Degree

Doctor of Philosophy (PhD)

Department

Department of Physics and Astronomy

Document Type

Dissertation

Abstract

Topological Insulators (TIs) have emerged as promising candidates for spintronics and quantum computing applications. Two properties that underlie these applications are the locking of spin and momentum of the surface electrons and topological protection of surface states. While the existence of gapless surface states is guaranteed by topology, as long as time-reversal symmetry is intact, other properties of the surface states are still tunable. Spin-momentum locking makes these states immune to scattering from non-magnetic point impurities. However, extended defects can scatter the states even with electrostatic potential. We show that a line defect with magnetic and electrostatic potential scattering hosts linearly dispersing bound states. These lead to spin accumulation near the defect and dissipation-less charge currents along the defect. We also show that these can be controlled via an in-plane magnetic field.

Several proposals for applications of TIs rely on forming interfaces with other materials, such as ferromagnets and superconductors. However, many of these proposals assume that the spin-momentum locking of the interface is identical to that of a bare TI surface. This is unlikely due to strain, lattice reconstruction, and other factors. Recent work has proposed possible interface states, which include velocity renormalization and symmetry-breaking effects. We propose a gated lateral double junction setup that can be used to detect these features using a conductance measurement.

Inspired by the above work, we discuss the possibility of bound states at the boundary between a surface state (helical) and an interface state (possibly non-helical). To explore this, we need to determine the boundary condition at the junction of the two. We show that this is far from trivial. We find a form of the boundary condition using current conservation and develop a regularization procedure to connect the boundary condition with boundary potentials formally. Then, we show that linearly dispersing bound states are present for most of the cases we consider. We also show that opening a gap on the TI surface will not destroy bound states in the presence of electrostatic potential, although they no longer be linearly dispersing.

Date

11-2-2023

Committee Chair

Vekhter, Ilya

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