Abstract
Topological insulators and topological semimetals are novel quantum states of matter, which have topologically nontrivial electronic structures. Due to the nontrivial topology of the energy band structures, these topological states show unique and exotic transport properties. For example, topological insulators possess unconventional electronic boundary states in the bulk energy gap and carry low-dissipation current at the boundaries. Topological semimetals, including topological Dirac and Weyl semimetals, have linear dispersion along all momentum directions around some degeneracy points and exhibit many intriguing transport phenomena associated with the chiral anomaly, such as negative longitudinal magneto-resistance, chiral magnetic effect and nonlocal transport. Both topological insulators and topological semimetals have been discovered in a growing number of realistic materials with strong spin-orbit coupling, and triggered a great deal of interest in recent years.
In this presentation, I will talk about my recent works on the low-temperature magneto-transport of topological insulators and topological semimetals. In the first part, I will talk about the topological invariant of the Bernevig-Hughes-Zhang model for HgTe or InAs/GaSb quantum wells when a perpendicular magnetic field is applied. The quantum spin Hall phase can persist up to a large perpendicular magnetic field. The critical magnetic field that breaks the quantum spin Hall phase in inverted HgTe or InAs/GaSb quantum wells is dominantly determined by the bulk energy gap at the Γ point. In the second part, the integer quantum Hall effect of the surface Dirac electrons in thin films of topological insulators is systematically investigated in an intuitive picture of edge states. No edge-state solution can be obtained for the half-integer Hall conductance. The interplay of structure inversion asymmetry and the perpendicular magnetic field leads to the breakdown of the quantum spin Hall phase of the surface electrons. In the last part, with the help of the general two-node model for topological semimetals, it is found that the magneto-transport of topological semimetals in the quantum limit is sensitive to the type of impurities. In the presence of a delta-correlated impurity potential, three scenarios with negative longitudinal magneto-conductance are unveiled. This indicates that the negative longitudinal magneto-resistance in the quantum limit is not a compelling signature of the chiral anomaly. In the presence of a finite-range Gaussian impurity potential, the Weyl semimetal has a linear, positive, and anisotropic magneto-conductance in weak fields. The longitudinal conductivity remains finite in the zero field limit, although the density of state vanishes at the Weyl nodes.
Anyone interested is welcome to attend.