Finite size effects on the helical edge states on the Lieb lattice

For a two-dimensional Lieb lattice,that is,a line-centered square lattice,the inclusion of the intrinsic spin–orbit(ISO)coupling opens a topologically nontrivial gap,and gives rise to the quantum spin Hall(QSH) effect characterized by two pairs of gapless helical edge states within the bulk gap.Generally,due to the finite size effect in QSH systems,the edge states on the two sides of a strip of finite width can couple together to open a gap in the spectrum.In this paper,we investigate the finite size effect of helical edge states on the Lieb lattice with ISO coupling under three different kinds of boundary conditions,i.e.,the straight,bearded and asymmetry edges.The spectrum and wave function of edge modes are derived analytically for a tight-binding model on the Lieb lattice.For a strip Lieb lattice with two straight edges,the ISO coupling induces the Dirac-like bulk states to localize at the edges to become the helical edge states with the same Dirac-like spectrum.Moreover,it is found that in the case with two straight edges the gapless Dirac-like spectrum remains unchanged with decreasing the width of the strip Lieb lattice,and no gap is opened in the edge band.It is concluded that the finite size effect of QSH states is absent in the case with the straight edges.However,in the other two cases with the bearded and asymmetry edges,the energy gap induced by the finite size effect is still opened with decreasing the width of the strip.It is also proposed that the edge band dispersion can be controlled by applying an on-site potential energy on the outermost atoms.     

Topological phase in one-dimensional Rashba wire

We study the possible topological phase in a one-dimensional(1D) quantum wire with an oscillating Rashba spin–orbital coupling in real space. It is shown that there are a pair of particle–hole symmetric gaps forming in the bulk energy band and fractional boundary states residing in the gap when the system has an inversion symmetry. These states are topologically nontrivial and can be characterized by a quantized Berry phase ±π or nonzero Chern number through dimensional extension. When the Rashba spin–orbital coupling varies slowly with time, the system can pump out 2 charges in a pumping cycle because of the spin flip effect. This quantized pumping is protected by topology and is robust against moderate disorders as long as the disorder strength does not exceed the opened energy gap.     

双带型光子晶体中双V型四能级原子自发辐射谱中的黑线研究

研究了双带型光子晶体中双V型四能级原子自发辐射的辐射谱.双V型四能级原子同时与真空热库和双带型光子晶体热库耦合.研究发现双V型四能级原子自发辐射谱中有三种不同原因可能引起的黑线：第一种是由于量子干涉效应;第二种是由于各向同性光子晶体带边处态密度具有奇异性;第三种是真空场中的量子干涉和光子晶体禁带内态密度为零共同作用的结果.通过移动光子晶体的带边位置,在各向同性光子晶体带边引入光滑因子,以及在光子晶体中引入缺陷等对这三种黑线的影响,对上述结果进行了分析和讨论. 关键词： 双带型光子晶体 双V型四能级原子 黑线     

硅量子点的形状及其弯曲表面效应

硅量子点的弯曲表面引起系统的对称性破缺, 致使某些表面键合在能带的带隙中形成局域电子态.计算结果表明:硅量子点的表面曲率不同形成的表面键合结合能和电子态分布明显不同. 例如, Si–O–Si桥键在曲率较大的表面键合能够在带隙中形成局域能级, 而在硅量子点曲率较小的近平台表面上键合不会形成任何局域态, 但此时的键合结合能较低. 用弯曲表面效应(CS)可以解释较小硅量子点的光致荧光光谱的红移现象. CS效应揭示了纳米物理中又一奇妙的特性. 实验证实, CS效应在带隙中形成的局域能级可以激活硅量子点发光. 关键词： 硅量子点 弯曲表面效应 表面键合 局域能级     

Higher-Order Topological Spin Hall Effect of Sound

We theoretically propose a reconfigurable two-dimensional(2 D) hexagonal sonic crystal with higher-order topology protected by the six-fold,C₆,rotation symmetry.The acoustic band gap and band topology can be controlled by rotating the triangular scatterers in each unit cell.In the nontrivial phase,the sonic crystal realizes the topological spin Hall effect in a higher-order fashion:(i) the edge states emerging in the bulk band gap exhibit partial spin-momentum correlation and are gapp...     

a-Si:H中本征缺陷引起的赝隙态

本文取原子集团模型Si₈H₁₈,Si₁₇及从连续无规网络中挖取的集团模型Si₂₉和Si₄₇,用CNDO LCAO-MO方法计算其电子结构,探讨了a-Si:H中由弱键、弯键和带电组态等本征缺陷引起的赝隙态分布。结果表明,当弱键拉伸时,两个弱键态移动并收缩至带隙中央;过剩电荷使弱键能级移至价带顶或导带底附近;弯键态主要出现在价带顶附近。当弯键曲率较大时,弯键态上移至带隙中央以下的区域。结构拓扑无序导致 关键词：     

The atom-photon entanglement of a two-level system embedded in double-band photonic band edge

The entanglement of a two-level atom with its spontaneously emitted photon embedded in double-band anisotropic photonic crystal has been investigated via the method of the quantum entropy. Different from the case in an isotropic crystal or in vacuum, the entanglement has symmetrical properties and much slower entanglement rate near the two band edges. Moreover, as a result of the atom-photon bound states by the virtue of the localization around the emitting atom, the degree of the entanglement gradually increases, achieves the maximum and then sharply reduces to zero on the boundary of forbidden band gap as the atomic frequency moves from the center of the band gap to either of the band edges.     

The emergence of bound states in a superconducting gap at the topological insulator edge

We consider the edge of a superconducting topological insulator with the impurity in the presence of the Zeeman field. We analytically prove that in the trivial phase two Andreev bound states (ABSs) arise with energies moving from the superconducting gap edges to zero forming two Majorana-like bound states, as the impurity strength varies from 0 to ±2. When the Zeeman field is locally perturbed, ABSs arise both in the trivial and topological phases, but in the topological phase ABSs with energy near the gap edges cannot transform into Majorana bound states and vice versa.     

Band gap and edge engineering via ferroic distortion and anisotropic strain: the case of SrTiO(3)

The effects of ferroic distortion and biaxial strain on the band gap and band edges of SrTiO(3) are calculated by using density functional theory and many-body perturbation theory. Anisotropic strains are shown to reduce the gap by breaking degeneracies at the band edges. Ferroic distortions are shown to widen the gap by allowing new band edge orbital mixings. Compressive biaxial strains raise band edge energies, while tensile strains lower them. To reduce the SrTiO(3) gap, one must lower the symmetry from cubic while suppressing ferroic distortions. Our calculations indicate that, for engineered orientation of the growth direction along [111], the SrTiO(3) gap can be controllably and considerably reduced at room temperature.     

Random Block Operators

We study fundamental spectral properties of random block operators that are common in the physical modelling of mesoscopic disordered systems such as dirty superconductors. Our results include ergodic properties, the location of the spectrum, existence and regularity of the integrated density of states, as well as Lifshits tails. Special attention is paid to the peculiarities arising from the block structure such as the occurrence of a robust gap in the middle of the spectrum. Without randomness in the off-diagonal blocks the density of states typically exhibits an inverse square-root singularity at the edges of the gap. In the presence of randomness we establish a Wegner estimate that is valid at all energies. It implies that the singularities are smeared out by randomness, and the density of states is bounded. We also show Lifshits tails at these band edges. Technically, one has to cope with a non-monotone dependence on the random couplings.     

A derivation for the energy dependence of the density of band tail states in disordered materials

Variational arguments are employed to present the first explicit derivation of the behavior of band tail states and localization length over most of the tail region of a random alloy. Exponential tails are shown to exist near the mobility edges, in conformity with the observation in disordered systems. The character of the states forming the band tails is discussed and, for random alloys, the localization length is shown to exhibit a minimum as a function of energy between band edge and mobility edge.     

The giant Stark effect in armchair-edge phosphorene nanoribbons under a transverse electric field

We study the variation of electronic properties for armchair-edge phosphorene nanoribbons (APNRs) modulated by a transverse electric field. Within the tight-binding model Hamiltonian, and by solving the differential Schrödinger equation, we find that a band gap closure appears at the critical field due to the giant Stark effect for an APNR. The gap closure has no field polarity, and the gap varies quadratically for small fields but becomes linear for larger ones. We attribute the giant Stark effect to the broken edge degeneracy, i.e., the charge redistributions of the conduction band minimum and valence band maximum states localized at opposite edges induced by the field. By combined with the Green's function approach, it is shown that in the presence of the critical field a gap of density of states (DOS) disappears and a high value DOS turns up at the energy position of the band gap closure. Finally, as the field increases, we find the band gap decreases more rapidly and the gap closure occurs at smaller fields for wider ribbons. Both the band gap and DOS variations with the field show an insulator-metal transition induced by a transverse electric field for the APNR. Our results show that wider APNRs are more appreciable to design field-effect transistors.     

Localized states in amorphous arsenic

Optical absorption and photoconductivity spectra of bulk amorphous arsenic are presented and analyzed using a general density of states model for amorphous semiconductors. It is concluded that localized states do exist in the forbidden gap and at the band edges and that transitions between localized states are significant.     

Defect modes of chiral photonic crystals with an isotropic defect

Specific features of the defect modes of cholesteric liquid crystals (CLCs) with an isotropic defect, as well as their photonic density of states, Q factor, and emission, have been investigated. The effect of the thicknesses of the defect layer and the system as a whole, the position of the defect layer, and the dielectric boundaries on the features of the defect modes have been analyzed. It is shown that when the CLC layer is thin the density of states and emission intensity are maximum for the defect mode, whereas when the CLC layer is thick, these peaks are observed at the edges of the photonic band gap. Similarly, when the gain is low, the density of states and emission intensity are maximum for the defect mode, whereas at high gains these peaks are also observed at the edges of the photonic band gap. The possibilities of low-threshold lasing and obtaining high-Q microcavities have been investigated.     

Band engineering of ZnS by codoping for visible-light photocatalysis

Codoping is demonstrated as an efficient approach to narrow the band gap of ZnS and enhance its photocatalytic activity. Herein, we perform the density-function theory calculations of ZnS by codoping of X (N, F) with transition metals (TM = V, Cu). The band gap is reduced in four different types of codoped ZnS. In particular, Cu_ZnF_S codoping, a charge-compensated donor–acceptor pair, leads to an about 32 % reduction of the energy gap, thus extending the absorption edge to visible-light region. The band gap reduction is due to the upshift of the top valence band comprised with the delocalized hybridizing levels of Cu 3d and S 3p states, and the downshift of the bottom conduction band consisting of F 2s states. Moreover, the larger value of m _e*/m _h* in Cu_ZnF_S–ZnS would result in a lower recombination rate of the electron–hole pairs. Both band gap reduction and low recombination rate are critical elements for efficient light-to-current conversion in codoped ZnS. These findings raise the prospect of using codoped ZnS with specifically engineered electronic properties in a variety of photocatalytic applications.     

Effect of the dangling bond on the electronic and magnetic properties of BN nanoribbon

The effect of the dangling bond on the electronic and magnetic properties of BN nanoribbon with zigzag edge (ZBNNR) and armchair edge (ABNNR) have been studied using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. Though ZBNNR or ABNNR with H atom terminated at both edges is nonmagnetic semiconductor, the dangling bond induces magnetism for the ZBNNR with bare N edge, bare B edge, bare N and B edges, the ABNNR with bare N edge and bare B edge. However, the ABNNR with bare N and B edges is still nonmagnetic semiconductor due to the strong coupling of the dangling bonds of dimeric N and B atoms at the same edge. The magnetic moment of ZBNNR with bare N(B) edge is nearly half the magnetic moment of ABNNR with bare N(B) edge. Such a half relationship is also existed in the number of the dangling bond states appeared around the Fermi level in the band structures. Furthermore, the dangling bond states also cause both ZBNNR and ABNNR with bare N edge a transition from semiconducting to half-metallic and thus a completely (100%) spin-polarization, while cause both ZBNNR and ABNNR with bare B edge as well as ABNNR with bare N and B edges only a decrease in their band gap.     

Observation of modulated spontaneous emission of Rhodamine 6G in low refractive index contrast 1D-periodic gelatin film

The modulation of the spontaneous emission of Rhodamine 6G has been observed in one-dimensional periodic dielectric structure of dichromated gelatin film with refractive index contrast as low as 0.01. The spontaneous emission is enhanced at the band edges and inhibits in the band gap, which agree well with the theoretical analysis on the redistribution of the fractional local density of optical states.     

Electronic structure of MgS and MgYb2S4: Electron Energy-Loss Spectroscopy and self-consistent multiple scattering calculations

The electronic structure of MgS and MgYb₂S₄ have been studied using the fine structure of the Mg-K, S-K, Mg-L_2,3, S-L_2,3 and Yb-N₅ edges measured by electron energy-loss spectroscopy (EELS). Our experimental results are compared with real-space full multiple scattering calculations as incorporated in the FEFF9.6 code. All edges are very well reproduced. Total and partial densities of states have been calculated. The calculated densities of states of Mg and S are similar in both compounds. The energy distribution of these states suggests a covalent nature for both materials. For MgYb₂S₄ a band gap smaller than for MgS is predicted. In this compound the top of the valence band and the bottom of the conduction band are dominated by Yb states.     

Indirect-direct band gap transition of two-dimensional arsenic layered semiconductors—cousins of black phosphorus

The monolayer arsenic in the puckered honeycomb structure was recently predicted to be a stable two-dimensional layered semiconductor and therefore named arsenene. Unfortunately, it has an indirect band gap, which limits its practical application. Using first-principles calculations, we show that the band gaps of few-layer arsenic have an indirect-direct transition as the number of arsenic layers(n) increases from n=1 to n=2. As n increases from n=2 to infinity, the stacking of the puckered honeycomb arsenic layers forms the orthorhombic arsenic crystal ??-As, arsenolamprite), which has a similar structure to the black phosphorus and also has a direct band gap. This indirect-direct transition stems from the distinct quantum-confinement effect on the indirect and direct band-edge states with different wavefunction distribution. The strain effect on these electronic states is also studied, showing that the in-plane strains can induce very different shift of the indirect and direct band edges, and thus inducing an indirect-direct band gap transition too. The band gap dependence on strain is non-monotonic, with both positive and negative deformation potentials. Although the gap of arsenene opens between As p-p bands, the spin-orbit interaction decreases the gap by only 0.02 e V, which is much smaller than the decrease in Ga As with an s-p band gap. The calculated band gaps of arsenene and ?-As using the hybrid functional are 1.4 and 0.4 e V respectively, which are comparable to those of phosphorene and black phosphorus.