Topological insulators induced by disorder and non-Hermiticity

正Searching for topological states of matter in real materials or engineered systems has been a fundamental theme in condensed matter physics in the past decade [1].It is known that the topological insulators are robust against certain disorders,but they usually become trivial under strong disorders due to the Anderson localization.In the year 2009,it was theoretically found that     

Observation of an acoustic non-Hermitian topological Anderson insulator

The interaction of band topology and disorder can give rise to intriguing phenomena. One paradigmatic example is the topological Anderson insulator, whose nontrivial topology is induced in a trivial system by disorders. In this study, we investigate the efect of purely non-Hermitian disorders on topological systems using a one-dimensional acoustic lattice with coupled resonators. Specifically, we construct a theoretical framework to describe the non-Hermitian topological Anderson insulator phase...     

Non-Hermitian topological Anderson insulators

Non-Hermitian systems can exhibit exotic topological and localization properties.Here we elucidate the non-Hermitian effects on disordered topological systems using a nonreciprocal disordered Su-Schrieffer-Heeger model.We show that the non-Hermiticity can enhance the topological phase against disorders by increasing bulk gaps.Moreover,we uncover a topological phase which emerges under both moderate non-Hermiticity and disorders,and is characterized by localized insulating bulk states with a disorder-averaged winding number and zero-energy edge modes.Such topological phases induced by the combination of non-Hermiticity and disorders are dubbed non-Hermitian topological Anderson insulators.We reveal that the system has unique non-monotonous localization behavior and the topological transition is accompanied by an Anderson transition.These properties are general in other non-Hermitian models.     

Band topologies of hexaborides CaB_6 and EuB_6

<正>In last decades,topological materials[1-3]have attracted lots of research interest.Topological Kondo insulator(TKI)[4],as an exotic quantum state,has been proposed theoretically in two hexaborides Sm B6and Yb B6,in which there is a d/f(p)band inversion at momentum point X[5].There are lots of experimental work supporting the TKI state and metallic surface state in typical Kondo insulator Sm B6[6-10].However,more and more experimental results demonstrated that Yb B6is trivial insulator at ambient pressure[11].Under high pressure,the band gap of Yb B6will close and the d/p band inversion can happen under pressure about 15 GPa[12].     

Making Axion Dynamical in Non-Centrosymmetric Magnetic Topological Insulators

“Axion”was predicted as a hypothetical elementary particle to resolve the strong conjugation-parity problem in particle physics,^[1]and it is also an attractive candidate for the as-yet-unobserved dark matter in cosmology.^[2]While the detection of axions still remains elusive in particle physics and cosmology,it was recently proposed that the elegant physics of axions,known“axion electrodynamics”,^[3]can emerge in certain condensed matter systems,particularly topological insulator materials,^[4]in which a variety of exotic physical phenomena(e.g.topological magnetoelectric effect)have been theoretically predicted.     

Observation of a Flat Band in Silicene

Flat bands that exhibit weak band dispersion in crystals can host heavy fermions that are cru- cial to many strongly correlated phenomena, such as ferromagnetism and superconductivity. For topologically trivial flat bands, theories have proven the stability of the ferromagnetic ground state i.e., flat band ferromagnetism. In the presence of weak disorders, systems with flat bands are expected to exhibit unconventional Anderson localization due to the lifting of degeneracy. More interestingly, flat bands with nontrivial topology, i.e., topological flat bands, are expected to host a fractional quantum Hall effect at zero magnetic field. Hence, searching for topologically trivial/nontrivial flat bands （TFB） has been an interesting while challenging task so far.     

Finite size effects on the quantum spin Hall state in HgTe quantum wells under two different types of boundary conditions

The finite size effect in a two-dimensional topological insulator can induce an energy gap Eg in the spectrum of helical edge states for a strip of finite width.In a recent work,it has been found that when the spin–orbit coupling due to bulk-inversion asymmetry is taken into account,the energy gap Eg of the edge states features an oscillating exponential decay as a function of the strip width of the inverted Hg Te quantum well.In this paper,we investigate the effects of the interface between a topological insulator and a normal insulator on the finite size effect in the Hg Te quantum well by means of the numerical diagonalization method.Two different types of boundary conditions,i.e.,the symmetric and asymmetric geometries,are considered.It is found that due to the existence of the interface between topological insulator and normal insulator this oscillatory pattern on the exponential decay induced by bulk-inversion asymmetry is modulated by the width of normal insulator regions.With the variation of the width of normal insulator regions,the shift of the Dirac point of the edge states in the spectrum and the energy gap Eg closing point in the oscillatory pattern can occur.Additionally,the effect of the spin–orbit coupling due to structure-inversion asymmetry on the finite size effects is also investigated.     

Proximity effects in topological insulator heterostructures

Topological insulators （Tls） are bulk insulators that possess robust helical conducting states along their interfaces with conventional insulators. A tremendous research effort has recently been devoted to TI-based heterostructures, in which con- ventional proximity effects give rise to a series of exotic physical phenomena. This paper reviews our recent studies on the potential existence of topological proximity effects at the interface between a topological insulator and a normal insu- lator or other topologically trivial systems. Using first-principles approaches, we have realized the tunability of the vertical location of the topological helical state via intriguing dual-proximity effects. To further elucidate the control parameters of this effect, we have used the graphene-based heterostructures as prototypical systems to reveal a more complete phase diagram. On the application side of the topological helical states, we have presented a catalysis example, where the topo- logical helical state plays an essential role in facilitating surface reactions by serving as an effective electron bath, These discoveries lay the foundation for accurate manipulation of the real space properties of the topological helical state in TI- based heterostructures and pave the way for realization of the salient functionality of topological insulators in future device applications.     

Efficiency of electrical manipulation in two-dimensional topological insulators

We investigate the efficiency of electrical manipulation in a two-dimensional topological insulator by inspecting the electronic states of a lateral electrical potential superlattice in the system. The spatial distribution of the electron density in the system can be tuned by changing the strength of the externally applied lateral electrical superlattice potential. This provides us the information about how efficiently one can manipulate the electron motion inside a two-dimensional topo- logical insulator. Such information is important in designing electronic devices, e.g., an electric field effect transistor made of the topological insulator. The electronic states under various conditions are examined carefully. It is found that the dispersion of the mini-band and the electron distribution in the potential well region both display an oscillatory behavior as the potential strength of the lateral superlattice increases. The probability of finding an electron in the potential well region can be larger or smaller than the average as the potential strength varies. These features can be attributed to the coupled multiple-band nature of the topological insulator. In addition, it is also found that these behaviors are not sensitive to the gap parameter of the two-dimensional topological insulator model. Our study suggests that the electron density manipulation via electrical gating in a two-dimensional topological insulator is less effective and more delicate than that in a traditional single-band semiconductor.     

Superconductivity of small metal grains

1 IntroductionThe superconductivity of small metallic grains has been attracting a lot of attentions. On one hand, Anderson[1] predicted that the superconductivity would disappear if a me- tallic grain was so small that the spaces between the nearest neighbor energy levels in the system became larger than the energy gap of the bulk metallic superconductor; On the other hand, it was found in experiments made in the 1960s[2] that the critical tem- perature of superconductivity of small metallic …     

考虑非傅立叶效应的固液相变分子动力学模拟

采用耦合双温度模型的分子动力学方法对飞秒激光照射金箔的固液相变过程进行了模拟研究,利用序参数法对固液原子进行判定从而确定了金箔发生相变时的固液界面位置和温度,对基于傅立叶定律的抛物线模型和考虑非傅立叶效应的双曲线模型模拟得到的结果进行对比分析,在此基础上采用耦合双曲线模型的分子动力学方法研究了激光能量密度和脉冲宽度对金箔相变过程的影响.结果表明,当激光作用于金箔时,金箔上表面首先熔化,固液界面随时间不断向金箔底部移动,并且在相同条件下,双曲线模型下的金箔熔化深度和固液界面温度均大于抛物线模型的结果.当考虑非傅里叶效应时,激光能量密度越大,固液界面温度越高,金箔熔化时间越短;激光脉冲宽度越小,固液界面温度越大,金箔熔化速度越快.     

Ab-initio simulations of the optical properties of warm dense gold

Using a combination of classical and ab-initio molecular dynamics simulations, we calculate the structure and the electrical conductivity of warm dense gold during the first picoseconds after a short-pulse laser illumination. We find that the ions remain in their initial fcc structure for several picoseconds, despite electron temperatures ranging from a few to several eV after the laser illumination. The electrical conductivities calculated under these nonequilibrium conditions and using the latter assumption are in remarkable agreement with recent measurements using a short-pulse laser interacting with gold thin films.     

Aluminum equation-of-state data in the warm dense matter regime

Isochore measurements were performed in the warm dense matter regime. Pressure and internal energy variation of aluminum plasma (density 0.1 g/cm(3) and 0.3 g/cm(3)) are measured using a homogeneous and thermally equilibrated media produced inside an isochoric plasma closed vessel in the internal energy range 20-50 MJ/kg. These data are compared to detailed calculations obtained from ab initio quantum molecular dynamics, average atom model within the framework of the density functional theory, and standard theories. A dispersion between theoretical isochore equation of state is found in the studied experimental thermodynamic regime.     

等离子屏蔽效应下飞秒激光照射金箔的分子动力学模拟

采用耦合了双温度模型的分子动力学方法对飞秒激光烧蚀金箔的传热过程进行了模拟研究,考虑了非傅里叶效应,探究了不同激光能流密度下等离子体羽流的屏蔽作用.根据密度分布将激光烧蚀过程中的金箔划分为过热液体层、熔融液体层和固体层,并比较了不同激光能量密度下过热液体层表面发生的相爆炸沸腾现象以及表面温度的变化情况.结果表明,随着激光能量密度的增大,等离子体的屏蔽比例几乎呈线性增大.在激光的烧蚀过程中,金箔的上表面最先经历液体层以及过热液体层,并且随着时间的推移,液体层和过热液体层逐渐向金箔底部移动.过热液体层发生体积移除的相爆炸沸腾是金箔烧蚀的主要方式,随着激光能量的增大,爆炸沸腾发生的时间提前,并且结束的时间相应延后,持续时间变长.     

Melting and resolidification of gold film irradiated by nano- to femtosecond lasers

Rapid melting and resolidification of a free-standing gold film subject to nano- to femtosecond laser pulses are investigated using the two-temperature model in conjunction with an interfacial tracking method. The interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, in the ultra-fast phase-change process are obtained by considering the interfacial energy balance and nucleation dynamics. A nonlinear electron heat capacity and a temperature-dependent electron–lattice coupling factor for the rapid phase change are taken into account. Effects of laser pulse width and fluence on melting and resolidification are also studied. PACS 42.62.Eh; 63.20.Kr; 64.70.Dv     

Thermoelastic modeling of microbump and nanojet formation on nanosize gold films under femtosecond laser irradiation

A model of elasto-plastic flow combined with the two-temperature model in a two-dimensional approximation has been developed to describe microbump and nanojet formation observed recently on nanosize gold films irradiated by femtosecond laser pulses. It has been shown that the effect of microbump and nanojet formation is conditioned by the elastic characteristics, yield stress, and other properties that are unique for gold. The numerical results are in excellent agreement with the experimental data with respect to a number of parameters, particularly the threshold fluence for nanojet formation at the tops of microbumps, which occurs nearby and above the melting threshold. The analysis of properties for a number of materials is performed and some other materials are discussed as possible candidates for nanotexturing of thin films by femtosecond laser pulses. PACS 79.20.Ds; 42.62.-b     

The role of electron-phonon coupling in femtosecond laser damage of metals

Femtosecond laser pulses were applied to study the energy deposition depth and transfer to the lattice for Au, Ni, and Mo films of varying thickness. The onset of melting, defined here as damage threshold, was detected by measuring changes in the scattering, reflection and transmission of the incident light. Experiments were done in multi-shot mode and single-shot threshold fluences were extracted by taking incubation into account. Since melting requires a well-defined energy density, we found the threshold depends on the film thickness whenever this is smaller than the range of electronic energy transport. The dependence of the threshold fluence on the pulse length and film thickness can be well described by the two-temperature model, proving that laser damage in metals is a purely thermal process even for femtosecond pulses. The importance of electron-phonon coupling is reflected by the great difference in electron diffusion depths of noble and transition metals.     

Thresholds for front-side ablation and rear-side spallation of metal foil irradiated by femtosecond laser pulse

The mechanical action of laser exposure on a foil may result in the ablation of irradiated front layer and the rear-side spallation. The dynamics of an Al foil is studied by means of two-temperature (2T) hydrodynamics and molecular dynamics (MD). It is found that the rear-side spallation threshold F _s exceeds the front-side ablation threshold F _a. We propose to extend the common approach in laser-matter experiments by pump–probe measuring of the rear-side displacement.     

Ultrafast solid–liquid–vapor phase change of a gold film induced by pico- to femtosecond lasers

Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature, deeper melting and ablation depths.