Disorder Effects on Surface States in Topological Materials
University of California Davis.
In this talk I will discuss numerical results how surface disorder affects the protected one electron states in novel quantum materials such as topological insulators. Charge and spin currents will be calculated and their dependence on surface disorder will be studied for a model of 3D topological insulator. In the low disorder limit, due to the momentum-spin lock up, both conductivity and surface spin accumulation are expected to be proportional to momentum relaxation time or the inverse of impurity concentration. Here we will show a new aspect of spin generation where the role of the insulating yet topologically non-trivial bulk becomes explicit: an external electric field creates a transverse pure spin current through the bulk of a 3D TI, which transports spins between the top and bottom surfaces and leads to spin accumulation on both. The surface spin density and charge current are then proportional to the spin relaxation time, which can be extended by nonmagnetic scattering via the Dyakonov-Perel spin relaxation mechanism for sufficiently high disorder level. Therefore, this new spin generation mechanism results in a distinct strategy for the enhancement of surface spin polarization by increasing nonmagnetic impurity concentration. At the end, studies of the surface disorder in a Weyl semimetal will be discussed and contrasted to the case of topological insulator.