Robust gap closing and reopening in topological-insulator Josephson junctions

TL;DR

This paper experimentally observes robust gap closing and reopening at specific phase differences in topological insulator Josephson junctions, confirming the topological nature of the Andreev states influenced by spin-momentum locking.

Contribution

It provides direct experimental evidence of phase-dependent gap closing in TI Josephson junctions, supporting the topological superconductivity theory involving TSS.

Findings

Observation of gap closing at phase difference (2n+1)π

Robustness of gap closing across various chemical potentials

Local gap closing at Josephson vortex cores under magnetic field

Abstract

In the seminal proposal by Fu and Kane, the superconducting proximity effect is used to realize topological superconductivity in the topological surface state (TSS) of a 3D topological insulator (TI). In a line Josephson junction made on the TI surface, the spin-momentum locking of the TSS guarantees the existence of a pair of spin-non-degenerate, perfectly transmitted Andreev modes. These modes lead to robust gap closing and parity alteration as a function of the superconducting phase difference $\varphi$ across the junction. Here, we report the observation of the predicted gap closing at $\varphi = (2n+1)\pi$ in a TI Josephson junction ($n$ integer), where the local density of states is probed via tunnel contacts and $\varphi$ is controlled by a flux loop. This phenomenon is robust for a wide range of chemical potentials, supporting its TSS origin. Under an applied perpendicular magnetic field, Josephson vortices form, making $\varphi$ position-dependent. In this case, the gap closing occurs locally at the Josephson vortex cores where $\varphi = (2n+1)\pi$, which we also observe. Our results confirm the fundamental role of spin-momentum locking in the Andreev physics in the TSS, which implies that the observed gap closing and reopening has a topological nature.