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The electronic properties of ZnIn2Se4 crystals have been studied experimentally by spectral ellipsometry and theoretically (from the first principles) using the density functional theory (DFT). Ellipsometric studies in the energy range of 0.7–6.5 eV made it possible to determine the imaginary and real parts of the dielectric function and optical conductivity, as well as the dispersion of the refractive indices, extinction coefficients, and absorption coefficients, and to estimate the values of the Urbach energy, plasma frequency, and nonlinear optical characteristics of ZnIn2Se4 crystals. The electronic band structure, origin of energy states, optical functions for incident light polarized along the crystal optic axis and perpendicular to it, and partial densities of states projected onto atoms are determined by ab initio calculations. The theoretical results are compared with the experimental data obtained by spectral ellipsometry. [ABSTRACT FROM AUTHOR]

Topological states of matter can be classified only in terms of global topological invariants. These global topological invariants are encoded in terms of global observable topological phase factors in the state vectors of electrons. In condensed matter, the energy spectrum of the Hamiltonian operator has a band structure, meaning that it is piecewise continuous. The energy in each continuous piece depends on the quasi-momentum which varies in the Brillouin zone. Thus, the Brillouin zone of quasi-momentum variables constitutes the base localization space of the energy eigenstates of electrons. This is a continuous topological parameter space bearing the homotopy of a torus. Since the base localization space has the homotopy of a torus, if we vary the quasi-momentum in a direction, when the edge of the zone is reached, we obtain a closed path. Then, if we lift this loop from the base space to the sections of the sheaf-theoretic fibration induced by the localization of the energy eigenfunctions, we obtain a global topological phase factor which encodes the topological structure of the Brillouin zone. Because it is homotopically equivalent to a torus, the global phase factor turns out to be quantized, taking integer values. The experimental significance of this model stems from the recent discovery that there are observable global topological phase factors in fairly ordinary materials. In this communication, we show that it is the unitary representation theory of the discrete Heisenberg group in terms of commutative modular symplectic variables, giving rise to a joint commutative representation space endowed with an integral and Z 2 -invariant symplectic form that articulates the specific form of the topological conditions characterizing both the quantum Hall effect and the spin quantum Hall effect under a unified sheaf-theoretic cohomological framework. [ABSTRACT FROM AUTHOR]

In this article, an exact strain gradient (SG) continuum model to capture the broadband bandgap characteristics of the prestressed diatomic lattice (PDL) with local/nonlocal interactions has been proposed. By considering the periodicity of wave motion, the wavelength-dependent Taylor expansion is established, leading to a desirable prediction when the wavelength is close to the lattice scale. For PDL, the dispersion curves of the proposed continuum model are always consistent with those of discrete model in the first Brillouin zone. The proposed SG continuum model is used to investigate the effects of structural parameters, orders of SG continua and various nonlocal interactions on bandgap properties of PDL. Continuum modelling of prestressed acoustic diatomic lattices. Perfect agreements achieved between dispersion diagrams by discrete and proposed continuum models. Effects of prestress, nonlocal interactions, mass and stiffness ratios on band gap structures analyzed. Proper strain gradient (SG) orders determined to guarantee a satisfactory accuracy. [ABSTRACT FROM AUTHOR]

The electronic properties of magnetic semiconductors TlFeS2 and TlFeSе2 have been investigated experimentally by spectral ellipsometry and theoretically ab initio using the density functional theory (DFT). The imaginary and real parts of the dielectric function and the dispersion of the refractive indices, extinction coefficients, and absorption coefficients are found from ellipsometric measurements in the energy range of 0.7−6.5 eV. The direct band gap is estimated. The band structure, the origin of energy states, the optical functions, and the partial densities of states (PDOS) projected on atoms are determined from ab initio calculations. The calculation results are compared with the spectral ellipsometry data obtained in this study. [ABSTRACT FROM AUTHOR]

The optical properties, electronic charge density, electronic structure of the new layered selenides materials, BaGdCuSe3, CsUCuSe3, CsZrCuSe3, and CsGdZnSe3 compounds have been calculated by using the full potential and linear augmented plane wave (FP‐LAPW) methods as applied in the WIEN2k package, which is based on the density functional theory. The ALnMSe3 compound's structure of these was (A = Cs, Ba; Ln = Zr, Gd, U; M = Cu, Zn) is composed of (n = 1, 2) layers, which might be separated by A atoms. It is to be observed that there is strong hybridization between the s, p, and d states of Zr, Gd, and Cu atoms. Around the gadolinium atom, the charge density contours are completely circular, but the Gadolinium "Gd" atom shows an ionic nature. To calculate the refractive index, we used Kramer's Kronig correlations with the imaginary part dielectric function. The decrease in the refractive index is due to the lack of probability for direct excitation of the electrons, resulting in a loss of energy. The value of the static refractive index for all reference compounds is about 1.75–2.25, which is indication that the material used in optoelectronic devices. [ABSTRACT FROM AUTHOR]

In this paper, we present the procedure of generalization and implementation of the Cauchy-Born approximation to the calculation of stress at finite temperature for alloy system in which the effects of inner displacement should be incorporated. With the help of quasi-harmonic approximation, a closed form of the first Piola-Kirchhoff stress is derived as a summation of pure deformation contribution and linear term due to thermal effects. For alloy system with periodic boundary condition, a further simplified formulation of stress based on some invariance constraints is derived in reciprocal space by using Fourier transformation, in which the temperature effect can be efficiently taking account. Several numerical examples are performed for various crystalline systems to validate our generalization procedure of finite temperature Cauchy-Born (FTCB) method for alloy. [ABSTRACT FROM AUTHOR]

We have proposed an "exact" strain gradient (SG) continuum model to properly predict the dispersive characteristics of diatomic lattice metamaterials with local and nonlocal interactions. The key enhancement is proposing a wavelength-dependent Taylor expansion to obtain a satisfactory accuracy when the wavelength gets close to the lattice spacing. Such a wavelength-dependent Taylor expansion is applied to the displacement field of the diatomic lattice, resulting in a novel SG model. For various kinds of diatomic lattices, the dispersion diagrams given by the proposed SG model always agree well with those given by the discrete model throughout the first Brillouin zone, manifesting the robustness of the present model. Based on this SG model, we have conducted the following discussions. (I) Both mass and stiffness ratios affect the band gap structures of diatomic lattice metamaterials, which is very helpful for the design of metamaterials. (II) The increase in the SG order can enhance the model performance if the modified Taylor expansion is adopted. Without doing so, the higher-order continuum model can suffer from a stronger instability issue and does not necessarily have a better accuracy. The proposed SG continuum model with the eighth-order truncation is found to be enough to capture the dispersion behaviors all over the first Brillouin zone. (III) The effects of the nonlocal interactions are analyzed. The nonlocal interactions reduce the workable range of the well-known long-wave approximation, causing more local extrema in the dispersive diagrams. The present model can serve as a satisfactory continuum theory when the wavelength gets close to the lattice spacing, i.e., when the long-wave approximation is no longer valid. For the convenience of band gap designs, we have also provided the design space from which one can easily obtain the proper mass and stiffness ratios corresponding to a requested band gap width. [ABSTRACT FROM AUTHOR]

A two-dimensional phononic crystals based hypersonic-wave switch is theoretically investigated. The temperature effect is considered for a two-dimensional solid–solid phononic crystal composed of a square array of Epoxy rods embedded in a Tungsten matrix. Using finite element method, temperature dependence of the band gap is examined. The numerical results show that the bandgap position and width strongly depend on the temperature and they can be modified when the temperature increases from 10 to 45 °C. Due to its good contrast with Tungsten and Epoxy in elastic properties and its high temperature dependence, Barium Strontium Titanate [Ba0.7Sr0.3TiO3] (BST) is used as the defect material. Defect modes could be tuned by modifying the radius of BST filled rods. The strong temperature dependence and phononic crystals periodicity prove the structure desirable for thermal switching. A hypersonic-wave 1 × 3 switch is designed for the specific frequency f = 1.50 GHz at room temperature. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) magnonic crystal (MC) with an asymmetric complex basis is proposed in this paper, and its band structures are calculated in the whole area of the first Brillouin zone (BZ). This kind of MCs is composed of two different atoms in the unit cell, and the symmetry of the unit cell is broken due to changes in the position of the second atom, so the irreducible part of the BZ is no longer the small area (Γ X M) for square lattice, and it must be expanded to the whole first BZ. Only by investigating the whole first BZ, can we get the true full band-gap for this kind of MCs. [ABSTRACT FROM AUTHOR]

We present a new method of gap control in two-dimensional periodic systems with the perturbation consisting of a second-order differential operator and a family of narrow potential "walls" separating the period cells in one direction. We show that under appropriate assumptions one can open gaps around points determined by dispersion curves of the associated "waveguide" system, in general any finite number of them, and to control their widths in terms of the perturbation parameter. Moreover, a distinctive feature of those gaps is that their edge values are attained by the corresponding band functions at internal points of the Brillouin zone. [ABSTRACT FROM AUTHOR]

A model of twisted bilayer graphene has been offered on the base of developed quasi-relativistic approach with high energy k ⋅ p -Hamiltonian. Monolayer-graphene twist is accounted as a perturbation of monolayer-graphene Hamiltonian in such a way that at a given point of the Brillouin zone there exists an external non-Abelian gauge field of another monolayer. Majorana-like resonances have been revealed in the band structure of model at a magic rotation angle 𝜃 M = 1. 0 5 ∘ . The simulations have also shown that a superlattice energy gap existing at a rotation angle 1. 0 8 ∘ vanishes at a rotation angle 1. 0 ∘ . [ABSTRACT FROM AUTHOR]

The spectroscopic ellipsometry method is used to investigate the optical properties of PbSe nanostructured thin films formed by the chemical deposition method. Function d2ε/dω2 formed by numerical differentiation of experimental data of the dielectric function ε/ω is used for better structural resolution of the interband transitions and the determination of critical points. Theoretical fitting is performed using the "Graphical Analysis" program. The best fit is found for the two-dimensional (2D) shape of the critical point (m = 0) for the energy region E = 2–3 eV, and one critical point corresponding to Eg = 2.5 eV is determined. This value is attributed to the L4 → L6 transition of the Brillouin zone. [ABSTRACT FROM AUTHOR]

The impurity energy spectrum of undoped n-type GaAs, InAs, CdSnAs2, CdGeAs2, and CdTe, ZnO bulk crystals is studied based on a quantitative analysis of the baric and temperature dependences of the kinetic coefficients. It is found that the deep-level donor center corresponds to the intrinsic vacancy defect in the anion sublattice in the above-listed semiconductors. The conclusion on the nature of the donor center, i.e., the vacancy in the anion sublattice, is based on the fact that, in contrast to shallow impurity centers which trace the intrinsic band under uniform pressure, to which they are genetically related, the energy of deep impurity centers with respect to absolute vacuum remains constant under isotropic compression of the lattice. Therefore, it seems favorable to study the evolution of the carrier energy spectrum in semiconductors under uniform-pressure conditions. The energy-level positions with respect to the conduction-band edge and pressure coefficients of the energy gaps between them and the corresponding conduction-band bottom are determined. The shift of the energy level of the deep donor center to the conduction-band depth with decreasing band gap is observed. [ABSTRACT FROM AUTHOR]