Symmetry Protected Josephson Supercurrents in Three-Dimensional Topological Insulators

TL;DR

This study demonstrates that Josephson supercurrents in three-dimensional topological insulators are predominantly carried by surface states and remain robust against disorder, especially when the bulk charge carriers are depleted, supporting the pursuit of Majorana quasiparticles.

Contribution

First experimental measurement of Josephson effects in topological insulators within the true topological regime with surface-only conduction, confirming surface state dominance.

Findings

Supercurrent is mainly carried by surface states due to topological protection.

Supercurrent remains robust against disorder in the topological regime.

Superconductivity is observed when the chemical potential is in the band gap.

Abstract

Coupling the surface state of a topological insulator (TI) to an s-wave superconductor is predicted to produce the long-sought Majorana quasiparticle excitations. However, superconductivity has not been measured in surface states when the bulk charge carriers are fully depleted, i.e., in the true topological regime that is relevant for investigating Majorana modes. Here, we report measurements of DC Josephson effects in TI-superconductor junctions as the chemical potential is moved from the bulk bands into the band gap, or through the true topological regime characterized by the presence of only surface currents. We examine the relative behavior of the system at different bulk/surface ratios, determining the effects of strong bulk/surface mixing, disorder, and magnetic field. We compare our results to 3D quantum transport simulations to conclude that the supercurrent is largely carried by surface states, due to the inherent topology of the bands, and that it is robust against disorder.