An Anderson Impurity Interacting with the Helical Edge States in a Quantum Spin Hall Insulator

Using the natural orbitals renormalization group（NORG）method,we investigate the screening of the local spin of an Anderson impurity interacting with the helical edge states in a quantum spin Hall insulator.It is found that there is a local spin formed at the impurity site and the local spin is completel.y screened by electrons in the quantum spin Hall insulator.Meanwhile,the local spin is screened dominantly by a single active natural orbital.We then show that the Kondo screening mechanism becomes transparent and simple in the framework of the natural orbitals formalism.We project the active natural orbital respectively into real space and momentum space to characterize its structure.We conilrm the spin-momentum locking property of the edge states based on the occupancy of a Bloch state on the edge to which the impurity couples.Furthermore,we study the dynamical property of the active natural orbital represented by the local density of states,from which we observe the Kondo resonance peak.

Wave-vector filtering effect of the electric-magnetic barrier in HgTe quantum wells

Because of the helicity of electrons in HgTe quantum wells（QWs） with inverted band structures,the electrons cannot be confined by electric barriers since electrons can tunnel the barriers perfectly without backscattering in the HgTe QWs.This behavior is similar to Dirac electrons in graphene.In this paper,we propose a scheme to confine carriers in HgTe QWs using an electric-magnetic barrier.We calculate the transmission of carriers in 2-dimensional HgTe QWs and find that the wave-vector filtering effect of local magnetic fields can confine the carriers.The confining effect will have a potential application in nanodevices based on HgTe QWs.

A computational investigation of topological insulator Bi2Se3 film

Topological insulators have a bulk band gap like a.n ordinary insulator and conducting states on their edge or surface which are formed by spin orbit coupling and protected by time-reversal sylnmetry. W＇e report theoretical analyses of tile electronic properties of three-dimensional topological insulator Bi2Sea film on different energies. We choose five different energies （ 123, -75, 0, 180. 350 meV） around the Dirac cone （ 113 meV）. When energy is close to the Dirac cone. the properties of wave function Inatch the topological insulator＇s halhnark perfectly. When energy is far way from tile Dirac cone, the halhnark of topological insulator is broken and the helical states disappear. The electronic properties of helical states are dug out fi＇om the calculation results. The spin-inomentum locking of the helical StatES are confirmed. A 3-fold symmetry of the helical states in Brillouin zone is also revealed. The penetration depth of tile helical states is two quintuple layers which can be identified from layer projection. Tile charge contribution on each quintuple layer depends on the energy, and has completely different behavior along K and M direction in Brillouin zone. From orbital projection, we can find that the maximum charge contribution of the helical states is p： orbit and the charge contribution on py and Px orbits have 2-fold symmetry.

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 Hamiltouian. 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.