Superlattice System Research Articles

Strain‐Mediated Inverse Photoresistivity in SrRuO3/La0.7Sr0.3MnO3 Superlattices

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.

Monte Carlo Investigation of the Magnetic and Thermal Properties of Four Sublattice Ferromagnetic– Antiferromagnetic Double-Layer Superlattices

Using Monte Carlo simulation, we have studied the magnetic and thermal properties of a four sublattice ferromagnetic (FM) and anti-FM (AFM) double-layer superlattices system in a longitudinal magnetic field. The effects of the intralayer AFM exchange coupling $J_{{2}}$ , the single-ion anisotropy ${D}$ and the longitudinal magnetic field ${h}$ on the magnetization ${M}$ , the susceptibility ${x}$ , the internal energy ${U}$ , and the specific heat ${c}$ of the system are discussed in detail. Particular attention has been paid to the variations of the hysteresis loops of the system with the AFM exchange coupling $J_{ {2}}$ and the anisotropy ${D}$ . Some interesting results, such as the triple hysteresis and the exchange bias behavior, have been found under certain conditions.

Identifying and Controlling the Behavior of Gold Nanoparticle-DNA Superlattice Systems

Identifying and Controlling the Behavior of Gold Nanoparticle-DNA Superlattice Systems

Effect of microscopic interface asymmetry on optical properties of short-period InAs/GaSb type-II superlattices

We theoretically investigate the effect of microscopic interface asymmetry (MIA) on the optical properties of short-period InAs/GaSb type-II superlattices (SLs) which can serve for the mid-infrared (mid-IR) detection. To calculate the band structures of such SLs, we use a modified eight-band Kane model which includes the MIA effect. With the band structures, we can evaluate the optical matrix elements and joint density of states for the corresponding SL systems. Based on these obtained results, we employ the Boltzmann equation approach to calculate the optical absorption coefficients for the corresponding SL systems. Theoretical calculations demonstrate that the MIA effect can greatly influence the electronic and optical properties of short-period InAs/GaSb type-II SLs. For the band structures, we find that the MIA effect causes strong interactions between different SL subbands and creates large spin splittings of these subbands. For the optical properties, the MIA effect activates the forbidden optical transition channels for TM polarization, enhances the absorption strength considerably and induces remarkable red-shifts for both transverse electric (TE) and transverse magnetic (TM) polarizations, where TE and TM refer to light polarizations along the in-plane and growth directions of SL structure. Different absorption features for TE and TM polarizations are well manifested by corresponding optical matrix elements. Moreover, in conjunction with recent experiment, we calculate the polarization-dependent absorption spectra for mid-IR InAs/GaSb type-II SLs. Our theoretical results can explain those observed experimentally. The present work sheds the significant importance of MIA effect in short-period InAs/GaSb type-II SLs.

Efficient photoinduced charge accumulation in reduced graphene oxide coupled with titania nanosheets to show highly enhanced and persistent conductance.

Tuning of the electrical properties of graphene via photoexcitation of a heteroassembled material has started to attract attention for electronic and optoelectronic applications. Actually photoinduced carrier doping from the hexagonal boron nitride (h-BN) substrate greatly modulated the transport property of the top layer graphene, showing promising potential for this approach. However, for practical applications, the large scale production of this two-dimensional heterostructure is needed. Here, a superlattice film constructed from reduced graphene oxide (rGO) and photoactive titania nanosheets (Ti0.87O2(0.52-)) was employed as a channel to construct a field effect transistor (FET) device, and its UV light response on the electrical transport property was examined. The UV light illumination induced significant improvement of the electrical conductance by ∼7 times on the basis of simultaneous enhancements of the electron carrier concentration and its mobility in rGO. Furthermore, the polarity of the FET response changed from ambipolar to n-type unipolar. Such modulated properties persisted in vacuum even after the UV light was turned off. These interesting behaviors may be explained in terms of photomodulation effects from Ti0.87O2(0.52-) nanosheets. The photoexcited electrons in Ti0.87O2(0.52-) are injected into rGO to increase the electron carrier concentration as high as 7.6×10(13) cm(-2). On the other hand, the holes are likely trapped in the Ti0.87O2(0.52-) nanosheets. These photocarriers undergo reduction and oxidation of oxygen and water molecules adsorbed in the film, respectively, which act as carrier scattering centers, contributing to the enhancement of the carrier mobility. Since the film likely contains more water molecules than oxygen, upon extinction of UV light, a major portion of electrons (∼80% of the concentration at the UV off) survives in rGO, showing the highly enhanced conductance for days. This surpassing photomodulated FET response and its persistency observed in the present superlattice system of rGO/Ti0.87O2(0.52-) are noteworthy compared with previous studies such as the device with a heteroassembly of graphene/h-BN.

Analytical formulas for carrier density and Fermi energy in semiconductors with a tight-binding band

Analytical formulas for evaluating the relation of carrier density and Fermi energy for semiconductors with a tight-binding band have been proposed. The series expansions for a carrier density with fast convergency have been obtained by means of a Bessel function. A simple and analytical formula for Fermi energy has been derived with the help of the Gauss integration method. The results of the proposed formulas are in good agreement with accurate numerical solutions. The formulas have been successfully used in the calculation of carrier density and Fermi energy in a miniband superlattice system. Their accuracy is in the order of 10−5.

Quantum phase diagram for Bosonic triangular superlattice system

We study the properties of the Bose-Hubbard model for the triangular superlattices. Firstly we give an easy experimental suggestion to produce a triangular supperlattice system. Then we use the generalized effective potential Landau theory to investigate the quantum phase diagram of the system. Our research provide reference data for the future study of the triangular superlattice system.

Kaleidoscope of symmetry-protected topological phases in one-dimensional periodically modulated lattices

We identify the existence of various symmetry-protected topological states in one-dimensional superlattices with periodically modulated hopping amplitudes or on-site potentials, which can be characterized by the quantized Berry phase $\pi$ or the emergence of a pair of degenerate boundary states. It is shown that there may exist three types of topological phases, which are protected by the inversion symmetry, the chiral symmetry, and both of them, respectively, depending on the modulations, the odd or even modulation period. The connection between the hopping and potential modulations is also discussed. Furthermore, we demonstrate that the topological phase protected by the inversion symmetry can be realized in the interacting boson systems trapped in the same superlattices. The results are very possibly studied experimentally in the superlattice systems engineered with state-of-art technologies.

Theory of interface and anharmonic phonon interactions in nanocomposite materials

We present a theory of phonon scattering rates resulting from mass smudging across interfaces and from anharmonicity in nanocomposite materials. We discuss the derivation of the anharmonic scattering term and present some recently published results indicating that if certain models of interface mass-mixing are adopted, the cross-planar thermal conductivity of an ultra-thin superlattice system will decrease dramatically with the number of bilayers n until n ≈ 4, above which value it begins to increase with n.

Effects of Thermal Annealing on the Thermoelectric and Optical Properties of SiO2/SiO2+Cu Nanolayer Thin Films

We have prepared multi-nanolayer superlattice thin-film systems comprising 36 alternating layers of SiO2 and SiO2+Cu nanolayers, of total thickness approximately 300 nm, by magnetron direct current–radio frequency sputtering. To modify their thermoelectric and optical properties, the films were placed in a furnace for annealing at temperatures between 500°C and 700°C, in air, for 1 h, to form quantum nano-dots and/or quantum clusters. Atomic force microscopy was used to analyze the surface of the thin-film systems. The thermoelectric and optical properties of the systems were characterized by study of ultraviolet–visible–near infrared absorption, fluorescence, and Raman spectroscopy, and by measurement of Seebeck coefficients. Seebeck coefficients increased from −70 μV/K to −100 μV/K when the temperature was increased from 0°C to 700°C. Optical absorption spectra showed that formation of nano-dots and/or nano-clustering also occurred as the temperature was increased. Thermal annealing affected the optical and thermal properties of the multi-nanolayer thermoelectric thin-film systems in the positive direction.

Investigations on Nonlinear Polariton Dispersion in Ferroelectric Superlattice System

AbstractSuperlattices have drawn considerable attention in the recent years. In this work, the behaviour of polaritons in a quantum well superlattice system is analysed both at the centre and at the edge of the brillouin zone using LiNbO3/ LiTaO3 as an example. The significance of the polariton modes are analysed. New modes due to nonlinearity on the polaritonic gap, where the propagation of electromagnetic wave is forbidden, are obtained in the system as suggested by some recent literature. The variation of frequency with the thickness is also studied.

Thermoelectric properties of Zn4Sb3/CeFe(4−)Co Sb12 nano-layered superlattices modified by MeV Si ion beam

We prepared multilayers of superlattice thin film system with 50 periodic alternating nano-layers of semiconducting half-Heusler β-Zn4Sb3 and skutterudite CeFe2Co2Sb12 compound thin films using ion beam assisted deposition (IBAD) with Au layers deposited on both sides as metal contacts. The deposited multilayer thin films have alternating layers about 5 nm thick. The total thickness of the multilayer system is 275 nm. The superlattices were then bombarded by 5 MeV Si ion at six different fluences to form nano-cluster structures. The film thicknesses and composition were monitored by Rutherford backscattering spectrometry (RBS) before and after MeV ion bombardment. We have measured the thermoelectric efficiency, Figure of Merit ZT, of the fabricated device by measuring the cross plane thermal conductivity by the 3rd harmonic (3ω) method, the cross plane Seebeck coefficient, and the electrical conductivity using the van der Pauw method before and after the MeV ion bombardments. We reached the remarkable thermoelectric Figure of Merit results at optimal fluences.

Doping effects in InN/GaN short-period quantum well structures—Theoretical studies based on density functional methods

Density functional theory studies were conducted to determine an influence of the carrier concentration on the optical and electronic properties of InN/GaN superlattice system. The oscillator strength values, energy gaps and the band profiles were obtained. The band profiles were found to be strongly affected for technically possible heavy n-type doping while for p-type doping the carrier influence, both screening and band shift, is negligible. Blue shift of the transition energy between conduction band minima and valence band maxima was observed for high concentrations of both type carriers.

Long-Period InAs/GaSb Type-II Superlattices for Terahertz Application

We present a theoretical study on the terahertz (THz) optoelectronic properties of long-period InAs/GaSb type-II super lattices (SLs). The eight-band k·p model is used to calculate the electronic structures of such SLs and on the basis of band structures, the Boltzmann equation approach is employed to calculate the optical absorption coefficients for the corresponding SL systems. It is found that long-period InAs/GaSb type-II SLs have a considerable absorption in the THz bandwidth. By examining the dependence of THz absorption coefficient on the InAs/GaSb layer widths, we demonstrate that with a proper choice of InAs/GaSb layer widths, an optimized THz absorption can be achieved. This study is pertinent to the potential application of InAs/GaSb type-II SLs as THz photo detectors.

Random Lasing over Gap States from a Quasi-One-Dimensional Amplifying Periodic-on-Average Random Superlattice

We report the experimental implementation and theoretical analysis of a random laser system that constitutes an amplifying periodic-on-average random superlattice (PARS). The stringent conditions on monodispersity required for a periodic-on-average random superlattice system are fulfilled using a linear array of spherical microresonators whose separation and size distribution can be controlled. Statistical studies of the lasing frequency reveal a frequency-controlled behavior. We perform transfer matrix calculations with gain to analyze the origin of the lasing modes, their thresholds, and frequency statistics. The results confirm that the experimentally observed lasing modes arise from states introduced into the stop gap of the underlying periodic system. On virtue of the fact that these high-quality gap states are restricted to a band of frequencies, the consequent random lasing exhibits significant reduction in frequency fluctuations.

Size dependent elastic moduli of CdSe nanocrystal superlattices predicted from atomistic and coarse grained models

Nanocrystal superlattices are materials formed by assembly of monodisperse nanocrystal building blocks that are tunable in composition, size, shape, and surface functionalization. Such materials offer the potential to realize unprecedented combinations of physical properties, but theoretical prediction of such properties, particularly elastic properties, remains a challenge. Here we report the Young's moduli, bulk moduli, and Poisson's ratios of CdSe nanocrystal superlattices computed from fully atomistic molecular dynamics simulations, coarse grained models, and effective medium theory. The atomistic simulations yield Young's moduli in the 4-5 GPa range, in agreement with previously reported results for similar nanocrystal superlattice systems. A clear increase of Young's modulus and bulk modulus with increasing nanocrystal core size is observed, while Poisson's ratio decreases slightly with core size. Effective medium theory overpredicts the moduli, and it is surmised that this arises from its neglect of the atomic-level details of the of the core-ligand interface. The coarse grained calculations, using existing nanocrystal interaction models from the literature, also show similar increases with core size but predict moduli that are two orders of magnitude lower than the present atomistic results and previous literature. It is concluded that coarse grained models, in their current form, are not appropriate for calculating elastic properties of nanocrystal superlattices and that fully atomistic models are better suited for this purpose.

Ultra-low switching power, crystallographic analysis, and switching mechanism for SnXTe100−X/Sb2Te3 diluted superlattice system

Ultra-low switching power (∼1/50th–1/2250th that of a Ge2Sb2Te5 device) was obtained in a SnXTe100−X/Sb2Te3 diluted superlattice (SL) device (X = 10, 20, and 35 at. %). XRD analysis showed that there was little coexistence of the SnTe/Sb2Te3 SL, Bi2Te3-type SnSbTe-alloy and Te phases. Detailed crystallographic analysis showed that there is a high probability that the SnSbTe-alloy phase independently changed into a SL structure. This self-assembled SL structure had a vacancy layer in a specific Te layer. Some phenomenon, such as Sn switching, in the self-assembled SL might lead to the ultra-low switching power.

Symmetry Protected Weak Topological Phases in a Superlattice

We explore novel topological phases realized in a superlattice system based on the Wilson-Dirac model. Our main focus is on a two-dimensional analogue of weak topological insulator phases. We find such phases as those characterized by gapless edge states that are protected by symmetry but sensitive to the orientation of the edge relative to the superlattice structure. We show that manifest and hidden reflection symmetries protect such weak topological phases, and propose bulk Z2 indices responsible for the topological protection of the edge states.

Topological Mott insulators of ultracold atomic mixtures induced by interactions in one-dimensional optical superlattices

We present exactly solvable examples that topological Mott insulators can emerge from topologically trivial states due to strong interactions between atoms for atomic mixtures trapped in one-dimensional optical superlattice systems. The topological Mott insulating state is characterized by nonzero Chern number and appears in the strongly interacting limit as long as the total band filling factor is an integer, which is not sensitive to the filling of each component. The topological nature of the Mott phase can be revealed by observing the density profile of the trapped system. Our results can be also generalized to the multi-component atomic systems.

Resonant tunneling: A method for simultaneous determination of resonance energy and energy eigenvalue

Assuming an effective mass approximation and using Bastard's boundary conditions, a simple method for simultaneous determination of the energy levels forming the sub-band structure and the transmissions coefficient of non-symmetrical, non-periodical potential semiconducting heterostructure is being proposed. The method can be applied on a multilayer system with varying thickness and effective mass of the layers, and with potential that is neither periodical nor symmetrical. To illustrate the feasibility of the proposed method, cases of symmetrical rectangular triple-barrier structure with constant effective mass, multi-barrier semiconductor heterostructure (nine barriers–eight-wells), and monomer height barrier superlattices (300 barriers) systems have been examined. Findings show very good agreements with previously published results obtained by different methods on similar systems. The proposed method was found to be useful for any number of semiconducting layers arranged in any random way making it more realistic, simple, and applicable to superlattice analysis and for devices design.