Anomalous Josephson supercurrent in bulk-insulating topological insulator BiSbTeSe₂

Three dimensional topological insulators (3D-TIs) have recently drawn a considerable interest due to their spin-helical surface band structure. The Josephson junctions made of these 3D-TIs, by proximitizing with s-wave superconductors, are predicted to harbour 1D- Majorana modes. While observations of supercurrent in 3D-TI-based Josephson junctions are recently reported, the Fraunhofer patters observed in such topological Josephson junctions, which sometimes present anomalous features, are still not well understood. In this talk, I will discuss about our study on highly gate-tunable topological Josephson junctions made of bulk-insulating TI materials, BiSbTeS₂. The Fermi level can be tuned by gating across the Dirac point, and the high transparency of the Al/BiSbTeSe₂ interface is evinced by high multiple Andreev reflections peak indices reaching n = 12. Anomalous Fraunhofer patterns with missing lobes were observed in the entire range of gate voltage. We find that, by employing an fitting procedure to use the maximum entropy method in a Monte Carlo algorithm, the anomalous Fraunhofer patterns can be explained as a result of inhomogeneous supercurrent distributions on the TI surface in the junction.