Measurement of the current-phase relation in superconductor-topological insulator-superconductor Josephson junctions

The spin-helical topological surface states (TSS) of topological insulators in proximity with an s-wave superconductor are predicted to demonstrate signatures of topological superconductivity and host Majorana fermions. Here, we report highly skewed current-phase relation (CPR) in gate-tunable Josephson junctions made from topological insulator BiSbTeSe₂ with superconducting Nb electrodes. Our observations provide strong evidences for the existence of low-energy states and the ballistic transport in the TI-based junctions. We also investigate the gate dependence of the CPR, where we observe that close to the Dirac point the transport becomes more diffusive, most likely due to the presence of electron/hole puddles.