Edge absorption and circular photogalvanic effect in 2D topological insulator edges

The electron absorption on the edge states and the edge photocurrent of a 2D topological insulator (TI) are studied. We consider the optical transitions within linear edge branches of the energy spectrum. The interaction with impurities is taken into account. The circular polarization is found to produce the edge photocurrent, the direction of which is determined by light polarization and edge orientation.     

Edge excitons in a 2D topological insulator in a magnetic field

Exciton edge states and the microwave edge exciton absorption of a 2D topological insulator subject to the in-plane magnetic field are studied. The magnetic field forms a narrow gap in electron edge states that allows the existence of edge exciton. The exciton binding energy is found to be much smaller than the energy of a 1D Coulomb state. Phototransitions exist on the exciton states with even numbers, while odd exciton states are dark.     

Photogalvanic effects in topological insulators

We discuss optical absorption in topological insulators and study possible photoelectric effects theoretically. We found that absorption of circularly polarized electromagnetic waves in two-dimensional topological insulators results in electric current in the conducting 1D edge channels, the direction of the current being determined by the light polarization. We suggest two ways of inducing such a current: due to magnetic dipole electron transitions stimulated by irradiation of frequency below the bulk energy gap, and due to electric dipole transitions in the bulk at frequencies larger than the energy gap with subsequent capture of the photogenerated carriers on conducting edge states.     

Design principles and coupling mechanisms in the 2D quantum well topological insulator HgTe/CdTe

We present atomistic band structure calculations revealing a different mechanism than recently surmised via k · p calculations about the evolution of the topological state (TS) in HgTe/CdTe. We show that 2D interface (not 1D edge) TSs are possible. We find that the transitions from a topological insulator at critical HgTe thickness of n = 23 ML (6.453 [corrected] ?) to a normal insulator at smaller n is due to the crossing between two interface-localized states: one derived from the S-like Γ?(c) and one derived from the P-like Γ?(v) light hole, not because of the crossing of an interface state and an extended quantum well state. These atomistic calculations suggest that a 2D TS can exist in a 2D system, even without truncating its symmetry to 1D, thus explaining the otherwise surprising similarity between the 2D dispersion curves of the TS in HgTe/CdTe with those of the TS in 3D bulk materials such as Bi?Se?.     

Photoinduced anisotropic edge states and signatures of nonequilibrium spin polarization in silicene nanoribbons

Most topological phase transitions are accompanied by the emergence of surface/edge states with spin dependence. Usually, the quantized Hall conductivity cannot characterize the anisotropic transports and spin dependence of topological states. Here, we study the intricate topological phase transition and the anisotropic behavior of edge states in silicene nanoribbon submitted to an electric field or/and a light irradiation. It is interesting to find that a circularly polarized light can induce a type-II quantum anomaly Hall phase, which is manifested as the high Chern number and the strong anisotropic edge states. Besides the measurement of the quantized Hall conductivity, we further propose to probe these topological phase transitions and the anisotropy of edge states by measuring the current-induced nonequilibrium spin polarization. It is found that the spin polarization exhibits more signatures about the behavior of surface/edge states, beyond the quantized Hall conductivity, especially for spin-dependent transports with different velocities.     

On the photon-drag effect of photocurrent of surface states of topological insulators

The photocurrent of surface states of topological insulator due to photon-drag effect is computed, being based on pure Dirac model of surface states. The scattering by disorder is taken into account to provide a relaxation mechanism for the photocurrent. The Keldysh–Schwinger formalism has been employed for the systematic calculation of photocurrent. The helicity dependent photocurrent of sizable magnitude transverse to the in-plane photon momentum is found, which is consistent with experimental data. Other helicity independent photocurrents with various polarization states are also calculated.     

Optically engineering the topological properties of a spin Hall insulator

Time-periodic perturbations can be used to engineer topological properties of matter by altering the Floquet band structure. This is demonstrated for the helical edge state of a spin Hall insulator in the presence of monochromatic circularly polarized light. The inherent spin structure of the edge state is influenced by the Zeeman coupling and not by the orbital effect. The photocurrent (and the magnetization along the edge) develops a finite, helicity-dependent expectation value and turns from dissipationless to dissipative with increasing radiation frequency, signalling a change in the topological properties. The connection with Thouless' charge pumping and nonequilibrium zitterbewegung is discussed, together with possible experiments.     

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,C_6,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 gapped due to the reduced spatial symmetry at the edges.(ii) The gapped edge states,on the other hand,stabilize the topological corner states emerging in the edge band gap.The partial spin-momentum correlation is manifested as pseudo-spin-polarization of edge states away from the time-reversal invariant momenta,where the pseudospin is emulated by the acoustic orbital angular momentum.We reveal the underlying topological mechanism using a corner topological index based on the symmetry representation of the acoustic Bloch bands.     

Theoretical prediction of topological insulators in two-dimensional ternary transition metal chalcogenides (MM'X4, M = Ta,Nb, or V; M'= Ir,Rh, or Co; X = Se or Te)

Ternary transition metal chalcogenides (TTMCs) have attracted interest due to the discovery of their Weyl semimetallic property and the recent synthesis of layered TTMCs which are regarded as potential candidates for two-dimensional (2D) topological insulators. Here, employing first-principles calculations, we predicted the emergence of non-trivial band topologies in the monolayer MM'X₄ family (M= V, Nb, or Ta; M' = Co, Rh, or Ir; and X = Se or Te) within hybrid functional calculations. Five of eighteen 2D materials were found to be topological insulators, while four of them are magnetic thin films. The nontrivial topologies were verified via the calculated Z₂ topological invariant and topologically protected edge states. Further calculations showed a strain-induced phase transition in VCoTe₄ from a magnetic phase to a nonmagnetic topological insulating phase. Our comprehensive study revealed a diverse family of monolayer ternary transition metal chalcogenides adding new members to the current catalog of 2D topological insulators and 2D magnetic materials.     

Anomalous Spin Textures in a 2D Topological Superconductor Induced by Point Impurities

Topological superconductors are foreseen as good candidates for the search of Majorana zero modes, where they appear as edge states and can be used for quantum computation. In this context, it becomes necessary to study the robustness and behavior of electron states in topological superconductors when a magnetic or non-magnetic impurity is present. The focus is on scattering resonances in the bands and on spin texture to know what the spin behavior of the electrons in the system will be. It is found that the scattering resonances appear outside the superconducting gap, thus providing evidence of topological robustness. Non-trivial and anisotropic spin textures related to the Dzyaloshinskii–Moriya interaction are also found. The spin textures show a Ruderman–Kittel–Kasuya–Yosida interaction governed by Friedel oscillations. It is believed that the results are useful for further studies which consider many-point-impurity scattering or a more structured impurity potential with a finite range.     

Coulomb edge states in electronic 2D systems

The details of the structure of edge electron states in bounded charged 2D systems (electron or hole) are discussed. These details are associated with the nonuniformities of the Coulomb electron (hole) density, which are involved in the formation of these states. It is shown that the Coulomb effects at the sharp edges of various 2D systems (in the Corbino and bar geometries) result in the considerable broadening of the conventional magnetic edge levels. In the same formalism, a threshold mechanism is proposed for the appearance of integer “shelves” in the central part of the screened 2D systems. The results are compared with the existing experimental data.     

Quench dynamics of edge states in 2-D topological insulator ribbons

We study the dynamics of edge states of the two dimensional BHZ Hamiltonian in a ribbon geometry following a sudden quench to the quantum critical point separating the topological insulator phase from the trivial insulator phase. The effective edge state Hamiltonian is a collection of decoupled qubit-like two-level systems which get coupled to bulk states following the quench. We notice a pronounced collapse and revival of the Lochschmidt echo for low-energy edge states illustrating the oscillation of the state between the two edges. We also observe a similar collapse and revival in the spin Hall current carried by these edge states, leading to a persistence of its time-averaged value.     

Interface states in two-dimensional electron systems with spin-orbital interaction

Interface states at a boundary between regions with different spin-orbit interactions (SOIs) in two-dimensional (2D) electron systems are investigated within the one-band effective mass method with generalized boundary conditions for envelope functions. We have found that the interface states unexpectedly exist even if the effective interface potential equals zero. Depending on the system parameters, the energy of these states can lie in either or both forbidden and conduction bands of bulk states. The interface states have chiral spin texture similar to that of the edge states in 2D topological insulators. However, their energy spectrum is more sensitive to the interfacial potential, the largest effect being produced by the spin-dependent component of the interfacial potential. We have also studied the size quantization of the interface states in a strip of 2D electron gas with SOI and found an unusual (non-monotonic) dependence of the quantization energy on the strip width.     

Magnetic quantum ring in two-dimensional topological insulators

The quantum properties of topological insulator magnetic quantum rings formed by inhomogeneous magnetic fields are investigated using a series expansion method for the modified Dirac equation. Cycloid-like and snake-like magnetic edge states are respectively found in the bulk gap for the normal and inverted magnetic field profiles. The energy spectra, current densities and classical trajectories of the magnetic edge states are discussed in detail. The bulk band inversion is found to manifest itself through the angular momentum transition in the ground state for the cycloid-like states and the resonance tunneling effect for the snake-like states.     

Spin injection from topological insulator tunnel-coupled to metallic leads

We study theoretically helical edge states of 2D and 3D topological insulators (TI) tunnel-coupled to metal leads and show that their transport properties are strongly affected by contacts as the latter play a role of a heat bath and induce damping and relaxation of electrons in the helical states of TI. A simple structure that produces a pure spin current in the external circuit is proposed. The current and spin current delivered to the external circuit depend on relation between characteristic lengths: damping length due to tunneling, contact length and, in case of 3D TI, mean free path and spin relaxation length caused by momentum scattering. If the damping length due to tunneling is the smallest one, then the electric and spin currents are proportional to the conductance quantum in 2D TI, and to the conductance quantum multiplied by the ratio of the contact width to the Fermi wavelength in 3D TI.     

Effective spin dephasing mechanism in confined two-dimensional topological insulators

A Kramers pair of helical edge states in quantum spin Hall effect (QSHE) is robust against normal dephasing but not robust to spin dephasing. In our work, we provide an effective spin dephasing mechanism in the puddles of two-dimensional (2D) QSHE, which is simulated as quantum dots modeled by 2D massive Dirac Hamiltonian. We demonstrate that the spin dephasing effect can originate from the combination of the Rashba spin-orbit coupling and electron-phonon interaction, which gives rise to inelastic backscattering in edge states within the topological insulator quantum dots, although the time-reversal symmetry is preserved throughout. Finally, we discuss the tunneling between extended helical edge states and local edge states in the QSH quantum dots, which leads to backscattering in the extended edge states. These results can explain the more robust edge transport in InAs/GaSb QSH systems.     

Photocarrier transport in 2D macroporous silicon structures

The mechanisms of photocarrier transport through a barrier in the surface space-charge region (SCR) of 2D macroporous silicon structures have been studied at photon energies comparable to that of the silicon indirect band-to-band transition. It was found that the photoconductivity relaxation time was determined by the light modulation of barrier on the macropore surface; as a result, the relaxation itself obeyed the logarithmic law. The temperature dependence of the photoconductivity relaxation time was determined by the thermionic emission mechanism of the current transport in the SCR at temperatures T > 180 K, and by the tunnel current flow at T < 100 K, with temperature-independent tunnelling probability. The photo-emf was found to become saturated or reverse its sign to negative at temperatures below 130 K because of light absorption due to optical transitions via surface electronic states close to the silicon conduction band. In this case, the surface band bending increases due to the growth of a negative charge of the semiconductor surface. The equilibrium electrons in the bulk and photoexcited holes on the macropore surface recombine through the channel of multistage tunnel recombination between the conduction and valence bands.     

掺杂对2H-MoTe2光电特性影响的第一性原理研究

MoTe₂是一种非空间反演对称性半导体,由线性偏振光照射,在无偏压条件下可以直接产生光电流,但是非常微弱.掺杂可以改变电子能带结构和降低空间反演对称性,从而有效的增强光电流.本文基于非平衡格林函数-密度泛函理论,采用第一性原理,计算了本征、Nb掺杂、Ti掺杂和W掺杂2H-MoTe₂的能带结构、透射谱和光电流.能带结构表明：Nb掺杂使半导体2H-MoTe₂能带穿越费米能级,转变为金属特性;Ti和W掺杂减小了2H-MoTe₂的带隙,能带没有穿越费米能级,依然为半导体.掺杂都降低2H-MoTe₂的反演对称对称性,从本征的D_3h转变为C_s.从而在线偏振光的照射下可以有效的提高2H-MoTe₂的光电流.同时,发现掺杂可以提高单层2H-MoTe₂在低光子能量下的消光比,如Nb和Ti掺杂单层2H-MoTe₂分别在光子能量1.1 eV和1.2 eV处取得39.48和28.48的高消光比...