Conductance spectroscopy of vertical topological Josephson junction Nb/(Bi0.5Sb0.5)2Te3/Nb

The surface states of a topological insulator (TI) can be induced with superconductivity through proximity with a conventional s-wave superconductor (S). To study the coupling between two superconducting TI surfaces, we report the growth and fabrication of vertical Josephson junctions with the topological insulators (Bi0.5Sb0.5)2Te3 sandwiched between Nb electrodes. We observed two Josephson critical currents on the I-V characteristic at 3.5 K, attributed to the bulk Nb and the proximity-induced superconducting (Bi0.5Sb0.5)2Te3 surfaces, respectively. The enhancement of conductance at these two critical currents leads to a bump and a plateau on the differential conductance spectroscopy. By further cooling down to 300 mK, a zero bias conductance peak (ZBCP) appears on the plateau, which is taken as a feature from the unconventional paring at the (Bi0.5Sb0.5)2Te3-Nb interface.