High Chern number phase in topological insulator multilayer structures: A Dirac cone model study

We employ the Dirac cone model to explore the high Chern number (C) phases that are realized in the magnetic-doped topological insulator (TI) multilayer structures by Zhao et al. [Nature 588 419 (2020)]. The Chern number is calculated by capturing the evolution of the phase boundaries with the parameters, then the Chern number phase diagrams of the TI multilayer structures are obtained. The high-C behavior is attributed to the band inversion of the renormalized Dirac cones, along with which the spin polarization at the $\varGamma$ point will get increased. Moreover, another two TI multilayer structures as well as the TI superlattice structures are studied.     

PT Symmetry Induced Rings of Lasing Threshold Modes Embedded with Discrete Bound States in the Continuum

It is well known that spatial symmetry in a photonic crystal(PhC) slab is capable of creating bound states in the continuum(BICs),which can be characterized by topological charges of polarization vortices.Here,we show that when a PT-symmetric perturbation is introduced into the PhC slab,a new type of BICs(pt-BICs) will arise from each ordinary BIC together with the creation of rings of lasing threshold modes with pt-BICs embedded in these rings.Different from ordinary BICs,the Q-factor divergence rate of a pt-BIC is reduced and anisotropic in momentum space.Also,pt-BICs can even appear at off-high symmetry lines of the Brillouin zone.The pt-BICs also carry topological charges and can be created or annihilated with the total charge conserved.A unified picture on pt-BICs and the associated lasing threshold modes is given based on the temporal coupled mode theory.Our findings reveal the new physics arising from the interplay between PT symmetry and BIC in PhC slabs.     

Bound states in the continuum in metal—dielectric photonic crystal with a birefringent defect

By using the difference of the band structure for the TE and TM waves in the metal—dielectric photonic crystals beyond the light cone and the birefringence of the anisotropic crystal, a one-dimensional photonic system is constructed to realize the bound states in the continuum (BICs). In addition to the BICs arising from the polarization incompatibility, the Friedrich—Wintgen BICs are also achieved when the leaking TM wave is eliminated due to the destructive interference of its ordinary and extraordinary wave components in the anisotropic crystal. A modified scheme favorable for practical application is also proposed. This scheme for BICs may help to suppress the radiation loss in the metal—dielectric photonic crystal systems.     

基于一维金属光子晶体平凹镜的柱矢量光束亚波长聚焦

柱矢量光束具有柱对称性的偏振分布,其独特的光场分布和聚焦特性被广泛应用于光学微操纵及光学成像等领域,并迅速向亚波长尺度拓展.通常,亚波长尺度聚焦采用等离激元透镜实现,但存在光场调控的偏振态局限性.而借助光子晶体的负折射效应,不仅能够实现亚波长聚焦或成像,而且应对正交偏振态同时有效.采用对电磁波具有更强调控能力的一维金属光子晶体结构,计算得到的能带结构和等频曲线表明其负折射效应在特定波段对正交偏振态同时有效.在此基础上设计出一维金属光子晶体柱对称平凹镜结构,通过有限元算法模拟显示了可见光波段的径向和旋向偏振光的同时亚波长聚焦行为.进一步的结果表明,改变柱矢量光束的偏振组分能够直接有效地调节焦场空间分布及偏振分布特性.所提出的平凹镜结构能够实现对任意偏振组分的柱矢量光束的亚波长尺度聚焦,且该结构的设计对于各波段情况均有参考意义.该研究结果对小尺度粒子的光学微操纵、超分辨率成像等相关领域具有潜在的应用价值.     

硅中硼离子注入的带电缺陷动力学模拟

为准确描述硼离子注入硅后缺陷/杂质的动力学物理过程,获得硼浓度空间分布及其演化行为,构建一个跨尺度带电缺陷动力学模型,考虑离子注入缺陷的产生及其演化的多种微观过程,包括缺陷电荷态和带电缺陷间的反应、硼-自间隙团簇（BICs）演化以及缺陷与载流子相互作用等物理过程。模拟得到与实验一致的硼浓度深度分布。结果表明：BICs对硼浓度的深度分布起主要作用,而间隙硼（B_I）导致硼浓度分布向深处扩展;计及缺陷的不同电荷态修正自间隙（I）和硼间隙（B_I）的扩散系数,从而更准确地描述硼浓度分布。模型揭示了硼离子注入硅发生的物理过程和微观机理,证明BICs和缺陷真实的电荷态是描述硼浓度分布的重要因素,为半导体器件制造与研发提供理论指导。     

Topological Nodal Line Semimetal in Non-Centrosymmetric PbTaS_2

Topological semimetals are a new type of matter with one-dimensional Fermi lines or zero-dimensional Weyl or Dirac points in momentum space. Here using first-principles calculations, we find that the non-centrosymmetric PbTaS2 is a topological nodal line semimetal. In the absence of spin-orbit coupling(SOC), one band inversion happens around a high symmetrical H point, which leads to forming a nodal line. The nodal line is robust and protected against gap opening by mirror reflection symmetry even with the inclusion of strong SOC. In addition, it also hosts exotic drumhead surface states either inside or outside the projected nodal ring depending on surface termination. The robust bulk nodal lines and drumhead-like surface states with SOC in PbTaS_2 make it a potential candidate material for exploring the freakish properties of the topological nodal line fermions in condensed matter systems.     

Band renormalization and spin polarization of MoS2 in graphene/MoS2 heterostructures

Transition metal dichalcogenides exhibit spin–orbit split bands at the K‐point that become spin polarized for broken crystal inversion symmetry. This enables simultaneous manipulation of valley and spin degrees of freedom. While the inversion symmetry is broken for monolayers, we show here that spin polarization of the MoS₂ surface may also be obtained by interfacing it with graphene, which induces a space charge region in the surface of MoS₂. Polarization induced symmetry breaking in the potential gradient of the space charge is considered to be responsible for the observed spin polarization. In addition to spin polarization we also observe a renormalization of the valence band maximum (VBM) upon interfacing of MoS₂ with graphene. The energy difference between the VBM at the Γ‐point and K‐point shifts by ～150 meV between the clean and graphene covered surface. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Spin-resolved photoemission of surface states of W(110)-(1 x 1)H

The surface electronic states of W(110)-(1 x 1)H have been measured using spin- and angle-resolved photoemission. We directly demonstrate that the surface bands are both split and spin-polarized by the spin-orbit interaction in association with the loss of inversion symmetry near a surface. We observe 100% spin polarization of the surface states, with the spins aligned in the plane of the surface and oriented in a circular fashion relative to the Smacr; symmetry point. In contrast, no measurable polarization of nearby bulk states is observed.     

Designing polarization insensitive negative index metamaterial for operation in near infrared

Hexagonal arrays of gold nanoelements with C₃ symmetry are studied as a metamaterial slab of reduced anisotropy. Tri-dimensional (3D) finite-difference time domain (FDTD) simulations are used to calculate the reflected and transmitted electromagnetic field for two waves normally incident to the slab with mutually perpendicular polarizations. S-parameter method is used to retrieve the constitutive parameters for each polarization. While dipolar plasmonic resonance in the case of one layer metamaterial slab leads only to negative values of permittivity, using two layers of asymmetric nanoelements leads to a negative refractive index metamaterial in the near infrared range (158–172 THz) as a result of hybridized plasmonic states inversion.     

Tuning the polarization states of optical spots at the nanoscale on the Poincaré sphere using a plasmonic nanoantenna

It is shown that the polarization states of optical spots at the nanoscale can be manipulated to various points on the Poincaré sphere using a plasmonic nanoantenna. Linearly, circularly, and elliptically polarized near-field optical spots at the nanoscale are achieved with various polarization states on the Poincaré sphere using a plasmonic nanoantenna. A novel plasmonic nanoantenna is illuminated with diffraction-limited linearly polarized light. It is demonstrated that the plasmonic resonances of perpendicular and longitudinal components of the nanoantenna and the angle of incident polarization can be tuned to obtain optical spots beyond the diffraction limit with a desired polarization and handedness.     

Bound states in the continuum in open Aharonov-Bohm rings

Using the formalism of the effective Hamiltonian, we consider bound states in a continuum (BIC). They are nonhermitian effective Hamiltonian eigenstates that have real eigenvalues. It is shown that BICs are orthogonal to open channels of the leads, i.e., disconnected from the continuum. As a result, BICs can be superposed to a transport solution with an arbitrary coefficient and exist in a propagation band. The one-dimensional Aharonov-Bohm rings that are opened by attaching single-channel leads to them allow exact consideration of BICs. BICs occur at discrete values of the energy and magnetic flux; however, it’s realization strongly depends on the way to the BIC point. The text was submitted by the authors in English.     

INTERNAL FRICTION AND ULTRASONIC ATTENUATION RELATED TO CARRIERS IN HIGH Tc SUPERCONDUCTORS

For both BiSrCaCuO and TlBaCaCuO samples, internal frictions (Q^-1) in the kHz range reveal a plateau (Q^-1_p) above T_c and a rapid drop below T_c with the turning points located just at T_c for various samples with different T_c. This anomaly cannot be observed in non-superconducting samples. The ultrasonic attenuation (α) in TlBaCaCuO displays similar results to the internal friction. Moreover, it is discovered that the Q^-1_p is nearly proportional to the carrier density for Y(Pr)BaCuO and Gd(Pr)BaCuO with different Pr contents. These results show that the drop of Q^-1 and a below T_c is closely related to superconducting condensation. The Q^-1_p and α_p can be explained using coupling-model of carriers with local dynamic distortion because high T_c superconductors are strong-correlation systems. Furthermore, by taking account of the smearing of superconducting gap structure resulting from the recombination of quasi-particles and by modifying the BCS relative jump rate as S(E, E', Γ) = Re{1-Δ²/[(E-iΓ)(E'-iΓ)]}, the calculated results of internal friction and ultrasonic attenuation below T_c are in good agreement with the experimental data. The superconducting gap Δ and the damping rate Γ for both BiSrCaCuO and TlBaCaCuO have also been obtained, they are in accordance with those got by tunneling spectrum and NMR methods, etc.     

Bound states of Dirac fermions in monolayer gapped graphene in the presence of local perturbations

In graphene,conductance electrons behave as massless relativistic particles and obey an analogue of the Dirac equation in two dimensions with a chiral nature.For this reason,the bounding of electrons in graphene in the form of geometries of quantum dots is impossible.In gapless graphene,due to its unique electronic band structure,there is a minimal conductivity at Dirac points,that is,in the limit of zero doping.This creates a problem for using such a highly motivated new material in electronic devices.One of the ways to overcome this problem is the creation of a band gap in the graphene band structure,which is made by inversion symmetry breaking(symmetry of sublattices).We investigate the confined states of the massless Dirac fermions in an impured graphene by the short-range perturbations for "local chemical potential" and "local gap".The calculated energy spectrum exhibits quite different features with and without the perturbations.A characteristic equation for bound states(BSs) has been obtained.It is surprisingly found that the relation between the radial functions of sublattices wave functions,i.e.,f_m~+(r),g_m~+(r),and f_m~-(r),g_m~-(r),can be established by SO(2) group.     

General demonstration of principal states of polarization and real-time monitoring of polarization mode dispersion in optical fibres

We investigated the general properties of polarization effects in optical fibres and demonstrated the existence of socalled principal states of polarization (PSP), which mean the fixed points in mathematics, in different polarization effects,such as birefringence and polarization mode dispersion, by using fixed point theory. Furthermore, a time evolution vector is defined to describe the time evolution of polarization state in optical fibres, which is used to investigate the time evolution of polarization mode dispersion vector (PDV), including differential group delay and PSP. The experimental results of real-time monitoring of PDV by using this method are reported. To our knowledge, this is the first report on monitoring PSP evolution in optical fibres.     

缀饰格子中时间反演对称破缺的量子自旋霍尔效应

<正>研究了缀饰格子中的量子自旋霍尔效应,模型中同时考虑了Rashba自旋轨道耦合和交换场的作用.缀饰格子具有简立方对称性,以零能平带和单狄拉克锥结构为主要特点.在缀饰格子中,不论是实现量子自旋霍尔效应还是量子反常霍尔效应,都需要一个不为零的内禀自旋轨道耦合作用来打开一个完全的体能隙,这与石墨烯等六角格子模型有着很大的不同.在交换场破坏了时间反演对称性的情况下,以自旋陈数为标志的量子自旋霍尔效应仍然能够存在,边缘态和极化率的相关结果也证明了这一结论.结果表明自旋陈数比z2拓扑数在表征量子自旋霍尔效应方面有着更广泛的适用范围,相应的结论为利用磁场控制量子自旋霍尔效应提出了一个理论模型和依据.     

人工光学微纳结构中的连续体束缚态:原理、发展及应用

人工微结构可以捕获特定频率的电磁波,其为增强光与物质相互作用以及调控光场的重要平台之一。连续体束缚态在能谱上位于辐射连续区域,其是开放波动系统中与辐射连续态完全正交的本征态。连续体束缚态源于波动的相干相消,可以极大地抑制微纳光子器件的辐射损耗,为解决人工微纳结构中的光束缚提供全新思路。本文回顾连续体束缚态的发展历程,着重阐述连续体束缚态的理论模型在不同人工光学微纳结构中的进展与应用。连续体束缚态有望促进光通信、集成光学及高效率光场调控等领域的发展。     

Plasma induced by pulsed laser and fabrication of silicon nanostructures

It is interesting that in preparing process of nanosilicon by pulsed laser, the periodic diffraction pattern from plasmonic lattice structure in the Purcell cavity due to interaction between plasmons and photons is observed. This kind of plasmonic lattice structure confined in the cavity may be similar to the Wigner crystal structure. Emission manipulation on Si nanostructures fabricated by the plasmonic wave induced from pulsed laser is studied by using photoluminescence spectroscopy.The electronic localized states and surface bonding are characterized by several emission bands peaked near 600nm and 700nm on samples prepared in oxygen or nitrogen environment. The electroluminescence wavelength is measured in the telecom window on silicon film coated by ytterbium. The enhanced emission originates from surface localized states in band gap due to broken symmetry from some bonds on surface bulges produced by plasmonic wave in the cavity.     

FIRST-PRINCIPLE SELF-CONSISTENT PSEUDOPOTENTIAL CALCULATION OF THE ELECTRONIC STRUCTURES OF SHORT-PERIOD (GaAs)m(AlAs)n SUPERLATT1CES

With the local density approximation, the band structares of the short-period (GaAs)₁(AlAs)₁ and (GaAs)₂(AlAs)₁ superlattices are calculated by using the first-principle self-consistent pseudopotential method. The results show that the (GaAs)₁(AlAs)₁ superlattice is an indirect semiconductor, and the lowest conduction band state is at point R in the Brillouin zone; the (GaAs)₂(AlAs)₁ superlattice is a direct semiconductor and the lowest conduction band state is at point Γ. The squared matrix elements of transition are calculated. The pressure coefficients of energy gaps of the (GaAs)₁(AlAs)₁ and (GaAs)₂(AlAs)₁ superlattices are calculated and compared with those obtained by hydrostatic pressure experiments.     

二维介电光子晶体中的赝自旋态与拓扑相变

基于背散射抑制且对缺陷免疫的传输性质,光子拓扑绝缘体为电磁传输调控提供了一种新颖的思路.类比电子体系中的量子自旋霍尔效应,本文设计出一种简单的二维介电光子晶体,以实现自旋依赖的光子拓扑边界态.该光子晶体是正三角环形硅柱子在空气中排列而成的蜂窝结构.将硅柱子绕各自中心旋转60°,可实现二重简并的偶极子态和四极子态之间的能带翻转.这两对二重简并态的平均能流密度围绕原胞中心的手性可充当赝自旋自由度,其点群对称性可用来构建赝时间反演对称.根据k·p微扰理论,给出了布里渊区中心附近的有效哈密顿量以及对应的自旋陈数,由此证实能带翻转的实质是拓扑相变.数值计算结果揭示,在拓扑非平庸和平庸的光子晶体分界面上可实现单向传输且对弯曲、空穴等缺陷免疫的拓扑边界态.本文中的光子晶体只由电介质材料组成并且晶格结构简单,实现拓扑相变时无需改变柱子的填充率或位置,只需转动一个角度.因此,这种结构在拓扑边界态的应用中更为有效.