grown on Si(111) substrates

    Institute of Scientific and Technical Information of China (English)

    Liu Zhe; Wang Xiao-Liang; Wang Jun-Xi; Hu Guo-Xin; Guo Lun-Chun; Li Jin-Min

    2007-01-01

    AIN/GaN superlattice buffer is inserted between GaN epitaxial layer and Si substrate before epitaxial growth of GaN layer. High-quality and crack-free GaN epitaxial layers can be obtained by inserting AIN/GaN superlattice buffer layer. The influence of AIN/GaN superlattice buffer layer on the properties of GaN films are investigated in this paper. One of the important roles of the superlattice is to release tensile strain between Si substrate and epilayer. Raman spectra show a substantial decrease of in-plane tensile strain in GaN layers by vising AIN/GaN superlattice buffer layer. Moreover, TEM cross-sectional images show that the densities of both screw and edge dislocations are significantly reduced. The GaN films grown on Si with the superlattice buffer also have better surface morphology and optical properties.

  8. Analytic model of deformation of construction interfaces of rolled control concrete dam

    Institute of Scientific and Technical Information of China (English)

    GU Chong-shi; HUANG Guang-ming; LAI Dao-ping

    2007-01-01

    The construction interfaces of RCCD have a distinct influence on the deformation of dams. The characters and rules on deformation of construction interfaces are studied. The methods simulating the deformation of the interfaces at different stages are proposed. A thickness analytic model and a no-thickness analytic model of construction interfaces are built. These models can reflect the elastic deformation, the attenuation creep deformation, the irreversible creep deformation and the accelerating creep deformation of interfaces. The example shows that these proposed models can simulate the deformation of the dam structure objectively. Especially, the results of the thickness analytic model which simulates the gradual changing regularities of interfaces can tally with those of monitoring in situ preferably. The methods proposed and the analytic models can be generalized and applied to general concrete dams, especially to the analysis on deformation rules of fault and interlayer in dam base.

  9. Resonance Raman Scattering Studies of Gallium - - Aluminum-Arsenide Superlattices.

    Science.gov (United States)

    Gant, Thomas Andrew

    We have made resonance Raman scattering studies of folded LA phonons and quantized LO phonons in several GaAs-AlAs superlattices. The motivation for this work was to study the electronic structure and the electron -phonon interaction in these structures. The samples were not intentionally doped. The Raman spectra of optic phonons were usually taken at a temperature of 10 K or less. The folded acoustic phonon work was taken at temperatures ranging from 200-300 K in order to enhance the scattering by the thermal factor. Two samples in particular have received very close attention: sample 2292 (50 A GaAs- 20 A AlAs) and sample 3250 (20 A GaAs- 50 A AlAs). In sample 2292 we have made resonance studies of the folded LA phonons and the GaAs -like confined LO_2 mode near the second heavy hole exciton. The results on the folded acoustic phonons show a very strong resonance enhancement for the second order folded phonons, but very little for the first order. An interference between two different scattering channels (the n = 1 light hole and the n = 2 heavy hole subbands) seems to be responsible for this effect. The resonance profile for the LO_2 confined optic phonon in sample 2292 shows 4 peaks in the region from 1.8 eV to 2.05 eV. We have studied the dependence of this resonance profile on the power density. A higher power density was achieved by using the same laser power with a tighter focus. At the higher power density the peak at 1.93 eV (formerly the strongest peak present) vanished. This "bleaching" effect is related to screening due to the higher carrier density. In sample 3250 we have studied the polarization dependence of the resonance profiles of four peaks (LO _2, LO_4, LO_6, and an interface mode) near the lowest direct gap. The A_1 symmetry confined LO modes are seen in both polarized and depolarized geometries, in violation of the usual selection rule (polarized). A mechanism is proposed to explain this result, which has been previously observed by other

  10. Thermal deformations of a glass spherical satellite

    Science.gov (United States)

    Vasiliev, V. P.; Nenadovich, V. D.; Murashkin, V. V.; Sokolov, A. L.

    2016-09-01

    The effect of the kind of the reflecting coating of a glass spherical satellite on thermal deformations caused by the solar irradiation is considered. Two types of coating deposited on one of the hemispheres are considered: aluminum with a protective layer of bakelite varnish and interference dielectric coating for two orientations of the satellite orbit. Structures of a multilayer dielectric coating and technologies of its deposition are described.

  11. Highly Polarized Electrons from GaAs-GaAsP and InGaAs-AlGaAs Strained Layer Superlattice Photocathodes

    CERN Document Server

    Nakanishi, T; Kuwahara, M; Naniwa, K; Nishitani, T; Okumi, S; Yamamoto, N; Yasui, K

    2004-01-01

    GaAs-GaAsP strained layer superlattice photocathode has been developed for highly polarized electron beams. This cathode achieved a maximum polarization of 92% with a quantum efficiency of 0.5%. Criteria for achieving the highest polarization together with high quantum efficiency using superlattice photocathodes are discussed based on experimental spin-resolved quantum efficiency spectra of GaAs-AlGaAs, InGaAs-AlGaAs and GaAs-GaAsP superlattice structures.

  12. Reflective optics

    CERN Document Server

    Korsch, Dietrich

    1991-01-01

    This is the first book dedicated exclusively to all-reflective imaging systems. It is a teaching tool as well as a practical design tool for anyone who specializes in optics, particularly for those interested in telescopes, infrared, and grazing-incidence systems. The first part of the book describes a unified geometric optical theory of all-reflective imaging systems (from near-normal to grazing incidence) developed from basic principles. The second part discusses correction methods and a multitude of closed-form solutions of well-corrected systems, supplemented with many conventional and unc

  13. Magnetocaloric effects in a freestanding and flexible graphene-based superlattice synthesized with a spatially confined reaction.

    Science.gov (United States)

    Zhu, Haiou; Xiao, Chong; Cheng, Hao; Grote, Fabian; Zhang, Xiaodong; Yao, Tao; Li, Zhou; Wang, Chengming; Wei, Shiqiang; Lei, Yong; Xie, Yi

    2014-06-03

    Superlattices have attracted great interest because of their tailorable electronic properties at the interface. However, the lack of an efficient and low-cost synthetic method represents a huge challenge to implement superlattices into practical applications. Herein, we report a space-confined nanoreactor strategy to synthesize flexible freestanding graphene-based superlattice nanosheets, which consist of alternately intercalated monolayered metal-oxide frameworks and graphene. Taking vanadium oxide as an example, clear-cut evidences in extended X-ray absorption fine structure, high-resolution transmission electron microscopy and infrared spectra have confirmed that the vanadium oxide frameworks in the superlattice nanosheets show high symmetry derived from the space-confinement and electron-donor effect of graphene layers, which enable the superlattice nanosheets to show emerging magnetocaloric effect. Undoubtedly, this freestanding and flexible superlattice synthesized from a low-cost and scalable method avoids complex transferring processes from growth substrates for final applications and thus should be beneficial to a wide variety of functionalized devices.

  14. Deformation quantization of principal bundles

    CERN Document Server

    Aschieri, Paolo

    2016-01-01

    We outline how Drinfeld twist deformation techniques can be applied to the deformation quantization of principal bundles into noncommutative principal bundles, and more in general to the deformation of Hopf-Galois extensions. First we twist deform the structure group in a quantum group, and this leads to a deformation of the fibers of the principal bundle. Next we twist deform a subgroup of the group of authomorphisms of the principal bundle, and this leads to a noncommutative base space. Considering both deformations we obtain noncommutative principal bundles with noncommutative fiber and base space as well.

  15. Reflective equilibrium

    NARCIS (Netherlands)

    van der Burg, W.; van Willigenburg, T.

    1998-01-01

    The basic idea of reflective equilibrium, as a method for theory construction and decision making in ethics, is that we should bring together a broad variety of moral and non-moral beliefs and, through a process of critical scrutiny and mutual adjustment, combine these into one coherent belief syste

  16. Reflective equilibrium

    NARCIS (Netherlands)

    van der Burg, W.; van Willigenburg, T.

    1998-01-01

    The basic idea of reflective equilibrium, as a method for theory construction and decision making in ethics, is that we should bring together a broad variety of moral and non-moral beliefs and, through a process of critical scrutiny and mutual adjustment, combine these into one coherent belief syste

  17. Jordanian deformation of the open XXX spin chain

    Science.gov (United States)

    Kulish, P. P.; Manojlović, N.; Nagy, Z.

    2010-05-01

    We find the general solution of the reflection equation associated with the Jordanian deformation of the SL(2)-invariant Yang R-matrix. A special scaling limit of the XXZ model with general boundary conditions leads to the same K-matrix. Following the Sklyanin formalism, we derive the Hamiltonian with the boundary terms in explicit form. We also discuss the structure of the spectrum of the deformed XXX model and its dependence on the boundary conditions.

  18. Deformable Simplicial Complexes

    DEFF Research Database (Denmark)

    Misztal, Marek Krzysztof

    In this dissertation we present a novel method for deformable interface tracking in 2D and 3D|deformable simplicial complexes (DSC). Deformable interfaces are used in several applications, such as fluid simulation, image analysis, reconstruction or structural optimization. In the DSC method......, the interface (curve in 2D; surface in 3D) is represented explicitly as a piecewise linear curve or surface. However, the domain is also subject to discretization: triangulation in 2D; tetrahedralization in 3D. This way, the interface can be alternatively represented as a set of edges/triangles separating...... demonstrate those strengths in several applications. In particular, a novel, DSC-based fluid dynamics solver has been developed during the PhD project. A special feature of this solver is that due to the fact that DSC maintains an explicit interface representation, surface tension is more easily dealt with...

  19. Autogenous Deformation of Concrete

    DEFF Research Database (Denmark)

    Autogenous deformation of concrete can be defined as the free deformation of sealed concrete at a constant temperature. A number of observed problems with early age cracking of high-performance concretes can be attributed to this phenomenon. During the last 10 years , this has led to an increased...... focus on autogenous deformation both within concrete practice and concrete research. Since 1996 the interest has been significant enough to hold international, yearly conferences entirely devoted to this subject. The papers in this publication were presented at two consecutive half-day sessions...... at the American Concrete Institute’s Fall Convention in Phoenix, Arizona, October 29, 2002. All papers have been reviewed according to ACI rules. This publication, as well as the sessions, was sponsored by ACI committee 236, Material Science of Concrete. The 12 presentations from 8 different countries indicate...

  20. Autogenous Deformation of Concrete

    DEFF Research Database (Denmark)

    Autogenous deformation of concrete can be defined as the free deformation of sealed concrete at a constant temperature. A number of observed problems with early age cracking of high-performance concretes can be attributed to this phenomenon. During the last 10 years , this has led to an increased...... focus on autogenous deformation both within concrete practice and concrete research. Since 1996 the interest has been significant enough to hold international, yearly conferences entirely devoted to this subject. The papers in this publication were presented at two consecutive half-day sessions...... at the American Concrete Institute’s Fall Convention in Phoenix, Arizona, October 29, 2002. All papers have been reviewed according to ACI rules. This publication, as well as the sessions, was sponsored by ACI committee 236, Material Science of Concrete. The 12 presentations from 8 different countries indicate...

  1. Post-laminectomy deformities

    Directory of Open Access Journals (Sweden)

    Fabiano Stumpf Lutz

    2014-12-01

    Full Text Available Objective: To present the deformities and evaluate the results of their treatment. Methods: Retrospective study of patients with deformity following surgical access to the spinal canal. Fifteen patients who met the inclusion criteria were included. Patients without complete data in medical records were excluded. Results: Fourteen patients underwent surgical treatment and one patient received conservative treatment with vest type TLSO. The average angle of kyphosis correction was 87° preoperatively to 38° postoperatively, while the associated scoliosis correction was 69° preoperatively to 23° postoperatively. Conclusions: The prevention of deformity should be emphasized to avoid laminectomy alone, while laminoplasty should be the procedure of choice for canal access in surgeries where there is no need for resection of the posterior elements.

  2. Deformation of C isotopes

    CERN Document Server

    Kanada-Enyo, Y

    2004-01-01

    Systematic analysis of the deformations of proton and neutron densities in even-even C isotopes was done based on the method of antisymmetrized molecular dynamics. The $E2$ transition strength was discussed in relation to the deformation. We analyze the $B(E2;2^+_1\\to 0^+_1)$ in $^{16}$C, which has been recently measured to be abnormally small. The results suggest the difference of the deformations between proton and neutron densities in the neutron-rich C isotopes. It was found that stable proton structure in C isotopes plays an important role in the enhancement the neutron skin structure as well as in the systematics of $B(E2)$ in the neutron-rich C.

  3. Shear wave splitting and subcontinental mantle deformation

    Science.gov (United States)

    Silver, Paul G.; Chan, W. Winston

    1991-09-01

    We have made measurements of shear wave splitting in the phases SKS and SKKS at 21 broadband stations in North America, South America, Europe, Asia, and Africa. Measurements are made using a retrieval scheme that yields the azimuth of the fast polarization direction ϕ and delay time δt of the split shear wave plus uncertainties. Detectable anisotropy was found at most stations, suggesting that it is a general feature of the subcontinental mantle. Delay times range from 0.65 s to 1.70 s and average about 1 s. Somewhat surprisingly, the largest delay time is found in the 2.7 b.y.-old Western Superior Province of the Canadian Shield. The splitting observations are interpreted in terms of the strain-induced lattice preferred orientation of mantle minerals, especially olivine. We consider three hypotheses concerning the origin of the continental anisotropy: (1) strain associated with absolute plate motion, as in the oceanic upper mantle, (2) crustal stress, and (3) the past and present internal deformation of the subcontinental upper mantle by tectonic episodes. It is found that the last hypothesis is the most successful, namely that the most recent significant episode of internal deformation appears to be the best predictor of ϕ. For stable continental regions, this is interpreted as "fossil" anisotropy, whereas for presently active regions, such as Alaska, the anisotropy reflects present-day tectonic activity. In the stable portion of North America there is a good correlation between delay time and lithospheric thickness; this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant. The acceptance of this hypothesis has several implications for subcontinental mantle deformation. First, it argues for coherent deformation of the continental lithosphere (crust and mantle) during orogenies. This implies that the anisotropic portion of the lithosphere was present since the

  4. Deformation in nanocrystalline metals

    Directory of Open Access Journals (Sweden)

    Helena Van Swygenhoven

    2006-05-01

    Full Text Available It is now possible to synthesize polycrystalline metals made up of grains that average less than 100 nm in size. Such nanocrystalline metals contain a significant volume fraction of interfacial regions separated by nearly perfect crystals. The small sizes involved limit the conventional operation of dislocation sources and thus a fundamental question arises: how do these materials deform plastically? We review the current views on deformation mechanisms in nanocrystalline, face-centered cubic metals based on insights gained by atomistic computer simulations. These insights are discussed with reference to recent striking experimental observations that can be compared with predictions made by the simulations.

  5. Heat treatment deformations

    Energy Technology Data Exchange (ETDEWEB)

    Bavaro, A. (Soliveri SpA, Caravaggio (Italy))

    1990-02-01

    Types and causes of heat treatement derived isotropic and anisotropic dilatancies in ferrous materials are reviewed. The concepts are developed in such a way as to allow extension to all materials exhibiting martensitic tempering behaviour. This paper intends to illustrate the basic processes of dimensional variations undergone by the materials under heat treatments. The parametric analysis includes an analysis of the interactions amongst the parameters themselves. The relative importance of each parameter is assessed in order to determine methods to attenuate deformation action. Simplified examples are offered to provide technicians explanations as to why specific deformations occur and indications on improved materials working techniques.

  6. Strain analysis of compositionally tailored interfaces in InAs/GaSb superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Mahalingam, Krishnamurthy; Haugan, Heather J.; Brown, Gail J.; Aronow, Andrew J. [AFRL/RXAN, Materials and Manufacturing Directorate Air Force Research Laboratory, Wright Patterson AFB, Ohio 45433-7707 (United States)

    2013-11-18

    The effect of interface composition control on interfacial strain distribution in InAs/GaSb superlattices on (100)-GaSb substrates is investigated by atomic resolution scanning transmission electron microscopy. The interface composition was controlled by either depositing InSb at each interface or soaking the GaSb-on-InAs interface under Sb{sub 2} atmosphere. The strain profiles reveal a distinct difference in the extent to which the superlattice strain is balanced using the two methods. In particular, they indicate that the degree of strain balance achievable with soaking is inherently limited by the arsenic surface coverage during GaSb-on-InAs interface formation, emphasizing the influence of V/III flux ratio at this interface. The results also explain observed X-ray diffraction profiles.

  7. Short-period InAs/GaSb superlattices for mid-infrared photodetectors

    Energy Technology Data Exchange (ETDEWEB)

    Haugan, H.J.; Szmulowicz, F.; Brown, G.J.; Munshi, S.R. [Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, Ohio 45433 (United States); Ullrich, B. [Department of Physics and Astronomy, Bowling Green State University, Ohio 45433 (United States); Wickett, J.C.; Stokes, D.W. [Department of Physics, University of Houston, Texas 77204 (United States)

    2007-04-15

    Using a newly developed envelope function approximation model that includes interface effects, several InAs/GaSb type-II superlattices (SLs) were designed for uncooled mid-infrared detector applications. The 4 micron cutoff could be achieved with several SL designs. Superlattices with shorter periods have larger intervalence band separations than larger-ones, which could increase the optical signal and reduce the detector noise, thus making room temperature operation possible. To test these possibilities, several short-period SLs were grown by molecular-beam epitaxy and their optical properties with reducing SL period were studied by band-edge absorption, photoconductivity and photoluminescence measurements. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  8. Strong coupling of Rydberg atoms and surface phonon polaritons on piezoelectric superlattices

    CERN Document Server

    Sheng, Jiteng; Shaffer, James P

    2016-01-01

    We propose a hybrid quantum system where the strong coupling regime can be achieved between a Rydberg atomic ensemble and propagating surface phonon polaritons on a piezoelectric superlattice. By exploiting the large electric dipole moment and long lifetime of Rydberg atoms as well as tightly confined surface phonon polariton modes, it is possible to achieve a coupling constant far exceeding the relevant decay rates. The frequency of the surface mode can be selected so it is resonant with a Rydberg transition by engineering the piezoelectric superlattice. We describe a way to observe the Rabi splitting associated with the strong coupling regime under realistic experimental conditions. The system can be viewed as a new type of optomechanical system.

  9. Electronic Structures and Giant Magnetoresistance of Co/Cu Superlattices with Different Orientations

    Institute of Scientific and Technical Information of China (English)

    SHANG Jia-Xiang; ZHAO Xiao-Dan

    2006-01-01

    @@ The electronic structures of Co3 Cu3 superlattices with the orientations of (100), (110) and (111) are calculated by the first-principle method within the framework of the density functional theory. It has been found that the spin-dependent scattering and charge transfers are prominent at interfaces compared to the interior layers for the three orientation superlattices. We also evaluate the magnetoresistance ratio by using the two-current model The results show that the giant magnetoresistance ratio decreases in the order of (110), (100), (111) orientations for Co3Cu3 models (49. 4%, 37. 7%, 29.3%, respectively). Further analysis shows that an expansion of average atomic volume would enhance the magnetic moment of Co, which is consistent with other calculation and experimental results. In addition, the giant magnetoresistance effect is analysed from the point of charge transfer.

  10. Collective excitations of dipolar gases based on local tunneling in ultracold superlattices

    CERN Document Server

    Cao, L; Deng, X; Schmelcher, P

    2016-01-01

    The collective dynamics of a dipolar fermionic quantum gas confined in a one-dimensional double-well superlattice is explored. The fermionic gas resides in a paramagnetic-like ground state in the weak interaction regime, upon which a new type of collective dynamics is found when applying a local perturbation. This dynamics is composed of the local tunneling of fermions in separate supercells, and is a pure quantum effect, with no classical counterpart. Due to the presence of the dipolar interactions the local tunneling transports through the entire superlattice, giving rise to a collective dynamics. A well-defined momentum-energy dispersion relation is identified in the ab-initio simulations demonstrating the phonon-like behavior. The phonon-like characteristic is also confirmed by an analytical description of the dynamics within a semiclassical picture.

  11. Layer thickness and period as design parameters to tailor pyroelectric properties in ferroelectric superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Misirlioglu, I. B., E-mail: burc@sabanciuniv.edu [Faculty of Engineering and Natural Sciences, Sabanci University, Tuzla/Orhanli, 34956 Istanbul (Turkey); Kesim, M. T. [Department of Materials Science and Engineering and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269 (United States); Alpay, S. P. [Department of Materials Science and Engineering and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269 (United States); Department of Physics, University of Connecticut, Storrs, Connecticut 06269 (United States)

    2014-10-27

    We theoretically examine the pyroelectric properties of ferroelectric-paraelectric superlattices as a function of layer thickness and configuration using non-linear thermodynamics coupled with electrostatic and electromechanical interactions between layers. We specifically study PbZr{sub 0.3}Ti{sub 0.7}O{sub 3}/SrTiO{sub 3} superlattices. The pyroelectric properties of such constructs consisting of relatively thin repeating units are shown to exceed the pyroelectric response of monolithic PbZr{sub 0.3}Ti{sub 0.7}O{sub 3} films. This is related to periodic internal electric fields generated due to the polarization mismatch between layers that allows tailoring of the shift in the transition temperature. Our results indicate that higher and electric field sensitive pyroresponse can be achieved from layer-by-layer engineered ferroelectric heterostructures.

  12. Long-Range Effects on the Pyroelectric Coefficient of Ferroelectric Superlattice

    Institute of Scientific and Technical Information of China (English)

    DONG Wen; YAO Dong-Lai; WU Yin-Zhong; LI Zhen-Ya

    2002-01-01

    Long-range effects on the pyroelectric coefficient of a ferroelectric superlattice consisting of two differentferroelectric materials are investigated based on the transverse Ising model. The effects of the interfacial coupling andthe thickness of one period on the pyroelectric coefficient of the ferroelectric superlattice are studied by taking intoaccount the long-range interaction. It is found that with the increase of the strength of the long-range interaction, thepyroelectric coefficient decreases when the temperature is lower than the phase transition temperature; the number ofthe pyroelectric peaks decreases gradually and the phase transition temperature increases. It is also found that with thedecrease of the interfacial coupling and the thickness of one period, the phase transition temperature and the number ofthe pyroelectric peaks decrease.

  13. Synthesis of In2S3 nanoplates and their self-assembly into superlattices.

    Science.gov (United States)

    Zhong, Haizheng; Ye, Mingfu; Zhou, Yi; Yang, Chunhe; Li, Yongfang

    2007-12-01

    Ultrathin nanoplates of beta-phase semiconductor In2S3 with the diameter of 50 - 80 nm and a thickness of 1.85 nm were synthesized via a simple hot injection solution method with Hexadecylamine (HDA) as ligand, and the nanoplates self-assembled into superlattices by face to face stacking during evaporation of the solvent. The nanoplates and the assembled superlattices were characterized by XRD, TEM, HRTEM, XPS etc. The TEM and XRD results display that the nanocrystals are highly crystalline, hexagonal shape, and uniform in thickness. A strong quantum confinement effect of the 2D nanoplates was observed from the UV-Vis and PL spectra for the first time in the In2S3 nanoplates. A possible mechanism of the synthesis and assembly was proposed.

  14. Investigation of thermal conductivity of nitride mixed crystals and superlattices by molecular dynamics

    Energy Technology Data Exchange (ETDEWEB)

    Kawamura, Takahiro [Graduate School of Engineering, Kyushu University (Japan); Kangawa, Yoshihiro; Kakimoto, Koichi [Research Institute for Applied Mechanics, Kyushu University, 6-1, Kasuga-Koen, Kasuga 816-8580 (Japan)

    2006-06-15

    The thermal conductivities of Al{sub x}Ga{sub 1-x}N and In{sub x}Ga{sub 1-x}N mixed crystal and AlN/GaN superlattices were investigated by molecular dynamics simulation. We used Stillinger-Weber potentials, and Green-Kubo's formula was employed to calculate thermal conductivity. The results showed that the value of thermal conductivity of Al{sub 0.5}Ga{sub 0.5}N was about 1/20 smaller than that of GaN. It was also found that the thermal conductivity of AlN/GaN superlattices along the c-axis, which is parallel to the growth direction, was much less than that of bulk GaN. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  15. Reprint of : Three-terminal heat engine and refrigerator based on superlattices

    Science.gov (United States)

    Choi, Yunjin; Jordan, Andrew N.

    2016-08-01

    We propose a three-terminal heat engine based on semiconductor superlattices for energy harvesting. The periodicity of the superlattice structure creates an energy miniband, giving an energy window for allowed electron transport. We find that this device delivers a large power, nearly twice than the heat engine based on quantum wells, with a small reduction of efficiency. This engine also works as a refrigerator in a different regime of the system's parameters. The thermoelectric performance of the refrigerator is analyzed, including the cooling power and coefficient of performance in the optimized condition. We also calculate phonon heat current through the system and explore the reduction of phonon heat current compared to the bulk material. The direct phonon heat current is negligible at low temperatures, but dominates over the electronic at room temperature and we discuss ways to reduce it.

  16. Epitaxial growth of Fe/Ag single crystal superlattices and their magnetic properties

    Institute of Scientific and Technical Information of China (English)

    Yu Gu; Fei Zeng; Fang Lv; Yuli Cu; Pei-yong Yang; Feng Pan

    2009-01-01

    Single crystal Fe/Ag(001) superlattices with various periodicities were fabricated using ultrahigh vacuum evaporation de-position.It was found that single crystal bcc Fe layers and single crystal fcc Ag layers can epitaxially grow on a single crystal Ag buffer layer alternately,which was deposited on NaCl single crystal chips by ion beam assisted deposition.The magnetic measure-ments of the superlattices reveal an oscillation coupling between ferromagnetism and antiferromagnetism as a function of the Ag layer thickness.The oscillation period,which is 1 nm (5 Ag layers),is in good agreement with the calculated values when the Ag thickness is greater than 1.5 nm.While the thickness of the Ag spacer layer decreases to 1 nm,the oscillation coupling varies from calculations,which can be attributed to the intermixing of the interlayers according to the annealing results.

  17. 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.

  18. Current density in generalized Fibonacci superlattices under a uniform electric field.

    Science.gov (United States)

    Panchadhyayee, P; Biswas, R; Khan, Arif; Mahapatra, P K

    2008-07-09

    We present an exhaustive study on tunneling and electrical conduction in an electrically biased GaAs-Al(y)Ga(1-y)As generalized Fibonacci superlattice. The study is based on transfer matrix formalism using an Airy function approach and provides an exact calculation of the current density in the case of quasi-periodic multibarrier systems. The results suggest the use of such quasi-periodic systems in perfect band-pass or band-eliminator (of extremely low width) circuitry. We have clearly demonstrated the resonance-type peaks and negative differential conductivity regimes in such systems. It has also been found that quasi-periodicity favors sharp negative differential conductivity peaks compared to those in periodic superlattices and thus have profound importance in device applications.

  19. Nonlinear dynamics and band transport in a superlattice driven by a plane wave

    Science.gov (United States)

    Apostolakis, A.; Awodele, M. K.; Alekseev, K. N.; Kusmartsev, F. V.; Balanov, A. G.

    2017-06-01

    A quantum particle transport induced in a spatially periodic potential by a propagating plane wave has a number of important implications in a range of topical physical systems. Examples include acoustically driven semiconductor superlattices and cold atoms in an optical crystal. Here we apply a kinetic description of the directed transport in a superlattice beyond standard linear approximation, and utilize exact path-integral solutions of the semiclassical transport equation. We show that the particle drift and average velocities have nonmonotonic dependence on the wave amplitude with several prominent extrema. Such nontrivial kinetic behavior is related to global bifurcations developing with an increase of the wave amplitude. They cause dramatic transformations of the system phase space and lead to changes of the transport regime. We describe different types of phase trajectories contributing to the directed transport and analyze their spectral content.

  20. Phase diagrams of a finite superlattice with two disordered interfaces: Monte Carlo simulation

    Science.gov (United States)

    Feraoun, A.; Zaim, A.; Kerouad, M.

    2015-11-01

    The phase diagrams and the magnetic properties of an Ising finite superlattice with two disordered interfaces are investigated using Monte Carlo simulations based on Metropolis algorithm. The superlattice consists of k unit cells, where the unit cell contains L layers of spin-1/2 (A atoms), L layers of spin-1 (B atoms), and a disordered interface, with two layers in between, which is characterized by a random arrangement of A and B atoms Ap B1-p A1-p Bp with a negative A-B coupling. The A-A and B-B exchange coupling are considered ferromagnetic. We have investigated the effects of the thickness of the film, the crystal field interactions and the surface exchanges coupling on the magnetic properties. The obtained results show that the number of compensation points and the number of first-order phase transition lines depend strongly on the thickness, the probability p and the exchange interactions in the surfaces.

  1. Simple and Superlattice Turing Patterns in Reaction-Diffusion Systems Bifurcation, Bistability, and Parameter Collapse

    CERN Document Server

    Judd, S L; Judd, Stephen L.; Silber, Mary

    1998-01-01

    This paper investigates the competition between both simple (e.g. stripes, hexagons) and ``superlattice'' (super squares, super hexagons) Turing patterns in two-component reaction-diffusion systems. ``Superlattice'' patterns are formed from eight or twelve Fourier modes, and feature structure at two different length scales. Using perturbation theory, we derive simple analytical expressions for the bifurcation equation coefficients on both rhombic and hexagonal lattices. These expressions show that, no matter how complicated the reaction kinectics, the nonlinear reaction terms reduce to just four effective terms within the bifurcation equation coefficients. Moreover, at the hexagonal degeneracy -- when the quadratic term in the hexagonal bifurcation equation disappears -- the number of effective system parameters drops to two, allowing a complete characterization of the possible bifurcation results at this degeneracy. The general results are then applied to specific model equations, to investigate the stabilit...

  2. Improvements in (112-bar2) semipolar GaN crystal quality by graded superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Xu, S.R., E-mail: shengruixidian@126.com [Key Lab of Wide Band-Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi' an 710071 (China); Zhang, J.C. [Key Lab of Wide Band-Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi' an 710071 (China); Cao, Y.R. [School of Electronical and Machanical Engineering, Xidian University, Xi' an, 710071 (China); Zhou, X.W.; Xue, J.S.; Lin, Z.Y.; Ma, J.C. [Key Lab of Wide Band-Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi' an 710071 (China); Bao, F. [Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215125 (China); Hao, Y. [Key Lab of Wide Band-Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi' an 710071 (China)

    2012-01-01

    We report on the use of graded superlattices (SLs) for defect reduction in semipolar (112-bar2) GaN films, grown by metal-organic chemical vapor deposition. High-resolution x-ray diffraction analysis revealed that there was a great reduction in the full width at half maximum, both on-axis and off-axis, with the SLs. Atomic force microscopy images revealed a significant decrease in slate features which was associated with the basal-plane stacking faults. The transmission electron microscopy images showed that the threading dislocation was greatly reduced after the graded superlattices. Room temperature photoluminescence measurement revealed that the band-edge emission intensity increased with the insertion of the SLs, which suggested reduction in the nonradiative recombination centers.

  3. Noble-metal Ag nanoparticle chains: annealing Ag/Bi superlattice nanowires in vacuum

    Science.gov (United States)

    Xu, Shao Hui; Fei, Guang Tao; You, Qiao; Gao, Xu Dong; Huo, Peng Cheng; De Zhang, Li

    2016-09-01

    One-dimensional noble-metal Ag nanoparticle chains have been prepared by electrodepositing Ag/Bi superlattice nanowires in a porous anodic alumina oxide (AAO) template and following an annealing process in vacuum. It is found that Bi, as a sacrificial metal, can be removed completely after annealing at 450 °C with a vacuum degree of 10-5 Torr. The regulation of particle size, shape and interparticle spacing of Ag NP chains has been realized by adjusting the segment length of the Ag/Bi superlattice nanowires and the annealing condition. With an extension of the annealing time, it is observed that Ag particles display the transform trend from ellipsoid to sphere. Our findings could inspire further investigation on the design and fabrication of metal nanoparticle chains.

  4. Spin-dependent tunneling time in periodic diluted-magnetic-semiconductor/nonmagnetic-barrier superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Yang, Ping-Fan; Guo, Yong, E-mail: guoy66@tsinghua.edu.cn [Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084 (China); Collaborative Innovation Center of Quantum Matter, Beijing (China)

    2016-02-01

    We investigate the tunneling time (dwell time) in periodic diluted-magnetic-semiconductor/nonmagnetic-barrier (DMS/NB) superlattices subjected to an external magnetic field. It is found that spin-dependent resonant bands form in the spectra of dwell time, which can be effectively manipulated by not only the external magnetic field but also the geometric parameters of the system. Moreover, an intuitive semiclassical delay is defined to illustrate the behavior of the dwell time, and the former one is shown to be the result of “smoothing out” the latter one. We also find that the dwell time in diluted-magnetic-semiconductor/semiconductor superlattices behaves surprisingly different from the DMS/NB case, especially for spin-down electrons.

  5. Interfacial Ferromagnetism in LaNiO3/CaMnO3 Superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Grutter, Alexander J.; Yang, Hao; Kirby, B. J.; Fitzsimmons, M. R.; Aguiar, Jeffery A.; Browning, Nigel D.; Jenkins, C. A.; Arenholz, Elke; Mehta, V. V.; Alaan, U. S.; Suzuki, Y.

    2013-08-01

    We observe interfacial ferromagnetism in superlattices of the paramagnetic metal LaNiO3 and the antiferromagnetic insulator CaMnO3. LaNiO3 exhibits a thickness dependent metal-insulator transition and we find the emergence of ferromagnetism to be coincident with the conducting state of LaNiO3. That is, only superlattices in which the LaNiO3 layers are metallic exhibit ferromagnetism. Using several magnetic probes, we have determined that the ferromagnetism arises in a single unit cell of CaMnO3 at the interface. Together these results suggest that ferromagnetism can be attributed to a double exchange interaction among Mn ions mediated by the adjacent itinerant metal.

  6. Interfacial ferromagnetism in LaNiO3/CaMnO3 superlattices.

    Science.gov (United States)

    Grutter, A J; Yang, H; Kirby, B J; Fitzsimmons, M R; Aguiar, J A; Browning, N D; Jenkins, C A; Arenholz, E; Mehta, V V; Alaan, U S; Suzuki, Y

    2013-08-23

    We observe interfacial ferromagnetism in superlattices of the paramagnetic metal LaNiO3 and the antiferromagnetic insulator CaMnO3. LaNiO3 exhibits a thickness dependent metal-insulator transition and we find the emergence of ferromagnetism to be coincident with the conducting state of LaNiO3. That is, only superlattices in which the LaNiO3 layers are metallic exhibit ferromagnetism. Using several magnetic probes, we have determined that the ferromagnetism arises in a single unit cell of CaMnO3 at the interface. Together these results suggest that ferromagnetism can be attributed to a double exchange interaction among Mn ions mediated by the adjacent itinerant metal.

  7. 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)

  8. THz elastic dynamics in finite-size CoFeB-MgO phononic superlattices

    Science.gov (United States)

    Ulrichs, Henning; Meyer, Dennis; Müller, Markus; Wittrock, Steffen; Mansurova, Maria; Walowski, Jakob; Münzenberg, Markus

    2016-10-01

    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.

  9. Synchrotron X—Ray Study on Structures of Ni80Fe20/Cu Superlattices

    Institute of Scientific and Technical Information of China (English)

    杨涛; 麦振洪; 赖武彦; 吴忠华; 王德武; XUMing; 罗光明; 柴春林

    2001-01-01

    We have shown that,in contrast to the results in the literature,the Bragg peak intensity of Ni80Fe20/Cu superlattices is enhanced at the incident x-ray energy slightly higher than the absorption edge of the heavier element(Cu).The atomic density at Ni80Fe20/Cu interface was analysed by the diffraction anomalous fine structure technology with the incident angle of x-ray fixed at the first Bragg peak.Our results demonstrate the epitaxy growth of Ni80Fe20/Cu superlattices.Upon annealing,the epitaxity of Ni80Fe20/Cu multilayers becomes poor but the local crystallinity in each layer is improved.

  10. 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.

  11. Shape-controlled synthesis of palladium and copper superlattice nanowires for high-stability hydrogen sensors

    Science.gov (United States)

    Yang, Dachi; Carpena-Núñez, Jennifer; Fonseca, Luis F.; Biaggi-Labiosa, Azlin; Hunter, Gary W.

    2014-01-01

    For hydrogen sensors built with pure Pd nanowires, the instabilities causing baseline drifting and temperature-driven sensing behavior are limiting factors when working within a wide temperature range. To enhance the material stability, we have developed superlattice-structured palladium and copper nanowires (PdCu NWs) with random-gapped, screw-threaded, and spiral shapes achieved by wet-chemical approaches. The microstructure of the PdCu NWs reveals novel superlattices composed of lattice groups structured by four-atomic layers of alternating Pd and Cu. Sensors built with these modified NWs show significantly reduced baseline drifting and lower critical temperature (259.4 K and 261 K depending on the PdCu structure) for the reverse sensing behavior than those with pure Pd NWs (287 K). Moreover, the response and recovery times of the PdCu NWs sensor were of ~9 and ~7 times faster than for Pd NWs sensors, respectively.

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

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Yu, E-mail: ywang@semi.ac.cn [Department of Physics, Faculty of Science, Kunming University of Science and Technology, Kunming 650500 (China)

    2014-10-28

    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.

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

    Science.gov (United States)

    Wang, Yu

    2014-10-01

    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.

  14. Investigation of the magnetoresistivity in compositional superlattices under the influence of an intense electromagnetic wave

    Science.gov (United States)

    Hoi, Bui Dinh; Bau, Nguyen Quang; Nam, Nguyen Dinh

    2016-01-01

    The magnetoresistivity (MR) is theoretically calculated in a compositional semiconductor superlattice (CSSL), subjected to a crossed DC electric field and magnetic field, in the presence of an intense electromagnetic wave (EMW). The magnetic field is oriented along the growth direction of the CSSL and the electron-acoustic phonon interaction is taken into account at low temperature. Numerical results for the GaN/AlGaN CSSL show the Shubnikov-de Haas (SdH) oscillations in the MR whose period does not depend on the temperature and amplitude decreases with increasing temperature. The temperature dependence of the relative amplitude of these oscillations is in good agreement with other theories and experiments in some two-dimensional (2D) electron systems. The influence of the EMW as well as superlattice structure on the MR is discussed and compared with available theoretical and experimental results.

  15. Can fractal-like spectra be experimentally observed in aperiodic superlattices?

    Science.gov (United States)

    Maciá, Enrique; Domínguez-Adame, Francisco

    1996-07-01

    We numerically investigate the effects of inhomogeneities in the energy spectrum of aperiodic semiconductor superlattices, focusing our attention on Thue-Morse and Fibonacci sequences. In the absence of disorder, the corresponding electronic spectra are self-similar. The presence of a certain degree of randomness, due to imperfections occurring during the growth processes, gives rise to a progressive loss of quantum coherence, smearing out the finer details of the energy spectra predicted for perfect aperiodic superlattices and spurring the onset of electron localization. However, depending on the degree of disorder introduced, a critical size for the system exists, below which peculiar transport properties, related to the pre-fractal nature of the energy spectrum, may be measured.

  16. Electronic structure and magnetic couplings in metallic superlattices with diffuse interfaces

    Science.gov (United States)

    Stoeffler, Daniel; Gautier, François

    1992-02-01

    The real-space tight-binding method allows one to describe the magnetic order and the interlayer magnetic couplings (IMC) in Fe 3Cr n superlattices. In this paper we extend our previous study on Fe 3Cr n to Co 3Cr n and Ni 6Cr n superlattices. We obtain similar results for the IMC except that for Co 3Cr n the oscillations are obtained only after a "preasymptotic" ferromagnetic coupling regime ( n > 8). Then, we explore the role of diffuse interfaces by assuming as a first step the existence of ordered interfacial compounds (OIC). We show that the IMC oscillations are strongly perturbed and can be strongly damped by frustration effects on the Cr-OIC interfaces.

  17. 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.

  18. Magnetic properties of a three layer superlattice with a crystal field

    Institute of Scientific and Technical Information of China (English)

    Jiang Wei; Li Xin; Wang Xi-Kun; Guo An-Bang

    2006-01-01

    In this paper the magnetic properties of a three layer superlattice with the crystal field on the honeycomb and square lattice have been studied based on the effective-field theory with self-spin correlations and the differential operator technique. The effects of the crystal field and longitudinal magnetic field on the susceptibility are discussed in detail. A number of interesting phenomena, originating from the competition between the longitudinal magnetic field, crystal-field,and coordination number, have been found.

  19. Geometric phase and topology of elastic oscillations and vibrations in model systems: Harmonic oscillator and superlattice

    Directory of Open Access Journals (Sweden)

    P. A. Deymier

    2016-12-01

    Full Text Available We illustrate the concept of geometric phase in the case of two prototypical elastic systems, namely the one-dimensional harmonic oscillator and a one-dimensional binary superlattice. We demonstrate formally the relationship between the variation of the geometric phase in the spectral and wave number domains and the parallel transport of a vector field along paths on curved manifolds possessing helicoidal twists which exhibit non-conventional topology.

  20. 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 on...... strongly on the strain conditions and SL geometry, but weakly on the applied external hydrostatic pressure....

  1. Helimagnetic order in bulk MnSi and CoSi/MnSi superlattices

    Science.gov (United States)

    Loh, G. C.; Khoo, K. H.; Gan, C. K.

    2017-01-01

    Skyrmions are nanoscopic whirls of spins that reside in chiral magnets. It is only fairly recent that a plethora of applications for these quasiparticles emerges, especially in data storage. On the other hand, spin spirals are the periodic analogs of skyrmions, and are equally imperative in the course of exploration to enhance our understanding of helimagnetism. In this study, a new infrastructure based on the B20 compound, MnSi is propounded as a hosting material for spin spirals; alternating thin layers of CoSi and MnSi in the superlattice form provides a facile way of varying the properties of the spin spirals across a continuum. Using first-principles calculations based on full-potential linearized augmented plane-wave (FLAPW)-based density functional theory (DFT), the spin order of bulk MnSi, MnSi film, and the CoSi/MnSi superlattice is investigated. Spin dispersion plots as a function of propagation vectors show that the spiral size changes in the presence of CoSi - we find that the size of the spiral is reduced in the superlattice with thin CoSi layers (CoSi:MnSi=1:1 thickness ratio), whilst at a larger CoSi:MnSi=2:1 thickness ratio, the material behaves as a ferromagnet. In a similar fashion, the spin moment and orbital occupancy depend significantly on the thickness of the CoSi layers. However, the exchange interaction between Mn atoms appears to be generally impervious to the presence of CoSi. Succinctly, the CoSi/MnSi superlattice could be an excellent functional material in data storage applications.

  2. Geometric phase and topology of elastic oscillations and vibrations in model systems: Harmonic oscillator and superlattice

    Science.gov (United States)

    Deymier, P. A.; Runge, K.; Vasseur, J. O.

    2016-12-01

    We illustrate the concept of geometric phase in the case of two prototypical elastic systems, namely the one-dimensional harmonic oscillator and a one-dimensional binary superlattice. We demonstrate formally the relationship between the variation of the geometric phase in the spectral and wave number domains and the parallel transport of a vector field along paths on curved manifolds possessing helicoidal twists which exhibit non-conventional topology.

  3. Design and Fabrication of High Performance LWIR Photodetectors Based on Type II Superlattices

    Science.gov (United States)

    2017-08-11

    Program Manager Technical Advisor, Space Based Advanced Sensing and Protection JOHN BEAUCHEMIN Chief Engineer, Spacecraft Technology Division Space...34Focal plane arrays in type II-superlattices," USA Patent No. 6864552 (2005). H. Mohseni, A. Tahraoui, J. Wojkowski, M. Razeghi, G. J. Brown , W. C...has reached a level comparable to state of the art MCT detectors (Figure 2-b). 7 8 9 B. M. Nguyen, M. Razeghi, V. Nathan, and Gail J. Brown , "Type-II

  4. Monolithic Integration of a DFB Superlattice Laser Using High Energy Ion Implantation

    Science.gov (United States)

    1991-02-13

    Considerations Relative to DFB/DBR Laser Fabrication .... 46 4.0 Materials Evaluation of Multi-Quantum Well/Superlattice Samples Fabricated by Molecular Beam...important to consider the disordering that can occur during annealing and its effects on device performance. 3.7 CONSIDERATIONS RELATIVE TO DFB/DBR LASER ... FABRICATION The structure examined in ts work is to be incorporated in a distributed feedback type laser. The resolution of the selective ion beam

  5. Layer by Layer Growth of 2D Quantum Superlattices (NBIT III)

    Science.gov (United States)

    2017-02-28

    AFRL-AFOSR-JP-TR-2017-0019 Layer-by-Layer Growth of 2D Quantum Superlattices (NBIT III) Jiwoong Park CORNELL UNIVERSITY Final Report 02/22/2017...NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) CORNELL UNIVERSITY 373 PINE TREE RD ITHACA, NY 14850-2820 US 8. PERFORMING ORGANIZATION...age) Hee Cheul Choi (44) Full Name (age) Jiwoong Park (41) Nationality Korea Nationality Korea (US Resident ) Affiliation (department) POSTECH

  6. Atomic Layering, Intermixing and Switching Mechanism in Ge-Sb-Te based Chalcogenide Superlattices

    OpenAIRE

    Xiaoming Yu; John Robertson

    2016-01-01

    GeSbTe-based chalcogenide superlattice (CSLs) phase-change memories consist of GeSbTe layer blocks separated by van der Waals bonding gaps. Recent high resolution electron microscopy found two types of disorder in CSLs, a chemical disorder within individual layers, and SbTe bilayer stacking faults connecting one block to an adjacent block which allows individual block heights to vary. The disorder requires a generalization of the previous switching models developed for CSL systems. Density fu...

  7. Atomic Layering, Intermixing and Switching Mechanism in Ge-Sb-Te based Chalcogenide Superlattices.

    OpenAIRE

    Yu, X.; Robertson, J

    2016-01-01

    GeSbTe-based chalcogenide superlattice (CSLs) phase-change memories consist of GeSbTe layer blocks separated by van der Waals bonding gaps. Recent high resolution electron microscopy found two types of disorder in CSLs, a chemical disorder within individual layers, and SbTe bilayer stacking faults connecting one block to an adjacent block which allows individual block heights to vary. The disorder requires a generalization of the previous switching models developed for CSL systems. Density fu...

  8. Lattice dynamics of a simple GaSe-InSe superlattice

    Science.gov (United States)

    Schwarcz, R.; Kanehisa, M. A.

    1994-11-01

    A simple linear chain model is used to calculate the phonon dispersion curves for a superlattice composed of one layer of GaSe and one layer of InSe alternatively stacked along the c-axis. The resultant acoustic modes are essentially averages of the two constituents. Most optical modes are found to be confined in either GaSe layers of InSe layers. However, some low energy-optical modes have contributions from the two constituents.

  9. Self-similarity and novel sample-length-dependence of conductance in quasiperiodic lateral magnetic superlattices

    OpenAIRE

    Zeng, Z. Y.; Claro, F.

    2001-01-01

    We study the transport of electrons in a Fibonacci magnetic superlattice produced on a two-dimensional electron gas modulated by parallel magnetic field stripes arranged in a Fibonacci sequence. Both the transmission coefficient and conductance exhibit self-similarity and the six-circle property. The presence of extended states yields a finite conductivity at infinite length, that may be detected as an abrupt change in the conductance as the Fermi energy is varied, much as a metal-insulator t...

  10. Self-stabilization of high-frequency oscillations in semiconductor superlattices by time-delay autosynchronization.

    Science.gov (United States)

    Schlesner, J; Amann, A; Janson, N B; Just, W; Schöll, E

    2003-12-01

    We present a scheme to stabilize high-frequency domain oscillations in semiconductor superlattices by a time-delayed feedback loop. Applying concepts from chaos control theory we propose to control the spatiotemporal dynamics of fronts of accumulation and depletion layers which are generated at the emitter and may collide and annihilate during their transit, and thereby suppress chaos. The proposed method only requires the feedback of internal global electrical variables, viz., current and voltage, which makes the practical implementation very easy.

  11. Phonon modes of MgB2: super-lattice structures and spectral response.

    Science.gov (United States)

    Alarco, Jose A; Chou, Alison; Talbot, Peter C; Mackinnon, Ian D R

    2014-11-28

    Micrometre-sized MgB2 crystals of varying quality, synthesized at low temperature and autogenous pressure, are compared using a combination of Raman and infra-red (IR) spectroscopy. These data, which include new peak positions in both spectroscopies for high quality MgB2, are interpreted using DFT calculations on phonon behaviour for symmetry-related structures. Raman and IR activity additional to that predicted by point group analyses of the P6/mmm symmetry are detected. These additional peaks, as well as the overall shapes of calculated phonon dispersion (PD) models are explained by assuming a double super-lattice, consistent with a lower symmetry structure for MgB2. A 2× super-lattice in the c-direction allows a simple correlation of the pair breaking energy and the superconducting gap by activation of corresponding acoustic frequencies. A consistent physical interpretation of these spectra is obtained when the position of a phonon anomaly defines a super-lattice modulation in the a-b plane.

  12. Dynamic tuning of DNA-nanoparticle superlattices by molecular intercalation of double helix.

    Science.gov (United States)

    Pal, Suchetan; Zhang, Yugang; Kumar, Sanat K; Gang, Oleg

    2015-04-01

    Nanoparticle (NP) assembly using DNA recognition has emerged as a powerful tool for the fabrication of 3D superlattices. In addition to the vast structural diversity, this approach provides an avenue for dynamic 3D NP assembly, which is promising for the modulation of interparticle distances and, hence, for example, for in situ tuning of optical properties. While several approaches have been explored for changing NP separations in the lattices using responsiveness of single-stranded DNA (ss-DNA), far less work has been done for the manipulation of most abundant double-stranded DNA (ds-DNA) motifs. Here, we present a novel strategy for modulation of interparticle distances in DNA linked 3D self-assembled NP lattices by molecular intercalator. We utilize ethidium bromide (EtBr) as a model intercalator to demonstrate selective and isotropic lattice expansion for three superlattice types (bcc, fcc, and AlB2) due to the intercalation of ds-DNA linking NPs. We further show the reversibility of the lattice parameter using n-butanol as a retrieving agent as well as an increased lattice thermal stability by 12-14 °C due to the inclusion of EtBr. The proposed intercalator-based strategy permits the creation of reconfigurable and thermally stable superlattices, which could lead to tunable and functionally responsive materials.

  13. 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.

  14. The influence of non-idealities on the thermoelectric power factor of nanostructured superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Thesberg, Mischa; Pourfath, Mahdi; Kosina, Hans [Institute for Microelectronics, Technical University of Vienna, Gußhausstraße 27-29/E360, A-1040 Wien (Austria); Neophytou, Neophytos, E-mail: N.Neophytou@warwick.ac.uk [School of Engineering, University of Warwick, Coventry CV4 7AL (United Kingdom)

    2015-12-14

    Cross-plane superlattices composed of nanoscale layers of alternating potential wells and barriers have attracted great attention for their potential to provide thermoelectric power factor improvements and higher ZT figure of merit. Previous theoretical works have shown that the presence of optimized potential barriers could provide improvements to the Seebeck coefficient through carrier energy filtering, which improves the power factor by up to 40%. However, experimental corroboration of this prediction has been extremely scant. In this work, we employ quantum mechanical electronic transport simulations to outline the detrimental effects of random variation, imperfections, and non-optimal barrier shapes in a superlattice geometry on these predicted power factor improvements. Thus, we aim to assess either the robustness or the fragility of these theoretical gains in the face of the types of variation one would find in real material systems. We show that these power factor improvements are relatively robust against: overly thick barriers, diffusion of barriers into the body of the wells, and random fluctuations in barrier spacing and width. However, notably, we discover that extremely thin barriers and random fluctuation in barrier heights by as little as 10% is sufficient to entirely destroy any power factor benefits of the optimized geometry. Our results could provide performance optimization routes for nanostructured thermoelectrics and elucidate the reasons why significant power factor improvements are not commonly realized in superlattices, despite theoretical predictions.

  15. 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.

  16. The influence of non-idealities on the thermoelectric power factor of nanostructured superlattices

    Science.gov (United States)

    Thesberg, Mischa; Pourfath, Mahdi; Kosina, Hans; Neophytou, Neophytos

    2015-12-01

    Cross-plane superlattices composed of nanoscale layers of alternating potential wells and barriers have attracted great attention for their potential to provide thermoelectric power factor improvements and higher ZT figure of merit. Previous theoretical works have shown that the presence of optimized potential barriers could provide improvements to the Seebeck coefficient through carrier energy filtering, which improves the power factor by up to 40%. However, experimental corroboration of this prediction has been extremely scant. In this work, we employ quantum mechanical electronic transport simulations to outline the detrimental effects of random variation, imperfections, and non-optimal barrier shapes in a superlattice geometry on these predicted power factor improvements. Thus, we aim to assess either the robustness or the fragility of these theoretical gains in the face of the types of variation one would find in real material systems. We show that these power factor improvements are relatively robust against: overly thick barriers, diffusion of barriers into the body of the wells, and random fluctuations in barrier spacing and width. However, notably, we discover that extremely thin barriers and random fluctuation in barrier heights by as little as 10% is sufficient to entirely destroy any power factor benefits of the optimized geometry. Our results could provide performance optimization routes for nanostructured thermoelectrics and elucidate the reasons why significant power factor improvements are not commonly realized in superlattices, despite theoretical predictions.

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

    Science.gov (United States)

    Naik, Gururaj V; Saha, Bivas; Liu, Jing; Saber, Sammy M; Stach, Eric A; Irudayaraj, Joseph M K; Sands, Timothy D; Shalaev, Vladimir M; Boltasseva, Alexandra

    2014-05-27

    Titanium nitride (TiN) is a plasmonic material having optical properties resembling gold. Unlike gold, however, TiN is complementary metal oxide semiconductor-compatible, mechanically strong, and thermally stable at higher temperatures. Additionally, TiN exhibits low-index surfaces with surface energies that are lower than those of the noble metals which facilitates the growth of smooth, ultrathin crystalline films. Such films are crucial in constructing low-loss, high-performance plasmonic and metamaterial devices including hyperbolic metamaterials (HMMs). HMMs have been shown to exhibit exotic optical properties, including extremely high broadband photonic densities of states (PDOS), which are useful in quantum plasmonic applications. However, the extent to which the exotic properties of HMMs can be realized has been seriously limited by fabrication constraints and material properties. Here, 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 enhancement than gold- or silver-based HMMs.

  18. Exchange coupling and superconductivity in light rare earth alloys and superlattices

    CERN Document Server

    Deen, P P M

    2003-01-01

    The complementary techniques of X-ray magnetic resonant scattering, neutron and X-ray diffraction and SQUID magnetometry have been employed to explore rare earth superlattices and thin films. In particular, this thesis concerns the complex magnetism of Cerium, exchange coupling in Nd/Pr superlattices and the interplay between magnetism and superconductivity in Gd/La superlattices. Molecular beam epitaxy enables the growth of high quality single-crystal structures that do not occur in nature with tailor-made physical properties. Fundamental problems in condensed matter physics can therefore be addressed. Through a study of CeY and CeLu alloys, the origin of diffuse scattering and intermediate valence behaviour observed in CeHo alloys was determined. XMRS was able to probe the behaviour of Ce in various environments. A higher energy resonance, indicative of intermediate valence behaviour, is present for the Lu and Ho based alloys but not for those containing Y. Since the lattice parameters of CeLu closely match...

  19. Phonon thermal transport in silicene-germanene superlattice: a molecular dynamics study

    Science.gov (United States)

    Wang, Xinyu; Hong, Yang; Chan, Paddy K. L.; Zhang, Jingchao

    2017-06-01

    Two-dimensional (2D) hybrid materials have drawn enormous attention in thermoelectric applications. In this work, we apply a molecular dynamics (MD) simulation to investigate the phonon thermal transport in silicene-germanene superlattice. A non-monotonic thermal conductivity of silicene-germanene superlattice with period length is revealed, which is due to the coherent-incoherent phonon conversion and phonon confinement mechanisms. We also calculate the thermal conductivity of a Si-Ge random mixing monolayer, showing a U-shaped trend. Because of the phonon mode localizations at Ge concentration of 80%, thermal conductivity varies dramatically at low doping regions. By changing the total length (L total), the infinite-length thermal conductivities of pure silicene, pure germanene, silicene-germanene superlattice, and Si-Ge random mixing monolayer are extracted as 16.08, 15.95, 5.60 and 4.47 W/m-K, respectively. The thermal boundary conductance (TBC) of the silicene-germanene is also evaluated, showing a small thermal rectification. At L total = 274.7 nm, the TBC of silicene to germanene is 620.49 MW/m2-K, while that of germanene to silicene is 528.76 MW/m2-K.

  20. Nanoengineering of an Si/MnGe quantum dot superlattice for high Curie-temperature ferromagnetism.

    Science.gov (United States)

    Nie, Tianxiao; Kou, Xufeng; Tang, Jianshi; Fan, Yabin; Lee, Shengwei; He, Qinglin; Chang, Li-Te; Murata, Koichi; Gen, Yin; Wang, Kang L

    2017-02-14

    The realization and application of spintronic devices would be dramatically advanced if room-temperature ferromagnetism could be integrated into semiconductor nanostructures, especially when compatible with mature silicon technology. Herein, we report the observation of such a system - an Si/MnGe superlattice with quantum dots well aligned in the vertical direction successfully grown by molecular beam epitaxy. Such a unique system could take full advantage of the type-II energy band structure of the Si/Ge heterostructure, which could trap the holes inside MnGe QDs, significantly enhancing the hole-mediated ferromagnetism. Magnetic measurements indeed found that the superlattice structure exhibited a Curie temperature of above 400 K. Furthermore, zero-field cooling and field cooling curves could confirm the absence of ferromagnetic compounds, such as Ge8Mn11 (Tc ∼ 270 K) and Ge3Mn5 (Tc ∼ 296 K) in our system. Magnetotransport measurement revealed a clear magnetoresistance transition from negative to positive and a pronounced anomalous Hall effect. Such a unique Si/MnGe superlattice sets a new stage for strengthening ferromagnetism due to the enhanced hole-mediation by quantum confinement, which can be exploited for realizing the room-temperature Ge-based spin field-effect transistors in the future.

  1. Performance degradation of superlattice MOSFETs due to scattering in the contacts

    Science.gov (United States)

    Long, Pengyu; Huang, Jun Z.; Jiang, Zhengping; Klimeck, Gerhard; Rodwell, Mark J. W.; Povolotskyi, Michael

    2016-12-01

    Ideal, completely coherent quantum transport calculations had predicted that superlattice MOSFETs (SL-MOSFET) may offer steep subthreshold swing performance below 60 mV/dec to around 39 mV/dec. However, the high carrier density in the superlattice source suggests that scattering may significantly degrade the ideal device performance. Such effects of electron scattering and decoherence in the contacts of SL-MOSFETs are examined through a multi-scale quantum transport model developed in NEMO5. This model couples the NEGF-based quantum ballistic transport in the channel to a quantum mechanical density of states dominated reservoir, which is thermalized through strong scattering with local quasi-Fermi levels determined by drift-diffusion transport. The simulations show that scattering increases the electron transmission in the nominally forbidden minigap, therefore, degrading the subthreshold swing (S.S.) and the ON/OFF DC current ratio. This degradation varies with both the scattering rate and the length of the scattering dominated regions. Different SL-MOSFET designs are explored to mitigate the effects of such deleterious scattering. Specifically, shortening the spacer region between the superlattice and the channel from 3.5 nm to 0 nm improves the simulated S.S. from 51 mV/dec. to 40 mV/dec.

  2. 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.

  3. Density functional theory investigation of titanium-tungsten superlattices: Structure and mechanical properties

    Science.gov (United States)

    Rudin, Sven P.

    2012-11-01

    Titanium (Ti) exhibits the body-centered crystal structure only at high temperatures. The temperature range of this so-called β-Ti phase can be expanded by alloying Ti with tungsten (W). Rather than placing the W atoms in the β-Ti crystal at random, this work applies density functional theory calculations to explore the consequences of an orderly placement in Ti/W superlattice structures. In all examples the W layer remains bcc-like. The stacking direction of the Ti/W superlattice drives the core of the Ti layer toward either a locally hcp- or ω-Ti structure, though the latter is mechanically unstable for all but the thinnest W layers. The relative thicknesses of the W and Ti layers as well as the stacking direction influence the formation energies, which consistently fall within a range corresponding roughly to room temperature. Superlattices allow a choice of stacking direction and layer thicknesses, both strongly influencing the material's strength, though not improving the mechanical properties as observed for Ti with randomly placed W particles.

  4. 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.

  5. Coherent phonon transport in short-period two-dimensional superlattices of graphene and boron nitride

    Science.gov (United States)

    da Silva, Carlos; Saiz, Fernan; Romero, David A.; Amon, Cristina H.

    2016-03-01

    Promoting coherent transport of phonons at material interfaces is a promising strategy for controlling thermal transport in nanostructures and an alternative to traditional methods based on structural defects. Coherent transport is particularly relevant in short-period heterostructures with smooth interfaces and long-wavelength heat-carrying phonons, such as two-dimensional superlattices of graphene and boron nitride. In this work, we predict phonon properties and thermal conductivities in these superlattices using a normal mode decomposition approach. We study the variation of the frequency dependence of these properties with the periodicity and interface configuration (zigzag and armchair) for superlattices with period lengths within the coherent regime. Our results showed that the thermal conductivity decreases significantly from the first period length (0.44 nm) to the second period length (0.87 nm), 13% across the interfaces and 16% along the interfaces. For greater periods, the conductivity across the interfaces continues decreasing at a smaller rate of 11 W/mK per period length increase (0.43 nm), driven by changes in the phonon group velocities (coherent effects). In contrast, the conductivity along the interfaces slightly recovers at a rate of 2 W/mK per period, driven by changes in the phonon relaxation times (diffusive effects). By changing the interface configuration from armchair to zigzag, the conductivities for all period lengths increase by approximately 7% across the interfaces and 19% along the interfaces.

  6. Marginally Deformed Starobinsky Gravity

    DEFF Research Database (Denmark)

    Codello, A.; Joergensen, J.; Sannino, Francesco

    2015-01-01

    We show that quantum-induced marginal deformations of the Starobinsky gravitational action of the form $R^{2(1 -\\alpha)}$, with $R$ the Ricci scalar and $\\alpha$ a positive parameter, smaller than one half, can account for the recent experimental observations by BICEP2 of primordial tensor modes....

  7. Ethical reflection and psychotherapy.

    Science.gov (United States)

    Vyskocilová, Jana; Prasko, Jan

    2013-01-01

    Theories of ethics and ethical reflection may be applied to both theory and practice in psychotherapy. There is a natural affinity between ethics and psychotherapy. Psychotherapy practice is concerned with human problems, dilemmas and emotions related to both one's own and other people's values. Ethics is also concerned with dilemmas in human thinking and with how these dilemmas reflect other individuals' values. Philosophical reflection itself is not a sufficient basis for the ethics of psychotherapy but it may aid in exploring attitudes related to psychotherapy, psychiatry and health care. PubMed, Web of Science and Scopus databases were searched for articles containing the keywords "psychotherapy", "ethics", "therapeutic relationship" and "supervision". The search was conducted by repeating the terms in various combinations without language or time restrictions. Also included were data from monographs cited in reviews. The resulting text is a review with conclusions concerning ethical aspects of psychotherapy. The ability to behave altruistically, sense for justice and reciprocity and mutual help are likely to be genetically determined as dispositions to be later developed by upbringing or to be formed or deformed by upbringing. Early experiences lead to formation of ethical attitudes which are internalized and then applied to both one's own and other people's behavior. Altruistic behavior has a strong impact on an individual's health and its acceptance may positively influence the pathophysiological mechanisms underlying numerous diseases. Ethical theory and reflection, however, may be applied to both theory and practice of psychotherapy in a conscious, targeted and thoughtful manner. In everyday practice, psychotherapists and organizations must necessarily deal with conscious conflicts between therapeutic possibilities, clients' wishes, their own as well as clients' ideas and the real world. Understanding one's own motives in therapy is one of the aims of a

  8. Deformed Algebras and Generalizations of Independence on Deformed Exponential Families

    Directory of Open Access Journals (Sweden)

    Hiroshi Matsuzoe

    2015-08-01

    Full Text Available A deformed exponential family is a generalization of exponential families. Since the useful classes of power law tailed distributions are described by the deformed exponential families, they are important objects in the theory of complex systems. Though the deformed exponential families are defined by deformed exponential functions, these functions do not satisfy the law of exponents in general. The deformed algebras have been introduced based on the deformed exponential functions. In this paper, after summarizing such deformed algebraic structures, it is clarified how deformed algebras work on deformed exponential families. In fact, deformed algebras cause generalization of expectations. The three kinds of expectations for random variables are introduced in this paper, and it is discussed why these generalized expectations are natural from the viewpoint of information geometry. In addition, deformed algebras cause generalization of independences. Whereas it is difficult to check the well-definedness of deformed independence in general, the κ-independence is always well-defined on κ-exponential families. This is one of advantages of κ-exponential families in complex systems. Consequently, we can well generalize the maximum likelihood method for the κ-exponential family from the viewpoint of information geometry.

  9. Deformation of chlorite in naturally deformed low-grade rocks

    NARCIS (Netherlands)

    Bons, A.J.

    1988-01-01

    The intracrystalline deformation of chlorite in naturally deformed low-grade rocks was investigated with transmission electron microscopy (TEM). As in other phyllosilicates, the deformation of chlorite is dominated by the (001) slip plane. Slip along this plane is very easy through the generation an

  10. Relationship between nano-scale deformation of coal structure and metamorphic-deformed environments

    Institute of Scientific and Technical Information of China (English)

    JU Yiwen; JIANG Bo; HOU Quanlin; WANG Guiliang

    2005-01-01

    There is a more consanguineous relation between nano-scale deformation of coal structure and metamorphic-deformed environment. In different metamorphic-deformed environments, deformation in the coal structure can occur not only at micro-scale, but also at nano-scale, and even leads to the change of molecular structure and nano-scale pore (<100 nm) structure. The latter is the main space absorbing coalbed methane. Through X-ray diffraction (XRD) and liquid-nitrogen absorption methods, the characteristics of macromolecular and nano-scale pore structures of coals in different metamorphic-deformed environments and deformational series of coals have been studied. By combining with high-resolution transmission electron microcopy (HRTEM), the macromolecular and nano-scale pore structures are also directly observed. These results demonstrate that the stacking Lc of the macromolecular BSU in tectonic coals increases quickly from the metamorphic-deformed environment of low rank coals to that of high rank coals. For different deformed tectonic coals, in the same metamorphic-deformed environment, the difference of Lc is obvious. These changes reflect chiefly the difference of different temperature and stress effect of nano-scale deformation in tectonic coals. The factor of temperature plays a greater role in the increase of macromolecular structure parameters Lc, the influence of stress factor is also important. With the stress strengthening, Lc shows an increasing trend, and La /Lc shows a decreasing trend. Therefore, Lc and La /Lc can be used as the indicator of nano-scale deformation degree of tectonic coals. With increasing temperature and pressure, especially oriented stress, the orientation of molecular structure becomes stronger, and ordering degree of C-nets and the arrangement of BSU are obviously enhanced. For the deformation of nano-scale pore structure, in the same metamorphic-deformed environment, along with the strengthening of stress, the ratio of mesopores to

  11. Postural deformities in Parkinson's disease

    NARCIS (Netherlands)

    Doherty, K.M.; Warrenburg, B.P.C. van de; Peralta, M.C.; Silveira-Moriyama, L.; Azulay, J.P.; Gershanik, O.S.; Bloem, B.R.

    2011-01-01

    Postural deformities are frequent and disabling complications of Parkinson's disease (PD) and atypical parkinsonism. These deformities include camptocormia, antecollis, Pisa syndrome, and scoliosis. Recognition of specific postural syndromes might have differential diagnostic value in patients prese

  12. Nonperturbative effects in deformation quantization

    CERN Document Server

    Periwal, V

    2000-01-01

    The Cattaneo-Felder path integral form of the perturbative Kontsevich deformation quantization formula is used to explicitly demonstrate the existence of nonperturbative corrections to na\\"\\i ve deformation quantization.

  13. Nanoscale deformation mechanisms in bone.

    Science.gov (United States)

    Gupta, Himadri S; Wagermaier, Wolfgang; Zickler, Gerald A; Raz-Ben Aroush, D; Funari, Sérgio S; Roschger, Paul; Wagner, H Daniel; Fratzl, Peter

    2005-10-01

    Deformation mechanisms in bone matrix at the nanoscale control its exceptional mechanical properties, but the detailed nature of these processes is as yet unknown. In situ tensile testing with synchrotron X-ray scattering allowed us to study directly and quantitatively the deformation mechanisms at the nanometer level. We find that bone deformation is not homogeneous but distributed between a tensile deformation of the fibrils and a shearing in the interfibrillar matrix between them.

  14. Photoluminescence in Er-implanted AlGaN/GaN superlattices and GaN epilayers

    Energy Technology Data Exchange (ETDEWEB)

    Sobolev, N.A.; Emel' yanov, A.M.; Sakharov, V.I.; Serenkov, I.T.; Shek, E.I.; Besyul' kin, A.I.; Lundin, W.V.; Shmidt, N.M.; Usikov, A.S.; Zavarin, E.E

    2003-12-31

    Photoluminescence (PL), structural and electrophysical properties of Al{sub 0.26}Ga{sub 0.74}N/GaN superlattices grown by metal-organic chemical vapor deposition, implanted by erbium (Er) ions with 1 MeV energy and 1x10{sup 15} cm{sup -2} dose as well as annealed at 700-1100 deg. C for 4 min in argon have been investigated. A comparison of the properties of the superlattices with that of the GaN epilayers grown, implanted and annealed at the same conditions is also given. The Er-related peak with a maximum at {lambda}{approx}1.54 {mu}m dominated and the defect-related emission band at {lambda}{approx}1-1.4 {mu}m was observed in the PL spectra of both types of samples. When the measurement temperature was increased from 80 to 300 K, practically the same temperature quenching of the Er-related intensity was observed in the superlattices and GaN epilayers implanted and annealed at the same conditions. The Er-related intensity at 300 K increased monotonically as the annealing temperature was raised from 700 to 1000 deg. C, but the intensity in the superlattices was higher by several times than that in the epilayers. A decrease of the Er-related PL intensity in the superlattice after annealing at 1100 deg. C is associated with the formation of non-radiative recombination centers.

  15. Absorption properties of type-II InAs/InAsSb superlattices measured by spectroscopic ellipsometry

    Energy Technology Data Exchange (ETDEWEB)

    Webster, P. T.; Riordan, N. A.; Liu, S.; Zhang, Y.-H.; Johnson, S. R., E-mail: shane.johnson@asu.edu [Center for Photonics Innovation and School of Electrical, Computer, and Energy Engineering, Arizona State University, Tempe, Arizona 85287 (United States); Steenbergen, E. H. [U.S. Air Force Research Laboratory, AFRL/RXAN, Wright Patterson, Ohio 45433 (United States); Synowicki, R. A. [J. A. Woollam Co., Inc., 645 M. Street, Suite 102, Lincoln, Nebraska 68508 (United States)

    2015-02-09

    Strain-balanced InAs/InAsSb superlattices offer access to the mid- to long-wavelength infrared region with what is essentially a ternary material system at the GaSb lattice constant. The absorption coefficients of InAs/InAsSb superlattices grown by molecular beam epitaxy on (100)-oriented GaSb substrates are measured at room temperature over the 30 to 800 meV photon energy range using spectroscopic ellipsometry, and the miniband structure of each superlattice is calculated using a Kronig-Penney model. The InAs/InAsSb conduction band offset is used as a fitting parameter to align the calculated superlattice ground state transition energy to the measured absorption onset at room temperature and to the photoluminescence peak energy at low temperature. It is observed that the ground state absorption coefficient and transition strength are proportional to the square of the wavefunction overlap and the ground state absorption coefficient approaches a maximum value of around 5780 cm{sup −1} as the wavefunction overlap approaches 100%. The absorption analysis of these samples indicates that the optical joint density of states is weakly dependent on the period thickness and Sb content of the superlattice, and that wavefunction overlap is the principal design parameter in terms of obtaining strong absorption in these structures.

  16. Interface-Induced Polarization in SrTiO 3 -LaCrO 3 Superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Comes, Ryan B. [Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland WA 99352 USA; Spurgeon, Steven R. [Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland WA 99352 USA; Heald, Steve M. [Advanced Photon Source, Argonne National Laboratory, Argonne IL 60439 USA; Kepaptsoglou, Despoina M. [SuperSTEM, SciTech Daresbury Campus, Daresbury WA44AD UK; Jones, Lewys [Department of Materials, University of Oxford, Oxford OX13PH UK; Ong, Phuong Vu [Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland WA 99352 USA; Bowden, Mark E. [Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland WA 99352 USA; Ramasse, Quentin M. [SuperSTEM, SciTech Daresbury Campus, Daresbury WA44AD UK; Sushko, Peter V. [Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland WA 99352 USA; Chambers, Scott A. [Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland WA 99352 USA

    2016-02-29

    Epitaxial interfaces and superlattices comprised of polar and non-polar perovskite oxides have generated a good deal of interest because of the variety of novel properties they possess. In this work, we examine superlattices comprised of SrTiO3 (STO) and LaCrO3 (LCO) layers; we demonstrate that the differing band alignment of the polar LCO layer and the non-polar STO layer produces a ferroelectric phase transition throughout the STO layers of the superlattice. Through x-ray absorption near edge spectroscopy and aberration-corrected scanning transmission electron microscopy we show that the Ti cations are displaced off-center in the TiO6 octahedra along the superlattice growth direction. We also demonstrate that a built-in potential gradient exists within the STO and LCO layers via in situ x-ray photoelectron spectroscopy measurements. Density functional theory models of the system are in excellent agreement with these results, predicting both the ferroelectric octahedral distortion and the built-in electric field. These results represent a new avenue for research in perovskite superlattices, as two non-ferroelectric phases are shown to induce a bulk ferroelectric response due to interfacial phenomena.

  17. On Reflection

    DEFF Research Database (Denmark)

    Blasco, Maribel

    2012-01-01

    This article explores how the concept of reflexivity is used in intercultural education. Reflexivity is often presented as a key learning goal in acquiring intercultural competence (ICC). Yet, reflexivity can be defined in different ways, and take different forms across time and space, depending...... on the concepts of selfhood that prevail and how notions of difference are constructed. First, I discuss how the dominant usages of reflexivity in intercultural education reflect and reproduce a Cartesian view of the self that shapes how ICC is conceptualized and taught. I discuss three assumptions that this view...... in designing learning objectives in intercultural education and in devising ways to attain them. Greater attention is also needed in intercultural education to the ways in which selfhood, and hence also reflexivity and constructions of difference, differ across space and time....

  18. Inspiring Reflections

    DEFF Research Database (Denmark)

    Muchie, Mammo

    2011-01-01

    contributions have been put together. There are a number of ways to continue Chris Freeman's legacy on innovation research. The first is to build in a critical tradition in the economics of innovation research by introducing fearlessly emancipatory epistemology. Second the economic system that dominates social......A numberof Chris Freeman's colleagues were asked to reflect on what they thought describes his life and work in a few words. Some of the colleagues replied including former SPRU students that were taught or supervised by Chris Freeman. Their views on what they thought were Chris Freeman's defining...... life is not free from fluctuations, cycles, disruptions, crises and destructions both human and ecological. Innovation research ought to position itself to address environmental, financial and economic crises. The third is innovation research for development by addressing not only poverty erdaication...

  19. Transverse thermoelectric effect in La{sub 0.67}Sr{sub 0.33}MnO{sub 3}|SrRuO{sub 3} superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Shiomi, Y. [Institute for Materials Research, Tohoku University, Sendai 980-8577 (Japan); Spin Quantum Rectification Project, ERATO, Japan Science and Technology Agency, Aoba-ku, Sendai 980-8577 (Japan); Handa, Y.; Kikkawa, T. [Institute for Materials Research, Tohoku University, Sendai 980-8577 (Japan); Saitoh, E. [Institute for Materials Research, Tohoku University, Sendai 980-8577 (Japan); Spin Quantum Rectification Project, ERATO, Japan Science and Technology Agency, Aoba-ku, Sendai 980-8577 (Japan); WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577 (Japan); CREST, Japan Science and Technology Agency, Tokyo 102-0076 (Japan); Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195 (Japan)

    2015-06-08

    Transverse thermoelectric effects in response to an out-of-plane heat current have been studied in an external magnetic field for ferromagnetic superlattices consisting of La{sub 0.67}Sr{sub 0.33}MnO{sub 3} and SrRuO{sub 3} layers. The superlattices were fabricated on SrTiO{sub 3} substrates by pulsed laser deposition. We found that the sign of the transverse thermoelectric voltage for the superlattices is opposite to that for La{sub 0.67}Sr{sub 0.33}MnO{sub 3} and SrRuO{sub 3} single layers at 200 K, implying an important role of spin Seebeck effects inside the superlattices. At 10 K, the magnetothermoelectric curves shift from the zero field due to an antiferromagnetic coupling between layers in the superlattices.

  20. Structural evidence for enhanced polarization in a commensurate short-period BaTiO3/SrTiO3 superlattice

    Science.gov (United States)

    Tian, W.; Jiang, J. C.; Pan, X. Q.; Haeni, J. H.; Li, Y. L.; Chen, L. Q.; Schlom, D. G.; Neaton, J. B.; Rabe, K. M.; Jia, Q. X.

    2006-08-01

    A short-period (BaTiO3)6/(SrTiO3)5 superlattice was characterized by x-ray diffraction and transmission electron microscopy. The superlattice is epitaxially oriented with the c axes of BaTiO3 and SrTiO3 normal to the (001) surface of the SrTiO3 substrate. Despite the large in-plane lattice mismatch between BaTiO3 and SrTiO3 (˜2.2%), the superlattice interfaces were found to be nearly commensurate. The crystallographic c /a ratio of the superlattice was measured and the results agree quantitatively with first-principles calculations and phase-field modeling. The agreement supports the validity of the enhanced spontaneous polarization predicted for short-period BaTiO3/SrTiO3 superlattices.