Surface step states and Majorana end states in profiled topological insulator thin films

TL;DR

This paper explores how surface steps in topological insulator thin films can host Majorana zero modes when coupled with superconductors, revealing a simple way to realize Majorana states in such systems.

Contribution

It introduces a novel mechanism where surface steps induce bound states and Majorana zero modes in topological insulator films with superconducting contacts.

Findings

Step bound states have linear dispersion and inherit spin locking.

Majorana zero modes can form at step ends with superconducting proximity.

Hybridisation gap oscillates and decays exponentially with film thickness.

Abstract

The protected helical surface states in thin films of topological insulators (TI) are subject to inter-surface hybridisation. This leads to gap opening and spin texture changes as witnessed in photoemission and quasiparticle interference investigations. Theoretical studies show that universally the hybridisation energy exhibits exponential decay as well as sign oscillations as a function of film thickness, depending on the effective band parameters of the material. When a step is introduced in the TI film e.g. by profiling the substrate such that the hybridisation has different signs on both sides of the step, 1D bound states appear within the hybridisation gap which decay exponentially with distance from the step. The step bound states have linear dispersion and inherit the helical spin locking from the surface states and are therefore non-degenerate. When the substrate becomes an s-wave superconductor Majorana zero modes located at the step ends are created inside the superconducting gap. The proposed scenario involves just a suitably stepped interface of superconductor and TI and therefore may be a most simple device being able to host Majorana zero modes.