Topological phases in Bi/Sb planar and buckled honeycomb monolayers

We investigate topological phases in two-dimensional Bi/Sb honeycomb crystals considering planar and buckled structures, both freestanding and deposited on a substrate. We use the multi-orbital tight-binding model and compare results with density functional theory calculations. We distinguish topological phases by calculating topological invariants, analyzing edge states properties of systems in a ribbon geometry and studying their entanglement spectra. We show that coupling to the substrate induces transition to the $Z_2$ topological insulator phase. It is observed that topological crystalline insulator (TCI) phase, found in planar crystals, exhibits an additional pair of edge states in both energy spectrum and entanglement spectrum. Transport calculations for TCI phase suggest robust quantized conductance even in the presence of crystal symmetry-breaking disorder.     

Ferromagnetic-insulators-modulated transport properties on the surface of a topological insulator

Transport properties on the surface of a topological insulator （TI） under the modulation of a two-dimensional （2D） ferromagnet/ferromagnet junction are investigated by the method of wave function matching. The single ferromagnetic barrier modulated transmission probability is expected to be a periodic function of the polarization angle and the planar rotation angle, that decreases with the strength of the magnetic proximity exchange increasing. However, the transmission probability for the double ferromagnetic insulators modulated n-n junction and n-p junction is not a periodic function of polarization angle nor planar rotation angle, owing to the combined effects of the double ferromagnetic insulators and the barrier potential. Since the energy gap between the conduction band and the valence band is narrowed and widened respectively in ranges of 0 ≤ 0 〈π/2 and r/2 〈 0 ≤ π, the transmission probability of the n-n junction first increases rapidly and then decreases slowly with the increase of the magnetic proximity exchange strength. While the transmission probability for the n-p junction demonstrates an opposite trend on the strength of the magnetic proximity exchange because the band gaps contrarily vary. The obtained results may lead to the possible realization of a magnetic/electric switch based on TIs and be useful in further understanding the surface states of TIs.     

Classification of “Real” Bloch-bundles: Topological quantum systems of type AI

We provide a classification of type AI topological quantum systems in dimension $d=1,2,3,4$ which is based on the equivariant homotopy properties of “Real” vector bundles. This allows us to produce a fine classification able to take care also of the non stable regime which is usually not accessible via $K$-theoretic techniques. We prove the absence of non-trivial phases for one-band AI free or periodic quantum particle systems in each spatial dimension by inspecting the second equivariant cohomology group which classifies “Real” line bundles. We also show that the classification of “Real” line bundles suffices for the complete classification of AI topological quantum systems in dimension $d⩽3$. In dimension $d=4$ the determination of different topological phases (for free or periodic systems) is fixed by the second “Real” Chern class which provides an even labeling identifiable with the degree of a suitable map. Finally, we provide explicit realizations of non trivial 4-dimensional free models for each given topological degree.     

半Heusler型拓扑绝缘体LaPtBi能带调控的研究

采用基于密度泛函理论的全势能线性缀加平面波的方法, 研究了化学替代和施加等体积单轴(拉、压)应力两种方式对半Heusler型拓扑绝缘体LaPtBi能带的影响. 计算结果表明, 通过在LaPtBi合金中用Sc元素替换La, 或者用Pd替换Pt, 都可以使得原本受立方对称性保护的Γ₈能带在费米能级附近打开一带隙; 而对于施加等体积单轴应力来扭曲立方晶格的方式, 在使得Γ₈ 能带打开的同时, 还可以实现对费米能级位置有规律地调控, 使 LaPtBi合金最终成为真正意义上的体材料是绝缘性而表面是金属性的拓扑绝缘体.     

Evolution of Bond-Order-Wave Phase in One-Dimensional Mott Insulators

In this paper, by using the level spectroscopy method and bosonization theory, we discuss the evolution of the bond-order-wave （BOW） phase in a one-dimensional half-filled extended Hubbard model wlth the on-site Coulomb repulsion U as well as the inter-site Coulomb repulsion V and antiferromagnetic exchange J. After clarifying the generic phase diagrams in three limiting cases with one of the parameters being fixed at zero individually, we find that the BOW phase in the U-V phase diagram is initially enlarged as J increases from zero but is eventually suppressed as J increases further in the strong-coupling regime. A three-dimensional phase diagram is suggested where the BOW phase exists in an extended region separated from the spin-density-wave and charge-density-wave phases.     

输电线路中均压环对复合绝缘子的研究

本文利用COMSOL Multiphysics有限元仿真软件仿真了绝缘子在加装均压环前后周围电场的情况,证实了复合绝缘子加装均压环确实能够解决输电线路中电场集中的问题,为输电线路安全稳定的运行提供理论基础.     

Electronic-transport properties of unhydrogenated amorphous gallium arsenide

Summary Hall mobility, μ_H, and electrical conductivity, σ, of unhydrogenated amorphous-gallium-arsenide films, prepared by r.f. sputtering, have been measured. Conductivity as a function of temperature shows a variable-range hopping mechanism atT<260 K, while at high temperature, conductivity and Hall mobility are both thermally activated. The results are interpreted in terms of the presence of defect complexes due to an excess of Ga. The stoichiometry and the structure of the films are used to explain the behaviour and the values of μ_H. The values of the activation energy of the conductivity seem in agreement with theoretical calculations on the position of electronic states created by defect complexes in the mobility-gap of a-GaAs.     

Semiempirical molecular orbital techniques applied to silicon dioxide: MINDO/3

We present a discussion of MINDO/3, a semiempirical molecular orbital program, which we have applied to clusters of atoms used to represent silicon dioxide. We have obtained MINDO/3 parameters for the Si-O interaction and have calculated a variety of observables for both molecules and solid-state clusters, for both open- and closed-shell systems. With some exceptions, we find very good agreement with other calculations and with experiment. In particular, our results for both E′₁ and E′₄ defects support existing models which feature asymmetric atomic relaxations.