Tunneling spectroscopy of topological insulator Josephson junctions

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 Josephson junction made on the TI surface, the spin-momentum locking of the TSS guarantees the existence of spin nondegenerate, perfectly transmitted Andreev modes. These modes lead to gap closing as a function of the superconducting phase difference φ. Here, we report the observation of highly transmissive Andreev modes in a TI Josephson junction, probed via tunnel contacts and φ controlled by a flux loop. Within our experimental resolution, they could indicate a gap closing and reopening. This phenomenon is observed for a wide range of chemical potentials, supporting its TSS origin. Under an applied magnetic field, highly transmissive modes are observed locally, matching theoretical predictions for Josephson vortices in the TSS. Our results support the fundamental role of spin-momentum locking in the Andreev physics in the TSS.