Topological magnetic insulators with corundum structure

Topological insulators are new states of quantum matter in which surface states residing in the bulk insulating gap are protected by time-reversal symmetry. When a proper kind of antiferromagnetic long-range order is established in a topological insulator, the system supports axionic excitations. In this Letter, we study theoretically the electronic states in a transition metal oxide of corundum structure, in which both spin-orbit interaction and electron-electron interaction play crucial roles. A tight-binding model analysis predicts that materials with this structure can be strong topological insulators. Because of the electron correlation, an antiferromagnetic order may develop, giving rise to a topological magnetic insulator phase with axionic excitations.     

拓扑绝缘体及其研究进展

拓扑绝缘体是当前凝聚态物理领域中的一个热点问题.这类材料的典型特征是体内元激发存在能隙,但在边界上具有受拓扑保护的无能隙边缘激发.从广义上讲,拓扑绝缘体可以分两大类:一类是破坏时间反演的量子霍尔体系,另一类是新近发现的时间反演不变的拓扑绝缘体.这些新材料的奇特物理性质和潜在的应用前景,使其倍受人们关注.文章对这种新奇物态的物理性质和研究进展做了简要的介绍.     

拓扑绝缘体及其研究进展

拓扑绝缘体是当前凝聚态物理领域中的一个热点问题.这类材料的典型特征是体内元激发存在能隙,但在边界上具有受拓扑保护的无能隙边缘激发.从广义上讲,拓扑绝缘体可以分两大类:一类是破坏时间反演的量子霍尔体系,另一类是新近发现的时间反演不变的拓扑绝缘体.这些新材料的奇特物理性质和潜在的应用前景,使其倍受人们关注.文章对这种新奇物态的物理性质和研究进展做了简要的介绍.     

激子绝缘体

激子绝缘体是20世纪60年代初由诺贝尔物理学奖获得者莫特提出的一种新物相。众所周知,激子是固态系统中最典型的集体激发之一,简单地可视为电子—空穴由于库仑相互作用而形成的束缚对。在常规绝缘体或半导体材料中,单粒子能隙远大于激子束缚能。而在某些特殊的材料体系,如窄能隙半导体和二维材料中,激子的束缚能可能大于体系的单粒子能隙,因此体系内会自发形成大量的激子,进入激子绝缘体相。激子绝缘体是体系的基态,低浓度下激子可视为组合玻色子,在低温下会形成宏观相干态——激子玻色—爱因斯坦凝聚。文章简要地回顾了激子绝缘体的发展历史,并介绍了激子绝缘体、玻色—爱因斯坦凝聚和自旋超流的最新进展。     

Optical properties for topological insulators with metamaterials

The two fields of topological insulators and metamaterials are independent. In this Letter, we firstly investigate the Fresnel coefficients for the reflected and refracted electromagnetic waves across the interface between topological insulators and left-handed metamaterials. Then, we derive the exact analytic expressions for Kerr and Faraday rotations. By way of multiple reflections method, we demonstrate that perfect lens with left-handed metamaterials slab and topological insulators can be designed. On the other hand, the processes of reflection and refraction are investigated in the case of topological insulator and chiral metamaterial. Then, we give the reflection and transmission coefficients of topological insulator with a chiral medium slab. Lastly, the potential applications of these results are discussed.     

Modification of Lexan polycarbonate induced by electron irradiation

An 8 MeV electron-induced modification of Lexan polycarbonate (Lexan) films has been studied systematically using UV–visible spectroscopy, LCR meter, X-ray diffractogram (XRD) and differential scanning calorimetry (DSC) techniques. The optical properties of the Lexan films showed a decrease in the optical energy gap with an increase in the electron dose. The AC conductivity and dielectric constant were found to change significantly due to irradiation, and the dielectric constant was found to obey the universal law of dielectric constant. The XRD results show that the crystallite size and the percentage of crystallinity of films decrease after irradiation. The decrease in glass transition temperature shown by DSC studies reveals that the polymeric system has changed towards a more disordered state.     

Charge accumulation by insulators irradiated with electrons

A model, more general than the existing ones, is proposed for the accumulation of charge by plane-parallel insulators irradiated with accelerated electrons. The model takes into account the spatial distribution of the charge injected by the irradiation and the coordinate and time dependence of the bulk electrical conductivity; it is valid for different boundary conditions imposed on the solution of the Poisson equation. The charge density distribution obtained by the light probe method in a sample of a lithium fluoride single crystal irradiated with 2-MeV electrons and current density 5·10^–4 A/m² is described qualitatively by the model.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 49–57, February, 1981.     

Tunable Casimir repulsion with three-dimensional topological insulators

In this Letter, we show that switching between repulsive and attractive Casimir forces by means of external tunable parameters could be realized with two topological insulator plates. We find two regimes where a repulsive (attractive) force is found at small (large) distances between the plates, canceling out at a critical distance. For a frequency range where the effective electromagnetic action is valid, this distance appears at length scales corresponding to 1 - ?(ω) ～ (2/π)αθ.     

Fragile Mott insulators

We prove that there exists a class of crystalline insulators, which we call "fragile Mott insulators," which are not adiabatically connected to any sort of band insulator provided time-reversal and certain point-group symmetries are respected, but which are otherwise unspectacular in that they exhibit no topological order nor any form of fractionalized quasiparticles. Different fragile Mott insulators are characterized by different nontrivial one-dimensional representations of the crystal point group. We illustrate this new type of insulators with two examples: the d Mott insulator discovered in the checkerboard Hubbard model at half-filling and the Affleck-Kennedy-Lieb-Tasaki insulator on the square lattice.     

Topological crystalline insulators

The recent discovery of topological insulators has revived interest in the band topology of insulators. In this Letter, we extend the topological classification of band structures to include certain crystal point group symmetry. We find a class of three-dimensional "topological crystalline insulators" which have metallic surface states with quadratic band degeneracy on high symmetry crystal surfaces. These topological crystalline insulators are the counterpart of topological insulators in materials without spin-orbit coupling. Their band structures are characterized by new topological invariants. We hope this work will enlarge the family of topological phases in band insulators and stimulate the search for them in real materials.     

Pseudo topological insulators

We predict pseudo topological insulators that have been previously overlooked. We determine some conditions under which robust pseudo topological edge states appear and illustrate our idea on the Su–Schrieffer–Heeger (SSH) model with extra chiral symmetry breaking potentials. We discuss that pseudo topological insulating phase transition occurs without band gap closing.     

Realistic time-reversal invariant topological insulators with neutral atoms

We lay out an experiment to realize time-reversal invariant topological insulators in alkali atomic gases. We introduce an original method to synthesize a gauge field in the near field of an atom chip, which effectively mimics the effects of spin-orbit coupling and produces quantum spin-Hall states. We also propose a feasible scheme to engineer sharp boundaries where the hallmark edge states are localized. Our multiband system has a large parameter space exhibiting a variety of quantum phase transitions between topological and normal insulating phases. Because of their remarkable versatility, cold-atom systems are ideally suited to realize topological states of matter and drive the development of topological quantum computing.     

Topological Mott insulators

We consider extended Hubbard models with repulsive interactions on a honeycomb lattice, and the transitions from the semimetal to Mott insulating phases at half-filling. Because of the frustrated nature of the second-neighbor interactions, topological Mott phases displaying the quantum Hall and the quantum spin Hall effects are found for spinless and spin fermion models, respectively. The mean-field phase diagram is presented and the fluctuations are treated within the random phase approximation. Renormalization group analysis shows that these states can be favored over the topologically trivial Mott insulating states.     

Doped Mott insulators are insulators: hole localization in the cuprates

We demonstrate that a Mott insulator lightly doped with holes is still an insulator at low temperature even without disorder. Hole localization obtains because the chemical potential lies in a pseudogap which has a vanishing density of states at zero temperature. The energy scale for the pseudogap is set by the nearest-neighbor singlet-triplet splitting. As this energy scale vanishes if transitions, virtual or otherwise, to the upper Hubbard band are not permitted, the fundamental length scale in the pseudogap regime is the average distance between doubly occupied sites. Consequently, the pseudogap is tied to the noncommutativity of the two limits U-->infinity (U the on-site Coulomb repulsion) and L -->infinity (the system size).     

Railgun using plasma initiation separated from the projectile

Preacceleration of a projectile is important for reducing the erosion of the bore surface in a railgun. Gas guns, electrothermal guns, and other railguns are commonly used to preaccelerate the projectile. A new method, called the plasma initiation separated from the projectile (PISP) method is proposed, and its effectiveness is confirmed experimentally. A thin copper wire is placed near the edge of the railgun, and it explodes and forms a plasma that has a fast flow velocity due to the Lorentz force. This fast flowing plasma collides with the projectile, which obtains an initial velocity mainly by the momentum transfer. Since the current increases while only the plasma is accelerated, the driving force of the projectile just after the collision of the plasma with the projectile is large. The PISP method works as an inductive energy storage circuit with an opening switch     

Two-dimensional topological insulators with large bulk energy gap

Two-dimensional(2D) topological insulators(TTs,or quantum spin Hall insulators) are special insulators that possess bulk 2D electronic energy gap and time-reversal symmetry protected one-dimensional(1D) edge state.Carriers in the edge state have the property of spin-momentum locking,enabling dissipation-free conduction along the 1D edge.The existence of 2D TIs was confirmed by experiments in semiconductor quantum wells.However,the 2D bulk gaps in those quantum wells are extremely small,greatly limiting potential application in future electronics and spintronics.Despite this limitation,2D TIs with a large bulk gap attracted plenty of interest.In this paper,recent progress in searching for TIs with a large bulk gap is reviewed briefly.We start by introducing some theoretical predictions of these new materials and then discuss some recent important achievements in crystal growth and characterization.     

Elastic three-dimensional phononic topological insulators with Dirac hierarchy

Three-dimensional(3D) phononic topological insulators(TIs) featuring two-dimensional(2D) surface states and one-dimensional(1D) hinge states have opened up a new route for multi-dimensional robust wave transport,providing unprecedented methods for integrated acoustic sensors and energy harvesting devices.However,aiming at the elastic 3D phononic TI with gapless surface states and hinge states,the realization of elastic 3D phononic TIs with gapless surface states and hinge states is a significant...     

电磁轨道炮刨削的形成机理及仿真分析

对电磁轨道炮发射过程中轨道表面的刨削现象进行了研究,根据其发射特点提出了微角度斜冲击引起的刨削形成机理,根据实际轨道炮的结构和载荷特点,应用LS-DYNA有限元软件在不同的参数条件下进行了数值仿真计算。计算表明,随着电枢前端倒角的增大轨道的刨削深度减小,电枢的质心位置后移也可以减小刨削深度,并且这些因素对刨削深度的影响并非是线性的。研究结果为进一步抑制轨道刨削现象的发生提供了依据。     

Ion implantation in insulators

The modifications of insulating materials by ion implantation can be of considerable interest for many applications. If ion implantation has considerable potential for changing the properties of insulators, only a few experiments have determined the critical parameters which are important in the behaviour of the implanted layer. As a matter of fact, the ion implantation process is, by its very nature, a non-equilibrium process and, in addition, the injection of charged particles in a lattice determines a concomitant creation of intrinsic lattice defects. These defects are associated with the nuclear part of the energy loss of the particles and sometimes with the electronic one (halide compounds). The characterization of these intrinsic defects and the knowledge of their spatial distribution in the lattice are necessary because the formation or dissolution of phases precipitated in the implanted layer are strongly influenced by point or extended defects.

The main parameters which determine the charge state and site location of the implanted atoms in insulating targets are: ionicity of the chemical bond of the lattice, electronegativity, thermal or radiation enhanced diffusion processes, and intrinsic defects. For heavily implanted insulators, phase changes of compositional type can occur which can lead to strong modifications of the physical properties of the implanted layer. In order to obtain information on the relative extent of the critical parameters of chemical implantation in insulators it is necessary to combine analysis with different techniques such as, nuclear techniques, TEM, ESR, optical spectroscopy, X-ray diffraction at glancing angle, etc.

To illustrate the effects of these parameters, typical experimental results obtained in implanted binary or ternary compounds are reported (alkali halides, silver halides, alkaline earth oxides or fluorides, transition metal oxides, natural minerals, etc.).