Local and Nonlocal Fraunhofer-like Pattern from an Edge-Stepped Topological Surface Josephson Current Distribution

We report a surface-dominant Josephson effect in superconductor–topological insulator–superconductor (S–TI–S) devices, where a Bi1.5Sb0.5Te1.7Se1.3 (BSTS) crystal flake was adopted as an intervening TI between Al superconducting electrodes. We observed a Fraunhofer-type critical current modulation in a perpendicular magnetic field in an Al–TI–Al junction for both local and nonlocal current biasing. Fraunhofer-type modulation of the differential resistance was also observed in a neighboring Au–TI–Au normal junction when it was nonlocally biased by the Al–TI–Al junction. In all cases, the Fraunhofer-like signal was highly robust to the magnetic field up to the critical field of the Al electrodes, corresponding to the edge-stepped nonuniform supercurrent density arising from the top and rough side surfaces of the BSTS flake, which strongly suggests that the Josephson coupling in a TI is established through the surface conducting channels that are topologically protected.