Surface electron structure of short-period semiconductor superlattice

International Nuclear Information System (INIS)

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

2004-01-01

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

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

International Nuclear Information System (INIS)

Arutyunov, N.Y.; Sekkal, N.

2008-08-01

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

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

Science.gov (United States)

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

2017-10-01

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

Designing Optical Properties in DNA-Programmed Nanoparticle Superlattices

Science.gov (United States)

Ross, Michael Brendan

can be used to influence the properties of mesoscale single crystal superlattices, such that they exhibit either plasmonic absorption or photonic scattering. This concept is generalized through simulation, which demonstrates that the crystal habit (size, shape, and morphology) is a powerful design parameter for optical properties in mesoscale nanoparticle assemblies. Finally, chapter 7 summarizes these data and their impact, and puts them in context regarding future opportunities. This work presents a comprehensive demonstration that the optical properties of nanoparticle-based architectures can be precisely controlled and deliberately designed a priori using the unique programmability of DNA and the use of several levels of predictive electromagnetic theory.

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

International Nuclear Information System (INIS)

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

1997-01-01

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

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

International Nuclear Information System (INIS)

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

2013-01-01

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

Photonic Crystal Laser-Driven Accelerator Structures

International Nuclear Information System (INIS)

Cowan, Benjamin M.

2007-01-01

Laser-driven acceleration holds great promise for significantly improving accelerating gradient. However, scaling the conventional process of structure-based acceleration in vacuum down to optical wavelengths requires a substantially different kind of structure. We require an optical waveguide that (1) is constructed out of dielectric materials, (2) has transverse size on the order of a wavelength, and (3) supports a mode with speed-of-light phase velocity in vacuum. Photonic crystals---structures whose electromagnetic properties are spatially periodic---can meet these requirements. We discuss simulated photonic crystal accelerator structures and describe their properties. We begin with a class of two-dimensional structures which serves to illustrate the design considerations and trade-offs involved. We then present a three-dimensional structure, and describe its performance in terms of accelerating gradient and efficiency. We discuss particle beam dynamics in this structure, demonstrating a method for keeping a beam confined to the waveguide. We also discuss material and fabrication considerations. Since accelerating gradient is limited by optical damage to the structure, the damage threshold of the dielectric is a critical parameter. We experimentally measure the damage threshold of silicon for picosecond pulses in the infrared, and determine that our structure is capable of sustaining an accelerating gradient of 300 MV/m at 1550 nm. Finally, we discuss possibilities for manufacturing these structures using common microfabrication techniques

Waves in man-made materials: superlattice to metamaterials

Science.gov (United States)

Tsu, Raphael; Fiddy, Michael A.

2014-07-01

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

Structure of highly perfect semiconductor strained-layer superlattices

International Nuclear Information System (INIS)

Vandenberg, J.M.

1989-01-01

High-resolution x-ray diffraction (HRXRD) measurements of strained-layer superlattices (SLS's) have been carried out using a four-crystal monochromator. A wide asymmetric range of sharp higher-order x-ray satellite peaks is observed indicating well-defined periodic structures. Using a kinematical diffraction step model very good agreement between measured and simulated x-ray satellite patterns could be achieved. These results show that this x- ray method is a powerful tool to evaluate the crystal quality of SLS's

Extraction from TEV-range accelerators using bent crystal channeling

International Nuclear Information System (INIS)

Carrigan, R.A. Jr.; Jackson, G.; Murphy, C.T.; Newberger, B.

1993-01-01

Plans and first results from Fermilab Experiment 853 are presented. E853 is an experiment to test the feasibility and efficiency of extracting a low-intensity beam from the halo of the Tevatron using channeling in a bent silicon crystal. The motivation of the experiment is to apply crystal extraction to trans-TeV accelerators like the SSC. Channeling developments related to crystal extraction and some early results from accelerator studies at the Tevatron are presented

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.

Magnon band structure and magnon density in one-dimensional magnonic crystals

International Nuclear Information System (INIS)

Qiu, Rong-ke; Huang, Te; Zhang, Zhi-dong

2014-01-01

By using Callen's Green's function method and the Tyablikov and Anderson–Callen decoupling approximations, we systematically study the magnon band structure and magnon density perpendicular to the superlattice plane of one-dimensional magnonic crystals, with a superlattice consisting of two magnetic layers with ferromagnetic (FM) or antiferromagnetic (AFM) interlayer exchange coupling. The effects of temperature, interlayer coupling, anisotropy and external magnetic field on the magnon-energy band and magnon density in the K x -direction are investigated in three situations: a) the magnon band of magnetic superlattices with FM interlayer coupling, b) separate and c) overlapping magnon bands of magnetic superlattices with AFM interlayer coupling. In the present work, a quantum approach is developed to study the magnon band structure and magnon density of magnonic crystals and the results are beneficial for the design of magnonic-crystal waveguides or gigahertz-range spin-wave filters. - Highlights: • A quantum approach has been developed to study the magnon band of magnonic crystals. • The separate and overlapping magnon bands of magnetic superlattices are investigated. • The results are beneficial for the design of gigahertz-range spin-wave filters

Magnon band structure and magnon density in one-dimensional magnonic crystals

Energy Technology Data Exchange (ETDEWEB)

Qiu, Rong-ke, E-mail: rkqiu@163.com [Shenyang University of Technology, Shenyang 110870 (China); Huang, Te [Shenyang University of Technology, Shenyang 110870 (China); Zhang, Zhi-dong [Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016 (China)

2014-11-15

By using Callen's Green's function method and the Tyablikov and Anderson–Callen decoupling approximations, we systematically study the magnon band structure and magnon density perpendicular to the superlattice plane of one-dimensional magnonic crystals, with a superlattice consisting of two magnetic layers with ferromagnetic (FM) or antiferromagnetic (AFM) interlayer exchange coupling. The effects of temperature, interlayer coupling, anisotropy and external magnetic field on the magnon-energy band and magnon density in the K{sub x}-direction are investigated in three situations: a) the magnon band of magnetic superlattices with FM interlayer coupling, b) separate and c) overlapping magnon bands of magnetic superlattices with AFM interlayer coupling. In the present work, a quantum approach is developed to study the magnon band structure and magnon density of magnonic crystals and the results are beneficial for the design of magnonic-crystal waveguides or gigahertz-range spin-wave filters. - Highlights: • A quantum approach has been developed to study the magnon band of magnonic crystals. • The separate and overlapping magnon bands of magnetic superlattices are investigated. • The results are beneficial for the design of gigahertz-range spin-wave filters.

Characterization of Crystals for Steering of Protons through Channelling in Hadronic Accelerators

CERN Document Server

Guidi, V; Boscolo-Marchi, E; Carnera, A; Chesnokov, Yu A; Della Mea, G; De Salvador, D; Fiorini, M; Ivanov, Y M; Martinelli, G; Mazzolari, A; Milan, E; Milan, R; Sambo, A; Scandale, Walter; Todros, S; Vomiero, A

2006-01-01

Channeling of relativistic particles through a crystal may be useful for many applications in accelerators, and particularly for collimation in hadronic colliders. Efficiency proved to be dependent on the state of the crystal surface and hence on the method used for preparation. We investigated the morphology and structure of the surface of the samples that have been used in accelerators with high efficiency. We found that crystal fabrication by only mechanical methods (dicing, lapping, and others) leads to a superficial damaged layer, which is correlated to performance limitation in accelerators. A planar chemical etching was studied and applied in order to remove the superficial damaged layer. RBS channeling analysis with low-energy protons and 4He+ highlighted better crystal perfection at surface, as a result of the etching. A protocol for preparation and characterization of crystal for channelling has been developed, which may be of interest for reliable operation with crystals in accelerators.

Modulating nanoparticle superlattice structure using proteins with tunable bond distributions

International Nuclear Information System (INIS)

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

2017-01-01

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

Magnetic profiles in ferromagnetic/superconducting superlattices.

Energy Technology Data Exchange (ETDEWEB)

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

2007-02-28

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

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

International Nuclear Information System (INIS)

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

1996-01-01

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

Substantial increase in acceleration potential of pyroelectric crystals

International Nuclear Information System (INIS)

Tornow, W.; Lynam, S. M.; Shafroth, S. M.

2010-01-01

We report on a substantial increase in the acceleration potential achieved with a LiTaO 3 pyroelectric crystal. With a single 2.5 cm diameter and 2.5 cm long z-cut crystal without electric field-enhancing nanotip we produced positive ion beams with maximal energies between 300 and 310 keV during the cooling phase when the crystal was exposed to 5 mTorr of deuterium gas. These values are about a factor of 2 larger than previously obtained with single pyroelectric crystals.

Planar channeling in superlattices: Theory

International Nuclear Information System (INIS)

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

1988-01-01

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

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

Science.gov (United States)

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

2017-07-06

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

Manipulation of hadron beams with bent crystals in circular accelerators

CERN Document Server

Rossi, R; Redaelli, S; Scandale, W

2016-01-01

Over the past years the understanding and use of coherent interactions of charged particles with ordered crystal lattices has achieved excellent results. Improving collimation of hadron beams in circular accelerators, like the Large Hadron Collider (LHC) of the European Council for Nuclear Research (CERN), it is one of the possible applications. The aim of the UA9 experiment is to demonstrate the feasibility of a two-stage collimation system in the CERN-SPS : the first stage is a bent crystal oriented for an optimal channeling of the incoming halo particles; the second stage is a massive absorber. Two crystals were installed in the LHC last year and a test of crystal assisted collimation at the highest energy will be possible as early as 2015. Finally, the UA9 Collaboration is investigating extraction of particles from a circular accelerator, based on bent crystals.

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

Energy Technology Data Exchange (ETDEWEB)

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

2016-03-14

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

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.

A CPA study of the phonon structure of disordered superlattices

International Nuclear Information System (INIS)

Shijie Xiong; Gendi Pang; Chienhua Tsai.

1985-08-01

The phonon structure of superlattices or modulated alloys with substitutional disorder is studied in the Coherent Phase Approximation (CPA). We consider first the case with diagonal disorder only, by adopting a virtual crystal approximation for the force constants. Then we treat the more complicated case with inclusion of off-diagonal disorder. Numerical examples are also studied in both cases. (author)

Superlattices in thermoelectric applications

International Nuclear Information System (INIS)

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

1994-08-01

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

Simulation and design of the photonic crystal microwave accelerating structure

International Nuclear Information System (INIS)

Song Ruiying; Wu Congfeng; He Xiaodong; Dong Sai

2007-01-01

The authors have derived the global band gaps for general two-dimensional (2D) photonic crystal microwave accelerating structures formed by square or triangular arrays of metal posts. A coordinate-space, finite-difference code was used to calculate the complete dispersion curves for the lattices. The fundamental and higher frequency global photonic band gaps were determined numerically. The structure formed by triangular arrays of metal posts with a missing rod at the center has advantages of higher-order-modes (HOM) suppression and main mode restriction under the condition of a/b<0.2. The relationship between the RF properties and the geometrical parameters have been studied for the 9.37 GHz photonic crystal accelerating structure. The Rs, Q, Rs/Q of the new structure may be comparable to the disk-loaded accelerating structure. (authors)

Atomic stacking and van-der-Waals bonding in GeTe-Sb2Te3 superlattices

NARCIS (Netherlands)

Momand, J.; Lange, F.R.L.; Wang, R.; Boschker, J.E.; Verheijen, M.A.; Calarco, R.; Wuttig, M.; Kooi, B.J.

2016-01-01

GeTe–Sb2Te3 superlattices have attracted major interest in the field of phase-change memories due to their improved properties compared with their mixed counterparts. However, their crystal structure and resistance-switching mechanism are currently not clearly understood. In this work epitaxial

Magnetic modes in superlattices

International Nuclear Information System (INIS)

Oliveira, F.A.

1990-04-01

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

Silicon carbide whiskers with superlattice structure: A precursor for a new type of nanoreactor

International Nuclear Information System (INIS)

Lutsenko, Vadym G.

2008-01-01

Silicon carbide whiskers exhibit growth predominantly in the direction. The high level of impurities, stacking faults and nanosized twins govern the formation of homojunctions and heterojunctions in crystals. The structure of the whiskers comprises a hybrid superlattice, i.e. contains elements of doped and composite superlattices. An individual SiC whisker can contain hundreds of quantum wells with anomalous chemical properties. This paper shows that it is possible to selectively etch quantum wells and to construct whiskers with quasi-regularly distributed slit-like nanopores (nanoreactors), which are bordered by polar planes {1 1 1}, {0 0 0 1} or a combination of them, and also to produce flat SiC nanocrystals bordered by polar planes

Superlattice formation lifting degeneracy protected by nonsymmorphic symmetry through a metal-insulator transition in RuAs

Science.gov (United States)

Kotegawa, Hisashi; Takeda, Keiki; Kuwata, Yoshiki; Hayashi, Junichi; Tou, Hideki; Sugawara, Hitoshi; Sakurai, Takahiro; Ohta, Hitoshi; Harima, Hisatomo

2018-05-01

A single crystal of RuAs obtained with the Bi-flux method shows obvious successive metal-insulator transitions at TMI 1˜255 K and TMI 2˜195 K. The x-ray diffraction measurement reveals the formation of a superlattice of 3 ×3 ×3 of the original unit cell below TMI 2, accompanied by a change of the crystal system from the orthorhombic structure to the monoclinic one. Simple dimerization of the Ru ions is not seen in the ground state. The multiple As sites observed in the nuclear quadrupole resonance spectrum also demonstrate the formation of the superlattice in the ground state, which is clarified to be nonmagnetic. The divergence in 1 /T1 at TMI 1 shows that a symmetry lowering by the metal-insulator transition is accompanied by strong critical fluctuations of some degrees of freedom. Using the structural parameters in the insulating state, the first-principles calculation reproduces successfully the reasonable size of nuclear quadrupole frequencies νQ for the multiple As sites, ensuring the high validity of the structural parameters. The calculation also gives a remarkable suppression in the density of states near the Fermi level, although the gap opening is insufficient. A coupled modulation of the calculated Ru d -electron numbers and the crystal structure proposes the formation of a charge density wave in RuAs. Some lacking factors remain, but it is shown that a lifting of degeneracy protected by the nonsymmorphic symmetry through the superlattice formation is a key ingredient for the metal-insulator transition in RuAs.

Exchange bias in Fe/Cr double superlattices

International Nuclear Information System (INIS)

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

1999-01-01

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

Exchange bias in Fe/Cr double superlattices

International Nuclear Information System (INIS)

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

2000-01-01

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

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

Science.gov (United States)

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

2017-06-01

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

Thermal conductivity and heat transfer in superlattices

Energy Technology Data Exchange (ETDEWEB)

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

1997-07-01

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

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....

Shape-Anisotropy Driven Symmetry Transformations in Nanocrystal Superlattice Polymorphs

KAUST Repository

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

2011-01-01

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

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.

Quantitative x-ray structure determination of superlattices and interfaces

International Nuclear Information System (INIS)

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

1990-01-01

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

Oblique surface waves at an interface between a metal-dielectric superlattice and an isotropic dielectric

International Nuclear Information System (INIS)

Vuković, Slobodan M; Miret, Juan J; Zapata-Rodriguez, Carlos J; Jakšić, Zoran

2012-01-01

We investigate the existence and dispersion characteristics of surface waves that propagate at an interface between a metal-dielectric superlattice and an isotropic dielectric. Within the long-wavelength limit, when the effective-medium (EM) approximation is valid, the superlattice behaves like a uniaxial plasmonic crystal with the main optical axes perpendicular to the metal-dielectric interfaces. We demonstrate that if such a semi-infinite plasmonic crystal is cut normally to the layer interfaces and brought into contact with a semi-infinite dielectric, a new type of surface mode can appear. Such modes can propagate obliquely to the optical axes if favorable conditions regarding the thickness of the layers and the dielectric permittivities of the constituent materials are met. We show that losses within the metallic layers can be substantially reduced by making the layers sufficiently thin. At the same time, a dramatic enlargement of the range of angles for oblique propagation of the new surface modes is observed. This can lead, however, to field non-locality and consequently to failure of the EM approximation.

Sb2Te3 and Its Superlattices: Optimization by Statistical Design.

Science.gov (United States)

Behera, Jitendra K; Zhou, Xilin; Ranjan, Alok; Simpson, Robert E

2018-05-02

The objective of this work is to demonstrate the usefulness of fractional factorial design for optimizing the crystal quality of chalcogenide van der Waals (vdW) crystals. We statistically analyze the growth parameters of highly c axis oriented Sb 2 Te 3 crystals and Sb 2 Te 3 -GeTe phase change vdW heterostructured superlattices. The statistical significance of the growth parameters of temperature, pressure, power, buffer materials, and buffer layer thickness was found by fractional factorial design and response surface analysis. Temperature, pressure, power, and their second-order interactions are the major factors that significantly influence the quality of the crystals. Additionally, using tungsten rather than molybdenum as a buffer layer significantly enhances the crystal quality. Fractional factorial design minimizes the number of experiments that are necessary to find the optimal growth conditions, resulting in an order of magnitude improvement in the crystal quality. We highlight that statistical design of experiment methods, which is more commonly used in product design, should be considered more broadly by those designing and optimizing materials.

Thermal Conductivity of Graphene-hBN Superlattice Ribbons.

Science.gov (United States)

Felix, Isaac M; Pereira, Luiz Felipe C

2018-02-09

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

Anisotropic behavior of quantum transport in graphene superlattices

DEFF Research Database (Denmark)

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

2014-01-01

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

Relativistic electron Wigner crystal formation in a cavity for electron acceleration

CERN Document Server

Thomas, Johannes; Pukhov, Alexander

2014-01-01

It is known that a gas of electrons in a uniform neutralizing background can crystallize and form a lattice if the electron density is less than a critical value. This crystallization may have two- or three-dimensional structure. Since the wake field potential in the highly-nonlinear-broken-wave regime (bubble regime) has the form of a cavity where the background electrons are evacuated from and only the positively charged ions remain, it is suited for crystallization of trapped and accelerated electron bunch. However, in this case, the crystal is moving relativistically and shows new three-dimensional structures that we call relativistic Wigner crystals. We analyze these structures using a relativistic Hamiltonian approach. We also check for stability and phase transitions of the relativistic Wigner crystals.

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.

Magnetic structure of holmium-yttrium superlattices

DEFF Research Database (Denmark)

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

1993-01-01

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

Plasmon Modes of Vertically Aligned Superlattices

DEFF Research Database (Denmark)

Filonenko, Konstantin; Duggen, Lars; Willatzen, Morten

2017-01-01

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

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

International Nuclear Information System (INIS)

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

1991-10-01

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

Current responsivity of semiconductor superlattice THz-photon detectors

DEFF Research Database (Denmark)

Ignatov, Anatoly A.; Jauho, Antti-Pekka

1999-01-01

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

Tunneling of electrons through semiconductor superlattices

Indian Academy of Sciences (India)

Unknown

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

Photostimulated attenuation of hypersound in superlattice

International Nuclear Information System (INIS)

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

1992-10-01

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

Formation of superlattice with aligned plane orientation of colloidal PbS quantum dots

Science.gov (United States)

Mukai, Kohki; Fujimoto, Satoshi; Suetsugu, Fumimasa

2018-04-01

We investigated a method of forming a perfect quantum dot (QD) superlattice, in which each QD has the same plane orientation, by depositing colloidal PbS QDs with clear facets in solution. QD facets were controlled by adjusting the synthesis temperature. X-ray evaluation showed that the crystal orientations of the film with QDs having clear facets were aligned. The slow deposition promoted this crystal alignment. The red shift of photoluminescence wavelength caused by the film formation was larger with QDs having facets than with spherical QDs, suggesting that the connection of the wave function between QDs was better so that the quantum size effect was further reduced.

Dissipative chaos in semiconductor superlattices

Directory of Open Access Journals (Sweden)

F. Moghadam

2008-03-01

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

XYO{sub 3} (X = K, Na; Y = Nb, Ta) based superlattices for photocatalysis

Energy Technology Data Exchange (ETDEWEB)

Wang, Guang-Zhao; Chen, Xiao-Rui; Yuan, Hong-Kuan; Kuang, An-Long [School of Physical Science and Technology, Southwest University, Chongqing 400715 (China); Chen, Hong [School of Physical Science and Technology, Southwest University, Chongqing 400715 (China); Key Laboratory of Luminescent and Real-Time Analytical Chemistry, Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715 (China)

2017-05-15

The photocatalytic activities of XYO{sub 3} (X = K, Na; Y = Ta, Nb) and XYO{sub 3}/X1Y1O{sub 3} (X, X1 = K, Na; Y, Y1 = Ta, Nb) systems are investigated by using hybrid density functional. All the XYO{sub 3} and XYO{sub 3}/X1Y1O{sub 3} systems are indirect band gap semiconductors, and the band gap of KNbO{sub 3}/KTaO{sub 3} is smaller than those of KNbO{sub 3} and KTaO{sub 3}, while the band gaps of KNbO{sub 3}/NaNbO{sub 3}, KNbO{sub 3}/NaTaO{sub 3}, KTaO{sub 3}/NaNbO{sub 3}, KTaO{sub 3}/NaTaO{sub 3}, and NaNbO{sub 3}/NaTaO{sub 3} are respectively between the band gaps of these two crystals which make up these superlattices. The electronic structure of KNbO{sub 3}/NaTaO{sub 3} is the same as that of KTaO{sub 3}/NaNbO{sub 3} since both have the same component and similar crystal structure. The band edges of all the considered superlattices are thermodynamically allowed for the water reduction and oxidation processes, and therefore, they could be used for photocatalytic water splitting. Band structures for (i) KNbO{sub 3}/KTaO{sub 3}, (ii) KNbO{sub 3}/NaNbO{sub 3}, (iii) KNbO{sub 3}/NaTaO{sub 3}, (iv) KTaO{sub 3}/NaNbO{sub 3}, (v) KTaO{sub 3}/NaTaO{sub 3}, and (vi) NaNbO{sub 3}/NaTaO{sub 3} superlattices. The horizontal dashed lines represent the Fermi levels. (copyright 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

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

Energy Technology Data Exchange (ETDEWEB)

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

1997-07-01

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

Ground state energy of a polaron in a superlattice

International Nuclear Information System (INIS)

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

2000-10-01

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

Investigation of switching region in superlattice phase change memories

Science.gov (United States)

Ohyanagi, T.; Takaura, N.

2016-10-01

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

The solitary electromagnetic waves in the graphene superlattice

International Nuclear Information System (INIS)

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

2013-01-01

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

Picosecond electron bunches from GaAs/GaAsP strained superlattice photocathode

International Nuclear Information System (INIS)

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

2013-01-01

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

Microwave absorption in YBCO/PrBCO superlattices

International Nuclear Information System (INIS)

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

1992-01-01

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

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.

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

KAUST Repository

Abutaha, Anas I.

2015-03-24

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

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

KAUST Repository

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

2015-01-01

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

Dielectric function of semiconductor superlattice

International Nuclear Information System (INIS)

Qin Guoyi.

1990-08-01

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

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.

Electrocrystallization and scanning probe microscopy of ceramic thin films and superlattices

Science.gov (United States)

Hung, Chen-Jen

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

1,2,3,4-bis(p-methylbenzylidene sorbitol) accelerates crystallization and improves hole mobility of poly(3-hexylthiophene)

Science.gov (United States)

Yuan, Nana; Huo, Hong

2016-02-01

The addition of 1,2,3,4-bis(p-methylbenzylidene sorbitol) (MDBS) does not change the nucleation mechanism or the crystal form of poly(3-hexylthiophene) (P3HT), but its presence increases the crystallization temperature (T c) of P3HT, decreases the crystallization half-time (t 1/2) and accelerates P3HT crystallization, which indicates that MDBS is an effective nucleating agent for P3HT. An acceleration of P3HT crystallization by the addition of MDBS decreases the crystalline size and crystallinity of P3HT, and enhances the connectivity between ordered regions of P3HT, leading to the hole mobility rising from 1.99 × 10-6 to 7.57 × 10-5 cm2 V-1s-1 in P3HT:PCBM blend based hole-only devices with sandwich configurations. Our results suggest that accelerating P3HT crystallization by adding a nucleating agent might be an important factor to improve the hole mobility and balance the electron and hole mobility in a photovoltaic blend.

Superlattice design for optimal thermoelectric generator performance

Science.gov (United States)

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

2018-05-01

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

DNA-nanoparticle superlattices formed from anisotropic building blocks

Science.gov (United States)

Jones, Matthew R.; Macfarlane, Robert J.; Lee, Byeongdu; Zhang, Jian; Young, Kaylie L.; Senesi, Andrew J.; Mirkin, Chad A.

2010-11-01

Directional bonding interactions in solid-state atomic lattices dictate the unique symmetries of atomic crystals, resulting in a diverse and complex assortment of three-dimensional structures that exhibit a wide variety of material properties. Methods to create analogous nanoparticle superlattices are beginning to be realized, but the concept of anisotropy is still largely underdeveloped in most particle assembly schemes. Some examples provide interesting methods to take advantage of anisotropic effects, but most are able to make only small clusters or lattices that are limited in crystallinity and especially in lattice parameter programmability. Anisotropic nanoparticles can be used to impart directional bonding interactions on the nanoscale, both through face-selective functionalization of the particle with recognition elements to introduce the concept of valency, and through anisotropic interactions resulting from particle shape. In this work, we examine the concept of inherent shape-directed crystallization in the context of DNA-mediated nanoparticle assembly. Importantly, we show how the anisotropy of these particles can be used to synthesize one-, two- and three-dimensional structures that cannot be made through the assembly of spherical particles.

Passive high-frequency devices based on superlattice ferromagnetic nanowires

International Nuclear Information System (INIS)

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

2007-01-01

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

Theoretical study of nitride short period superlattices

Science.gov (United States)

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

2018-02-01

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

Mixing of III-V compound semiconductor superlattices

International Nuclear Information System (INIS)

Mei, Ping.

1989-01-01

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

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.

Sm cluster superlattice on graphene/Ir(111)

Science.gov (United States)

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

2017-12-01

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

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

Science.gov (United States)

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

2013-09-11

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

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

Science.gov (United States)

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

2010-05-21

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

Superlattice configurations in linear chain hydrocarbon binary mixtures

Indian Academy of Sciences (India)

Unknown

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

Characterization of the Nb-B superlattice system

Energy Technology Data Exchange (ETDEWEB)

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

2016-12-15

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

Characterization of the Nb-B superlattice system

International Nuclear Information System (INIS)

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

2016-01-01

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

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...

Enhancement of broadband optical absorption in photovoltaic devices by band-edge effect of photonic crystals.

Science.gov (United States)

Tanaka, Yoshinori; Kawamoto, Yosuke; Fujita, Masayuki; Noda, Susumu

2013-08-26

We numerically investigate broadband optical absorption enhancement in thin, 400-nm thick microcrystalline silicon (µc-Si) photovoltaic devices by photonic crystals (PCs). We realize absorption enhancement by coupling the light from the free space to the large area resonant modes at the photonic band-edge induced by the photonic crystals. We show that multiple photonic band-edge modes can be produced by higher order modes in the vertical direction of the Si photovoltaic layer, which can enhance the absorption on multiple wavelengths. Moreover, we reveal that the photonic superlattice structure can produce more photonic band-edge modes that lead to further optical absorption. The absorption average in wavelengths of 500-1000 nm weighted to the solar spectrum (AM 1.5) increases almost twice: from 33% without photonic crystal to 58% with a 4 × 4 period superlattice photonic crystal; our result outperforms the Lambertian textured structure.

Epitaxial rare-earth superlattices and films

International Nuclear Information System (INIS)

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

1992-01-01

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

Antiferromagnetic spinor condensates in a bichromatic superlattice

Science.gov (United States)

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

2017-04-01

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

Scaling properties of optical reflectance from quasi-periodic superlattices

International Nuclear Information System (INIS)

Wu Xiang; Yao Hesheng; Feng Weiguo

1991-08-01

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

Superlattice to nanoelectronics

CERN Document Server

Tsu, Raphael

2005-01-01

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

Novel electronic structures of superlattice composed of graphene and silicene

International Nuclear Information System (INIS)

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

2014-01-01

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

Band structure of superlattice with δ-like potential

International Nuclear Information System (INIS)

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

1993-08-01

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

The reflected amplitude ratio of multilayers and superlattice describe the dynamical diffraction of x-rays

International Nuclear Information System (INIS)

Bhatti, Q.A.; Mangi, F.A.

2006-01-01

Calculating the rocking curves of complicated layered structures, such as non-ideal super lattices on perfect crystals are clearly exposed with observed diffraction profile. Recursion formulas for calculating reflected amplitude ratio of multilayer and super lattices have been involved from the Takagi-Taupin differential equation, which describes the dynamical diffraction of X-rays in deformed crystal. The Kinematical theory can computing time only in case of ideal superlattice for which geometric series can be used but the reflectivity must be below 10 % so that multiple reflections can be neglected for a perfect crystal of arbitrary thickness the absorption at the centre of the dynamical reflection is found to be proportional to the square root of the reflectivity. Sputter- deposited periodic multilayers of tungsten and carbon can be considered as an artificial crystal, for which dynamical X-rays diffraction calculations give the result very similar to those of macroscopic optical description in terms of the complex index of refraction and Frensnel relation coefficient. (author)

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

Science.gov (United States)

2017-06-05

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

Thermal transport through Ge-rich Ge/Si superlattices grown on Ge(0 0 1)

Science.gov (United States)

Thumfart, L.; Carrete, J.; Vermeersch, B.; Ye, N.; Truglas, T.; Feser, J.; Groiss, H.; Mingo, N.; Rastelli, A.

2018-01-01

The cross-plane thermal conductivities of Ge-rich Si/Ge superlattices have been measured using both time-domain thermoreflectance and the differential 3ω method. The superlattices were grown by molecular beam epitaxy on Ge(0 0 1) substrates. Crystal quality and structural information were investigated by x-ray diffractometry and transmission electron microscopy. The influence of segregation during growth on the composition profiles was modeled using the experimental growth temperatures and deposition rates. Those profiles were then employed to obtain parameter-free theoretical estimates of the thermal conductivity by combining first-principles calculations, Boltzmann transport theory and phonon Green’s functions. Good agreement between theory and experiment is observed. The thermal conductivity shows a strong dependence on the composition and the thickness of the samples. Moreover, the importance of the composition profile is reflected in the fact that the thermal conductivity of the superlattices is considerably lower than predicted values for alloys with the same average composition and thickness. Measurement on different samples with the same Si layer thickness and number of periods, but different Ge layer thickness, show that the thermal resistance is only weakly dependent on the Ge layers. We analyze this phenomenon based on the first-principles mode, and build an approximate parametrization showing that, in this regime, the resistivity of a SL is roughly linear on the amount of Si.

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

CERN Document Server

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

1990-01-01

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

Control of the interparticle spacing in gold nanoparticle superlattices

Energy Technology Data Exchange (ETDEWEB)

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

2000-04-06

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

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.

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.

Electronic structure of superlattices of graphene and hexagonal boron nitride

KAUST Repository

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

2011-01-01

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

Superlattice doped layers for amorphous silicon photovoltaic cells

Science.gov (United States)

Arya, Rajeewa R.

1988-01-12

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

Chronic skin inflammation accelerates macrophage cholesterol crystal formation and atherosclerosis

Science.gov (United States)

Ng, Qimin; Sanda, Gregory E.; Dey, Amit K.; Teague, Heather L.; Sorokin, Alexander V.; Dagur, Pradeep K.; Silverman, Joanna I.; Harrington, Charlotte L.; Rodante, Justin A.; Rose, Shawn M.; Varghese, Nevin J.; Belur, Agastya D.; Goyal, Aditya; Gelfand, Joel M.; Springer, Danielle A.; Bleck, Christopher K.E.; Thomas, Crystal L.; Yu, Zu-Xi; Winge, Mårten C.G.; Kruth, Howard S.; Marinkovich, M. Peter; Joshi, Aditya A.; Playford, Martin P.; Mehta, Nehal N.

2018-01-01

Inflammation is critical to atherogenesis. Psoriasis is a chronic inflammatory skin disease that accelerates atherosclerosis in humans and provides a compelling model to understand potential pathways linking these diseases. A murine model capturing the vascular and metabolic diseases in psoriasis would accelerate our understanding and provide a platform to test emerging therapies. We aimed to characterize a new murine model of skin inflammation (Rac1V12) from a cardiovascular standpoint to identify novel atherosclerotic signaling pathways modulated in chronic skin inflammation. The RacV12 psoriasis mouse resembled the human disease state, including presence of systemic inflammation, dyslipidemia, and cardiometabolic dysfunction. Psoriasis macrophages had a proatherosclerotic phenotype with increased lipid uptake and foam cell formation, and also showed a 6-fold increase in cholesterol crystal formation. We generated a triple-genetic K14-RacV12–/+/Srb1–/–/ApoER61H/H mouse and confirmed psoriasis accelerates atherogenesis (~7-fold increase). Finally, we noted a 60% reduction in superoxide dismutase 2 (SOD2) expression in human psoriasis macrophages. When SOD2 activity was restored in macrophages, their proatherogenic phenotype reversed. We demonstrate that the K14-RacV12 murine model captures the cardiometabolic dysfunction and accelerates vascular disease observed in chronic inflammation and that skin inflammation induces a proatherosclerotic macrophage phenotype with impaired SOD2 function, which associated with accelerated atherogenesis. PMID:29321372

InN/GaN Superlattices: Band Structures and Their Pressure Dependence

DEFF Research Database (Denmark)

Gorczyca, Iza; Suski, Tadek; Staszczak, Grzegorz

2013-01-01

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

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

OpenAIRE

Kappus, Wolfgang

2012-01-01

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

Electronic structure of superlattices

International Nuclear Information System (INIS)

Altarelli, M.

1987-01-01

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

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

International Nuclear Information System (INIS)

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

2000-01-01

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

Binding of biexcitons in GaAs/AlxGa1-xAs superlattices

DEFF Research Database (Denmark)

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

1997-01-01

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

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

International Nuclear Information System (INIS)

Wolf, D.

1992-10-01

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

pi-phase magnetism in ferromagnetic-superconductor superlattices

CERN Document Server

Khusainov, M G; Proshin, Y N

2001-01-01

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

Fabrication of C60/amorphous carbon superlattice structures

International Nuclear Information System (INIS)

Kojima, Nobuaki; Ohshita, Yoshio; Yamaguchi, Masafumi

2001-01-01

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

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

International Nuclear Information System (INIS)

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

1995-01-01

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

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

DEFF Research Database (Denmark)

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

2014-01-01

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

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...

Ordered patterns and structures via interfacial self-assembly: superlattices, honeycomb structures and coffee rings.

Science.gov (United States)

Ma, Hongmin; Hao, Jingcheng

2011-11-01

Self-assembly is now being intensively studied in chemistry, physics, biology, and materials engineering and has become an important "bottom-up" approach to create intriguing structures for different applications. Self-assembly is not only a practical approach for creating a variety of nanostructures, but also shows great superiority in building hierarchical structures with orders on different length scales. The early work in self-assembly focused on molecular self-assembly in bulk solution, including the resultant dye aggregates, liposomes, vesicles, liquid crystals, gels and so on. Interfacial self-assembly has been a great concern over the last two decades, largely because of the unique and ingenious roles of this method for constructing materials at interfaces, such as self-assembled monolayers, Langmuir-Blodgett films, and capsules. Nanocrystal superlattices, honeycomb films and coffee rings are intriguing structural materials with more complex features and can be prepared by interfacial self-assembly on different length scales. In this critical review, we outline the recent development in the preparation and application of colloidal nanocrystal superlattices, honeycomb-patterned macroporous structures by the breath figure method, and coffee-ring-like patterns (247 references). This journal is © The Royal Society of Chemistry 2011

Polarized photons from a silicon crystal in a 31 GeV electron beam at the Serpukhov proton accelerator

International Nuclear Information System (INIS)

Frolov, A.M.; Maisheev, V.A.; Arakelyan, E.A.; Armaganyan, A.A.; Avakyan, R.O.; Bayatyan, G.L.; Grigoryan, N.K.; Kechechyan, A.O.; Knyazyan, S.G.; Margaryan, A.T.

1980-01-01

Tagged photons coherently emitted in a silicon crystal by the 31 GeV electron beam of intensity 4 x 10 4 ppp and beam pulse duration of up to 1.7 s have been obtained at the Serpukhov proton accelerator. The photon intensities were I approx. 10 -1 - 10 -2 γ/e - in five almost equal energy bins within the total range k = (8.2-24.2) GeV. The calculated linear polarizations were P approx. 50-20%, respectively. Narrow peaks in the radiation intensity were observed when varying the orientation of a silicon crystal which could not be explained. The method for the experimental alignment of a crystal in electron beams at the proton accelerator has been described. (orig.)

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

International Nuclear Information System (INIS)

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

2006-01-01

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

Coupled polaritonic band gaps in the anisotropic piezoelectric superlattices

Science.gov (United States)

Tang, Zheng-Hua; Jiang, Zheng-Sheng; Chen, Tao; Jiang, Chun-Zhi; Lei, Da-Jun; Huang, Jian-Quan; Qiu, Feng; Yao, Min; Huang, Xiao-Yi

2018-01-01

Anisotropic piezoelectric superlattices (APSs) with the periodic arrangement of polarized anisotropic piezoelectric domains in a certain direction are presented, in which the coupled polaritonic band gaps (CPBGs) can be obtained in the whole Brillouin Zone and the maximum relative bandwidth (band-gap sizes divided by their midgap frequencies) of 5.1% can be achieved. The general characteristics of the APSs are similar to those of the phononic crystals composed of two types of materials, with the main difference being the formation mechanism of the CPBGs, which originate from the couplings between lattice vibrations along two different directions and electromagnetic waves rather than from the periodical modulation of density and elastic constants. In addition, there are no lattice mismatches because the APSs are made of the same material. Thus, the APSs can also be extended to the construction of novel acousto-optic devices.

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

International Nuclear Information System (INIS)

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

2012-01-01

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

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

KAUST Repository

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

2013-01-01

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

Phonon-induced optical superlattice.

Science.gov (United States)

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

2005-04-01

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

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...

Magneto-transport studies of InAs/GaSb short period superlattices

International Nuclear Information System (INIS)

Broadley, Victoria Jane

2002-01-01

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

Magnetic structures of holmium-lutetium alloys and superlattices

DEFF Research Database (Denmark)

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

1996-01-01

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

Second harmonic generation in generalized Thue-Morse ferroelectric superlattices

International Nuclear Information System (INIS)

Wang Longxiang; Yang Xiangbo; Chen Tongsheng

2009-01-01

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

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

OpenAIRE

Monticelli, C.; Zucchi, F.

2009-01-01

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

Transmission of electrons with flat passbands in finite superlattices

International Nuclear Information System (INIS)

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

2013-01-01

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

GeTe sequences in superlattice phase change memories and their electrical characteristics

Energy Technology Data Exchange (ETDEWEB)

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

2014-06-23

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

Superlattices of platinum and palladium nanoparticles

Energy Technology Data Exchange (ETDEWEB)

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

2000-04-06

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

TeV/m Nano-Accelerator: Current Status of CNT-Channeling Acceleration Experiment

Energy Technology Data Exchange (ETDEWEB)

Shin, Young Min [Northern Illinois U.; Lumpkin, Alex H. [Fermilab; Thangaraj, Jayakar Charles [Fermilab; Thurman-Keup, Randy Michael [Fermilab; Shiltsev, Vladimir D. [Fermilab

2014-09-17

Crystal channeling technology has offered various opportunities in the accelerator community with a viability of ultrahigh gradient (TV/m) acceleration for future HEP collider. The major challenge of channeling acceleration is that ultimate acceleration gradients might require a high power driver in the hard x-ray regime (~ 40 keV). This x-ray energy exceeds those for x-rays as of today, although x-ray lasers can efficiently excite solid plasma and accelerate particles inside a crystal channel. Moreover, only disposable crystal accelerators are possible at such high externally excited fields which would exceed the ionization thresholds destroying the atomic structure, so acceleration will take place only in a short time before full dissociation of the lattice. Carbon-based nanostructures have great potential with a wide range of flexibility and superior physical strength, which can be applied to channeling acceleration. This paper presents a beam- driven channeling acceleration concept with CNTs and discusses feasible experiments with the Advanced Superconducting Test Accelerator (ASTA) in Fermilab.

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.

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

International Nuclear Information System (INIS)

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

2003-01-01

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

Laser-controlled stress of Si nanocrystals in a free-standing Si /SiO2 superlattice

Science.gov (United States)

Khriachtchev, Leonid; Räsänen, Markku; Novikov, Sergei

2006-01-01

We report laser manipulations with stress at the nanoscale level. The continuous-wave Ar+ laser radiation melts Si nanocrystals in a free-standing Si /SiO2 superlattice. Silicon crystallization from the liquid phase leads to a compressive stress, which can be accurately tuned in the 3GPa range using laser annealing below the Si melting temperature and then recovered by laser annealing above the melting temperature. This allows investigations of various phenomena as a function of stress and makes a case of Si-nanocrystal memory with very long retention time, which can be written, erased, and read by optical means.

Laser-controlled stress of Si nanocrystals in a free-standing Si/SiO2 superlattice

International Nuclear Information System (INIS)

Khriachtchev, Leonid; Raesaenen, Markku; Novikov, Sergei

2006-01-01

We report laser manipulations with stress at the nanoscale level. The continuous-wave Ar + laser radiation melts Si nanocrystals in a free-standing Si/SiO 2 superlattice. Silicon crystallization from the liquid phase leads to a compressive stress, which can be accurately tuned in the 3 GPa range using laser annealing below the Si melting temperature and then recovered by laser annealing above the melting temperature. This allows investigations of various phenomena as a function of stress and makes a case of Si-nanocrystal memory with very long retention time, which can be written, erased, and read by optical means

Dielectric enhancement of BaTiO3/SrTiO3 superlattices with embedded Ni nanocrystals

International Nuclear Information System (INIS)

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

2012-01-01

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

Possible THz gain in superlattices at a stable operation point

DEFF Research Database (Denmark)

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

1997-01-01

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

Optical Tamm states in one-dimensional superconducting photonic crystal

Energy Technology Data Exchange (ETDEWEB)

El Abouti, O. [LPMR, Département de Physique, Faculté des Sciences, Université Mohammed 1, 60000 Oujda (Morocco); El Boudouti, E. H. [LPMR, Département de Physique, Faculté des Sciences, Université Mohammed 1, 60000 Oujda (Morocco); IEMN, UMR-CNRS 8520, UFR de Physique, Université de Lille 1, 59655 Villeneuve d' Ascq (France); El Hassouani, Y. [ESIM, Département de Physique, Faculté des Sciences et Techniques, Université Moulay Ismail, Boutalamine BP 509, 52000 Errachidia (Morocco); Noual, A. [LPMR, Département de Physique, Faculté des Sciences, Université Mohammed 1, 60000 Oujda (Morocco); Ecole Normale Supérieur de Tétouan, Université Abdelmalek Essaadi, Tétouan (Morocco); Djafari-Rouhani, B. [IEMN, UMR-CNRS 8520, UFR de Physique, Université de Lille 1, 59655 Villeneuve d' Ascq (France)

2016-08-15

In this study, we investigate localized and resonant optical waves associated with a semi-infinite superlattice made out of superconductor-dielectric bilayers and terminated with a cap layer. Both transverse electric and transverse magnetic waves are considered. These surface modes are analogous to the so-called Tamm states associated with electronic states found at the surface of materials. The surface guided modes induced by the cap layer strongly depend on whether the superlattice ends with a superconductor or a dielectric layer, the thickness of the surface layer, the temperature of the superconductor layer as well as on the polarization of the waves. Different kinds of surface modes are found and their properties examined. These structures can be used to realize the highly sensitive photonic crystal sensors.

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.

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

International Nuclear Information System (INIS)

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

2014-01-01

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

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

Science.gov (United States)

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

2014-09-01

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

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

International Nuclear Information System (INIS)

Tynell, Tommi; Karppinen, Maarit

2014-01-01

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

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

Energy Technology Data Exchange (ETDEWEB)

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

2014-01-31

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

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

Energy Technology Data Exchange (ETDEWEB)

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

2015-05-11

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

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

International Nuclear Information System (INIS)

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

2015-01-01

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

Surface phonon polaritons in semi-infinite semiconductor superlattices

International Nuclear Information System (INIS)

Nkoma, J.S.

1986-07-01

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

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.

TeV/m nano-accelerator: Investigation on feasibility of CNT-channeling acceleration at Fermilab

Energy Technology Data Exchange (ETDEWEB)

Shin, Y. M. [Northern Illinois Univ., DeKalb, IL (United States); Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Lumpkin, A. H. [Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Thurman-Keup, R. M. [Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)

2015-03-23

The development of high gradient acceleration and tight phase-space control of high power beams is a key element for future lepton and hadron colliders since the increasing demands for higher energy and luminosity significantly raise costs of modern HEP facilities. Atomic channels in crystals are known to consist of 10–100 V/Å potential barriers capable of guiding and collimating a high energy beam providing continuously focused acceleration with exceptionally high gradients (TeV/m). However, channels in natural crystals are only angstrom-size and physically vulnerable to high energy interactions, which has prevented crystals from being applied to high power accelerators. Carbon-based nano-crystals such as carbon-nanotubes (CNTs) and graphenes have a large degree of dimensional flexibility and thermo-mechanical strength, which could be suitable for channeling acceleration of MW beams. Nano-channels of the synthetic crystals can accept a few orders of magnitude larger phase-space volume of channeled particles with much higher thermal tolerance than natural crystals. This study presents the current status of CNT-channeling acceleration research at the Advanced Superconducting Test Accelerator (ASTA) in Fermilab.

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

International Nuclear Information System (INIS)

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

2006-01-01

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

In Situ Visualization of the Growth and Fluctuations of Nanoparticle Superlattice in Liquids

Science.gov (United States)

Ou, Zihao; Shen, Bonan; Chen, Qian

We use liquid phase transmission electron microscopy to image and understand the crystal growth front and interfacial fluctuation of a nanoparticle superlattice. With single particle resolution and hundreds of nanoscale building blocks in view, we are able to identify the interface between ordered lattice and disordered structure and visualize the kinetics of single building block attachment at the lattice growth front. The spatial interfacial fluctuation profiles support the capillary wave theory, from which we derive a surface stiffness value consistent with scaling analysis. Our experiments demonstrate the potential of extending model study on collective systems to nanoscale with single particle resolution and testing fundamental theories of condensed matter at a length scale linking atoms and micron-sized colloids.

Phonon dispersion and thermal conductivity of nanocrystal superlattices using three-dimensional atomistic models

International Nuclear Information System (INIS)

Zanjani, Mehdi B.; Lukes, Jennifer R.

2014-01-01

A computational study of thermal conductivity and phonon dispersion of gold nanocrystal superlattices is presented. Phonon dispersion curves, reported here for the first time from combined molecular dynamics and lattice dynamics calculations, show multiple phononic band gaps and consist of many more dispersion branches than simple atomic crystals. Fully atomistic three dimensional molecular dynamics calculations of thermal conductivity using the Green Kubo method are also performed for the first time on these materials. Thermal conductivity is observed to increase for increasing nanocrystal core size and decrease for increasing surface ligand density. Our calculations predict values in the range 0.1–1 W/m K that are consistent with reported experimental results

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.

Natural nanostructure and superlattice nanodomains in AgSbTe{sub 2}

Energy Technology Data Exchange (ETDEWEB)

Carlton, Christopher E.; De Armas, Ricardo; Shao-Horn, Yang, E-mail: delaireoa@ornl.gov, E-mail: shaohorn@mit.edu [Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States); Ma, Jie [Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (United States); May, Andrew F.; Delaire, Olivier, E-mail: delaireoa@ornl.gov, E-mail: shaohorn@mit.edu [Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (United States)

2014-04-14

AgSbTe{sub 2} has long been of interest for thermoelectric applications because of its favorable electronic properties and its low lattice thermal conductivity of ∼0.7 W/mK. In this work, we report new findings from a high-resolution transmission electron microscopy study revealing two nanostructures in single crystal Ag{sub 1−x}Sb{sub 1+x}Sb{sub 2+x} (with x = 0, 0.1, 0.2); (i) a rippled natural nanostructure with a period of ∼2.5–5 nm and (ii) superlattice ordered nanodomains consistent with cation ordering predicted in previous density functional theory studies. These nanostructures, combined with point-defects, probably serve as sources of scattering for phonons, thereby yielding a low lattice thermal conductivity over a wide temperature range.

Interface properties of superlattices with artificially broken symmetry

International Nuclear Information System (INIS)

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

2007-01-01

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

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

Science.gov (United States)

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

2018-03-01

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

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

International Nuclear Information System (INIS)

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

2011-01-01

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

Molecular dynamics simulation of thermal conductivities of superlattice nanowires

Institute of Scientific and Technical Information of China (English)

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

2003-01-01

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

Interface disorder and transport properties in HTC/CMR superlattices

International Nuclear Information System (INIS)

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

2004-01-01

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

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

International Nuclear Information System (INIS)

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

2011-01-01

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

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.

Simulation studies of crystal-photodetector assemblies for the Turkish accelerator center particle factory electromagnetic calorimeter

Energy Technology Data Exchange (ETDEWEB)

Kocak, F., E-mail: fkocak@uludag.edu.tr

2015-07-01

The Turkish Accelerator Center Particle Factory detector will be constructed for the detection of the produced particles from the collision of a 1 GeV electron beam against a 3.6 GeV positron beam. PbWO{sub 4} and CsI(Tl) crystals are considered for the construction of the electromagnetic calorimeter part of the detector. The generated optical photons in these crystals are detected by avalanche or PIN photodiodes. Geant4 simulation code has been used to estimate the energy resolution of the calorimeter for these crystal–photodiode assemblies.

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

Science.gov (United States)

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

2018-06-01

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

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.

Electron dynamics in intentionally disordered semiconductor superlattices

International Nuclear Information System (INIS)

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

1996-01-01

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

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.

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

International Nuclear Information System (INIS)

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

1993-01-01

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

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.

Theory of transmission through disordered superlattices

DEFF Research Database (Denmark)

Wacker, Andreas; Hu, Ben Yu-Kuang

1999-01-01

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

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

Directory of Open Access Journals (Sweden)

C. C. Fan

2017-08-01

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

Engineering the oxygen coordination in digital superlattices

Science.gov (United States)

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

2017-12-01

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

High-electric-field quantum transport theory for semiconductor superlattices

International Nuclear Information System (INIS)

Nguyen Hong Shon; Nazareno, H.N.

1995-12-01

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

Transport in a magnetic field modulated graphene superlattice.

Science.gov (United States)

Li, Yu-Xian

2010-01-13

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

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

Science.gov (United States)

Huang, Juyang

2002-08-01

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

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

Science.gov (United States)

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

2018-04-01

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

Two-dimensional plasma photonic crystals in dielectric barrier discharge

International Nuclear Information System (INIS)

Fan Weili; Dong Lifang; Zhang Xinchun

2010-01-01

A series of two-dimensional plasma photonic crystals have been obtained by filaments' self-organization in atmospheric dielectric barrier discharge with two water electrodes, which undergo the transition from square to square superlattice and finally to the hexagon. The spatio-temporal behaviors of the plasma photonic crystals in nanosecond scale have been studied by optical method, which show that the plasma photonic crystal is actually an integration of different transient sublattices. The photonic band diagrams of the transverse electric (TE) mode and transverse magnetic mode for each sublattice of these plasma photonic crystals have been investigated theoretically. A wide complete band gap is formed in the hexagonal plasma photonic crystal with the TE mode. The changes of the band edge frequencies and the band gap widths in the evolvement of different structures are studied. A kind of tunable plasma photonic crystal which can be controlled both in space and time is suggested.

Dynamic localization and negative absolute conductance in terahertz driven semiconductor superlattices

International Nuclear Information System (INIS)

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

1995-01-01

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

Stability and dynamic of strain mediated adatom superlattices on Cu

Science.gov (United States)

Kappus, Wolfgang

2013-03-01

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

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)

Simultaneous microwave photonic and phononic band gaps in piezoelectric–piezomagnetic superlattices with three types of domains in a unit cell

Energy Technology Data Exchange (ETDEWEB)

Tang, Zheng-hua [Xiangnan University-Gospell Joint Laboratory of Microwave Communication Technology, Xiangnan University, Chenzhou 423000 (China); Jiang, Zheng-Sheng [National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093 (China); Chen, Tao [Laboratory of Quantum Information and Quantum Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006 (China); Lei, Da-Jun [Xiangnan University-Gospell Joint Laboratory of Microwave Communication Technology, Xiangnan University, Chenzhou 423000 (China); Yan, Wen-Yan, E-mail: yanwenyan88@126.com [School of Software and Communication Engineering, Xiangnan University, Chenzhou 423000 (China); Qiu, Feng; Huang, Jian-Quan; Deng, Hai-Ming; Yao, Min [Xiangnan University-Gospell Joint Laboratory of Microwave Communication Technology, Xiangnan University, Chenzhou 423000 (China)

2016-04-29

A novel phoxonic crystal using the piezoelectric (PMN-PT) and piezomagnetic (CoFe{sub 2}O{sub 4}) superlattices with three types of domains in a unit cell (PPSUC) is present, in which dual microwave photonic and phononic band gaps can be obtained simultaneously. Two categories of phononic band gaps, originating from both the Bragg scattering of acoustic waves in periodic structures at the Brillouin zone boundary and the electromagnetic wave-lattice vibration couplings near the Brillouin zone center, can be observed in the phononic band structures. The general characteristics of the microwave photonic band structures are similar to those of pure piezoelectric or piezomagnetic superlattices, with the major discrepancy being the appearance of nearly dispersionless branches within the microwave photonic band gaps, which show an extremely large group velocity delay. Thus, the properties may also be applied to compact acoustic-microwave devices. - Highlights: • Dual microwave photonic and phononic band gaps can coexist in the PPSUC. • Two categories of phononic band gaps with different mechanism can be obtained. • Nearly dispersionless branches appear in the microwave photonic band gaps.

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

International Nuclear Information System (INIS)

Isohätälä, J; Alekseev, K N

2012-01-01

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

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

Energy Technology Data Exchange (ETDEWEB)

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

2012-01-05

We investigate the structural, magnetic, and electrical properties of superlattices composed of the ferromagnetic/metal La_0.7Sr_0.3MnO₃ and non-magnetic/metal La_0.5Sr_0.5TiO₃ grown on (001)-oriented SrTiO₃ substrates. Using a combination of bulk magnetometry, soft x-ray magnetic spectroscopy, and scanning transmission electron microscopy, we demonstrate that robust ferromagnetic properties can be maintained in this superlattice system where charge transfer at the interfaces is minimized. Thus, ferromagnetism can be controlled effectively through the chemical identity and the thickness of the individual superlattice layers.

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

International Nuclear Information System (INIS)

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

2015-01-01

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

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

KAUST Repository

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

2011-01-01

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

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.

Superlattice Microstructured Optical Fiber

Science.gov (United States)

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

2014-01-01

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

Large negative differential resistance in graphene nanoribbon superlattices

Science.gov (United States)

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

2018-05-01

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

A possible radiation-resistant solar cell geometry using superlattices

Science.gov (United States)

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

1985-01-01

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

Organic p-n heterostructures and superlattices

Energy Technology Data Exchange (ETDEWEB)

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

2009-07-01

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

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

International Nuclear Information System (INIS)

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

2015-01-01

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

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

Science.gov (United States)

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

2017-12-01

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

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.

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

Science.gov (United States)

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

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

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

Energy Technology Data Exchange (ETDEWEB)

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

2015-12-01

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

Thermoelectric transport in superlattices

Energy Technology Data Exchange (ETDEWEB)

Reinecke, T L; Broido, D A

1997-07-01

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

Crystal structure of ferroelectric Bi{sub 2}VO{sub 5.5}

Energy Technology Data Exchange (ETDEWEB)

Sooryanarayana, K.; Guru Row, T.N.; Varma, K.B.R. [Indian Inst. of Science, Bangalore (India)

1997-12-01

The structure of the {alpha}-phase of bismuth vanadate Bi{sub 2}VO{sub 5.5} has been determined using single crystal X-ray diffraction data in the space group Aba2. The refinement involves a well defined disorder at the vanadium site, which incorporates the features of the superlattice structure with vanadium tetrahedra and oxygen-deficient octahedra that is displaced about the twofold axis.

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

OpenAIRE

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

1987-01-01

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

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

DEFF Research Database (Denmark)

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

2012-01-01

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

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

International Nuclear Information System (INIS)

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

1998-01-01

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

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

Science.gov (United States)

Hsing, Greg Hsiang-Chun

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

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

Science.gov (United States)

Lach, Marcel; Künzle, Matthias; Beck, Tobias

2017-12-11

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

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

Science.gov (United States)

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

1987-06-08

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

Thermoelectric properties of strontium titanate superlattices incorporating niobium oxide nanolayers

KAUST Repository

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

2014-01-01

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

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.

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

Energy Technology Data Exchange (ETDEWEB)

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

2017-06-14

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

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.

Quality regularities of dynamic X-ray diffraction in superlattices and films with variable gradient of deformation based on analysis of types of Takagi equation solutions

International Nuclear Information System (INIS)

Dyshekov, A.A.; Khapachev, Yu.P.

1997-01-01

It is proposed to use qualitative investigation methods of the differential Takagi equation solutions for the analysis of general properties of wave fields in deformed crystals. The physical interpretation of possible types of the Takagi equation solutions is considered briefly from the viewpoint of the stability theory. The type of solutions are defined by ratios between parameters involved in the equations set. For the Takagi equation these parameters are prescribed by the angular tuning from the precise Bragg angle as well as structural characteristics of the crystal and the deformation profile. The qualitative analysis for the problem of the dynamic X-ray diffraction is carried out for films with the variable deformation gradient and superlattices [ru

Band structure of ABC-trilayer graphene superlattice

International Nuclear Information System (INIS)

Uddin, Salah; Chan, K. S.

2014-01-01

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

Energy minibands degeneration induced by magnetic field effects in graphene superlattices

Science.gov (United States)

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

2017-12-01

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

Ab initio study of thermoelectric properties of doped SnO_2 superlattices

International Nuclear Information System (INIS)

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

2015-01-01

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

Propagation and generation of Josephson radiation in superconductor/insulator superlattices

International Nuclear Information System (INIS)

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

1987-01-01

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

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)

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

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.

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

International Nuclear Information System (INIS)

Choi, Taekjib; Lee, Jaichan

2005-01-01

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

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

International Nuclear Information System (INIS)

Bouchard, A.M.

1994-01-01

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

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

International Nuclear Information System (INIS)

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

2016-01-01

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

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.

InGaAs/InAlAs superlattice detector for THz radiation

CERN Document Server

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

2002-01-01

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

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

Energy Technology Data Exchange (ETDEWEB)

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

2014-01-15

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

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

International Nuclear Information System (INIS)

Min, Li; Xian-Wu, Mi

2009-01-01

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

Two-dimensional thermoelectric Seebeck coefficient of SrTiO3-based superlattices

International Nuclear Information System (INIS)

Ohta, Hiromichi

2008-01-01

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

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

KAUST Repository

Dai, Qingqing

2018-05-04

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

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

Data.gov (United States)

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

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.

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

Energy Technology Data Exchange (ETDEWEB)

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

2017-02-15

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

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

Energy Technology Data Exchange (ETDEWEB)

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

2015-11-15

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

Tunable Noncollinear Antiferromagnetic Resistive Memory through Oxide Superlattice Design

Science.gov (United States)

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

2018-04-01

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

Lateral surface superlattices in strained InGaAs layers

International Nuclear Information System (INIS)

Milton, B.

2000-08-01

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

Design of band pass filter in a modulated magnetic graphene superlattice

International Nuclear Information System (INIS)

Lu, Wei-Tao; Li, Wen

2015-01-01

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

Quenching long range magnetic excitations in oxygen sub-lattice reconstructed thin films of (SrCuO{sub 2}){sub n}/(SrTiO{sub 2}){sub 2} superlattices

Energy Technology Data Exchange (ETDEWEB)

Dantz, Marcus; Pelliciari, Jonathan; Huang, Yaobo; Bisogni, Valentina; Olalde-Velasco, Paul; Strocov, Vladimir; Schmitt, Thorsten [Paul Scherrer Institut, Villigen PSI (Switzerland); Samal, Debakanta; Koster, Gertjan [MESA+ Institute for Nanotechnology, University of Twente (Netherlands)

2015-07-01

Multi-layered thin cuprate films allow inducing subtle structural changes with which local crystal field and long range magnetic excitations can be finely tuned. In (SrCuO{sub 2}){sub n}/(SrTiO{sub 2}){sub 2} cuprate superlattices, in particular, a structural transformation from a bulk infinite planar to a sheet-like local domain environment has recently been predicted and subsequently observed. Here we present results on the influence of this subtle structural reconstruction on the collective magnetic excitations using high-resolution resonant inelastic x-ray scattering (RIXS) at the Cu L{sub 3} edge. While bulk like infinite layer films exhibit magnon excitations throughout the whole Brillouin zone, decreasing the thickness of the cuprate layers leads to quenching of the magnons starting from the Gamma point successively to the zone boundary, allowing us to study the coherence length of the collective long range magnetic excitations in these cuprate superlattices.

Surface magnetic phase transitions in Dy/Lu superlattices

International Nuclear Information System (INIS)

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

1999-01-01

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

Multilayer Photonic Crystal for Spectral Narrowing of Emission

Directory of Open Access Journals (Sweden)

Zhanfang LIU

2017-08-01

Full Text Available Multilayer colloidal crystal has been prepared by the layer-by-layer deposition of silica microspheres on a glass slide. Each layer is a slab consisting of a fcc close-packed colloidal arrays. By properly choosing the sizes of spheres, the whole spectral feature of multilayer colloidal crystal can be tuned. Here, we engineered a multilayer superlattice structure with an effective passband between two stop bands. This gives a strong narrowing effect on emission spectrum. With the stop bands at the shortwave and longwave edges of emission spectrum, the passband in the central wavelength region can be regarded as a strong decrease of suppression effect and enhancement of a narrow wavelength region of emission. The spectral narrowing modification effect of suitably engineered colloidal crystals shows up their importance in potential application as optical filters and lasing devices.DOI: http://dx.doi.org/10.5755/j01.ms.23.3.16320

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.

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.

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

KAUST Repository

Liu, Heng-Jui

2015-12-09

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

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

International Nuclear Information System (INIS)

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

2009-01-01

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

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

Directory of Open Access Journals (Sweden)

Aeberhard Urs

2011-01-01

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

Crystals in the LHC

CERN Multimedia

Antonella Del Rosso

2012-01-01

Bent crystals can be used to deflect charged particle beams. Their use in high-energy accelerators has been investigated for almost 40 years. Recently, a bent crystal was irradiated for the first time in the HiRadMat facility with an extreme particle flux, which crystals would have to withstand in the LHC. The results were very encouraging and confirmed that this technology could play a major role in increasing the beam collimation performance in future upgrades of the machine.   UA9 bent crystal tested with a laser. Charged particles interacting with a bent crystal can be trapped in channelling states and deflected by the atomic planes of the crystal lattice (see box). The use of bent crystals for beam manipulation in particle accelerators is a concept that has been well-assessed. Over the last three decades, a large number of experimental findings have contributed to furthering our knowledge and improving our ability to control crystal-particle interactions. In modern hadron colliders, su...

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

KAUST Repository

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

2018-01-01

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

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

Science.gov (United States)

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

2015-11-24

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

Quantum ratchets for quantum communication with optical superlattices

International Nuclear Information System (INIS)

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

2007-01-01

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

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

International Nuclear Information System (INIS)

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

2009-01-01

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

Excitation on breather (bion) in superlattice

International Nuclear Information System (INIS)

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

1999-09-01

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

InGaNAs/GaAs multi-quantum wells and superlattices solar cells

International Nuclear Information System (INIS)

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

2011-01-01

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

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.

Time and space domain separation of pulsed X-ray beams diffracted from vibrating crystals

Energy Technology Data Exchange (ETDEWEB)

Nosik, V. L., E-mail: v-nosik@yandex.ru, E-mail: nosik@ns.crys.ras.ru [Russian Academy of Sciences, Shubnikov Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics,” (Russian Federation)

2016-11-15

It is known that a set of additional reflections (satellites) may arise on rocking curves in the case of X-ray diffraction in the Bragg geometry from crystals where high-frequency ultrasonic vibrations are excited. It is shown that, under certain conditions, the pulse wave fields of the satellites and main reflection may be intersected in space (playing the role of pump and probe beams) and in time (forming interference superlattices).

Transport properties of YBa2Cu3O7/PrBa2Cu3O7 superlattices

International Nuclear Information System (INIS)

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

1992-01-01

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

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

International Nuclear Information System (INIS)

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

2016-01-01

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

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

Data.gov (United States)

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

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

International Nuclear Information System (INIS)

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

2008-01-01

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

Superlattice electroabsorption radiation detector

International Nuclear Information System (INIS)

Cooke, B.J.

1993-06-01

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

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...

Quantum Transport: The Link between Standard Approaches in Superlattices

DEFF Research Database (Denmark)

Wacker, Andreas; Jauho, Antti-Pekka

1998-01-01

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

The Role of Repulsion in Colloidal Crystal Engineering with DNA

Energy Technology Data Exchange (ETDEWEB)

Seo, Soyoung E. [Department; Li, Tao [X-ray; Senesi, Andrew J. [X-ray; Mirkin, Chad A. [Department; Lee, Byeongdu [X-ray

2017-11-07

Hybridization interactions between DNA-functionalized nanoparticles (DNA-NPs) can be used to program the crystallization behavior of superlattices, yielding access to complex three-dimensional structures with more than 30 different lattice symmetries. The first superlattice structures using DNA-NPs as building blocks were identified almost two decades ago, yet the role of repulsive interactions in guiding structure formation is still largely unexplored. Here, a com-prehensive approach is taken to study the role of repulsion in the assembly behavior of DNA-NPs, enabling the calculation of interparticle interaction potentials based on experimental results. In this work, we used two different means to assemble DNA-NPs—Watson-Crick base pairing interactions and depletion interactions—and systematically varied the salt concen-tration to study the effective interactions in DNA-NP superlattices. A comparison between the two systems allows us to decouple the repulsive forces from the attractive hybridization interactions that are sensitive to the ionic environment. We find that the gap distance between adjacent DNA-NPs follows a simple power law dependence on solution ionic strength regardless of the type of attractive forces present. This result suggests that the observed trend is driven by repulsive inter-actions. To better understand such behavior, we propose a mean-field model that provides a mathematical description for the observed trend. This model shows that the trend is due to the variation in the effective cross-sectional diameter of DNA duplex and the thickness of DNA shell.

Electronic structure of silicon superlattices

International Nuclear Information System (INIS)

Krishnamurthy, S.; Moriarty, J.A.

1984-01-01

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

Monte Carlo simulation of continuous-space crystal growth

International Nuclear Information System (INIS)

Dodson, B.W.; Taylor, P.A.

1986-01-01

We describe a method, based on Monte Carlo techniques, of simulating the atomic growth of crystals without the discrete lattice space assumed by conventional Monte Carlo growth simulations. Since no lattice space is assumed, problems involving epitaxial growth, heteroepitaxy, phonon-driven mechanisms, surface reconstruction, and many other phenomena incompatible with the lattice-space approximation can be studied. Also, use of the Monte Carlo method circumvents to some extent the extreme limitations on simulated timescale inherent in crystal-growth techniques which might be proposed using molecular dynamics. The implementation of the new method is illustrated by studying the growth of strained-layer superlattice (SLS) interfaces in two-dimensional Lennard-Jones atomic systems. Despite the extreme simplicity of such systems, the qualitative features of SLS growth seen here are similar to those observed experimentally in real semiconductor systems

Ferroelectric domain structures in -oriented K0.15Na0.85NbO3 lead-free single crystal

Directory of Open Access Journals (Sweden)

Yan Chen

2015-03-01

Full Text Available In this work, ferroelectric domain structures of -oriented K0.15Na0.85NbO3 single crystal are characterized. Transmission electron microscopy (TEM observation revealed high-density of laminate domain structures in the crystal and the lattices of the neighboring domains are found to be twisted in a small angle. Superlattice diffraction spots of 1 2 { eeo } and 1 2 { ooe } in electron diffraction patterns are observed in the crystal, revealing the a+a+c− tilting of oxygen octahedral in the perovskite structure. The piezoresponse of domains and in-situ poling responses of K0.15Na0.85NbO3 crystal are observed by piezoresponse force microscopy (PFM, and the results assure its good ferroelectric properties.

Characterization Of Graphene-Ferroelectric Superlattice Hybrid Devices

Science.gov (United States)

Yusuf, Mohammed; Du, Xu; Dawber, Matthew

2013-03-01

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

Electron drag by solitons in superlattices in an external magnetic field

International Nuclear Information System (INIS)

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

1996-01-01

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

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.

Analysis of polariton dispersion in metal nanocomposite based novel superlattice system

Science.gov (United States)

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

2018-06-01

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

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

International Nuclear Information System (INIS)

Shuleiko, D V; Ilin, A S

2016-01-01

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

Ge/SiGe superlattices for nanostructured thermoelectric modules

International Nuclear Information System (INIS)

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

2013-01-01

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

Transfer matrix theory of monolayer graphene/bilayer graphene heterostructure superlattice

International Nuclear Information System (INIS)

Wang, Yu

2014-01-01

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

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

International Nuclear Information System (INIS)

Feng Weiguo; Yao Hesheng; Xu Xiang

1989-12-01

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

Structural study of multilayered vanadium/nickel superlattices

International Nuclear Information System (INIS)

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

1985-07-01

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

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

International Nuclear Information System (INIS)

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

1993-01-01

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

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

Energy Technology Data Exchange (ETDEWEB)

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

2016-10-15

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

IPNS time-of-flight single crystal diffractometer

International Nuclear Information System (INIS)

Schultz, A.J.; Teller, R.G.; Williams, J.M.

1983-01-01

The single crystal diffractometer (SCD) at the Argonne Intense Pulsed Neutron Source (IPNS) utilizes the time-of-flight (TOF) Laue technique to provide a three-dimensional sampling of reciprocal space during each pulse. The instrument contains a unique neutron position-sensitive 6 Li-glass scintillation detector with an active area of 30 x 30 cm. The three-dimensional nature of the data is very useful for fast, efficient measurement of Bragg intensities and for the studies of superlattice and diffuse scattering. The instrument was designed to achieve a resolution of 2% or better (R = δQ/Q) with 2 THETA > 60 0 and lambda > 0.7A

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

International Nuclear Information System (INIS)

Khayatzadeh Mahani, Mohammad Reza; Faizabadi, Edris

2008-01-01

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

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

International Nuclear Information System (INIS)

Ulloa, S.E.

1988-01-01

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

Morphological Instability in InAs/GaSb Superlattices due to Interfacial Bonds

International Nuclear Information System (INIS)

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

2005-01-01

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

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

International Nuclear Information System (INIS)

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

2013-01-01

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

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

Science.gov (United States)

Mughnetsyan, Vram; Kirakosyan, Albert

2017-12-01

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

Localization in superlattices with randomness in layer thickness

International Nuclear Information System (INIS)

Yuan Jian; Tsai Chienhua.

1987-08-01

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

Future device applications of low-dimensional carbon superlattice structures

Science.gov (United States)

Bhattacharyya, Somnath

2005-03-01

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

Optical transmission through multi-component generalized Thue-Morse superlattices

Energy Technology Data Exchange (ETDEWEB)

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

2010-09-01

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

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

International Nuclear Information System (INIS)

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

2005-01-01

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

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

Science.gov (United States)

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

2016-02-01

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

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

International Nuclear Information System (INIS)

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

2013-01-01

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

Recent results on heterojunctions and superlattices: transport and optics

International Nuclear Information System (INIS)

Voos, M.

1983-01-01

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

Chemically Triggered Formation of Two-Dimensional Epitaxial Quantum Dot Superlattices

NARCIS (Netherlands)

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

2016-01-01

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

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

International Nuclear Information System (INIS)

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

2011-01-01

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

Epitaxial crystal growth by sputter deposition: Applications to semiconductors. Part 2

International Nuclear Information System (INIS)

Greene, J.E.

1984-01-01

The understanding of the physics of ion-surface interactions has progressed sufficiently to allow sputter depositinn to be used as a crystal growth technique for depositing a wide variety of single crystal elemental, compound, alloy, and superlattice semiconductors. In many cases, films with essentially bulk values of carrier concentrations and mobilities have been obtained. The controlled use of low energy particle bombardment of the growing film during sputter deposition has been shown to affect all stages of crystal growth ranging from adatom mobilities and nucleation kinetics to elemental incorporation probabilities. Such effects provide inherent advantages for sputter deposition over other vapor phase techniques for the low temperature growth of compound and alloy semiconductors and are essential in allowing the growth of new and unique single crystal metastable semiconductors. Part 1 of this review includes sections on experimental techniques, the physics of ion-surface interactions, and ion bombardment effects on film nucleation and growth, while Part 2 presents a discussion of recent results in the growth of elemental, III-V, II-VI, IV-VI, metastable, and other compound semiconductors

Surface acoustic waves in acoustic superlattice lithium niobate coated with a waveguide layer

Science.gov (United States)

Yang, G. Y.; Du, J. K.; Huang, B.; Jin, Y. A.; Xu, M. H.

2017-04-01

The effects of the waveguide layer on the band structure of Rayleigh waves are studied in this work based on a one-dimensional acoustic superlattice lithium niobate substrate coated with a waveguide layer. The present phononic structure is formed by the periodic domain-inverted single crystal that is the Z-cut lithium niobate substrate with a waveguide layer on the upper surface. The plane wave expansion method (PWE) is adopted to determine the band gap behavior of the phononic structure and validated by the finite element method (FEM). The FEM is also used to investigate the transmission of Rayleigh waves in the phononic structure with the interdigital transducers by means of the commercial package COMSOL. The results show that, although there is a homogeneous waveguide layer on the surface, the band gap of Rayleigh waves still exist. It is also found that increasing the thickness of the waveguide layer, the band width narrows and the band structure shifts to lower frequency. The present approach can be taken as an efficient tool in designing of phononic structures with waveguide layer.

Surface acoustic waves in acoustic superlattice lithium niobate coated with a waveguide layer

Directory of Open Access Journals (Sweden)

G. Y. Yang

2017-04-01

Full Text Available The effects of the waveguide layer on the band structure of Rayleigh waves are studied in this work based on a one-dimensional acoustic superlattice lithium niobate substrate coated with a waveguide layer. The present phononic structure is formed by the periodic domain-inverted single crystal that is the Z-cut lithium niobate substrate with a waveguide layer on the upper surface. The plane wave expansion method (PWE is adopted to determine the band gap behavior of the phononic structure and validated by the finite element method (FEM. The FEM is also used to investigate the transmission of Rayleigh waves in the phononic structure with the interdigital transducers by means of the commercial package COMSOL. The results show that, although there is a homogeneous waveguide layer on the surface, the band gap of Rayleigh waves still exist. It is also found that increasing the thickness of the waveguide layer, the band width narrows and the band structure shifts to lower frequency. The present approach can be taken as an efficient tool in designing of phononic structures with waveguide layer.

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

International Nuclear Information System (INIS)

Fong, C Y; Qian, M C

2004-01-01

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

Accelerated rogue waves generated by soliton fusion at the advanced stage of supercontinuum formation in photonic-crystal fibers.

Science.gov (United States)

Driben, Rodislav; Babushkin, Ihar

2012-12-15

Soliton fusion is a fascinating and delicate phenomenon that manifests itself in optical fibers in case of interaction between copropagating solitons with small temporal and wavelength separation. We show that the mechanism of acceleration of a trailing soliton by dispersive waves radiated from the preceding one provides necessary conditions for soliton fusion at the advanced stage of supercontinuum generation in photonic-crystal fibers. As a result of fusion, large-intensity robust light structures arise and propagate over significant distances. In the presence of small random noise the delicate condition for the effective fusion between solitons can easily be broken, making the fusion-induced giant waves a rare statistical event. Thus oblong-shaped giant accelerated waves become excellent candidates for optical rogue waves.

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

International Nuclear Information System (INIS)

Casagrande, D.; Ferraz, A.C.

1996-01-01

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

Electronic structure of a graphene superlattice with massive Dirac fermions

International Nuclear Information System (INIS)

Lima, Jonas R. F.

2015-01-01

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

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

KAUST Repository

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

2017-01-01

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

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

KAUST Repository

Cossu, Fabrizio

2017-08-01

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

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

Science.gov (United States)

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

2010-08-16

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

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...

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.

Effect of InSb/In0.9Al0.1Sb superlattice buffer layer on the structural and electronic properties of InSb films

Science.gov (United States)

Zhao, Xiaomeng; Zhang, Yang; Guan, Min; Cui, Lijie; Wang, Baoqiang; Zhu, Zhanping; Zeng, Yiping

2017-07-01

The effect of InSb/In0.9Al0.1Sb buffer layers on InSb thin films grown on GaAs (0 0 1) substrate by molecular beam epitaxy (MBE) is investigated. The crystal quality and the surface morphology of InSb are characterized by XRD and AFM. The carrier transport property is researched through variable temperature hall test. The sharp interface between InSb/In0.9Al0.1Sb is demonstrated important for the high quality InSb thin film. We try different superlattice buffer layers by changing ratios, 2-0.5, thickness, 300-450 nm, and periods, 20-50. According to the function of the dislocation density to the absolute temperature below 150 K with different periods of SL buffers, we can find that the number of periods of superlattice is a major factor to decrease the density of threading dislocations. With the 50 periods SL buffer layer, the electron mobility of InSb at the room temperature and liquid nitrogen cooling temperature is ∼63,000 and ∼4600 cm2/V s, respectively. We deduce that the interface in the SL structure works as a filter layer to prevent the dislocation propagating to the upper InSb thin films.

Analysis of photonic band gap in novel piezoelectric photonic crystal

Science.gov (United States)

Malar Kodi, A.; Doni Pon, V.; Joseph Wilson, K. S.

2018-03-01

The transmission properties of one-dimensional novel photonic crystal having silver-doped novel piezoelectric superlattice and air as the two constituent layers have been investigated by means of transfer matrix method. By changing the appropriate thickness of the layers and filling factor of nanocomposite system, the variation in the photonic band gap can be studied. It is found that the photonic band gap increases with the filling factor of the metal nanocomposite and with the thickness of the layer. These structures possess unique characteristics enabling one to operate as optical waveguides, selective filters, optical switches, integrated piezoelectric microactuators, etc.

One-way mode transmission in one-dimensional phononic crystal plates

Science.gov (United States)

Zhu, Xuefeng; Zou, Xinye; Liang, Bin; Cheng, Jianchun

2010-12-01

We investigate theoretically the band structures of one-dimensional phononic crystal (PC) plates with both antisymmetric and symmetric structures, and show how unidirectional transmission behavior can be obtained for either antisymmetric waves (A modes) or symmetric waves (S modes) by exploiting mode conversion and selection in the linear plate systems. The theoretical approach is illustrated for one PC plate example where unidirectional transmission behavior is obtained in certain frequency bands. Employing harmonic frequency analysis, we numerically demonstrate the one-way mode transmission for the PC plate with finite superlattice by calculating the steady-state displacement fields under A modes source (or S modes source) in forward and backward direction, respectively. The results show that the incident waves from A modes source (or S modes source) are transformed into S modes waves (or A modes waves) after passing through the superlattice in the forward direction and the Lamb wave rejections in the backward direction are striking with a power extinction ratio of more than 1000. The present structure can be easily extended to two-dimensional PC plate and efficiently encourage practical studies of experimental realization which is believed to have much significance for one-way Lamb wave mode transmission.

Transport and spin effects in homogeneous magnetic superlattice

International Nuclear Information System (INIS)

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

2000-09-01

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

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

International Nuclear Information System (INIS)

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

2006-01-01

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

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

International Nuclear Information System (INIS)

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

2007-01-01

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

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

KAUST Repository

Kaushik, Ananth P.; Clancy, Paulette

2012-01-01

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

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...

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

Energy Technology Data Exchange (ETDEWEB)

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

2016-06-15

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

Superlattice-like SnSb{sub 4}/Ga{sub 3}Sb{sub 7} thin films for ultrafast switching phase-change memory application

Energy Technology Data Exchange (ETDEWEB)

Hu, Yifeng [Tongji University, Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Functional Materials Research Laboratory, School of Materials Science and Engineering, Shanghai (China); Jiangsu University of Technology, School of Mathematics and Physics, Changzhou (China); He, Zifang; Zhai, Jiwei [Tongji University, Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Functional Materials Research Laboratory, School of Materials Science and Engineering, Shanghai (China); Wu, Pengzhi; Lai, Tianshu [Sun Yat-Sen University, State Key Laboratory of Optoelectronic Materials and Technology, Department of Physics, Guangzhou (China); Song, Sannian; Song, Zhitang [Chinese Academy of Sciences, State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-system and Information Technology, Shanghai (China)

2015-11-15

The carrier concentration of Sb-rich phase SnSb{sub 4}, Ga{sub 3}Sb{sub 7} and superlattice-like [SnSb{sub 4}(3.5 nm)/Ga{sub 3}Sb{sub 7}(4 nm)]{sub 7} (SLL-7) thin films as a function of annealing temperature was investigated to explain the reason of resistance change. The activation energy for crystallization was calculated with a Kissinger equation to estimate the thermal stability. In order to illuminate the transition mechanisms, the crystallization kinetics of SLL-7 were explored by using Johnson-Mehl-Avrami theory. The obtained values of Avrami indexes indicate that a one-dimensional growth-dominated mechanism is responsible for the set transition of SLL-7 thin film. X-ray diffractometer and Raman scattering spectra were recorded to investigate the change of crystalline structure. The measurement of atomic force microscopy indicated that SLL-7 thin film has a good smooth surface. A picosecond laser pump-probe system was used to test and verify phase-change speed of the SLL-7 thin film. (orig.)

Atomic structure of CaF2/MnF2-Si(1 1 1) superlattices from X-ray diffraction

International Nuclear Information System (INIS)

Alcock, Simon G.; Nicklin, C.L.; Howes, P.B.; Norris, C.A.; Kyutt, R.N.; Sokolov, N.S.; Yakovlev, N.L.

2007-01-01

X-ray reflectivity and non-specular crystal truncation rod scans have been used to determine the three-dimensional atomic structure of the buried CaF 2 -Si(1 1 1) interface and ultrathin films of MnF 2 and CaF 2 within a superlattice. We show that ultrathin films of MnF 2 , below a critical thickness of approximately four monolayers, are crystalline, pseudomorphic, and adopt the fluorite structure of CaF 2 . High temperature deposition of the CaF 2 buffer layer produces a fully reacted, CaF 2 -Si(1 1 1) type-B interface. The mature, 'long' interface is shown to consist of a partially occupied layer of CaF bonded to the Si substrate, followed by a distorted CaF layer. Our atomistic, semi-kinematical scattering method extends the slab reflectivity method by providing in-plane structural information

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

Science.gov (United States)

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

2017-12-01

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

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

Science.gov (United States)

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

2017-12-01

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

CAS CERN Accelerator School second advanced accelerator physics course

International Nuclear Information System (INIS)

Turner, S.

1989-01-01

The advanced course on general accelerator physics given in West Berlin closely followed that organised by the CERN Accelerator School at Oxford in September 1985 and whose proceedings were published as CERN Yellow Report 87-03 (1987). However, certain subjects were treated in a different way, improved or extended, while some new ones were introduced and it is all of these which are included in the present proceedings. The lectures include particle-photon interactions, high-brilliance lattices and single/multiple Touschek effect, while the seminars are on the major accelerators presently under construction or proposed for the near future, applications of synchrotron radiation, free-electron lasers, cosmic accelerators and crystal beams. Also included are errata, and addenda to some of the lectures, of CERN 87-03. (orig.)

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.

Dopant-free twinning superlattice formation in InSb and InP nanowires

Energy Technology Data Exchange (ETDEWEB)

Yuan, Xiaoming [School of Physics and Electronics, Hunan Key Laboratory for Supermicrostructure and Ultrafast Process, Central South University, Changsha, Hunan (China); Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT (Australia); Guo, Yanan; Caroff, Philippe; Tan, Hark Hoe; Jagadish, Chennupati [Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT (Australia); He, Jun [School of Physics and Electronics, Hunan Key Laboratory for Supermicrostructure and Ultrafast Process, Central South University, Changsha, Hunan (China)

2017-11-15

Periodic arrangement of twin planes creates a controllable polytype that can affect both the electronic and optical properties of nanowires. The approach that is most used for inducing twinning superlattice (TSL) formation in III-V nanowires is introducing impurity dopants during growth. Here, we demonstrate that controlling the growth parameters is sufficient to produce regular twinning planes in Au-catalysed InSb and InP nanowires. Our results show that TSL formation in InSb nanowires only exists in a very narrow growth window. We suggest that growth conditions induce a high concentration of In (or Sb) in the Au droplet, which plays a similar role to that of surfactant impurities such as Zn, and increases the droplet wetting angle to yield a geometry that is favorable for TSL formation. The demonstration of TSL structure in InSb and InP nanowires by controlling the input of In (or Sb) further enhances fundamental understanding of TSL formation in III-V nanowires and allows us to tune the properties of these nanowires by crystal phase engineering. (copyright 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

Dopant-free twinning superlattice formation in InSb and InP nanowires

International Nuclear Information System (INIS)

Yuan, Xiaoming; Guo, Yanan; Caroff, Philippe; Tan, Hark Hoe; Jagadish, Chennupati; He, Jun

2017-01-01

Periodic arrangement of twin planes creates a controllable polytype that can affect both the electronic and optical properties of nanowires. The approach that is most used for inducing twinning superlattice (TSL) formation in III-V nanowires is introducing impurity dopants during growth. Here, we demonstrate that controlling the growth parameters is sufficient to produce regular twinning planes in Au-catalysed InSb and InP nanowires. Our results show that TSL formation in InSb nanowires only exists in a very narrow growth window. We suggest that growth conditions induce a high concentration of In (or Sb) in the Au droplet, which plays a similar role to that of surfactant impurities such as Zn, and increases the droplet wetting angle to yield a geometry that is favorable for TSL formation. The demonstration of TSL structure in InSb and InP nanowires by controlling the input of In (or Sb) further enhances fundamental understanding of TSL formation in III-V nanowires and allows us to tune the properties of these nanowires by crystal phase engineering. (copyright 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

Tunneling time and Hartman effect in a ferromagnetic graphene superlattice

Directory of Open Access Journals (Sweden)

Farhad Sattari

2012-03-01

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

Progress in thin-film silicon solar cells based on photonic-crystal structures

Science.gov (United States)

Ishizaki, Kenji; De Zoysa, Menaka; Tanaka, Yoshinori; Jeon, Seung-Woo; Noda, Susumu

2018-06-01

We review the recent progress in thin-film silicon solar cells with photonic crystals, where absorption enhancement is achieved by using large-area resonant effects in photonic crystals. First, a definitive guideline for enhancing light absorption in a wide wavelength range (600–1100 nm) is introduced, showing that the formation of multiple band edges utilizing higher-order modes confined in the thickness direction and the introduction of photonic superlattice structures enable significant absorption enhancement, exceeding that observed for conventional random scatterers. Subsequently, experimental evidence of this enhancement is demonstrated for a variety of thin-film Si solar cells: ∼500-nm-thick ultrathin microcrystalline silicon cells, few-µm-thick microcrystalline silicon cells, and ∼20-µm-thick thin single-crystalline silicon cells. The high short-circuit current densities and/or efficiencies observed for each cell structure confirm the effectiveness of using multiple band-edge resonant modes of photonic crystals for enhancing broadband absorption in actual solar cells.

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

Energy Technology Data Exchange (ETDEWEB)

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

2016-01-04

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

Accelerating action of stresses on crystallization kinetics in silicon ion-implanted layers during pulsed heating

International Nuclear Information System (INIS)

Aleksandrov, L.N.

1985-01-01

Numerical simulation of the effect of stressed in ion-implanted layers on kinetics of amorphous phase transformations is performed. The suggested model of accounting stresses including concentration ones is based on the locality of action of interstitial addition atoms and on general structural inhomogeneity of amorphous semiconductor leading to the formation of areas of the facilitated phase transition. Accounting of effect of energy variation of silicon atoms interaction on probability of displacement events and atoms building in lattice points or atomic bonds disintegration allows one to trace the accelerating action of introduced by ion implantation stresses on the kinetics of layer crystallization during pulsed heating

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

NARCIS (Netherlands)

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

2016-01-01

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

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

NARCIS (Netherlands)

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

2016-01-01

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

Control of Multiple Exciton Generation and Electron-Phonon Coupling by Interior Nanospace in Hyperstructured Quantum Dot Superlattice.

Science.gov (United States)

Chang, I-Ya; Kim, DaeGwi; Hyeon-Deuk, Kim

2017-09-20

The possibility of precisely manipulating interior nanospace, which can be adjusted by ligand-attaching down to the subnanometer regime, in a hyperstructured quantum dot (QD) superlattice (QDSL) induces a new kind of collective resonant coupling among QDs and opens up new opportunities for developing advanced optoelectric and photovoltaic devices. Here, we report the first real-time dynamics simulations of the multiple exciton generation (MEG) in one-, two-, and three-dimensional (1D, 2D, and 3D) hyperstructured H-passivated Si QDSLs, accounting for thermally fluctuating band energies and phonon dynamics obtained by finite-temperature ab initio molecular dynamics simulations. We computationally demonstrated that the MEG was significantly accelerated, especially in the 3D QDSL compared to the 1D and 2D QDSLs. The MEG acceleration in the 3D QDSL was almost 1.9 times the isolated QD case. The dimension-dependent MEG acceleration was attributed not only to the static density of states but also to the dynamical electron-phonon couplings depending on the dimensionality of the hyperstructured QDSL, which is effectively controlled by the interior nanospace. Such dimension-dependent modifications originated from the short-range quantum resonance among component QDs and were intrinsic to the hyperstructured QDSL. We propose that photoexcited dynamics including the MEG process can be effectively controlled by only manipulating the interior nanospace of the hyperstructured QDSL without changing component QD size, shape, compositions, ligand, etc.

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

Science.gov (United States)

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

2018-05-01

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

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

International Nuclear Information System (INIS)

Fahmi, A.; Qachaou, A.

2009-01-01

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

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

Science.gov (United States)

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

2018-05-01

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

Anisotropic critical fields in superconducting superlattices

International Nuclear Information System (INIS)

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

1983-01-01

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

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

Energy Technology Data Exchange (ETDEWEB)

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

2015-03-20

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

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

Science.gov (United States)

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

2006-02-08

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

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

KAUST Repository

Tian, Y. F.

2014-04-14

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

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

KAUST Repository

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

2014-01-01

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

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

Science.gov (United States)

2016-01-05

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

Soliton excitation in superlattice

International Nuclear Information System (INIS)

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

1995-10-01

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

Multi-level storage and ultra-high speed of superlattice-like Ge50Te50/Ge8Sb92 thin film for phase-change memory application.

Science.gov (United States)

Wu, Weihua; Chen, Shiyu; Zhai, Jiwei; Liu, Xinyi; Lai, Tianshu; Song, Sannian; Song, Zhitang

2017-10-06

Superlattice-like Ge 50 Te 50 /Ge 8 Sb 92 (SLL GT/GS) thin film was systematically investigated for multi-level storage and ultra-fast switching phase-change memory application. In situ resistance measurement indicates that SLL GT/GS thin film exhibits two distinct resistance steps with elevated temperature. The thermal stability of the amorphous state and intermediate state were evaluated with the Kissinger and Arrhenius plots. The phase-structure evolution revealed that the amorphous SLL GT/GS thin film crystallized into rhombohedral Sb phase first, then the rhombohedral GeTe phase. The microstructure, layered structure, and interface stability of SLL GT/GS thin film was confirmed by using transmission electron microscopy. The transition speed of crystallization and amorphization was measured by the picosecond laser pump-probe system. The volume variation during the crystallization was obtained from x-ray reflectivity. Phase-change memory (PCM) cells based on SLL GT/GS thin film were fabricated to verify the multi-level switching under an electrical pulse as short as 30 ns. These results illustrate that the SLL GT/GS thin film has great potentiality in high-density and high-speed PCM applications.

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

Energy Technology Data Exchange (ETDEWEB)

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

2016-03-01

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

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.

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.

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

Energy Technology Data Exchange (ETDEWEB)

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

2016-05-15

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

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

NARCIS (Netherlands)

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

2001-01-01

We report on the preparation of CuBa2(SrxCa1¿x)nCun¿1Oy compounds by fabrication of (Ba,Sr,Ca)CuO2 superlattices with pulsed laser deposition (PLD). A technique called interval deposition is used to suppress multi-level or island growth resulting in high-quality superlattice structures. Both, the

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

Science.gov (United States)

Fertig, H A; Brey, Luis

2010-12-13

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

Optical properties of graphene superlattices.

Science.gov (United States)

Le, H Anh; Ho, S Ta; Nguyen, D Chien; Do, V Nam

2014-10-08

In this work, the optical responses of graphene superlattices, i.e. graphene subjected to a periodic scalar potential, are theoretically reported. The optical properties were studied by investigating the optical conductivity, which was calculated using the Kubo formalism. It was found that the optical conductivity becomes dependent on the photon polarization and is suppressed in the photon energy range of (0, Ub), where Ub is the potential barrier height. In the higher photon energy range, i.e. Ω > Ub, the optical conductivity is, however, almost identical to that of pristine graphene. Such behaviors of the optical conductivity are explained microscopically through the analysis of the elements of optical matrices and effectively through a simple model, which is based on the Pauli blocking mechanism.

Competing interactions in ferromagnetic/antiferromagnetic perovskite superlattices

Energy Technology Data Exchange (ETDEWEB)

Takamura, Y.; Biegalski, M.B.; Christen, H.M.

2009-10-22

Soft x-ray magnetic dichroism, magnetization, and magnetotransport measurements demonstrate that the competition between different magnetic interactions (exchange coupling, electronic reconstruction, and long-range interactions) in La{sub 0.7}Sr{sub 0.3}FeO{sub 3}(LSFO)/La{sub 0.7}Sr{sub 0.3}MnO{sub 3}(LSMO) perovskite oxide superlattices leads to unexpected functional properties. The antiferromagnetic order parameter in LSFO and ferromagnetic order parameter in LSMO show a dissimilar dependence on sublayer thickness and temperature, illustrating the high degree of tunability in these artificially layered materials.

Advanced applications of ion channeling for the study of imperfections in crystals

Energy Technology Data Exchange (ETDEWEB)

Swanson, M L [North Carolina Univ., Chapel Hill, NC (United States)

1997-03-01

A review will be given of the applications of medium energy ion channeling for the studies of imperfections in the near-surface regions of crystals. The following topics will be discussed: (1.) epitaxial layers, including elemental depositions of a few monolayers, strained-layer superlattices, and compound layers; (2.) lattice defects, including ion damage in diamond, dislocation networks in Si, and anomalous lattice vibrations in high temperature superconductors; (3.) lattice sites of solute atoms, including substitutional sites in compounds (LiNbO{sub 3} and GaP), and interstitial sites produced by association with point defects. (author)

Touching points in the energy band structure of bilayer graphene superlattices

International Nuclear Information System (INIS)

Pham, C Huy; Nguyen, V Lien

2014-01-01

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

Correlating interfacial octahedral rotations with magnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices.

Science.gov (United States)

Zhai, Xiaofang; Cheng, Long; Liu, Yang; Schlepütz, Christian M; Dong, Shuai; Li, Hui; Zhang, Xiaoqiang; Chu, Shengqi; Zheng, Lirong; Zhang, Jing; Zhao, Aidi; Hong, Hawoong; Bhattacharya, Anand; Eckstein, James N; Zeng, Changgan

2014-07-09

Lattice distortion due to oxygen octahedral rotations have a significant role in mediating the magnetism in oxides, and recently attracts a lot of interests in the study of complex oxides interface. However, the direct experimental evidence for the interrelation between octahedral rotation and magnetism at interface is scarce. Here we demonstrate that interfacial octahedral rotation are closely linked to the strongly modified ferromagnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices. The maximized ferromagnetic moment in the N=6 superlattice is accompanied by a metastable structure (space group Imcm) featuring minimal octahedral rotations (a(-)a(-)c(-), α~4.2°, γ~0.5°). Quenched ferromagnetism for Nmagnetism. Our study demonstrates that engineering superlattices with controllable interfacial structures can be a feasible new route in realizing functional magnetic materials.

Bottom-up meets top-down: tailored raspberry-like Fe3O4-Pt nanocrystal superlattices.

Science.gov (United States)

Qiu, Fen; Vervuurt, René H J; Verheijen, Marcel A; Zaia, Edmond W; Creel, Erin B; Kim, Youngsang; Urban, Jeffrey J; Bol, Ageeth A

2018-03-29

Supported catalysts are widely used in industry and can be optimized by tuning the composition, chemical structure, and interface of the nanoparticle catalyst and oxide support. Here we firstly combine a bottom up colloidal synthesis method with a top down atomic layer deposition (ALD) process to achieve a raspberry-like Pt-decorated Fe3O4 (Fe3O4-Pt) nanoparticle superlattices. This nanocomposite ensures the precision of the catalyst/support interface, improving the catalytic efficiency of the Fe3O4-Pt nanocomposite system. The morphology of the hybrid nanocomposites resulting from different cycles of ALD was monitored by scanning transmission electron microscopy, giving insight into the nucleation and growth mechanism of the ALD process. X-ray photoelectron spectroscopy studies confirm the anticipated electron transfer from Fe3O4 to Pt through the nanocomposite interface. Photocurrent measurement further suggests that Fe3O4 superlattices with controlled decoration of Pt have substantial promise for energy-efficient photoelectrocatalytic oxygen evolution reaction. This work opens a new avenue for designing supported catalyst architectures via precisely controlled decoration of single component superlattices with noble metals.

Anisotropic magnetoresistance and spin polarization of La0.7Sr0.3MnO3/SrTiO3 superlattices

International Nuclear Information System (INIS)

Wang, L.M.; Guo, C.-C.

2005-01-01

The crystalline structure, anisotropic magnetoresistance (AMR), and magnetization of La 0.7 Sr 0.3 MnO 3 /SrTiO 3 (LSMO/STO) superlattices grown by a rf sputtering system are systematically analyzed to study the spin polarization of manganite at interfaces. The presence of positive low-temperature AMR in LSMO/STO superlattices implies that two bands of majority and minority character contribute to the transport properties, leading to a reduced spin polarization. Furthermore, the magnetization of superlattices follows the T 3/2 law and decays more quickly as the thickness ratio d STO /d LSMO increases, corresponding to a reduced exchange coupling. The results clearly show that the spin polarization is strongly correlated with the influence of interface-induced strain on the structure

The Luttinger liquid in superlattice structures: atomic gases, quantum dots and the classical Ising chain

International Nuclear Information System (INIS)

Bhattacherjee, Aranya B; Jha, Pradip; Kumar, Tarun; Mohan, Man

2011-01-01

We study the physical properties of a Luttinger liquid in a superlattice that is characterized by alternating two tunneling parameters. Using the bosonization approach, we describe the corresponding Hubbard model by the equivalent Tomonaga-Luttinger model. We analyze the spin-charge separation and transport properties of the superlattice system. We suggest that cold Fermi gases trapped in a bichromatic optical lattice and coupled quantum dots offer the opportunity to measure these effects in a convenient manner. We also study the classical Ising chain with two tunneling parameters. We find that the classical two-point correlator decreases as the difference between the two tunneling parameters increases.

Theoretical modelling of electron transport in InAs/GaAs quantum dot superlattices

International Nuclear Information System (INIS)

Vukmirovic, Nenad; Ikonic, Zoran; Savic, Ivana; Indjin, Dragan; Harrison, Paul

2006-01-01

A theoretical model describing the electron transport in InAs/GaAs quantum dot infrared photodetectors, modelled as ideal quantum dot superlattices, is presented. The carrier wave functions and energy levels were evaluated using the strain dependent 8-band k.p Hamiltonian and used to calculate all intra- and inter-period transition rates due to interaction with phonons and electromagnetic radiation. The interaction with longitudinal acoustic phonons and electromagnetic radiation was treated perturbatively within the framework of Fermi's golden rule, while the interaction with longitudinal optical phonons was considered taking into account their strong coupling to electrons. The populations of energy levels were then found from a system of rate equations, and the electron current in the superlattice was subsequently extracted. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Types of defect ordering in undoped and lanthanum-doped Bi2201 single crystals

International Nuclear Information System (INIS)

Martovitsky, V. P.

2006-01-01

Undoped and lanthanum-doped Bi2201 single crystals having a perfect average structure have been comparatively studied by x-ray diffraction. The undoped Bi2201 single crystals exhibit very narrow satellite reflections; their half-width is five to six times smaller than that of Bi2212 single crystals grown by the same technique. This narrowness indicates three-dimensional defect ordering in the former crystals. The lanthanumdoped Bi2201 single crystals with x = 0.7 and T c = 8-10 K exhibit very broad satellite reflections consisting of two systems (modulations) misoriented with respect to each other. The modulation-vector components of these two modulations are found to be q 1 = 0.237b* + 0.277c* and q 2 = 0.238b* + 0.037c*. The single crystals having a perfect average structure and a homogeneous average distribution of doping lanthanum consist of 70-to 80-A-thick layers that alternate along the c axis and have two different types of modulated superlattice. The crystals having a less perfect average structure also consist of alternating layers, but they have different lanthanum concentrations. The low value of T c in the undoped Bi2201 single crystals (9.5 K) correlates with three-dimensional defect ordering in them, and an increase in T c to 33 K upon lanthanum doping can be related to a thin-layer structure of these crystals and to partial substitution of lanthanum for the bismuth positions

Neutron and x-ray scattering study of phonon dispersion and diffuse scattering in (Na ,Bi ) Ti O3-x BaTi O3 single crystals near the morphotropic phase boundary

Science.gov (United States)

Luo, Chengtao; Bansal, Dipanshu; Li, Jiefang; Viehland, Dwight; Winn, Barry; Ren, Yang; Li, Xiaobing; Luo, Haosu; Delaire, Olivier

2017-11-01

Neutron and x-ray scattering measurements were performed on (N a1 /2B i1 /2 ) Ti O3-x at %BaTi O3 (NBT-x BT ) single crystals (x =4 , 5, 6.5, and 7.5) across the morphotropic phase boundary (MPB), as a function of both composition and temperature, and probing both structural and dynamical aspects. In addition to the known diffuse scattering pattern near the Γ points, our measurements revealed new, faint superlattice peaks, as well as an extensive diffuse scattering network, revealing a short-range ordering of polar nanoregions (PNR) with a static stacking morphology. In samples with compositions closest to the MPB, our inelastic neutron scattering investigations of the phonon dynamics showed two unusual features in the acoustic phonon branches, between the superlattice points, and between the superlattice points and Γ points, respectively. These critical elements are not present in the other compositions away from the MPB, which suggests that these features may be related to the tilt modes coupling behavior near the MPB.

Non-linear spin transport in magnetic semiconductor superlattices

International Nuclear Information System (INIS)

Bejar, Manuel; Sanchez, David; Platero, Gloria; MacDonald, A.H.

2004-01-01

The electronic spin dynamics in DC-biased n-doped II-VI semiconductor multiquantum wells doped with magnetic impurities is presented. Under certain range of electronic doping, conventional semiconductor superlattices present self-sustained oscillations. Magnetically doped wells (Mn) present large spin splittings due to the exchange interaction. The interplay between non-linear interwell transport, the electron-electron interaction and the exchange between electrons and the magnetic impurities produces interesting time-dependent features in the spin polarization current tuned by an external magnetic field

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

International Nuclear Information System (INIS)

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

2003-01-01

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

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

International Nuclear Information System (INIS)

Saber, A.; Ez-Zahraouy, H.

2000-09-01

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

A proposed GaAs-based superlattice solar cell structure with high efficiency and high radiation tolerance

Science.gov (United States)

Goradia, Chandra; Clark, Ralph; Brinker, David

1985-01-01

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

Microwave properties of YBa2Cu3O7-δ/PrBa2Cu3O7-δ superlattices

International Nuclear Information System (INIS)

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

1992-01-01

We have used non-resonant microwave absorption to study c-axis YBa 2 Cu 3 O 7-δ /PrBa 2 Cu 3 O 7-δ superlattices and compare the response to a film of similarly grown YBa 2 Cu 3 O 7-δ (YBCO). Near the respective transition temperatures, the response of the superlattice samples and the YBCO film have similar amplitudes and orientation dependences. This is consistent with the microwave loss being related to magnetic flux penetration at (110) slip planes. At lower temperatures, the response of the superlattices is much stronger than that of the YBCO film and, while both responses are hysteretic at low temperatures, the widths of the hysteresis have opposite orientation dependences, which we attribute to the role of the PrBa 2 Cu 3 O 7-δ layers. (orig.)

Phonon dispersion relations in monoatomic superlattices: a transfer matrix theory

International Nuclear Information System (INIS)

Albuquerque, E.L. de; Fulco, P.

1986-01-01

We present a lattice dynamical theory for monoatomic superlattices consisting of alternating layers of two different materials. Using a transfer matrix method we obtain explicit the equation for dispersion of the phonon's bulk modes, including the well known result in the long wave-length limit which can be obtained by elasticity theory. An illustation is shown and its features discussed. (Author) [pt

Improved structural and electrical properties in native Sb2Te3/GexSb2Te3+x van der Waals superlattices due to intermixing mitigation

Directory of Open Access Journals (Sweden)

Stefano Cecchi

2017-02-01

Full Text Available Superlattices made of Sb2Te3/GeTe phase change materials have demonstrated outstanding performance with respect to GeSbTe alloys in memory applications. Recently, epitaxial Sb2Te3/GeTe superlattices were found to feature GexSb2Te3+x blocks as a result of intermixing between constituting layers. Here we present the epitaxy and characterization of Sb2Te3/GexSb2Te3+x van der Waals superlattices, where GexSb2Te3+x was intentionally fabricated. X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, and lateral electrical transport data are reported. The intrinsic 2D nature of both sublayers is found to mitigate the intermixing in the structures, significantly improving the interface sharpness and ultimately the superlattice structural and electrical properties.

Optical properties of graphene superlattices

International Nuclear Information System (INIS)

Le, H Anh; Do, V Nam; Ho, S Ta; Nguyen, D Chien

2014-01-01

In this work, the optical responses of graphene superlattices, i.e. graphene subjected to a periodic scalar potential, are theoretically reported. The optical properties were studied by investigating the optical conductivity, which was calculated using the Kubo formalism. It was found that the optical conductivity becomes dependent on the photon polarization and is suppressed in the photon energy range of (0, U b ), where U b is the potential barrier height. In the higher photon energy range, i.e. Ω > U b , the optical conductivity is, however, almost identical to that of pristine graphene. Such behaviors of the optical conductivity are explained microscopically through the analysis of the elements of optical matrices and effectively through a simple model, which is based on the Pauli blocking mechanism. (paper)

Fine structure of the exciton electroabsorption in semiconductor superlattices

Energy Technology Data Exchange (ETDEWEB)

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

2017-02-15

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

Accelerated Physical Stability Testing of Amorphous Dispersions.

Science.gov (United States)

Mehta, Mehak; Suryanarayanan, Raj

2016-08-01

The goal was to develop an accelerated physical stability testing method of amorphous dispersions. Water sorption is known to cause plasticization and may accelerate drug crystallization. In an earlier investigation, it was observed that both the increase in mobility and decrease in stability in amorphous dispersions was explained by the "plasticization" effect of water (Mehta et al. Mol. Pharmaceutics 2016, 13 (4), 1339-1346). In this work, the influence of water concentration (up to 1.8% w/w) on the correlation between mobility and crystallization in felodipine dispersions was investigated. With an increase in water content, the α-relaxation time as well as the time for 1% w/w felodipine crystallization decreased. The relaxation times of the systems, obtained with different water concentration, overlapped when the temperature was scaled (Tg/T). The temperature dependencies of the α-relaxation time as well as the crystallization time were unaffected by the water concentration. Thus, the value of the coupling coefficient, up to a water concentration of 1.8% w/w, was approximately constant. Based on these findings, the use of "water sorption" is proposed to build predictive models for crystallization in slow crystallizing dispersions.

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.

Transport properties of YBa2Cu3O7/PrBa2Cu3O7-superlattices

International Nuclear Information System (INIS)

El Tahan, Ayman Mohamed Moussa

2010-01-01

The understanding of the coupling between superconducting YBa 2 Cu 3 O 7 (YBCO) layers decoupled by non superconducting PrBa 2 Cu 3 O 7 (PBCO) layers in c-axis oriented superlattices was the aim of this thesis. For this purpose two conceptually different kind of transport experiments have been performed. In the first type of transport experiments the current is flowing parallel to the layers. Here the coupling is probed indirectly using magnetic vortex lines, which are penetrating the superlattice. Movement of the vortex segments in neighbouring YBCO layers is more or less coherent depending on the thickness of both the superconducting and non superconducting layers. This in-plane transport was measured either by sending an external current through bridges patterned in the superlattice or by an induced internal current. The vortex-creep activation energy U was determined by analysis of the in-plane resistive transition in an external magnetic field B oriented along the c-axis. The activation energies for two series of superlattices were investigated. In one series the thickness of the YBCO layers was constant (n Y =4 unit cells) and the number of the PBCO unit cells was varied, while in the other the number of PBCO layers was constant (n P =4) and n Y varied. The correlation length of the vortex system was determined to be 80 nm along the c-axis direction. It was found that even a single PBCO unit cell in a superlattice effectively cuts the flux lines into shorter weakly coupled segments, and the coupling of the vortex systems in neighbouring layers is negligible already for a thickness of four unit cells of the PBCO layers. A characteristic variation of the activation energy for the two series of superlattices was found, where U 0 is proportional to the YBCO thickness. A change in the variation of U 0 with the current I in the specimen was observed. The analysis of standard dc magnetization relaxation data obtained for a series superlattices revealed the

Matter-Wave Solitons In Optical Superlattices

International Nuclear Information System (INIS)

Louis, Pearl J. Y.; Ostrovskaya, Elena A.; Kivshar, Yuri S.

2006-01-01

In this work we show that the properties of both bright and dark Bose-Einstein condensate (BEC) solitons trapped in optical superlattices can be controlled by changing the shape of the trapping potential whilst maintaining a constant periodicity and lattice height. Using this method we can control the properties of bright gap solitons by dispersion management. We can also control the interactions between dark lattice solitons. In addition we demonstrate a method for controlled generation of matter-wave gap solitons in stationary optical lattices by interfering two condensate wavepackets, producing a single wavepacket at a gap edge with properties similar to a gap soliton. As this wavepacket evolves, it forms a bright gap soliton

Phase transitions of a spin-one Ising ferromagnetic superlattice

International Nuclear Information System (INIS)

Saber, A.

2001-09-01

Using the effective field theory with a probability distribution technique, the magnetic properties in an infinite superlattice consisting of two different ferromagnets are studied in a spin-one Ising model. The dependence of the Curie temperatures are calculated as a function of two slabs in one period and as a function of the intra- and interlayer exchange interactions. A critical value of the exchange reduced interaction above which the interface magnetism appears is found. (author)

Theoretical modelling of electron transport in InAs/GaAs quantum dot superlattices

Energy Technology Data Exchange (ETDEWEB)

Vukmirovic, Nenad; Ikonic, Zoran; Savic, Ivana; Indjin, Dragan; Harrison, Paul [School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT (United Kingdom)

2006-07-01

A theoretical model describing the electron transport in InAs/GaAs quantum dot infrared photodetectors, modelled as ideal quantum dot superlattices, is presented. The carrier wave functions and energy levels were evaluated using the strain dependent 8-band k.p Hamiltonian and used to calculate all intra- and inter-period transition rates due to interaction with phonons and electromagnetic radiation. The interaction with longitudinal acoustic phonons and electromagnetic radiation was treated perturbatively within the framework of Fermi's golden rule, while the interaction with longitudinal optical phonons was considered taking into account their strong coupling to electrons. The populations of energy levels were then found from a system of rate equations, and the electron current in the superlattice was subsequently extracted. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Photoinduced Domain Pattern Transformation in Ferroelectric-Dielectric Superlattices

Energy Technology Data Exchange (ETDEWEB)

Ahn, Youngjun; Park, Joonkyu; Pateras, Anastasios; Rich, Matthew B.; Zhang, Qingteng; Chen, Pice; Yusuf, Mohammed H.; Wen, Haidan; Dawber, Matthew; Evans, Paul G.

2017-07-01

The nanodomain pattern in ferroelectric/dielectric superlattices transforms to a uniform polarization state under above-bandgap optical excitation. X-ray scattering reveals a disappearance of domain diffuse scattering and an expansion of the lattice. The reappearance of the domain pattern occurs over a period of seconds at room temperature, suggesting a transformation mechanism in which charge carriers in long-lived trap states screen the depolarization field. A Landau-Ginzburg-Devonshire model predicts changes in lattice parameter and a critical carrier concentration for the transformation.

Maximum Entropy Closure of Balance Equations for Miniband Semiconductor Superlattices

Directory of Open Access Journals (Sweden)

Luis L. Bonilla

2016-07-01

Full Text Available Charge transport in nanosized electronic systems is described by semiclassical or quantum kinetic equations that are often costly to solve numerically and difficult to reduce systematically to macroscopic balance equations for densities, currents, temperatures and other moments of macroscopic variables. The maximum entropy principle can be used to close the system of equations for the moments but its accuracy or range of validity are not always clear. In this paper, we compare numerical solutions of balance equations for nonlinear electron transport in semiconductor superlattices. The equations have been obtained from Boltzmann–Poisson kinetic equations very far from equilibrium for strong fields, either by the maximum entropy principle or by a systematic Chapman–Enskog perturbation procedure. Both approaches produce the same current-voltage characteristic curve for uniform fields. When the superlattices are DC voltage biased in a region where there are stable time periodic solutions corresponding to recycling and motion of electric field pulses, the differences between the numerical solutions produced by numerically solving both types of balance equations are smaller than the expansion parameter used in the perturbation procedure. These results and possible new research venues are discussed.

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

Science.gov (United States)

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

2015-11-16

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

Raman analysis of phonon modes in a short period AlN/GaN superlattice

Science.gov (United States)

Sarkar, Ketaki; Datta, Debopam; Gosztola, David J.; Shi, Fengyuan; Nicholls, Alan; Stroscio, Michael A.; Dutta, Mitra

2018-03-01

AlN/GaN-based optoelectronic devices have been the subject of intense research underlying the commercialization of efficient devices. Areas of considerable interest are the study of their lattice dynamics, phonon transport, and electron-phonon interactions specific to the interface of these heterostructures which results in additional optical phonon modes known as interface phonon modes. In this study, the framework of the dielectric continuum model (DCM) has been used to compare and analyze the optical phonon modes obtained from experimental Raman scattering measurements on AlN/GaN short-period superlattices. We have observed the localized E2(high), A1(LO) and the E1(TO) modes in superlattice measurements at frequencies shifted from their bulk values. To the best of our knowledge, the nanostructures used in these studies are among the smallest yielding useful Raman signatures for the interface modes. In addition, we have also identified an additional spread of interface phonon modes in the TO range resulting from the superlattice periodicity. The Raman signature contribution from the underlying AlxGa1-xN ternary has also been observed and analyzed. A temperature calibration was done based on Stokes/anti-Stokes ratio of A1(LO) using Raman spectroscopy in a broad operating temperature range. Good agreement between the experimental results and theoretically calculated calibration plot predicted using Bose-Einstein statistics was obtained.

The electronic band parameters calculated by the Kronig-Penney method for Cd1-xZnxS quantum dot superlattices

International Nuclear Information System (INIS)

Sakly, A.; Safta, N.; Mejri, H.; Lamine, A. Ben

2009-01-01

This work reports on a theoretical study of superlattices based on Cd 1-x Zn x S quantum dots embedded in an insulating material. We show, in particular, how this system can be assumed to a series of flattened cylindrical quantum dots with a finite barrier height at the boundary. In this paper, are also reviewed the approximations needed to calculate the band edges of the Cd 1-x Zn x S superlattices with use of the Kronig-Penney model. The electronic states and the electron effective masses of both Γ 1 - and Γ 2 -minibands have been computed as a function of zinc composition for different inter-quantum dot separations. As is found, the CdS system is appropriate to give rise a superlattice behavior for conduction electrons in a relatively large range of inter-sheet separations. An attempt to explain the electron band parameters calculated will be presented.

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

International Nuclear Information System (INIS)

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

2013-01-01

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

Theoretical Study of the Transverse Dielectric Constant of Superlattices and Their Alloys. Ph.D Thesis

Science.gov (United States)

Kahen, K. B.

1986-01-01

The optical properties of III to V binary and ternary compounds and GaAs-Al(x)Ga(1-x)As superlattices are determined by calculating the real and imaginary parts of the transverse dielectric constant. Emphasis is given to determining the influence of different material and superlattice parameters on the values of the index of refraction and absorption coefficient. In order to calculate the optical properties of a material, it is necessary to compute its electronic band structure. This was accomplished by introducing a partition band structure approach based on a combination of the vector k x vector p and nonlocal pseudopotential techniques. The advantages of this approach are that it is accurate, computationally fast, analytical, and flexible. These last two properties enable incorporation of additional effects into the model, such as disorder scattering, which occurs for alloy materials and excitons. Furthermore, the model is easily extended to more complex structures, for example multiple quantum wells and superlattices. The results for the transverse dielectric constant and absorption coefficient of bulk III to V compounds compare well with other one-electron band structure models and the calculations show that for small frequencies, the index of refraction is determined mainly by the contibution of the outer regions of the Brillouin zone.

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

Science.gov (United States)

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

2018-04-01

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

The new single crystal diffractometer SC3

International Nuclear Information System (INIS)

Schefer, J.; Koch, M.; Keller, P.; Fischer, S.; Thut, R.

1996-01-01

Single crystal diffraction is a powerful method for the determination of precise structure parameters, superlattices, stress. Neutron single crystal diffraction gives additionally to X-rays information on magnetic structures, both commensurate and incommensurate, hydrogen positions, hydrogen bonding behavior and accurate bondlengths, e.g. important in cuprates. The method is therefore especially powerful if combined with X-ray diffraction results. The new instrument at SINQ has been designed for inorganic materials and is positioned at a thermal beam tube, pointing on a water scatterer. This scatterer is presently operating with H 2 O at ambient temperature, but a change to another medium at different temperature is possible. The instrument will be equipped with three area detectors, moving at fixed difference in 2Θ. each detector may be individually moved around a vertical circle (tilting angle γ), allowing to use not only 4-circle geometry in the temperature range from 1.5 to 380 K, but also any equipment from a dilution refrigerator (7 mK) to a heavy magnet. A high temperature furnace for 4-circle geometry is foreseen as a future option. (author) 6 figs., 1 tab., 7 refs

The new single crystal diffractometer SC3

Energy Technology Data Exchange (ETDEWEB)

Schefer, J; Koch, M; Keller, P; Fischer, S; Thut, R [Lab. for Neutron Scattering ETH Zurich, Zurich (Switzerland) and Paul Scherrer Institute, Villigen (Switzerland)

1996-11-01

Single crystal diffraction is a powerful method for the determination of precise structure parameters, superlattices, stress. Neutron single crystal diffraction gives additionally to X-rays information on magnetic structures, both commensurate and incommensurate, hydrogen positions, hydrogen bonding behavior and accurate bondlengths, e.g. important in cuprates. The method is therefore especially powerful if combined with X-ray diffraction results. The new instrument at SINQ has been designed for inorganic materials and is positioned at a thermal beam tube, pointing on a water scatterer. This scatterer is presently operating with H{sub 2}O at ambient temperature, but a change to another medium at different temperature is possible. The instrument will be equipped with three area detectors, moving at fixed difference in 2{Theta}. each detector may be individually moved around a vertical circle (tilting angle {gamma}), allowing to use not only 4-circle geometry in the temperature range from 1.5 to 380 K, but also any equipment from a dilution refrigerator (7 mK) to a heavy magnet. A high temperature furnace for 4-circle geometry is foreseen as a future option. (author) 6 figs., 1 tab., 7 refs.

Total spectrum of photon emission by an ultra-relativistic positron channelling in a periodically bent crystal

International Nuclear Information System (INIS)

Krause, W.; Korol, A.V.; Department of Physics, St Petersburg State Maritime Technical University, Leninskii prospect 101, St Petersburg 198262; Solov'yov, A.V.; AF Ioffe Physical-Technical Institute of the Academy of Sciences of Russia, Polytechnicheskaya 26, St Petersburg 194021; Greiner, W.

2000-01-01

We present the results of numerical calculations of the channelling and undulator radiation generated by ultra-relativistic positron channelling along a crystal plane, which is periodically bent. The bending might be due to either the propagation of a transverse acoustic wave through the crystal, or the static strain as it occurs in superlattices. The periodically bent crystal serves as an undulator. We investigate the dependence of the intensities of both the ordinary channelling and the undulator radiations on the parameters of the periodically bent channel with simultaneous account for the de-channelling effect of the positrons. We demonstrate that there is a range of parameters in which the undulator radiation dominates over the channelling one and the characteristic frequencies of both types of radiation are well separated. This result is important, because the undulator radiation can be used to create a tunable source of x-ray and γ-radiation. (author). Letter-to-the-editor

Optical and vibrational properties of (ZnO){sub k} In{sub 2}O{sub 3} natural superlattice nanostructures

Energy Technology Data Exchange (ETDEWEB)

Margueron, Samuel [Laboratoire Matériaux Optiques, Photonique et Systèmes, Université de Lorraine et CentraleSupélec, 57070 Metz (France); John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Maryland 02138 (United States); Pokorny, Jan; Skiadopoulou, Stella; Kamba, Stanislav [Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21 Prague 8 (Czech Republic); Liang, Xin [School of Materials Science and Engineering, Changzhou University, Changzhou, Jiangsu Province 213164 (China); Clarke, David R. [John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Maryland 02138 (United States)

2016-05-21

A thermodynamically stable series of superlattices, (ZnO){sub k}In{sub 2}O{sub 3}, form in the ZnO-In{sub 2}O{sub 3} binary oxide system for InO{sub 1.5} concentrations from about 13 up to about 33 mole percent (m/o). These natural superlattices, which consist of a periodic stacking of single, two-dimensional sheets of InO{sub 6} octahedra, are found to give rise to systematic changes in the optical and vibrational properties of the superlattices. Low-frequency Raman scattering provides the evidence for the activation of acoustic phonons due to the folding of Brillouin zone. New vibrational modes at 520 and 620 cm{sup −1}, not present in either ZnO or In{sub 2}O{sub 3}, become Raman active. These new modes are attributed to collective plasmon oscillations localized at the two-dimensional InO{sub 1.5} sheets. Infrared reflectivity experiments, and simulations taking into account a negative dielectric susceptibility due to electron carriers in ZnO and interface modes of the dielectric layer of InO{sub 2}, explain the occurrence of these new modes. We postulate that a localized electron gas forms at the ZnO/InO{sub 2} interface due to the electron band alignment and polarization effects. All our observations suggest that there are quantum contributions to the thermal and electrical conductivity in these natural superlattices.

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�

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

KAUST Repository

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

2015-01-01

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

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

KAUST Repository

Feng, Nan

2015-04-30

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

Summary of Working Group 7 on 'Exotic acceleration schemes'

International Nuclear Information System (INIS)

Tajima, T.

2001-01-01

Exotic concepts of advanced acceleration technologies have been explored by Group 7 under the leadership of T. Tajima and T. Smith (who could not attend) at the AAC. Explored concepts are: (1) proton (ion) acceleration by laser, (2) additional ion acceleration methods, (3) crystal x-rays and acceleration, (4) vacuum acceleration, (5) active medium acceleration, and (6) some advanced methods in laser wakefield. The first subject of laser photon acceleration was discussed jointly with Group 1 and in the end the participants came to an agreement on the mechanism of proton acceleration by laser irradiation

Full Polarization Analysis of Resonant Superlattice and Forbidden x-ray Reflections in Magnetite

International Nuclear Information System (INIS)

Wilkins, S.B.; Bland, S.R.; Detlefs, B.; Beale, T.A.W.; Mazzoli, C.; Joly, Y.; Hatton, P.D.; Lorenzo, J.E.; Brabers, V.A.M.

2009-01-01

Despite being one of the oldest known magnetic materials, and the classic mixed valence compound, thought to be charge ordered, the structure of magnetite below the Verwey transition is complex and the presence and role of charge order is still being debated. Here, we present resonant x-ray diffraction data at the iron K-edge on forbidden (0, 0, 2n+1) C and superlattice (0, 0, 2n+1/2)C reflections. Full linear polarization analysis of the incident and scattered light was conducted in order to explore the origins of the reflections. Through simulation of the resonant spectra we have confirmed that a degree of charge ordering takes place, while the anisotropic tensor of susceptibility scattering is responsible for the superlattice reflections below the Verwey transition. We also report the surprising result of the conversion of a significant proportion of the scattered light from linear to nonlinear polarization.

THz elastic dynamics in finite-size CoFeB-MgO phononic superlattices

Energy Technology Data Exchange (ETDEWEB)

Ulrichs, Henning, E-mail: hulrich@gwdg.de; Meyer, Dennis; Müller, Markus; Wittrock, Steffen; Mansurova, Maria [I. Physical Institute, Georg-August University of Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen (Germany); Walowski, Jakob; Münzenberg, Markus [Institute of Physics, Ernst-Moritz-Arndt University of Greifswald, Felix-Hausdorff-Str. 6, 17489 Greifswald (Germany)

2016-10-14

In this article, we present the observation of coherent elastic dynamics in a nano-scale phononic superlattice, which consists of only 4 bilayers. We demonstrate how ultra-short light pulses with a length of 40 fs can be utilized to excite a coherent elastic wave at 0.535 THz, which persist over about 20 ps. In later steps of the elastic dynamics, modes with frequency of 1.7 THz and above appear. All these modes are related to acoustic band gaps. Thus, the periodicity strongly manifests in the wave physics, although the system under investigation has only a small number of spatial periods. To further illustrate this, we show how by breaking the translational invariance of the superlattice, these features can be suppressed. Discussed in terms of phonon blocking and radiation, we elucidate in how far our structures can be considered as useful building blocks for phononic devices.

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)

Optical properties of metallic Fibonacci quasi-superlattice

International Nuclear Information System (INIS)

Feng Weiguo; Liu Nianhua; Wu Xiang

1990-06-01

Within the approximation of hydrodynamic model, the optical properties of the metallic Fibonacci quasi-superlattice have been studied for the region of s-polarized soft x-rays and extreme ultraviolet. By using the transfer-matrix method and taking account of damping effects, we have discussed the electromagnetic normal modes for the quasisuperlattice in the rational approximation. The related dispersion curves explain the reflection spectra well, and we found that similar to the reflectivities, both real part and imagine part of the dispersion relation pattern has a rich structure of self-similarity. With the increasing of the generation number, the electromagnetic modes all become critical. (author). 13 refs, 3 figs

Engineering the oxygen coordination in digital superlattices

Energy Technology Data Exchange (ETDEWEB)

Cook, Seyoung [Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA; Department of Materials Science, Northwestern University, Evanston, Illinois 60202, USA; Andersen, Tassie K. [Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA; Department of Materials Science, Northwestern University, Evanston, Illinois 60202, USA; Hong, Hawoong [X-Ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA; Rosenberg, Richard A. [X-Ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA; Marks, Laurence D. [Department of Materials Science, Northwestern University, Evanston, Illinois 60202, USA; Fong, Dillon D. [Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

2017-12-01

The oxygen sublattice in the complex oxides is typically composed of corner-shared polyhedra, with transition metals at their centers. The electronic and chemical properties of the oxide depend on the type and geometric arrangement of these polyhedra, which can be controlled through epitaxial synthesis. Here, we use oxide molecular beam epitaxy to create SrCoOx:SrTiO3 superlattices with tunable oxygen coordination environments and sublattice geometries. Using soft X-ray spectroscopy, we find that the chemical state of Co can be varied with the polyhedral arrangement, demonstrating a new strategy for achieving unique electronic properties in the transition metal oxides.

Novel Si-Ge-C Superlattices for More than Moore CMOS

Science.gov (United States)

2016-03-31

Keywords: Silicon ; Germanium; Carbon; Superlattices; Direct Band-Gaps; Silicon - Photonics ; Image Sensors. Introduction Materials with direct band-gaps and...Wang Z., B. Tian, M. Pantouvaki, et al., “Room- temperature InP distributed feedback laser array directly grown on silicon ”, Nature Photonics 9, 2015...Creek Blvd. Suite 284 San Jose, CA 95129 Contact author: Lynn.Forester@Quantumsemi.com Abstract: The search for Silicon -based direct band-gap

Crystal size control of sulfathiazole using high pressure carbon dioxide

Science.gov (United States)

Kitamura, M.; Yamamoto, M.; Yoshinaga, Y.; Masuoka, H.

1997-07-01

The effect of the pressurization method of carbon dioxide on the crystallization behavior and crystal size of sulphathiazole (SUT) was investigated. In the "stepwise pressurization" method exceptionally large pillar-like crystals of 2-6 mm were obtained as mainly a scaling on the wall of the crystallizer. In the "rapid pressurization" method, crystals with a size one third to half of that obtained in the stepwise method precipitated, indicating the accelerated nucleation rate by the rapid increase of the supersaturation degree with a vigorous bubbling. With the new method of "two-step pressurization", in the first step the nucleation is accelerated with a much larger pressure instantly created, and in the second step the growth rate is retarded with the lower pressure. By this method much more fine crystals (from a few tens to several hundred micrometers) were produced and the scaling was suppressed. In this method a large supersaturation degree at an interface between the gas (bubble) and liquid phase under a vigorous bubbling may play an important role in accelerating the nucleation. The average size of the crystals tended to become smaller with increase of the first pressure and the expansion ratio at a decompression point, and it tended to get larger with increase of the second pressure. These results show that the GAS method is very useful for the control of crystal size over a wide range.

Proton beam spatial distribution and Bragg peak imaging by photoluminescence of color centers in lithium fluoride crystals at the TOP-IMPLART linear accelerator

Science.gov (United States)

Piccinini, M.; Ronsivalle, C.; Ampollini, A.; Bazzano, G.; Picardi, L.; Nenzi, P.; Trinca, E.; Vadrucci, M.; Bonfigli, F.; Nichelatti, E.; Vincenti, M. A.; Montereali, R. M.

2017-11-01

Solid-state radiation detectors based on the photoluminescence of stable point defects in lithium fluoride crystals have been used for advanced diagnostics during the commissioning of the segment up to 27 MeV of the TOP-IMPLART proton linear accelerator for proton therapy applications, under development at ENEA C.R. Frascati, Italy. The LiF detectors high intrinsic spatial resolution and wide dynamic range allow obtaining two-dimensional images of the beam transverse intensity distribution and also identifying the Bragg peak position with micrometric precision by using a conventional optical fluorescence microscope. Results of the proton beam characterization, among which, the estimation of beam energy components and dynamics, are reported and discussed for different operating conditions of the accelerator.

Fe/V and Fe/Co (0 0 1) superlattices: growth, anisotropy, magnetisation and magnetoresistance

International Nuclear Information System (INIS)

Nordblad, P.; Broddefalk, A.; Mathieu, R.; Blomqvist, P.; Eriksson, O.; Waeppling, R.

2003-01-01

Some physical properties of BCC Fe/V and Fe/Co (0 0 1) superlattices are reviewed. The dependence of the magnetic anisotropy on the in-plane strain introduced by the lattice mismatch between Fe and V is measured and compared to a theoretical derivation. The dependence of the magnetic anisotropy (and saturation magnetisation) on the layer thickness ratio Fe/Co is measured and a value for the anisotropy of BCC Co is derived from extrapolation. The interlayer exchange coupling of Fe/V superlattices is studied as a function of the V layer thickness (constant Fe thickness) and layer thickness of Fe (constant V thickness). A region of antiferromagnetic coupling and GMR is found for V thicknesses 12-14 monolayers. However, surprisingly, a 'cutoff' of the antiferromagnetic coupling and GMR is found when the iron layer thickness exceeds about 10 monolayers

Transmission spectra of electrons through the Thue-Morse graphene superlattice

International Nuclear Information System (INIS)

Korol', A.M.

2014-01-01

The transmission spectra of the Thue-Morse superlattice (SL) based on a monolayer gapped graphene are investigated. The SL consists of rectangular barriers located along the Ox axis. The Thue-Morse aperiodic modulation is proposed to be realized due to the difference in values of the gap width in different SL elements. It is shown that the effective splitting of the allowed bands (and thereby the arising of a series of gaps) is observed under the influence of the aperiodic factor in the case of normal incidence of the electron wave on the SL as well as in the case of oblique incidence. The spectra are periodical with potential barrier height. In some regions of the spectra, the splitting of bands is subjected to the Fibonacci inflation rule in every new Thue-Morse generation. As in the periodical graphene-based SL, in every Thue-Morse sequence a superlattice Dirac point is created. The width of the gap associated with this point depends on SL parameters substantially; at the same time the energy location of this gap depends weakly on mass term in the Hamiltonian and it does not depend on SL period. The spectra dependence on the angle of electron wave incidence is not substantial

Indium-bump-free antimonide superlattice membrane detectors on silicon substrates

Energy Technology Data Exchange (ETDEWEB)

Zamiri, M., E-mail: mzamiri@chtm.unm.edu, E-mail: skrishna@chtm.unm.edu; Klein, B.; Schuler-Sandy, T.; Dahiya, V.; Cavallo, F. [Center for High Technology Materials, Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, New Mexico 87106 (United States); Myers, S. [SKINfrared, LLC, Lobo Venture Lab, 801 University Blvd., Suite 10, Albuquerque, New Mexico 87106 (United States); Krishna, S., E-mail: mzamiri@chtm.unm.edu, E-mail: skrishna@chtm.unm.edu [Center for High Technology Materials, Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, New Mexico 87106 (United States); SKINfrared, LLC, Lobo Venture Lab, 801 University Blvd., Suite 10, Albuquerque, New Mexico 87106 (United States)

2016-02-29

We present an approach to realize antimonide superlattices on silicon substrates without using conventional Indium-bump hybridization. In this approach, PIN superlattices are grown on top of a 60 nm Al{sub 0.6}Ga{sub 0.4}Sb sacrificial layer on a GaSb host substrate. Following the growth, the individual pixels are transferred using our epitaxial-lift off technique, which consists of a wet-etch to undercut the pixels followed by a dry-stamp process to transfer the pixels to a silicon substrate prepared with a gold layer. Structural and optical characterization of the transferred pixels was done using an optical microscope, scanning electron microscopy, and photoluminescence. The interface between the transferred pixels and the new substrate was abrupt, and no significant degradation in the optical quality was observed. An Indium-bump-free membrane detector was then fabricated using this approach. Spectral response measurements provided a 100% cut-off wavelength of 4.3 μm at 77 K. The performance of the membrane detector was compared to a control detector on the as-grown substrate. The membrane detector was limited by surface leakage current. The proposed approach could pave the way for wafer-level integration of photonic detectors on silicon substrates, which could dramatically reduce the cost of these detectors.

First results from bent crystal extraction at the Fermilab Tevatron

International Nuclear Information System (INIS)

1996-07-01

First results from Fermilab Experiment 953 are presented. E853 is an experiment to test the feasibility and efficiency of extracting a low intensity beam from the halo of the Tevatron using channeling in a bent silicon crystal. The motivation for the experiment is to apply crystal extraction to trans-TeV accelerators like the LHC. Extensive simulation work has been carried out. Two accelerator operating modes have been developed for crystal studies, ''kick'' mode and diffusion mode. Results from the first successful extraction in kick mode are presented

The Peculiarities of Strain Relaxation in GaN/AlN Superlattices Grown on Vicinal GaN (0001) Substrate: Comparative XRD and AFM Study.

Science.gov (United States)

Kuchuk, Andrian V; Kryvyi, Serhii; Lytvyn, Petro M; Li, Shibin; Kladko, Vasyl P; Ware, Morgan E; Mazur, Yuriy I; Safryuk, Nadiia V; Stanchu, Hryhorii V; Belyaev, Alexander E; Salamo, Gregory J

2016-12-01

Superlattices (SLs) consisting of symmetric layers of GaN and AlN have been investigated. Detailed X-ray diffraction and reflectivity measurements demonstrate that the relaxation of built-up strain in the films generally increases with an increasing number of repetitions; however, an apparent relaxation for subcritical thickness SLs is explained through the accumulation of Nagai tilt at each interface of the SL. Additional atomic force microscopy measurements reveal surface pit densities which appear to correlate with the amount of residual strain in the films along with the appearance of cracks for SLs which have exceeded the critical thickness for plastic relaxation. These results indicate a total SL thickness beyond which growth may be limited for the formation of high-quality coherent crystal structures; however, they may indicate a growth window for the reduction of threading dislocations by controlled relaxation of the epilayers.

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...

Dualband MW/LW Strained Layer Superlattice Focal Plane Arrays For Satellite-Based Wildfire Detection, Phase I

Data.gov (United States)

National Aeronautics and Space Administration — Dualband focal plane arrays (FPAs) based on gallium-free Type-II strained layer superlattice (SLS) photodiodes have recently experienced significant advances. We...

Illumination-induced changes of the Fermi surface topology in three-dimensional superlattices

Czech Academy of Sciences Publication Activity Database

Goncharuk, Natalya; Smrčka, Ludvík; Svoboda, Pavel; Vašek, Petr; Kučera, Jan; Krupko, Yu.; Wegscheider, W.

2007-01-01

Roč. 75, č. 24 (2007), 245322/1-245322/7 ISSN 1098-0121 R&D Projects: GA MŠk LC510; GA AV ČR KAN400100652 Institutional research plan: CEZ:AV0Z10100521 Keywords : persistent photoconductivity * superlattice * Fermi surface Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.172, year: 2007

Visualizing period fluctuations in strained-layer superlattices with scanning tunneling microscopy

Science.gov (United States)

Kanedy, K.; Lopez, F.; Wood, M. R.; Gmachl, C. F.; Weimer, M.; Klem, J. F.; Hawkins, S. D.; Shaner, E. A.; Kim, J. K.

2018-01-01

We show how cross-sectional scanning tunneling microscopy (STM) may be used to accurately map the period fluctuations throughout epitaxial, strained-layer superlattices based on the InAs/InAsSb and InGaAs/InAlAs material systems. The concept, analogous to Bragg's law in high-resolution x-ray diffraction, relies on an analysis of the [001]-convolved reciprocal-space satellite peaks obtained from discrete Fourier transforms of individual STM images. Properly implemented, the technique enables local period measurements that reliably discriminate vertical fluctuations localized to within ˜5 superlattice repeats along the [001] growth direction and orthogonal, lateral fluctuations localized to within ˜40 nm along directions in the growth plane. While not as accurate as x-ray, the inherent, single-image measurement error associated with the method may be made as small as 0.1%, allowing the vertical or lateral period fluctuations contributing to inhomogeneous energy broadening and carrier localization in these structures to be pinpointed and quantified. The direct visualization of unexpectedly large, lateral period fluctuations on nanometer length scales in both strain-balanced systems supports a common understanding in terms of correlated interface roughness.

Predicting Chiral Nanostructures, Lattices and Superlattices in Complex Multicomponent Nanoparticle Self-Assembly

KAUST Repository

Hur, Kahyun

2012-06-13

"Bottom up" type nanoparticle (NP) self-assembly is expected to provide facile routes to nanostructured materials for various, for example, energy related, applications. Despite progress in simulations and theories, structure prediction of self-assembled materials beyond simple model systems remains challenging. Here we utilize a field theory approach for predicting nanostructure of complex and multicomponent hybrid systems with multiple types of short- and long-range interactions. We propose design criteria for controlling a range of NP based nanomaterial structures. In good agreement with recent experiments, the theory predicts that ABC triblock terpolymer directed assemblies with ligand-stabilized NPs can lead to chiral NP network structures. Furthermore, we predict that long-range Coulomb interactions between NPs leading to simple NP lattices, when applied to NP/block copolymer (BCP) assemblies, induce NP superlattice formation within the phase separated BCP nanostructure, a strategy not yet realized experimentally. We expect such superlattices to be of increasing interest to communities involved in research on, for example, energy generation and storage, metamaterials, as well as microelectronics and information storage. © 2012 American Chemical Society.

Electronic band structure of magnetic bilayer graphene superlattices

International Nuclear Information System (INIS)

Pham, C. Huy; Nguyen, T. Thuong; Nguyen, V. Lien

2014-01-01

Electronic band structure of the bilayer graphene superlattices with δ-function magnetic barriers and zero average magnetic flux is studied within the four-band continuum model, using the transfer matrix method. The periodic magnetic potential effects on the zero-energy touching point between the lowest conduction and the highest valence minibands of pristine bilayer graphene are exactly analyzed. Magnetic potential is shown also to generate the finite-energy touching points between higher minibands at the edges of Brillouin zone. The positions of these points and the related dispersions are determined in the case of symmetric potentials.

Tunable electronic transmission gaps in a graphene superlattice

International Nuclear Information System (INIS)

Lu Weitao; Wang Shunjin; Li Wen; Wang Yonglong; Jiang Hua

2012-01-01

The transmission in graphene superlattices with adjustable barrier height is investigated using transfer-matrix method. It is found that one could control the angular range of transmission by changing the ratio of incidence energy and barrier height. The transmission as a function of incidence energy has more than one gaps, due to the appearance of evanescent waves in different barriers. Accordingly, more than one conductivity minimums are induced. The transmission gaps could be controlled by adjusting the incidence angle, the barrier height, and the barrier number, which gives the possibility to construct an energy-dependent wavevector filter.

Dualband MW/LW Strained Layer Superlattice Focal Plane Arrays for Satellite-Based Wildfire Detection, Phase II

Data.gov (United States)

National Aeronautics and Space Administration — Infrared focal plane arrays (FPAs) based on Type-II strained layer superlattice (SLS) photodiodes have recently experienced significant advances. In Phase I we...

Crystal nucleation and dendrite growth of metastable phases in undercooled melts

International Nuclear Information System (INIS)

Herlach, Dieter

2011-01-01

Research highlights: → Homogenous nucleation. → Effects of convection on dendrite growth kinetics. → Description of disorder trapping validated by experiment. - Abstract: An undercooled melt possesses an enhanced free enthalpy that opens up the possibility to crystallize metastable crystalline solids in competition with their stable counterparts. Crystal nucleation selects the crystallographic phase whereas the growth dynamics controls microstructure evolution. We apply containerless processing techniques such as electromagnetic and electrostatic levitation to containerlesss undercool and solidify metallic melts. Owing to the complete avoidance of heterogeneous nucleation on container-walls a large undercooling range becomes accessible with the extra benefit that the freely suspended drop is direct accessible for in situ observation of crystallization far away from equilibrium. Results of investigations of maximum undercoolability on pure zirconium are presented showing the limit of maximum undercoolability set by the onset of homogeneous nucleation. Rapid dendrite growth is measured as a function of undercooling by a high-speed camera and analysed within extended theories of non-equilibrium solidification. In such both supersaturated solid solutions and disordered superlattice structure of intermetallics are formed at high growth velocities. A sharp interface theory of dendrite growth is capable to describe the non-equilibrium solidification phenomena during rapid crystallization of deeply undercooled melts. Eventually, anomalous growth behaviour of Al-rich Al-Ni alloys is presented, which may be caused by forced convection.

Ultra-large single crystals by abnormal grain growth.

Science.gov (United States)

Kusama, Tomoe; Omori, Toshihiro; Saito, Takashi; Kise, Sumio; Tanaka, Toyonobu; Araki, Yoshikazu; Kainuma, Ryosuke

2017-08-25

Producing a single crystal is expensive because of low mass productivity. Therefore, many metallic materials are being used in polycrystalline form, even though material properties are superior in a single crystal. Here we show that an extraordinarily large Cu-Al-Mn single crystal can be obtained by abnormal grain growth (AGG) induced by simple heat treatment with high mass productivity. In AGG, the sub-boundary energy introduced by cyclic heat treatment (CHT) is dominant in the driving pressure, and the grain boundary migration rate is accelerated by repeating the low-temperature CHT due to the increase of the sub-boundary energy. With such treatment, fabrication of single crystal bars 70 cm in length is achieved. This result ensures that the range of applications of shape memory alloys will spread beyond small-sized devices to large-scale components and may enable new applications of single crystals in other metallic and ceramics materials having similar microstructural features.Growing large single crystals cheaply and reliably for structural applications remains challenging. Here, the authors combine accelerated abnormal grain growth and cyclic heat treatments to grow a superelastic shape memory alloy single crystal to 70 cm.

High mobility half-metallicity in the (LaMnO3)2/(SrTiO3)8 superlattice

KAUST Repository

Cossu, Fabrizio

2013-01-28

First principles calculations have been performed to investigate the LaMnO3/SrTiO3 superlattice. Structural relaxation within the generalized gradient approximation results in no significant tiltings or rotations of oxygen octahedra, but in distinct distortions in the SrTiO3 region. Taking into account the onsite Coulomb interaction, we find that the Mn spins order ferromagnetically, in contrast to the antiferromagnetic state of bulk LaMnO3. Most importantly, the interface strain combined with charge transfer across the interface induces half-metallicity within the MnO2 layers. The superlattice is particulary interesting for spintronics applications because the half-metallic states are characterized by an extraordinary high mobility.

High mobility half-metallicity in the (LaMnO3)2/(SrTiO3)8 superlattice

KAUST Repository

Cossu, Fabrizio; Schwingenschlö gl, Udo; Singh, Nirpendra

2013-01-01

First principles calculations have been performed to investigate the LaMnO3/SrTiO3 superlattice. Structural relaxation within the generalized gradient approximation results in no significant tiltings or rotations of oxygen octahedra, but in distinct distortions in the SrTiO3 region. Taking into account the onsite Coulomb interaction, we find that the Mn spins order ferromagnetically, in contrast to the antiferromagnetic state of bulk LaMnO3. Most importantly, the interface strain combined with charge transfer across the interface induces half-metallicity within the MnO2 layers. The superlattice is particulary interesting for spintronics applications because the half-metallic states are characterized by an extraordinary high mobility.

Absence of confinement in (SrTiO3)/( SrTi0.8Nb0.2O3 ) superlattices

Science.gov (United States)

Bouzerar, G.; Thébaud, S.; Bouzerar, R.; Pailhès, S.; Adessi, Ch.

2018-03-01

The reduction of dimensionality is considered an efficient pathway to boost the performances of thermoelectric materials. Quantum confinement of the carriers is expected to induce large Seebeck coefficients (S ) and it also suppresses the thermal conductivity by increasing the phonon scattering processes. However, quantum confinement in superlattices is not always easy to achieve and needs to be carefully validated. In the past decade, large values of S have been measured in (SrTiO3)/(SrTi0.8Nb0.2O3 ) superlattices [H. Ohta et al., Nat. Mater. 6, 129 (2007), 10.1038/nmat1821; Y. Mune et al., Appl. Phys. Lett. 91, 192105 (2007), 10.1063/1.2809364]. In the δ -doped compound, the reported S was almost six times larger than that of the bulk material. This huge increase has been attributed to the two-dimensional carrier confinement in the doped regions. Here, we demonstrate that the experimental data are well explained quantitatively assuming delocalized electrons in both in-plane and growth directions. Moreover, we rule out the confined electron hypothesis whose signature would be the suppression of the Seebeck coefficient. This strongly suggests that the presupposed confinement picture in these superlattices is unlikely.

Improved structural and electrical properties in native Sb2Te3/GexSb2Te3+x van der Waals superlattices due to intermixing mitigation

NARCIS (Netherlands)

Cecchi, Stefano; Zallo, Eugenio; Momand, Jamo; Wang, Ruining; Kooi, Bart J.; Verheijen, Marcel A.; Calarco, Raffaella

Superlattices made of Sb2Te3/GeTe phase change materials have demonstrated outstanding performance with respect to GeSbTe alloys in memory applications. Recently, epitaxial Sb2Te3/GeTe superlattices were found to feature GexSb2Te3+x blocks as a result of intermixing between constituting layers.

Improved structural and electrical properties in native Sb2Te3/GexSb2Te3+x van der Waals superlattices due to intermixing mitigation

NARCIS (Netherlands)

Cecchi, S.; Zallo, E.; Momand, J.; Wang, R.; Kooi, B.J.; Verheijen, M.A.; Calarco, R.

Superlattices made of Sb2Te3/GeTe phase change materials have demonstrated outstanding performance with respect to GeSbTe alloys in memory applications. Recently, epitaxial Sb2Te3/GeTe superlattices were found to feature GexSb2Te3+x blocks as a result of intermixing between constituting layers. Here

The hyperfine properties of a hydrogenated Fe/V superlattice

Energy Technology Data Exchange (ETDEWEB)

Elzain, M., E-mail: elzain@squ.edu.om; Al-Barwani, M.; Gismelseed, A.; Al-Rawas, A.; Yousif, A.; Widatallah, H.; Bouziane, K.; Al-Omari, I. [Sultan Qaboos University, Department of Physics, College of Science (Oman)

2012-03-15

We study the effect of hydrogen on the electronic, magnetic and hyperfine structures of an iron-vanadium superlattice consisting of three Fe monolayers and nine V monolayers. The contact charge density ({rho}), the contact hyperfine field (B{sub hf}) and the electronic field gradient (EFG) at the Fe sites for different H locations and H fillings are calculated using the first principle full-potential linear-augmented-plane-wave (FP-LAPW) method. It is found that sizeable changes in the hyperfine properties are obtained only when H is in the interface region.

Moiré superlattice-level stick-slip instability originated from geometrically corrugated graphene on a strongly interacting substrate

Science.gov (United States)

Shi, Ruoyu; Gao, Lei; Lu, Hongliang; Li, Qunyang; Ma, Tian-Bao; Guo, Hui; Du, Shixuan; Feng, Xi-Qiao; Zhang, Shuai; Liu, Yanmin; Cheng, Peng; Hu, Yuan-Zhong; Gao, Hong-Jun; Luo, Jianbin

2017-06-01

Two dimensional (2D) materials often exhibit novel properties due to various coupling effects with their supporting substrates. Here, using friction force microscopy (FFM), we report an unusual moiré superlattice-level stick-slip instability on monolayer graphene epitaxially grown on Ru(0 0 0 1) substrate. Instead of smooth friction modulation, a significant long-range stick-slip sawtooth modulation emerges with a period coinciding with the moiré superlattice structure, which is robust against high external loads and leads to an additional channel of energy dissipation. In contrast, the long-range stick-slip instability reduces to smooth friction modulation on graphene/Ir(1 1 1) substrate. The moiré superlattice-level slip instability could be attributed to the large sliding energy barrier, which arises from the morphological corrugation of graphene on Ru(0 0 0 1) surface as indicated by density functional theory (DFT) calculations. The locally steep humps acting as obstacles opposing the tip sliding, originates from the strong interfacial electronic interaction between graphene and Ru(0 0 0 1). This study opens an avenue for modulating friction by tuning the interfacial atomic interaction between 2D materials and their substrates.

Investigation of the structural and electronic properties of quantum well superlattices made of CuCl, CuBr and CuI

International Nuclear Information System (INIS)

Badi, F.; Louhibi, S.; Aced, M.R.; Mehnane, N.; Sekkal, N.

2008-05-01

This paper reports first principle investigations of structural and electronic properties of both bulk and superlattice (SL) systems of cuprous halides. The obtained structural data for bulks compares well with literature while data for SL is not compared because unavailable (to our knowledge). The electronic structure of bulks and also of (001) and (110) growth axis superlattices (SLs) are calculated and it is shown that at the reverse of the IV-IV and III-V systems, the (110) growth axis impacts the electronic structure which indicates a great influence of the d-orbital. (author)

Data on a new beta titanium alloy system reinforced with superlattice intermetallic precipitates

Directory of Open Access Journals (Sweden)

Alexander J. Knowles

2018-04-01

Full Text Available The data presented in this article are related to the research article entitled “a new beta titanium alloy system reinforced with superlattice intermetallic precipitates” (Knowles et al., 2018 [1]. This includes data from the as-cast alloy obtained using scanning electron microscopy (SEM and x-ray diffraction (XRD as well as SEM data in the solution heat treated condition. Transmission electron microscopy (TEM selected area diffraction patterns (SADPs are included from the alloy in the solution heat treated condition, as well as the aged condition that contained < 100 nm B2 TiFe precipitates [1], the latter of which was found to exhibit double diffraction owing to the precipitate and matrix channels being of a similar width to the foil thickness (Williams and Carter, 2009 [2]. Further details are provided on the macroscopic compression testing of small scale cylinders. Of the micropillar deformation experiment performed in [1], SEM micrographs of focused ion beam (FIB prepared 2 µm micropillars are presented alongside those obtained at the end of the in-situ SEM deformation as well as videos of the in-situ deformation. Further, a table is included that lists the Schmidt factors of all the possible slip systems given the crystal orientations and loading axis of the deformed micropillars in the solution heat treated and aged conditions.

Tuning the electronic structure of SrTiO3/SrFeO3−x superlattices via composition and vacancy control

Directory of Open Access Journals (Sweden)

Robert F. Berger

2014-04-01

Full Text Available Using density functional theory-based calculations, we explore the effects of oxygen vacancies and epitaxial layering on the atomic, magnetic, and electronic structure of (SrTiO3n(SrFeO3−x1 superlattices. While structures without oxygen vacancies (x = 0 possess small or non-existent band gaps and ferromagnetic ordering in their iron layers, those with large vacancy concentrations (x = 0.5 have much larger gaps and antiferromagnetic ordering. Though the computed gaps depend numerically on the delicate energetic balance of vacancy ordering and on the value of Hubbard U eff used in the calculations, we demonstrate that changes in layering can tune the band gaps of these superlattices below that of SrTiO3 (3.2 eV by raising their valence band maxima. This suggests the possibility that these superlattices could absorb in the solar spectrum, and could serve as water-splitting photocatalysts.

Designing artificial 2D crystals with site and size controlled quantum dots.

Science.gov (United States)

Xie, Xuejun; Kang, Jiahao; Cao, Wei; Chu, Jae Hwan; Gong, Yongji; Ajayan, Pulickel M; Banerjee, Kaustav

2017-08-30

Ordered arrays of quantum dots in two-dimensional (2D) materials would make promising optical materials, but their assembly could prove challenging. Here we demonstrate a scalable, site and size controlled fabrication of quantum dots in monolayer molybdenum disulfide (MoS 2 ), and quantum dot arrays with nanometer-scale spatial density by focused electron beam irradiation induced local 2H to 1T phase change in MoS 2 . By designing the quantum dots in a 2D superlattice, we show that new energy bands form where the new band gap can be controlled by the size and pitch of the quantum dots in the superlattice. The band gap can be tuned from 1.81 eV to 1.42 eV without loss of its photoluminescence performance, which provides new directions for fabricating lasers with designed wavelengths. Our work constitutes a photoresist-free, top-down method to create large-area quantum dot arrays with nanometer-scale spatial density that allow the quantum dots to interfere with each other and create artificial crystals. This technique opens up new pathways for fabricating light emitting devices with 2D materials at desired wavelengths. This demonstration can also enable the assembly of large scale quantum information systems and open up new avenues for the design of artificial 2D materials.

Terahertz radiation on the base of accelerated charge carriers in GaAs; Terahertz-Strahlung auf der Basis beschleunigter Ladungstraeger in GaAs

Energy Technology Data Exchange (ETDEWEB)

Dreyhaupt, Andre

2008-07-01

Electromagnetic radiation in the frequency range between about 100 GHz and 5 THz can be used for spectroscopy and microscopy, but it is also promising for security screening and even wireless communication. In the present thesis a planar photoconducting large-area THz radiation source is presented. The device exhibits outstanding properties, in particular high THz field strength and generation efficiency and large spectral bandwidth with short THz pulse length. The THz emission is based on acceleration and deceleration of photoexcited carriers in semiconductor substrates. A metallic interdigitated structure at the surface of semi-insulating GaAs provides the electrodes of an Auston switch. In a biased structure photoexcited charge carriers are accelerated. Hence electromagnetic waves are emitted. An appropriately structured second metallization, electrically isolated from the electrodes, prevents destructive interference of the emitted waves. The structure investigated here combines several advantages of different conventional photoconducting THz sources. First, it provides high electric acceleration fields at moderate voltages owing to the small electrode separation. Second, the large active area in the mm2 range allows excitation by large optical powers of some mW. Optical excitation with near-infrared femtosecond lasers is possible with repetition rates in the GHz range. The presented results point out the excellent characteristics regarding the emitted THz field strength, average power, spectral properties, and easy handling of the interdigitated structure in comparison to various conventional emitter structures. Various modifications of the semiconductor substrate and the optimum excitation conditions were investigated. In the second part of this thesis the dynamic conductivity of GaAs/Al{sub x}Ga{sub 1-x}As superlattices in an applied static electric field was investigated with time-resolved THz spectroscopy. The original goal was to explore whether the

Coherent light from E-field induced quantum coupling of exciton states in superlattice-like quantum wells

DEFF Research Database (Denmark)

Lyssenko, V. G.; Østergaard, John Erland; Hvam, Jørn Märcher

1999-01-01

Summary form only given. We focus on the ability to control the electronic coupling in coupled quantum wells with external E-fields leading to a strong modification of the coherent light emission, in particular at a bias where a superlattice-like miniband is formed. More specifically, we investig......Summary form only given. We focus on the ability to control the electronic coupling in coupled quantum wells with external E-fields leading to a strong modification of the coherent light emission, in particular at a bias where a superlattice-like miniband is formed. More specifically, we...... investigate a MBE-grown GaAs sample with a sequence of 15 single quantum wells having a successive increase of 1 monolayer in width ranging from 62 A to 102 A and with AlGaAs barriers of 17 Å....

Effect of disorders on topological phases in one-dimensional optical superlattices

International Nuclear Information System (INIS)

Wang Zhizhou; Wu Yidong; Du Huijing; Jing Xili

2016-01-01

In a recent paper, Lang et al. proposed that edge states and topological phases can be observed in one-dimensional optical superlattices. They showed that the topological phases can be revealed by observing the density profile of a trapped fermion system, which displays plateaus with their positions. However, disorders are not considered in their model. To study the effect of disorders on the topological phases, we introduce random potentials to the model for optical superlattcies. Our calculations show that edge states are robust against the disorders. We find the edge states are very sensitive to the number of the sites in the optical superlattice and we propose a simple rule to describe the relationship between the edge states and the number of sites. The density plateaus are also robust against weak disorders provided that the average density is calculated over a long interval. The widths of the plateaus are proportional to the widths of the bulk energy gaps when there are disorders. The disorders can diminish the bulk energy gaps. So the widths of the plateaus decrease with the increase of disorders and the density plateaus disappear when disorders are too strong. The results in our paper can be used to guide the experimental detection of topological phases in one-dimensional systems. (paper)

Spectroscopic ellipsometry characterization of interface reactivity in GaAs-based superlattices

Energy Technology Data Exchange (ETDEWEB)

Losurdo, M.; Giuva, D.; Giangregorio, M.M.; Bruno, G.; Brown, A.S

2004-05-01

Pseudodielectric function spectra of GaAs/GaSb{sub 1-y}As{sub y}, GaSb/GaAs{sub y}Sb{sub 1-y} and GaAs/GaP{sub y}As{sub 1-y} superlattices have been measured by spectroscopic ellipsometry in the 0.75-5.5 eV photon energy range. The analysis of the E{sub 1} interband critical point and modeling of spectra has been carried out to investigate the chemistry of the anion exchange reaction and abruptness of interface composition in the superlattices. It has been found that a ternary compound GaP{sub y}As{sub 1-y} forms in the case of the P-for-As anion exchange reaction. In the case of As-for-Sb anion exchange reaction for (GaSb/GaAs{sub y}Sb{sub 1-y}){sub 20} SLs, SE data show that this anion exchange results in the formation not only of a ternary alloy GaAs{sub y}Sb{sub 1-y}, but also in the formation of isoelectronic compounds AsSb{sub x} that segregate at the GaSb/GaAs interface. In the case of Sb-for-As anion exchange for (GaAs/GaSbyAs{sub 1-y}){sub 20} SLs, Sb segregates at the GaAs surface.

Superconducting superlattices. Les super reseaux de supraconducteurs

Energy Technology Data Exchange (ETDEWEB)

Triscone, J M; Fischer, O [Geneva Univ. (Switzerland)

1993-03-01

By piling up ultra-thin layers of discrete materials, physicists now have a choice method for the study of superconductivity at high temperature. These superlattices are prepared by successive layers of YBaCuO and PrBaCuO deposited by cathode sputtering to study the variation of superconductivity with layer thickness. The transition temperature decreases rapidly when the distance between two layers increases. Current vortices are created, without a magnetic field, widening the transition temperature. The variation of resistivity near critical temperature in a magnetic field shows that the energy required to displace vortices is increasing with the thickness of the YBaCuO layer, with thin layers anisotropy is high and energy dissipation is important. (G.R.). refs., figs.

Development of bipolar-pulse accelerator for intense pulsed ion beam acceleration

Energy Technology Data Exchange (ETDEWEB)

Masugata, Katsumi [Department of Electrical and Electronic System Engineering, Toyama University, 3190 Gofuku, Toyama 930-8555 (Japan)]. E-mail: masugata@eng.toyama-u.ac.jp; Shimizu, Yuichro [Department of Electrical and Electronic System Engineering, Toyama University, 3190 Gofuku, Toyama 930-8555 (Japan); Fujioka, Yuhki [Department of Electrical and Electronic System Engineering, Toyama University, 3190 Gofuku, Toyama 930-8555 (Japan); Kitamura, Iwao [Department of Electrical and Electronic System Engineering, Toyama University, 3190 Gofuku, Toyama 930-8555 (Japan); Tanoue, Hisao [National Institute of Advanced Industry Science and Technology, 1-1-1, Umezono, Tsukuba-shi, Ibaraki 305-8568 (Japan); Arai, Kazuo [National Institute of Advanced Industry Science and Technology, 1-1-1, Umezono, Tsukuba-shi, Ibaraki 305-8568 (Japan)

2004-12-21

To improve the purity of intense pulsed ion beams, a new type of pulsed ion beam accelerator named 'bipolar pulse accelerator' was proposed. To confirm the principle of the accelerator a prototype of the experimental system was developed. The system utilizes By type magnetically insulated acceleration gap and operated with single polar negative pulse. A coaxial gas puff plasma gun was used as an ion source, which was placed inside the grounded anode. Source plasma (nitrogen) of current density {approx}25A/cm2, duration {approx}1.5{mu}s was injected into the acceleration gap by the plasma gun. The ions were successfully accelerated from the grounded anode to the drift tube by applying negative pulse of voltage 240kV, duration 100ns to the drift tube. Pulsed ion beam of current density {approx}40A/cm2, duration {approx}50ns was obtained at 41mm downstream from the anode surface. To evaluate the irradiation effect of the ion beam to solid material, an amorphous silicon thin film of thickness {approx}500nm was used as the target, which was deposited on the glass substrate. The film was found to be poly-crystallized after 4-shots of the pulsed nitrogen ion beam irradiation.

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

CERN Document Server

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

1999-01-01

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

Response functions of a superlattice with a basis: A model for oxide superconductors

International Nuclear Information System (INIS)

Griffin, A.

1988-01-01

The new high-T/sub c/ oxide superconductors appear to be superlattice structures with a basis composed of metallic sheets as well as metallic chains. Using a simple free-electron-gas model for the sheets and chains, we obtain the dielectric function ε(q,ω) of such a multilayer system within the random-phase approximation (RPA). We give results valid for arbitrary wave vector q appropriate to sheets and chains (as in the orthorhombic phase of Y-Ba-Cu-O) as well as for two different kinds of sheets (such as may be present in the Bi-Ca-Sr-Cu-O superconductors). The occurrence of acoustic plasmons is a general phenomenon in such superlattices, as shown by an alternative formulation based on the exact response functions for the individual sheets and chains, in which only the interchain (sheet) Coulomb interaction is treated in the RPA. These results generalize the long-wavelength expressions recently given in the literature. We also briefly discuss the analogous results for two arrays of mutually perpendicular chains, such as found in Hg chain compounds

Nonlinear elasticity in wurtzite GaN/AlN planar superlattices and quantum dots

International Nuclear Information System (INIS)

Lepkowski, S.P.; Majewski, J.A.; Jurczak, G.

2005-01-01

The elastic stiffness tensor for wurtzite GaN and AlN show a significant hydrostatic pressure dependence, which id the evidence of nonlinear elasticity of these compounds. We have examined how the pressure dependence of elastic constants for wurtzite nitrides influences elastic and piezoelectric properties of GaN/AlN planar superlattices and quantum dots. Particularly we show that built-in hydrostatic pressure, present in both quantum wells of the GaN/AlN superlattices and GaN/AlN quantum dots, increases significantly by 0.3-0.7 GPa when nonlinear elasticity is used. Consequently, the compressive volumetric strain in quantum wells and quantum dots decreases in comparison to the case of the linear elastic theory, However, the-component of the built-in electric field in the quantum wells and quantum dots increases considerably when nonlinear elasticity is taken into account. Both effects, i.e., a decrease in the compressive volumetric strain as well as an increase in the built-in electric field, decrease the band-to-band transition energies in the quantum wells and quantum dots. (author)

Phase coexistence and electric-field control of toroidal order in oxide superlattices.

Science.gov (United States)

Damodaran, A R; Clarkson, J D; Hong, Z; Liu, H; Yadav, A K; Nelson, C T; Hsu, S-L; McCarter, M R; Park, K-D; Kravtsov, V; Farhan, A; Dong, Y; Cai, Z; Zhou, H; Aguado-Puente, P; García-Fernández, P; Íñiguez, J; Junquera, J; Scholl, A; Raschke, M B; Chen, L-Q; Fong, D D; Ramesh, R; Martin, L W

2017-10-01

Systems that exhibit phase competition, order parameter coexistence, and emergent order parameter topologies constitute a major part of modern condensed-matter physics. Here, by applying a range of characterization techniques, and simulations, we observe that in PbTiO 3 /SrTiO 3 superlattices all of these effects can be found. By exploring superlattice period-, temperature- and field-dependent evolution of these structures, we observe several new features. First, it is possible to engineer phase coexistence mediated by a first-order phase transition between an emergent, low-temperature vortex phase with electric toroidal order and a high-temperature ferroelectric a 1 /a 2 phase. At room temperature, the coexisting vortex and ferroelectric phases form a mesoscale, fibre-textured hierarchical superstructure. The vortex phase possesses an axial polarization, set by the net polarization of the surrounding ferroelectric domains, such that it possesses a multi-order-parameter state and belongs to a class of gyrotropic electrotoroidal compounds. Finally, application of electric fields to this mixed-phase system permits interconversion between the vortex and the ferroelectric phases concomitant with order-of-magnitude changes in piezoelectric and nonlinear optical responses. Our findings suggest new cross-coupled functionalities.

Ferroic properties in bi-component perovskites: artificial superlattices and naturally forming compounds

International Nuclear Information System (INIS)

Saha-Dasgupta, T

2014-01-01

The use of four different metal cations in a bi-component perovskite ABO 3 structure with 50 : 50 substitution at A sublattice as well as B sublattice, opens up the door for materials designing, with the aim to improve ferroic properties. This can be achieved following two different routes; one using the concept of artificially grown superlattices with alternating layers of ABO 3 and A′B′O 3 perovskites in a periodic set-up and another, through synthesis of naturally grown bulk double perovskites with ordered arrangement of A and A′ cations, simultaneously with that of B and B′ cations. The tremendous progress in layered deposition techniques as well as advances in solid state chemistry methods, has made both routes equally plausible and an area of much activity. This review summarizes some of the recent progress in this field, with a special emphasis on two computational studies, (i) one on ultra-thin 1–1 superlattices built out of paraelectric and ferroelectric components, showing tunable piezoelectric properties, and (ii) another on CrOs-based double perovskites which show multiferroic behavior, achieved through layered ordering of A and A′ cations. (topical review)

Phase coexistence and electric-field control of toroidal order in oxide superlattices

International Nuclear Information System (INIS)

Damodaran, A. R.; Clarkson, J. D.; Hong, Z.

2017-01-01

Systems that exhibit phase competition, order parameter coexistence, and emergent order parameter topologies constitute a major part of modern condensed-matter physics. Here, by applying a range of characterization techniques, and simulations, we observe that in PbTiO 3 /SrTiO 3 superlattices all of these effects can be found. By exploring superlattice period-, temperature- and field-dependent evolution of these structures, we observe several new features. First, it is possible to engineer phase coexistence mediated by a first-order phase transition between an emergent, low-temperature vortex phase with electric toroidal order and a high-temperature ferroelectric a 1 /a 2 phase. At room temperature, the coexisting vortex and ferroelectric phases form a mesoscale, fibre-textured hierarchical superstructure. The vortex phase possesses an axial polarization, set by the net polarization of the surrounding ferroelectric domains, such that it possesses a multi-order-parameter state and belongs to a class of gyrotropic electrotoroidal compounds. Finally, application of electric fields to this mixed-phase system permits interconversion between the vortex and the ferroelectric phases concomitant with order-of-magnitude changes in piezoelectric and nonlinear optical responses. Here, our findings suggest new cross-coupled functionalities.

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...

Quantification of flexoelectricity in PbTiO3/SrTiO3 superlattice polar vortices using machine learning and phase-field modeling.

Science.gov (United States)

Li, Q; Nelson, C T; Hsu, S-L; Damodaran, A R; Li, L-L; Yadav, A K; McCarter, M; Martin, L W; Ramesh, R; Kalinin, S V

2017-11-13

Flexoelectricity refers to electric polarization generated by heterogeneous mechanical strains, namely strain gradients, in materials of arbitrary crystal symmetries. Despite more than 50 years of work on this effect, an accurate identification of its coupling strength remains an experimental challenge for most materials, which impedes its wide recognition. Here, we show the presence of flexoelectricity in the recently discovered polar vortices in PbTiO 3 /SrTiO 3 superlattices based on a combination of machine-learning analysis of the atomic-scale electron microscopy imaging data and phenomenological phase-field modeling. By scrutinizing the influence of flexocoupling on the global vortex structure, we match theory and experiment using computer vision methodologies to determine the flexoelectric coefficients for PbTiO 3 and SrTiO 3 . Our findings highlight the inherent, nontrivial role of flexoelectricity in the generation of emergent complex polarization morphologies and demonstrate a viable approach to delineating this effect, conducive to the deeper exploration of both topics.

Light-hole conduction in InGaAs/GaAs strained-layer superlattices

International Nuclear Information System (INIS)

Schirber, J.E.; Fritz, I.J.; Dawson, L.R.

1985-01-01

We report the first observation of light-hole band carriers in In/sub 0.2/Ga/sub 0.8/As/GaAs strained-layer superlattices by direct measurements of their effective mass (m*m/sub o/ = 0.14) using oscillatory magnetoresistance data. Preferential population of light-hole states, due to splitting of the degenerate bulk valence bands by built-in strain, allows this direct observation

Ultra-High Gradient Channeling Acceleration in Nanostructures: Design/Progress of Proof-of-Concept (POC) Experiments

Energy Technology Data Exchange (ETDEWEB)

Shin, Young Min [Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Northern Illinois Univ., DeKalb, IL (United States). Northern Illinois Center for Accelerator & Detector Development; Green, A. [Northern Illinois Univ., DeKalb, IL (United States). Northern Illinois Center for Accelerator & Detector Development; Lumpkin, A. H. [Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Thurman-Keup, R. M. [Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Shiltsev, V. [Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Zhang, X. [Shanhai Inst. of Optics and Fine Mechanics, Shanghai (China); Farinella, D. M. [Univ. of California, Irvine, CA (United States); Taborek, P. [Univ. of California, Irvine, CA (United States); Tajima, T. [Univ. of California, Irvine, CA (United States); Wheeler, J. A. [Univ. of Michigan, Ann Arbor, MI (United States). Center for Ultrafast Optical Science and FOCUS Center; Ecole Polytechnique, CNRS, Palaiseau (France). Lab. d' Optique Appliquee; Mourou, G. [Univ. of Michigan, Ann Arbor, MI (United States). Center for Ultrafast Optical Science and FOCUS Center; Ecole Polytechnique, CNRS, Palaiseau (France). Lab. d' Optique Appliquee

2016-09-16

A short bunch of relativistic particles or a short-pulse laser perturbs the density state of conduction electrons in a solid crystal and excites wakefields along atomic lattices in a crystal. Under a coupling condition the wakes, if excited, can accelerate channeling particles with TeV/m acceleration gradients in principle since the density of charge carriers (conduction electrons) in solids n₀ = ~ 10²⁰ – 10²³ cm^-3 is significantly higher than what can be obtained in gaseous plasma. Nanostructures have some advantages over crystals for channeling applications of high power beams. The dechanneling rate can be reduced and the beam acceptance increased by the large size of the channels. For beam-driven acceleration, a bunch length with a sufficient charge density would need to be in the range of the plasma wavelength to properly excite plasma wakefields, and channeled particle acceleration with the wakefields must occur before the ions in the lattices move beyond the restoring threshold. In the case of the excitation by short laser pulses, the dephasing length is appreciably increased with the larger channel, which enables channeled particles to gain sufficient amounts of energy. This paper describes simulation analyses on beam- and laser (X-ray)-driven accelerations in effective nanotube models obtained from Vsim and EPOCH codes. Experimental setups to detect wakefields are also outlined with accelerator facilities at Fermilab and NIU. In the FAST facility, the electron beamline was successfully commissioned at 50 MeV and it is being upgraded toward higher energies for electron accelerator R&D. The 50 MeV injector beamline of the facility is used for X-ray crystal-channeling radiation with a diamond target. It has been proposed to utilize the same diamond crystal for a channeling acceleration POC test. Another POC experiment is also designed for the NIU accelerator lab with time-resolved electron diffraction. Recently, a

Advances in the characterization of InAs/GaSb superlattice infrared photodetectors

Science.gov (United States)

Wörl, A.; Daumer, V.; Hugger, T.; Kohn, N.; Luppold, W.; Müller, R.; Niemasz, J.; Rehm, R.; Rutz, F.; Schmidt, J.; Schmitz, J.; Stadelmann, T.; Wauro, M.

2016-10-01

This paper reports on advances in the electro-optical characterization of InAs/GaSb short-period superlattice infrared photodetectors with cut-off wavelengths in the mid-wavelength and long-wavelength infrared ranges. To facilitate in-line monitoring of the electro-optical device performance at different processing stages we have integrated a semi-automated cryogenic wafer prober in our process line. The prober is configured for measuring current-voltage characteristics of individual photodiodes at 77 K. We employ it to compile a spatial map of the dark current density of a superlattice sample with a cut-off wavelength around 5 μm patterned into a regular array of 1760 quadratic mesa diodes with a pitch of 370 μm and side lengths varying from 60 to 350 μm. The different perimeter-to-area ratios make it possible to separate bulk current from sidewall current contributions. We find a sidewall contribution to the dark current of 1.2×10-11 A/cm and a corrected bulk dark current density of 1.1×10-7 A/cm2, both at 200 mV reverse bias voltage. An automated data analysis framework can extract bulk and sidewall current contributions for various subsets of the test device grid. With a suitable periodic arrangement of test diode sizes, the spatial distribution of the individual contributions can thus be investigated. We found a relatively homogeneous distribution of both bulk dark current density and sidewall current contribution across the sample. With the help of an improved capacitance-voltage measurement setup developed to complement this technique a residual carrier concentration of 1.3×1015 cm-3 is obtained. The work is motivated by research into high performance superlattice array sensors with demanding processing requirements. A novel long-wavelength infrared imager based on a heterojunction concept is presented as an example for this work. It achieves a noise equivalent temperature difference below 30 mK for realistic operating conditions.

Hydrogenation of Very Long Wavelength Infrared Focal Plane Arrays Based on Type II Superlattices, Phase I

Data.gov (United States)

National Aeronautics and Space Administration — We propose to advance the Ga-free InAs/InAsSb type II superlattice (T2SL) materials technology for very long wavelength infrared (VLWIR) focal plane arrays (FPAs) by...

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

Science.gov (United States)

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

2016-09-01

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

Magnetic quantum oscillations of diagonal conductivity in a two-dimensional conductor with a weak square superlattice modulation under conditions of the integer quantum Hall effect

International Nuclear Information System (INIS)

Gvozdikov, V M; Taut, M

2009-01-01

We report on analytical and numerical studies of the magnetic quantum oscillations of the diagonal conductivity σ xx in a two-dimensional conductor with a weak square superlattice modulation under conditions of the integer quantum Hall (IQHE) effect. The quantum Hall effect in such a system differs from the conventional IQHE, in which the finite width of the Landau bands is due to disorder only. The superlattice modulation potential yields a fractal splitting of the Landau levels into Hofstadter minibands. For rational flux through a unit cell, the minibands have a finite width and intrinsic dispersion relations. We consider a regime, now accessible experimentally, in which disorder does not wash out the fractal internal gap structure of the Landau bands completely. We found the following distinctions from the conventional IQHE produced by the superlattice: (i) the peaks in diagonal conductivity are split due to the Hofstadter miniband structure of Landau bands; (ii) the number of split peaks in the bunch, their positions and heights depend irregularly on the magnetic field and the Fermi energy; (iii) the gaps between the split Landau bands (and related quantum Hall plateaus) become narrower with the superlattice modulation than without it.

Valley-chiral quantum Hall state in graphene superlattice structure

Science.gov (United States)

Tian, H. Y.; Tao, W. W.; Wang, J.; Cui, Y. H.; Xu, N.; Huang, B. B.; Luo, G. X.; Hao, Y. H.

2016-05-01

We theoretically investigate the quantum Hall effect in a graphene superlattice (GS) system, in which the two valleys of graphene are coupled together. In the presence of a perpendicular magnetic field, an ordinary quantum Hall effect is found with the sequence σxy=ν e^2/h(ν=0,+/-1,+/-2,\\cdots) . At the zeroth Hall platform, a valley-chiral Hall state stemming from the single K or K' valley is found and it is localized only on one sample boundary contributing to the longitudinal conductance but not to the Hall conductivity. Our findings may shed light on the graphene-based valleytronics applications.

Minority Carrier Lifetime Studies of Narrow Bandgap Antimonide Superlattices

Science.gov (United States)

Hoglund, Linda; Ting, David Z.; Khoshakhlagh, Arezou; Soibel, Alexander; Hill, Cory J.; Fisher, Anita; Keo, Sam; Gunapala, Sarath D.

2014-01-01

In this study optical modulation response and photoluminescence spectroscopy were used to study mid-wave Ga-free InAs/InAsSb superlattices. The minority carrier lifetimes in the different samples varied from 480 ns to 4700 ns, partly due to different background doping concentrations. It was shown that the photoluminescence intensity can be used as a fast non-destructive tool to predict the material quality. It was also demonstrated that it is crucial to use a low excitation power in the photoluminescence measurements in order to get a good correlation between the photoluminescence intensity and the minority carrier lifetime.

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

Energy Technology Data Exchange (ETDEWEB)

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

2017-01-15

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

Ferroelectricity driven magnetism at domain walls in LaAlO3/PbTiO3 superlattices

Science.gov (United States)

Zhou, P. X.; Dong, S.; Liu, H. M.; Ma, C. Y.; Yan, Z. B.; Zhong, C. G.; Liu, J. -M.

2015-01-01

Charge dipole moment and spin moment rarely coexist in single-phase bulk materials except in some multiferroics. Despite the progress in the past decade, for most multiferroics their magnetoelectric performance remains poor due to the intrinsic exclusion between charge dipole and spin moment. As an alternative approach, the oxide heterostructures may evade the intrinsic limits in bulk materials and provide more attractive potential to realize the magnetoelectric functions. Here we perform a first-principles study on LaAlO3/PbTiO3 superlattices. Although neither of the components is magnetic, magnetic moments emerge at the ferroelectric domain walls of PbTiO3 in these superlattices. Such a twist between ferroelectric domain and local magnetic moment, not only manifests an interesting type of multiferroicity, but also is possible useful to pursuit the electrical-control of magnetism in nanoscale heterostructures. PMID:26269322

First-principles modeling of titanate/ruthenate superlattices

Science.gov (United States)

Junquera, Javier

2013-03-01

The possibility to create highly confined two-dimensional electron gases (2DEG) at oxide interfaces has generated much excitement during the last few years. The most widely studied system is the 2DEG formed at the LaO/TiO2 polar interface between LaAlO3 and SrTiO3, where the polar catastrophe at the interface has been invoked as the driving force. More recently, partial or complete delta doping of the Sr or Ti cations at a single layer of a SrTiO3 matrix has also been used to generate 2DEG. Following this recipe, we report first principles characterization of the structural and electronic properties of (SrTiO3)5/(SrRuO3)1 superlattices, where all the Ti of a given layer have been replaced by Ru. We show that the system exhibits a spin-polarized two-dimensional electron gas extremely confined to the 4 d orbitals of Ru in the SrRuO3 layer, a fact that is independent of the level of correlation included in the simulations. For hybrid functionals or LDA+U, every interface in the superlattice behaves as minority-spin half-metal ferromagnet, with a magnetic moment of μ = 2.0 μB/SrRuO3 unit. The shape of the electronic density of states, half metallicity and magnetism are explained in terms of a simplified tight-binding model, considering only the t2 g orbitals plus (i) the bi-dimensionality of the system, and (ii) strong electron correlations. Possible applications are discussed, from their eventual role in thermoelectric applications to the possible tuning of ferromagnetic properties of the 2DEG with the polarization of the dielectric. Work done in collaboration with P. García, M. Verissimo-Alves, D. I. Bilc, and Ph. Ghosez. Financial support provided by MICINN Grant FIS2009-12721-C04-02, and by the European Union Grant No. CP-FP 228989-2 ``OxIDes.'' The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the BSC/RES.

Physics of radiation effects in crystals

CERN Document Server

Johnson, RA

1986-01-01

``Physics of Radiation Effects in Crystals'' is presented in two parts. The first part covers the general background and theory of radiation effects in crystals, including the theory describing the generation of crystal lattice defects by radiation, the kinetic approach to the study of the disposition of these defects and the effects of the diffusion of these defects on alloy compositions and phases. Specific problems of current interest are treated in the second part and include anisotropic dimensional changes in x-uranium, zirconium and graphite, acceleration of thermal creep in reactor ma

2D Electron Gas with 100% Spin-Polarization in the (LaMnO3)2/(SrTiO3)2 Superlattice under Uniaxial Strain

KAUST Repository

Cossu, Fabrizio

2014-07-28

By first-principles calculations we investigate the structural, electronic, and magnetic properties of the (LaMnO3)2/(SrTiO3)2 superlattice. We find that a monoclinic C2h symmetry is energetically favorable and that the spins order ferromagnetically. Under both compressive and tensile uniaxial strain the electronic structure of the superlattice shows a half-metallic character. In particular, a fully spin-polarized two-dimensional electron gas, which traces back to the Ti 3dxy orbitals, is achieved under compressive uniaxial strain. The (LaMnO3)2/(SrTiO3)2 superlattice is analysed with respect to its structure, magnetism, and electronic properties. Our results demonstrate that uniaxial strain in an experimentally accessible range, both tensile and compressive, can be used to induce half-metallicity. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Molecular dynamics growth modeling of InAs1-xSbx-based type-II superlattice

Science.gov (United States)

Ciani, Anthony J.; Grein, Christoph H.; Irick, Barry; Miao, Maosheng; Kioussis, Nicholas

2017-09-01

Type-II strained-layer superlattices (T2SL) based on InAs1-xSbx are a promising photovoltaic detector material technology for thermal imaging; however, Shockley-Read-Hall recombination and generation rates are still too high for thermal imagers based on InAs1-xSbx T2SL to reach their ideal performance. Molecular dynamics simulations using the Stillinger-Weber (SW) empirical potentials are a useful tool to study the growth of tetrahedral coordinated crystals and the nonequilibrium formation of defects within them, including the long-range effects of strain. SW potentials for the possible atomic interactions among {Ga, In, As, Sb} were developed by fitting to ab initio calculations of elastically distorted zinc blende and diamond unit cells. The SW potentials were tested against experimental observations of molecular beam epitaxial (MBE) growth and then used to simulate the MBE growth of InAs/InAs0.5Sb0.5 T2SL on GaSb substrates over a range of processes parameters. The simulations showed and helped to explain Sb cross-incorporation into the InAs T2SL layers, Sb segregation within the InAsSb layers, and identified medium-range defect clusters involving interstitials and their induction of interstitial-vacancy pairs. Defect formation was also found to be affected by growth temperature and flux stoichiometry.

CAS CERN Accelerator School. 5. Advanced accelerator physics course. Proceedings. Vol. 2

International Nuclear Information System (INIS)

Turner, S.

1995-01-01

The fifth CERN Accelerator School (CAS) advanced course on Accelerator Physics was given at the Paradise Hotel, Rhodes, Greece from 20 September to 1 October 1993. Its syllabus was based on the previous similar courses held at Oxford 1985, Berlin 1987, Uppsala 1989 and Noordwijkerhout 1991, and whose proceedings were published as CERN Reports 97-03, 89-01, 90-04 and 92-01, respectively. The present volumes are intended to replace and to bring up to date all the material in earlier publications. They contain not only all the lectures given in the Rhodes course but a number of important contributions to previous courses which are thought to be essential for a complete understanding of all aspects of the design and construction of particle accelerators at an advanced level. They include sections on Hamiltonian equations and accelerator optics, chromaticity and dynamic beam aperture, particle tracking, the kinetic theory, longitudinal beam optics, coherent instabilities, beam-beam dynamics, intra-beam scattering, beam cooling, Schottky noise, beam radiation, neutralisation, beam polarisation, radio-frequency quadrupoles, as well as chapters on space charge, superconducting magnets, crystal bending, beam-beam measurement and accelerator medical applications. (orig.)

CAS CERN Accelerator School. 5. Advanced accelerator physics course. Proceedings. Vol. 1

Energy Technology Data Exchange (ETDEWEB)

Turner, S [ed.

1995-11-22

The fifth CERN Accelerator School (CAS) advanced course on Accelerator Physics was given at the Paradise Hotel, Rhodes, Greece from 20 September to 1 October 1993. Its syllabus was based on the previous similar courses held at Oxford 1985, Berlin 1987, Uppsala 1989 and Noordwijkerhout 1991, and whose proceedings were published as CERN Reports 87-03, 89-01, 90-04 and 92-01, respectively. The present volumes are intended to replace and to bring up to date all the material in earlier publications. They contain not only all the lectures given in the Rhodes course but a number of important contributions to previous courses which are thought to be essential for a complete understanding of all aspects of the design and construction of particle accelerators at an advanced level. They include sections on Hamiltonian equations and accelerator optics, chromaticity and dynamic beam aperture, particle tracking, the kinetic theory, longitudinal beam optics, coherent instabilities, beam-beam dynamics, intra-beam scattering, beam cooling, Schottky noise, beam radiation, neutralisation, beam polarisation, radio-frequency quadrupoles, as well as chapters on space charge, superconducting magnets, crystal bending, beam-beam measurement and accelerator medical applications. (orig.).

CAS CERN Accelerator School. 5. Advanced accelerator physics course. Proceedings. Vol. 1

International Nuclear Information System (INIS)

Turner, S.

1995-01-01

The fifth CERN Accelerator School (CAS) advanced course on Accelerator Physics was given at the Paradise Hotel, Rhodes, Greece from 20 September to 1 October 1993. Its syllabus was based on the previous similar courses held at Oxford 1985, Berlin 1987, Uppsala 1989 and Noordwijkerhout 1991, and whose proceedings were published as CERN Reports 87-03, 89-01, 90-04 and 92-01, respectively. The present volumes are intended to replace and to bring up to date all the material in earlier publications. They contain not only all the lectures given in the Rhodes course but a number of important contributions to previous courses which are thought to be essential for a complete understanding of all aspects of the design and construction of particle accelerators at an advanced level. They include sections on Hamiltonian equations and accelerator optics, chromaticity and dynamic beam aperture, particle tracking, the kinetic theory, longitudinal beam optics, coherent instabilities, beam-beam dynamics, intra-beam scattering, beam cooling, Schottky noise, beam radiation, neutralisation, beam polarisation, radio-frequency quadrupoles, as well as chapters on space charge, superconducting magnets, crystal bending, beam-beam measurement and accelerator medical applications. (orig.)

CAS CERN Accelerator School. 5. Advanced accelerator physics course. Proceedings. Vol. 2

Energy Technology Data Exchange (ETDEWEB)

Turner, S [ed.

1995-11-22

The fifth CERN Accelerator School (CAS) advanced course on Accelerator Physics was given at the Paradise Hotel, Rhodes, Greece from 20 September to 1 October 1993. Its syllabus was based on the previous similar courses held at Oxford 1985, Berlin 1987, Uppsala 1989 and Noordwijkerhout 1991, and whose proceedings were published as CERN Reports 97-03, 89-01, 90-04 and 92-01, respectively. The present volumes are intended to replace and to bring up to date all the material in earlier publications. They contain not only all the lectures given in the Rhodes course but a number of important contributions to previous courses which are thought to be essential for a complete understanding of all aspects of the design and construction of particle accelerators at an advanced level. They include sections on Hamiltonian equations and accelerator optics, chromaticity and dynamic beam aperture, particle tracking, the kinetic theory, longitudinal beam optics, coherent instabilities, beam-beam dynamics, intra-beam scattering, beam cooling, Schottky noise, beam radiation, neutralisation, beam polarisation, radio-frequency quadrupoles, as well as chapters on space charge, superconducting magnets, crystal bending, beam-beam measurement and accelerator medical applications. (orig.).

Magnetoresistance oscillations in GaAs/AlGaAs superlattices subject to in-plane magnetic fields

Czech Academy of Sciences Publication Activity Database

Smrčka, Ludvík; Vašek, Petr; Svoboda, Pavel; Goncharuk, Natalya; Pacherová, Oliva; Krupko, Yuriy; Sheikin, Y.; Wegscheider, W.

2006-01-01

Roč. 34, - (2006), s. 632-635 ISSN 1386-9477 R&D Projects: GA AV ČR(CZ) IAA1010408 Institutional research plan: CEZ:AV0Z10100521 Keywords : superlattice * Fermi surface * magnetoresistance Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.084, year: 2006

Hydrostatic pressure and strain effects in short period InN/GaN superlattices

DEFF Research Database (Denmark)

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

2012-01-01

The electronic structures of short-period pseudomorphically grown superlattices (SLs) of the form mInN/nGaN are calculated and the band gap variation with the well and the barrier thicknesses is discussed including hydrostatic pressure effects. The calculated band gap shows a strong dependence...... strongly on the strain conditions and SL geometry, but weakly on the applied external hydrostatic pressure....

Raman Scattering and Surface Photovoltage Spectroscopy Studies of InGaAs/GaAs Radial Superlattices

Science.gov (United States)

Angelova, T.; Cros, A.; Ivanov, Ts.; Donchev, V.; Cantarero, A.; Shtinkov, N.; Deneke, Ch.; Schmidt, O. G.

2011-12-01

In this work we get insight into the multilayer structure of rolled-up microtube radial superlattices (RSLs) by the study of the optical and folded acoustic phonon modes of individual microtubes. Raman results show shifts of the InGaAs and GaAs related longitudinal optical modes that can be related to the strain state of the tubes. The folding of the acoustic modes has been related with the periodicity of the artificial superlattice formed by the multiple turns of the heterostructures. Information on the electronic structure and optical transitions of RSLs has been obtained by surface photovoltage spectroscopy. Room temperature spectra reveal several electronic transitions with energies below 1.3 eV. These transitions have been identified as originating from defect levels at the interfaces, as well as from the RSLs and the In0.215Ga0.785As/GaAs quantum well in the unfolded regions of the sample.

The phase diagrams and the order parameters of the diluted superlattice with antiferromagnetic interface coupling

International Nuclear Information System (INIS)

Oubelkacem, A.; El Aouad, N.; Bentaleb, M.; Laaboudi, B.; Saber, M.

2004-01-01

Using the effective field theory with a probability distribution technique that accounts for the self-spin correlation functions, the magnetic properties of the diluted Ising superlattice consisting of two ferromagnetic materials A and B, with L a layers of diluted spins S a =((1)/(2)) and L b layers of diluted spins S b =1 with antiferromagnetic interface coupling are examined. For fixed values of the reduced exchange interactions and the concentration c of magnetic atoms, the phase diagrams, the two sublattice magnetizations and the total magnetization for the superlattice with the same spin S a =S b =((1)/(2)) and for S a =((1)/(2)), S b =1 are studied as a function of the temperature. We find a number of characteristic phenomena. In particular, the effect of the concentration c of magnetic atoms, the interlayer coupling and the layer thickness on both the compensation temperature and the magnetization profiles are clarified

Modelling and measurement of bandgap behaviour in medium-wavelength IR InAs/InAs0.815Sb0.185 strained-layer superlattices

Science.gov (United States)

Letka, Veronica; Keen, James; Craig, Adam; Marshall, Andrew R. J.

2017-10-01

InAs/InAs1-xSbx type-II strained-layer superlattices (SLS) are a structure with potential infrared detection applications, owing to its tunable bandgap and suppressed Auger recombination. A series of medium-wavelength infrared (MWIR) InAs/InAs0.815Sb0.185 SLS structures, grown as undoped absorption epilayers on GaAs, were fabricated using molecular beam epitaxy in order to study the dependence of the ground state transitions on temperature and superlattice period thickness. Photoluminescence peaks at 4 K were obtained with the use of a helium-cooled micro-PL system and an InSb detector, and temperature-dependent absorption spectra were measured in the range 77 K - 300 K on a Fourier Transform Infrared (FTIR) spectrometer, equipped with a 1370 K blackbody source and a DTGS detector. An nBn device sample with the absorber structure identical to one of the undoped samples was also grown and processed with the goal of measuring temperature-dependent spectral response. A model for superlattice band alignment was also devised, incorporating the Bir-Pikus transformation results for uniaxial and biaxial strain, and the Einstein oscillator model for bandgap temperature dependence. Absorption coefficients of several 1000 cm-1 throughout the entire MWIR range are found for all samples, and temperature dependence of the bandgaps is extracted and compared to the model. This and photoluminescence data also demonstrate bandgap shifts consistent with the different superlattice periods of the three samples.

Neutron interrogation systems using pyroelectric crystals and methods of preparation thereof

Science.gov (United States)

Tang, Vincent; Meyer, Glenn A.; Falabella, Steven; Guethlein, Gary; Rusnak, Brian; Sampayan, Stephen; Spadaccini, Christopher M.; Wang, Li-Fang; Harris, John; Morse, Jeff

2017-08-01

According to one embodiment, an apparatus includes a pyroelectric crystal, a deuterated or tritiated target, an ion source, and a common support coupled to the pyroelectric crystal, the deuterated or tritiated target, and the ion source. In another embodiment, a method includes producing a voltage of negative polarity on a surface of a deuterated or tritiated target in response to a temperature change of a pyroelectric crystal, pulsing a deuterium ion source to produce a deuterium ion beam, accelerating the deuterium ion beam to the deuterated or tritiated target to produce a neutron beam, and directing the ion beam onto the deuterated or tritiated target to make neutrons using a voltage of the pyroelectric crystal and/or an HGI surrounding the pyroelectric crystal. The directionality of the neutron beam is controlled by changing the accelerating voltage of the system. Other apparatuses and methods are presented as well.

Charge superlattice effects on the electronic structure of a model acceptor graphite intercalation compound

International Nuclear Information System (INIS)

Campagnoli, G.; Tosatti, E.

1981-08-01

In the present attempt we have considered a model ordered situation (a super-superlattice) where starting from a basic stoichiometry C 8 X, a fraction 1/3 of the molecules acquire one electron, the remaining 2/3 being left neutral. We have performed an electronic structure calculation using tight-binding plus electrostatic (Hartree) self-consistency

Parallelization for X-ray crystal structural analysis program

Energy Technology Data Exchange (ETDEWEB)

Watanabe, Hiroshi [Japan Atomic Energy Research Inst., Tokyo (Japan); Minami, Masayuki; Yamamoto, Akiji

1997-10-01

In this report we study vectorization and parallelization for X-ray crystal structural analysis program. The target machine is NEC SX-4 which is a distributed/shared memory type vector parallel supercomputer. X-ray crystal structural analysis is surveyed, and a new multi-dimensional discrete Fourier transform method is proposed. The new method is designed to have a very long vector length, so that it enables to obtain the 12.0 times higher performance result that the original code. Besides the above-mentioned vectorization, the parallelization by micro-task functions on SX-4 reaches 13.7 times acceleration in the part of multi-dimensional discrete Fourier transform with 14 CPUs, and 3.0 times acceleration in the whole program. Totally 35.9 times acceleration to the original 1CPU scalar version is achieved with vectorization and parallelization on SX-4. (author)

Influence of substrate quality on structural properties of AlGaN/GaN superlattices grown by molecular beam epitaxy

Energy Technology Data Exchange (ETDEWEB)

Schubert, F. [NaMLab gGmbH, Nöthnitzer Straße 64, 01187 Dresden (Germany); Merkel, U.; Schmult, S. [TU Dresden, Institute of Semiconductors and Microsystems, Nöthnitzer Straße 64, 01187 Dresden (Germany); Mikolajick, T. [NaMLab gGmbH, Nöthnitzer Straße 64, 01187 Dresden (Germany); TU Dresden, Institute of Semiconductors and Microsystems, Nöthnitzer Straße 64, 01187 Dresden (Germany)

2014-02-28

Short-period AlGaN/GaN superlattices were established as versatile test structures to investigate the structural properties of molecular beam epitaxy (MBE)-grown GaN and AlGaN layers and their dependence on the GaN substrate quality. X-ray diffractometry data of the investigated superlattices allow access to relevant structural parameters such as aluminum mole fraction and layer thicknesses. The occurrence of theoretically predicted intense high-order satellite peaks and pronounced interface fringes in the diffraction pattern reflects abrupt interfaces and perfect 2-dimensional growth resulting in smooth surfaces. The data unambiguously demonstrate that the structural quality of the MBE grown layers is limited by the structural properties of the GaN substrate.

Electron Backscatter Diffraction Studies on the Formation of Superlattice Metal Hydride Alloys

Directory of Open Access Journals (Sweden)

Shuli Yan

2017-12-01

Full Text Available Microstructures of a series of La-Mg-Ni-based superlattice metal hydride alloys produced by a novel method of interaction of a LaNi5 alloy and Mg vapor were studied using a combination of X-ray energy dispersive spectroscopy and electron backscatter diffraction. The conversion rate of LaNi5 increased from 86.8% into 98.2%, and the A2B7 phase abundance increased from 42.5 to 45.8 wt % and reduced to 39.2 wt % with the increase in process time from four to 32 h. During the first stage of reaction, Mg formed discrete grains with the same orientation, which was closely related to the orientation of the host LaNi5 alloy. Mg then diffused through the ab-phase of LaNi5 and formed the AB2, AB3, and A2B7 phases. Diffusion of Mg stalled at the grain boundary of the host LaNi5 alloy. Good alignments in the c-axis between the newly formed superlattice phases and LaNi5 were observed. The density of high-angle grain boundary decreased with the increase in process time and was an indication of lattice cracking.

Intrinsic to extrinsic phonon lifetime transition in a GaAs–AlAs superlattice

International Nuclear Information System (INIS)

Hofmann, F; Garg, J; Chen, G; Maznev, A A; Nelson, K A; Jandl, A; Bulsara, M; Fitzgerald, E A

2013-01-01

We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon–phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the ‘interfacial atomic disorder’ model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness. (paper)

Intrinsic to extrinsic phonon lifetime transition in a GaAs-AlAs superlattice.

Science.gov (United States)

Hofmann, F; Garg, J; Maznev, A A; Jandl, A; Bulsara, M; Fitzgerald, E A; Chen, G; Nelson, K A

2013-07-24

We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon-phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the 'interfacial atomic disorder' model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness.

Characterization and crystal defects of the new YBa2Cu3O7-x superconductor

International Nuclear Information System (INIS)

Boulesteix, C.; Ben Salem, M.; Mokrani, R.

1987-01-01

YBa 2 Cu 3 O 7-x superconductors from different origins have been studied by electron microscopy and X ray emission. This material has an easy clevage plane parallel to (001) making its observation easy. It has been shown that the preparation reaction was not complete for some superconducting materials. YBa 2 Cu 3 O 7-x crystals can be twinned (mechanical twins, ferroelastic material) but they are generally not, or have few twins. Another oxide has been encountered in epitaxial growth on YBa 2 Cu 3 O 7-x . A superlattice perpendicular to the common c axis has been observed probably due to a regular stacking of the both oxides [fr

MSM optical detector on the basis of II-type ZnSe/ZnTe superlattice

Energy Technology Data Exchange (ETDEWEB)

Kuznetzov, P. I., E-mail: pik218@ire216.msk.su; Averin, S. V., E-mail: sva278@ire216.msk.su; Zhitov, V. A.; Zakharov, L. Yu.; Kotov, V. M. [Russian Academy of Sciences, Kotel’nikov Institute of Radioengineering and Electronics (Fryazino Branch) (Russian Federation)

2017-02-15

On the basis of a type-II ZnSe/ZnTe superlattice, a MSM (metal—semiconductor–metal) photodetector is fabricated and investigated. The detector features low dark currents and a high sensitivity. The spectral characteristic of the detector provides the possibility of the selective detection of three separate spectral portions of visible and near-infrared radiation.

Manipulation of resonant tunneling by substrate-induced inhomogeneous energy band gaps in graphene with square superlattice potentials

International Nuclear Information System (INIS)

Li, Guanqiang; Chen, Guangde; Peng, Ping; Cao, Zhenzhou; Ye, Honggang

2013-01-01

We investigate the resonant transmission of Dirac electrons through inhomogeneous band gap graphene with square superlattice potentials by transfer matrix method. The effects of the incident angle of the electrons, Fermi energy and substrate-induced Dirac gaps on the transmission are considered. It is found that the Dirac gap of graphene adds another degree of freedom with respect to the incident angle, the Fermi energy and the parameters of periodic superlattice potentials (i.e., the number, width and height of the barriers) for the transmission. In particular, the inhomogeneous Dirac gap induced by staggered substrates can be used to manipulate the transmission. The properties of the conductance and Fano factor at the resonant peaks are found to be affected by the gaps significantly. The results may be helpful for the practical application of graphene-based electronic devices

Fiscal 1974 Sunshine Project result report. R and D on photovoltaic power generation system (R and D on particle non-accelerating growth Si thin film crystal); 1974 nendo taiyoko hatsuden system no kenkyu kaihatsu seika hokokusho. Ryushi hikasoku seichogata silicon usumaku kessho no kenkyu kaihatsu

Energy Technology Data Exchange (ETDEWEB)

NONE

1975-05-28

This research aims at development of the technology for producing photovoltaic power generation systems at a cost less than 1/100 of those by current technology. In fiscal 1974, basic study was made on formation technology of particle non-accelerating growth Si thin film crystals. In addition, evaluation was made on formed thin film crystal characteristics, and studies were also made on junction formation for thin film crystals, and on thin film formation and junction formation for indium phosphide compound semiconductor thin films. The research includes (1) study on formation technology for particle non-accelerating growth Si thin film crystals, (2) evaluation on Si thin film crystals, (3) study on junction formation technology for Si thin film crystals, and (4) study on indium phosphide compound semiconductors. Evaluations were made on thin film formation technology by CVD, and on crystallographical and electrical characteristics of the formed thin films. The evaluation results clarified the compatibility between substrates and Si thin films, the formation condition of columnar structure films, and the effect of growth conditions on a carrier density or mobility. (NEDO)

Isothermal crystallization and melting behavior of polypropylene/layered double hydroxide nanocomposites

International Nuclear Information System (INIS)

Lonkar, Sunil P.; Singh, R.P.

2009-01-01

The effect of layered double hydroxide (LDH) nanolayers on the crystallization behavior of polypropylene (PP) was studied based on the preparation of nanocomposites by a melt intercalation method. The isothermal crystallization kinetics and subsequent melting behavior of PP/LDH hybrids were studied with differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray diffraction (WAXD). Studies revealed that the LDH promoted heterogeneous nucleation, accelerating the crystallization of PP. The Avrami equation successfully describes the isothermal crystallization kinetics of PP/LDH hybrids and signifies heterogeneous nucleation in crystal growth of PP. The varying values of Avrami exponent (n) and half crystallization time (t 1/2 ) of PP and PP/LDH hybrids describes overall crystallization behavior. The crystallite size (D hkl ) and distribution of different crystallites in PP varied in presence of LDH. A significant increase in melting temperature is observed for PP/LDH hybrids. The POM showed that smaller and less perfect crystals were formed in nanocomposites because of molecular interaction between PP chains and LDH. The value of fold surface free energy (σ e ) of PP chains decreased with increasing LDH content. Finally, the overall results signify that LDH at nanometer level acted as nucleating agent and accelerate the overall crystallization process of PP.

Laser induced structural transformation in chalcogenide based superlattices

Energy Technology Data Exchange (ETDEWEB)

Zallo, Eugenio, E-mail: zallo@pdi-berlin.de; Wang, Ruining; Bragaglia, Valeria; Calarco, Raffaella [Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin (Germany)

2016-05-30

Superlattices made of alternating layers of nominal GeTe and Sb{sub 2}Te{sub 3} have been studied by micro-Raman spectroscopy. A structural irreversible transformation into ordered GeSbTe alloy is induced by high power laser light exposure. The intensity ratio of anti-Stokes and Stokes scattering under laser illumination gives a maximum average temperature in the sample of 177 °C. The latter is lower than the growth temperature and of 400 °C necessary by annealing to transform the structure in a GeSbTe alloy. The absence of this configuration after in situ annealing even up to 300 °C evidences an electronic excitation induced-transition which brings the system into a different and stable crystalline state.

Laser induced structural transformation in chalcogenide based superlattices

International Nuclear Information System (INIS)

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

2016-01-01

Superlattices made of alternating layers of nominal GeTe and Sb 2 Te 3 have been studied by micro-Raman spectroscopy. A structural irreversible transformation into ordered GeSbTe alloy is induced by high power laser light exposure. The intensity ratio of anti-Stokes and Stokes scattering under laser illumination gives a maximum average temperature in the sample of 177 °C. The latter is lower than the growth temperature and of 400 °C necessary by annealing to transform the structure in a GeSbTe alloy. The absence of this configuration after in situ annealing even up to 300 °C evidences an electronic excitation induced-transition which brings the system into a different and stable crystalline state.

Emergent chirality in the electric polarization texture of titanate superlattices.

Science.gov (United States)

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

2018-01-30

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

Coherent Phonon Dynamics in Short-Period InAs/GaSb Superlattices

OpenAIRE

Noe, G. T.; Haugan, H. J.; Brown, G. J.; Sanders, G. D.; Stanton, C. J.; Kono, J.

2011-01-01

We have performed ultrafast pump-probe spectroscopy studies on a series of InAs/GaSb-based short-period superlattice (SL) samples with periods ranging from 46 \\AA to 71 \\AA. We observe two types of oscillations in the differential reflectivity with fast ($\\sim$ 1- 2 ps) and slow ($\\sim$ 24 ps) periods. The period of the fast oscillations changes with the SL period and can be explained as coherent acoustic phonons generated from carriers photoexcited within the SL. This mode provides an accura...

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

International Nuclear Information System (INIS)

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

2010-01-01

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

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

Energy Technology Data Exchange (ETDEWEB)

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

2014-10-13

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

Theory of the negative differential conductivity effect in semiconductor superlattices

International Nuclear Information System (INIS)

Vo Hong Anh; Nguyen Hong Shon; Le Vu Ky

1990-01-01

A new mechanism of the negative differential conductivity (NDC) effect in semiconductor superlattices (SL) is proposed and analysed that is due to the conduction electron trapping by donor centers. It is shown that the NDC effect occurs for sufficently high (but reasonable) impurity concentration and not too large value of the τ ε /τ c ratio (where τ ε is the electron energy relaxation time and τ c the electron life time in the conduction band) when the applied d.c. electric field reaches certain critical value defined by the physical parameters of the sample. (author). 8 refs, 2 figs

Emergence of the sub-THz central peak at phase transitions in artificial BaTiO3/(Ba,Sr)TiO3 superlattices

International Nuclear Information System (INIS)

Tikhonov, Yu.A.; Razumnaya, A.G.; Torgashev, V.I.; Zakharchenko, I.N.; Yuzyuk, Yu.I.; El Marssi, M.; Ortega, N.; Kumar, A.; Katiyar, R.S.

2015-01-01

A prominent central peak in the sub-THz frequency range was observed in the Raman spectra of BaTiO 3 /(Ba,Sr)TiO 3 (BT/BST) superlattice grown on (001)MgO substrate. Both soft and central mode show an anomaly around 200 K and 280 K, which can be correlated with orthorhombic to monoclinic phase transition of BST and BT, respectively. The observed temperature dependence of the central mode enabled us to explain rather broad temperature dependence of the dielectric permittivity previously observed in BT/BST superlattices. (copyright 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

The phase diagrams and the order parameters of the diluted transverse superlattice with antiferromagnetic interface coupling

International Nuclear Information System (INIS)

Oubelkacem, A.; El Aouad, N.; Benaboud, A.; Saber, M.

2004-01-01

Using the effective field theory with a probability distribution technique that accounts for the self-spin correlation functions, the magnetic properties of the Ising superlattice consisting of two ferromagnetic materials A and B, with L a layers of diluted spins S a =((1)/(2)) and L b layers of diluted spins S b =1 in an applied transverse field Ω with antiferromagnetic interface coupling are examined. For fixed values of the reduced exchange interactions and the concentration c of magnetic atoms, the phase diagrams and the total magnetization for the superlattice are studied as a function of the transverse field and the temperature. We find a number of characteristic phenomena. In particular, the effect of the concentration c of magnetic atoms, the interlayer coupling and the transverse field on both the compensation temperature and the magnetization profiles are clarified. Some of them may be related to the experimental works of rare-earth (RE)/transition metal (TM) multilayer films

Creep deformation-induced antiphase boundaries in L12-containing single-crystal cobalt-base superalloys

International Nuclear Information System (INIS)

Eggeler, Yolita M.; Titus, Michael S.; Suzuki, Akane; Pollock, Tresa M.

2014-01-01

Creep-induced antiphase boundaries (APBs) in new Co-base single-crystal superalloys with coherent embedded L1 2 -γ′ precipitates have been observed. APBs formed during single-crystal tensile creep tests performed at 900 °C under vacuum at stresses between 275 and 310 MPa. The alloys investigated contained 30–39 at.% Ni, which was added to the Co–Al–W ternary system to expand the γ–γ′ phase field and increase the γ′-solvus. Transmission electron microscopy (TEM) using two-beam conditions with fundamental and superlattice reflections was performed for defect characterization. The Burgers vector b of dislocations associated with the APBs was determined to be of type b = a 0 /2[011] and a 0 /2[011 ¯ ]. The displacement vectors, R, of the APBs matched the dislocation Burgers vectors, with R = b = a 0 /2[011]. APBs were observed in nearly every precipitate beyond 0.5% creep strain for the compositions investigated. The implications for high-temperature properties are discussed

Spin echo dynamics under an applied drift field in graphene nanoribbon superlattices

Energy Technology Data Exchange (ETDEWEB)

Prabhakar, Sanjay, E-mail: sprabhakar@wlu.ca [M 2NeT Laboratory, Wilfrid Laurier University, 75 University Avenue West, Waterloo, Ontario N2L 3C5 (Canada); Melnik, Roderick [M 2NeT Laboratory, Wilfrid Laurier University, 75 University Avenue West, Waterloo, Ontario N2L 3C5 (Canada); Gregorio Millan Institute, Universidad Carlos III de Madrid, 28911 Leganes (Spain); Bonilla, Luis L. [Gregorio Millan Institute, Universidad Carlos III de Madrid, 28911 Leganes (Spain); Raynolds, James E. [Drinker Biddle and Reath LLP, Washington, DC 20005 (United States)

2013-12-02

We investigate the evolution of spin dynamics in graphene nanoribbon superlattices (GNSLs) with armchair and zigzag edges in the presence of a drift field. We determine the exact evolution operator and show that it exhibits spin echo phenomena due to rapid oscillations of the quantum states along the ribbon. The evolution of the spin polarization is accompanied by strong beating patterns. We also provide detailed analysis of the band structure of GNSLs with armchair and zigzag edges.

Spin echo dynamics under an applied drift field in graphene nanoribbon superlattices

International Nuclear Information System (INIS)

Prabhakar, Sanjay; Melnik, Roderick; Bonilla, Luis L.; Raynolds, James E.

2013-01-01

We investigate the evolution of spin dynamics in graphene nanoribbon superlattices (GNSLs) with armchair and zigzag edges in the presence of a drift field. We determine the exact evolution operator and show that it exhibits spin echo phenomena due to rapid oscillations of the quantum states along the ribbon. The evolution of the spin polarization is accompanied by strong beating patterns. We also provide detailed analysis of the band structure of GNSLs with armchair and zigzag edges

Planar channeling and quasichanneling oscillations in a bent crystal

International Nuclear Information System (INIS)

Sytov, A.I.; Guidi, V.; Bagli, E.; Bandiera, L.; Germogli, G.; Mazzolari, A.; Tikhomirov, V.V.

2016-01-01

Particles passing through a crystal under planar channeling are captured by a continuous potential and experience transverse oscillations in their motion. As channeled particles approach the atomic planes, they are likely to be dechanneled. This effect is being used in ion-beam analysis with MeV energy. We study this effect in a bent crystal for positive and negative particles within a wide range of energies in sight of application of such crystals at accelerators. We look for the conditions for the observation or not of channeling oscillations in the deflection angle distribution in experiments where the beam passes through the bent crystal. Indeed a new kind of oscillations in the deflection angle distribution, strictly related to the motion of over-barrier particles, i.e. quasichanneled particles, is predicted. Such oscillations, named planar quasichanneling oscillations, possess a different nature than channeling oscillations. Through computer simulation, we study this effect and provided a theoretical interpretation for them. We show that channeling oscillations can be observed only for positive particles while quasichanneling oscillations can exist for particles with either sign. The conditions for experimental observation of channeling and quasichanneling oscillations at existing accelerators with available crystal are found and optimized. (orig.)

Planar channeling and quasichanneling oscillations in a bent crystal

Energy Technology Data Exchange (ETDEWEB)

Sytov, A.I. [Universita di Ferrara, Dipartimento di Fisica e Scienze della Terra, Ferrara (Italy); Belarusian State University, Research Institute for Nuclear Problems, Minsk (Belarus); INFN, Ferrara (Italy); Guidi, V.; Bagli, E.; Bandiera, L.; Germogli, G.; Mazzolari, A. [Universita di Ferrara, Dipartimento di Fisica e Scienze della Terra, Ferrara (Italy); INFN, Ferrara (Italy); Tikhomirov, V.V. [Belarusian State University, Research Institute for Nuclear Problems, Minsk (Belarus); INFN, Ferrara (Italy)

2016-02-15

Particles passing through a crystal under planar channeling are captured by a continuous potential and experience transverse oscillations in their motion. As channeled particles approach the atomic planes, they are likely to be dechanneled. This effect is being used in ion-beam analysis with MeV energy. We study this effect in a bent crystal for positive and negative particles within a wide range of energies in sight of application of such crystals at accelerators. We look for the conditions for the observation or not of channeling oscillations in the deflection angle distribution in experiments where the beam passes through the bent crystal. Indeed a new kind of oscillations in the deflection angle distribution, strictly related to the motion of over-barrier particles, i.e. quasichanneled particles, is predicted. Such oscillations, named planar quasichanneling oscillations, possess a different nature than channeling oscillations. Through computer simulation, we study this effect and provided a theoretical interpretation for them. We show that channeling oscillations can be observed only for positive particles while quasichanneling oscillations can exist for particles with either sign. The conditions for experimental observation of channeling and quasichanneling oscillations at existing accelerators with available crystal are found and optimized. (orig.)

Further results on cerium fluoride crystals

International Nuclear Information System (INIS)

Anderson, S.; Auffray, E.; Aziz, T.; Baccaro, S.; Banerjee, S.; Bareyre, P.; Barone, L.E.; Borgia, B.; Boutet, D.; Burq, J.P.; Chemarin, M.; Chipaux, R.; Dafinei, I.; D'Atanasio, P.; De Notaristefani, F.; Dezillie, B.; Dujardin, C.; Dutta, S.; Faure, J.L.; Fay, J.; Ferrere, D.; Francescangeli, O.; Fuchs, B.A.; Ganguli, S.N.; Gillespie, G.; Goyot, M.; Gupta, S.K.; Gurtu, A.; Heck, J.; Herve, A.; Hillemanns, H.; Holdener, F.; Ille, B.; Joensson, L.; Kierstead, J.; Krenz, W.; Kway, W.; Le Goff, J.M.; Lebeau, M.; Lebrun, P.; Lecoq, P.; Lemoigne, Y.; Loomis, G.; Lubelsmeyer, K.; Madjar, N.; Majni, G.; El Mamouni, H.; Mangla, S.; Mares, J.A.; Martin, J.P.; Mattioli, M.; Mauger, G.J.; Mazumdar, K.; Mengucci, P.; Merlo, J.P.; Moine, B.; Nikl, N.; Pansart, J.P.; Pedrini, C.; Poinsignon, J.; Polak, K.; Raghavan, R.; Rebourgeard, P.; Rinaldi, D.; Rosa, J.; Rosowsky, A.; Sahuc, P.; Samsonov, V.; Sarkar, S.; Schegelski, V.; Schmitz, D.; Schneegans, M.; Seliverstov, D.; Stoll, S.; Sudhakar, K.; Svensson, A.; Tonwar, S.C.; Topa, V.; Vialle, J.P.; Vivargent, M.; Wallraff, W.; Weber, M.J.; Winter, N.; Woody, C.; Wuest, C.R.; Yanovski, V.

1993-01-01

A systematic investigation of the properties of cerium fluoride monocrystals has been performed by the 'Crystal Clear' collaboration in view of a possible use of such crystals for the construction of high precision electromagnetic calorimeters for the future generation of high luminosity accelerators. A large sample of different crystals grown by several producers has been studied. The spectroscopic characteristics, the transmission, luminescence and excitation spectra and the decay time curves are analysed. The light yield of the different crystals is measured with photomultipliers and Si photodiodes and compared to reference standards like BGO and NaI(Tl). The radiation damage behaviour is then presented for γ and neutron irradiations, at different doses and dose rates, including thermal and optical bleaching. (orig.)

The Polarized Electron Source at ELSA

International Nuclear Information System (INIS)

Drachenfels, Wolther von; Frommberger, Frank; Gowin, Michael; Hillert, Wolfgang; Hoffmann, Markus; Neff, Bernhold

2003-01-01

At the electron stretcher accelerator ELSA in Bonn a pulsed 50 kV inverted gun of polarized electrons has been in operation since February 2000. A strained-layer superlattice crystal is used to deliver a beam with a polarization of about 80 %. A flashlamp-pumped Ti-Sapphire laser with a pulse repetition rate of 50 Hz serves as source of light. The gun is operated in space charge limitation. The current can be chosen by varying the distance between cathode and anode. With 1 μs pulses of 100 mA the source was particularly used together with a polarized target for a GDH sum rule experiment. The high photocathode lifetime allows continuous operation at 100 mA typically for periods of about two weeks without maintenance. So far no change of the crystal was necessary

Generation of intense spin-polarized electron beams at the electron accelerator facility ELSA

International Nuclear Information System (INIS)

Heiliger, Dominik

2014-08-01

The inverted source of polarized electrons at the electron accelerator ELSA in Bonn routinely provides a pulsed and low energetic beam of polarized electrons (100 mA, 48 keV) by irradiating a GaAs strained-layer superlattice photocathode with laser light. Due to the beam energy of 48 keV the beam transport to the linear accelerator is strongly space charge dominated and the actual beam current has an impact on the beam dynamics. Thus, the optics of the transfer line to the linear accelerator must be optimized with respect to the chosen beam intensity. An intensity upgrade including numerical simulations of the beam transport as well as a generation and a transport of a beam current of nearly 200 mA was successfully operated. In order to enhance the reliability and uptime of the source, a new extreme high vacuum load lock system was installed and commissioned. It consists of an activation chamber for heat cleaning of the photocathodes and activation with cesium and oxygen, a storage in which different types of photocathodes can be stored and a loading chamber in which an atomic hydrogen source is used to remove nearly any remaining surface oxidation. The new cleaning procedure with atomic hydrogen was investigated regarding its potential to restore the initial quantum efficiency of the photocathode after many activations.

Highly constrained ferroelectric [BaTiO3](1−x)Λ/[BaZrO3]xΛ superlattices: X-ray diffraction and Raman spectroscopy

International Nuclear Information System (INIS)

Belhadi, J.; El Marssi, M.; Gagou, Y.; El Mendili, Y.; Bouyanfif, H.; Yuzyuk, Yu. I.; Raevski, I. P.; Wolfman, J.

2014-01-01

We report an x-ray diffraction (XRD) and a Raman-scattering investigation of ferroelectric/paraelectric superlattices [BaTiO 3 ] (1−x)Λ /[BaZrO 3 ] xΛ for which the composition varied, 0.15 ≤ x ≤ 0.85, while the superlattice (SL) modulation period Λ was kept constant at about 100 Å. The samples were epitaxially grown by pulsed laser deposition on MgO substrates buffered with La 0.5 Sr 0.5 CoO 3 . Based on the XRD analysis and on polarized Raman spectra, we have showed that the large strain in SLs induced ferroelectricity in BaZrO 3 (BZ) for all SLs, a material that is paraelectric in the bulk form at any temperature and in the single film. The induced polar axis in BZ layers is perpendicular to the plane of substrate while BaTiO 3 (BT) layers exhibit in-plane polar orientation. Raman spectroscopy revealed a lattice ordering in SLs due to the misfit strain generated by the large lattice mismatch between the alternating BZ and BT layers. This strain induced a huge upward frequency of the lowest E(1TO) soft mode from 60 cm −1 in the BT single film to 215 cm −1 in the SL with x = 0.85. These results show that in spite of relatively large periodicity of SLs, they are highly constrained and the variation of BZ ratio allowed modifying strains between layers. The temperature dependence of the Raman spectra for BT 0.3Λ /BZ 0.7Λ and BT 0.7Λ /BZ 0.3Λ samples revealed giant shift of the ferroelectric phase transition. The phase transition temperature was found to be upshifted by about 300 °C with respect to BT single crystal.

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

Science.gov (United States)

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

2017-06-01

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

Report of the joint seminar on solid state physics, atomic and molecular physics, and materials science in the energy region of tandem accelerators

International Nuclear Information System (INIS)

Kazumata, Yukio

1993-01-01

The joint seminar on Solid State Physics, Atomic and Molecular Physics and Materials Science in the Energy Region of Tandem Acceleration was held at Tokai Research Establishment of JAERI, for two days from January 22 to 23, 1991. About 60 physicists and material scientists participated and 18 papers were presented in this seminar. The topics presented in this seminar included lattice defects in semiconductors, ion-solid collisions, atomic collisions by high energy particles, radiation effects on high T c superconducting materials and FCC metals, radiation effects on materials of space and fusion reactors, uranium compounds and superlattice. (J.P.N.)

Ionic Potential and Band Narrowing as a Source of Orbital Polarization in Nickelate/Insulator Superlattices

Science.gov (United States)

Georgescu, Alexandru B.; Disa, Ankit S.; Kumah, Divine P.; Ismail-Beigi, Sohrab; Walker, Frederick J.; Ahn, Charles H.

Nickelate interfaces display complex, interacting electronic properties such as thickness dependent metal-insulator transitions. One large body of effort involving nickelates has aimed to split the energies of the Ni 3d orbitals (orbital polarization) to make the resulting band structure resemble that of cuprate superconductors. The most commonly studied interfacial system involves superlattices of alternating nickelate and insulating perovksite-structure layers; the resulting orbital polarization at the nickelate-insulator interface is understood as being due to confinement or structural symmetry breaking. By using first principles theory on the NdNiO3/NdAlO3 superlattice, we show that another important source of orbital polarization stems from electrostatic effects: the more ionic nature of the cations in the insulator (when compared to the nickelate) can shift the relative orbital energies of the Ni. We use density functional theory (DFT) and add electronic correlations via slave-bosons to describe the effect of correlation-induced band narrowing on the orbital polarization. Work supported by NSF Grant MRSEC DMR-1119826.

The electronic states calculated using the sinusoidal potential for Cd1-xZnxS quantum dot superlattices

International Nuclear Information System (INIS)

Sakly, A.; Safta, N.; Mejri, H.; Lamine, A. Ben

2011-01-01

Research highlights: → This paper is dedicated to structures based on Cd 1-x Zn x S. - Abstract: The present work reports on a theoretical investigation of superlattices based on Cd 1-x Zn x S quantum dots embedded in an insulating material. The system to model is assumed to be a series of flattened cylindrical quantum dots with a finite barrier at the boundary and is studied using a sinusoidal potential. The electronic states of both Γ 1 - (ground) and Γ 2 - (first excited) minibands have been computed as a function of inter-quantum dot separation and Zn composition. An analysis of the results shows that the widths of Γ 1 - and Γ 2 - minibands decrease as the superlattice period and Zn content increase separately. Moreover, the sinusoidal shape of the confining potential accounts for the coupling between quantum dots quantitatively less than the Kronig-Penney potential model.

Bulk-like-phonon polaritons in one-dimensional photonic superlattices

Science.gov (United States)

Gómez-Urrea, H. A.; Duque, C. A.; Mora-Ramos, M. E.

2017-05-01

We investigate the properties of a one-dimensional photonic superlattice made of alternating layers of air and wurtzite aluminum nitride. The Maxwell equations are solved for any admissible values of the angle of incidence by means of the transfer matrix formalism. The band structure of the frequency spectrum is obtained, as well as the density of states and transmittance associated to both the TM and TE modes. The dispersion relations indicate that for oblique incidence and TM modes there is a component of the electric field oriented along the growth direction of the structure that couples with the longitudinal optical phonon oscillations of the aluminum nitride thus leading to the appearance of longitudinal phonon polaritons in the system.

Sequential magnetic switching in Fe/MgO(001) superlattices

Science.gov (United States)

Magnus, F.; Warnatz, T.; Palsson, G. K.; Devishvili, A.; Ukleev, V.; Palisaitis, J.; Persson, P. O. Å.; Hjörvarsson, B.

2018-05-01

Polarized neutron reflectometry is used to determine the sequence of magnetic switching in interlayer exchange coupled Fe/MgO(001) superlattices in an applied magnetic field. For 19.6 Å thick MgO layers we obtain a 90∘ periodic magnetic alignment between adjacent Fe layers at remanence. In an increasing applied field the top layer switches first followed by its second-nearest neighbor. For 16.4 Å MgO layers, a 180∘ periodic alignment is obtained at remanence and with increasing applied field the layer switching starts from the two outermost layers and proceeds inwards. This sequential tuneable switching opens up the possibility of designing three-dimensional magnetic structures with a predefined discrete switching sequence.

Compact and intense parametric x-ray radiation source based on a linear accelerator with cryogenic accelerating and decelerating copper structures

Science.gov (United States)

Hyun, J.; Satoh, M.; Yoshida, M.; Sakai, T.; Hayakawa, Y.; Tanaka, T.; Hayakawa, K.; Sato, I.; Endo, K.

2018-01-01

This paper describes a proposal for a compact x-ray source based on parametric x-ray radiation (PXR). The PXR, which is produced when a single crystal is bombarded with relativistic electrons, has good monochromaticity and spatial coherence, and is expected to be well suited for imaging of low-Z materials and medical application. The proposed system employs a pair of copper accelerating structures which are operated at a cryogenic temperature of 20 K and arranged to form a resonant ring configuration. The electron beam is once accelerated up to 75 MeV in one of the structures, being decelerated down to lower than 7 MeV in the other structure after generating PXR at a single crystal, and then dumped. The expected x-ray yield is 1 09 photons /s at a center energy of 15 keV or higher.

Superlattices assembled through shape-induced directional binding

Science.gov (United States)

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

2015-04-01

Organization of spherical particles into lattices is typically driven by packing considerations. Although the addition of directional binding can significantly broaden structural diversity, nanoscale implementation remains challenging. Here we investigate the assembly of clusters and lattices in which anisotropic polyhedral blocks coordinate isotropic spherical nanoparticles via shape-induced directional interactions facilitated by DNA recognition. We show that these polyhedral blocks--cubes and octahedrons--when mixed with spheres, promote the assembly of clusters with architecture determined by polyhedron symmetry. Moreover, three-dimensional binary superlattices are formed when DNA shells accommodate the shape disparity between nanoparticle interfaces. The crystallographic symmetry of assembled lattices is determined by the spatial symmetry of the block's facets, while structural order depends on DNA-tuned interactions and particle size ratio. The presented lattice assembly strategy, exploiting shape for defining the global structure and DNA-mediation locally, opens novel possibilities for by-design fabrication of binary lattices.

Temperature Dependence of Short-Range Order in β-Brass

DEFF Research Database (Denmark)

Dietrich, O.W.; Als-Nielsen, Jens Aage

1967-01-01

Critical scattering of neutrons around the superlattice reflections (1, 0, 0) and (1, 1, 1) from a single crystal of beta-brass has been measured at temperatures from 2 to 25deg C above the transition temperature. The temperature dependence of the critical peak intensity, proportional to the susc......Critical scattering of neutrons around the superlattice reflections (1, 0, 0) and (1, 1, 1) from a single crystal of beta-brass has been measured at temperatures from 2 to 25deg C above the transition temperature. The temperature dependence of the critical peak intensity, proportional...