James C. McGroddy Prize for New Materials (2021): Proximity induced superconductivity in topological surface states on bulk insulating topological insulators.

Our group has explored the proximity effect from superconducting niobium into topological insulators using ARPES and Josephson junction-based transport samples. The goal is to make an artificial topological superconductor with a hard gap that may support Majorana Zero Modes (MZM) with long parity lifetimes. Using ARPES we have studied the free side of samples of some topological insulators on niobium substrates. We have found that TIs with bulk carriers (Bi2Se3) exhibit strongly thickness dependent superconductivity penetrating away from the surface into the bulk without a hard gap. On TIs without bulk carriers (BiSbTe3) this is absent. However, the topological carriers on the surface directly in contact with the superconductor still become superconducting. To study this, we have made transport samples by putting square arrays of small niobium islands (50 nm radius) on top of bulk-insulating BiSbTe3 thin films. The island size and spacing is smaller than emergent superconducting length scales and similar in size to the topological carrier mean free path. These samples exhibit a superconducting transition at 300 mK with a phase stiff supercurrent that saturates to a low temperature limit below 150 mK. The current-voltage characteristics of the array are similar to those of an overdamped SNS Josephson junction. The superconductivity occurs only in the topological surface layer. This kind of artificial topological superconductor may exhibit a hard gap and be suitable for exploring MZMs. This work was carried out in collaboration with Yang Bai, Joseph Hlevyack, David Floetotto, and Tai Chiang Chang.