Tunable superconducting diode effect in a topological nano-SQUID

A Josephson diode passes current with zero resistance in one direction but is resistive in the other direction. While such an effect has been observed in several platforms, a large and tunable Josephson diode effect has been rare. Here, we report that a simple device consisting of a topological-insulator (TI) nanowire side-contacted by superconductors to form a lateral Josephson junction presents a large diode effect with the efficiency η reaching 0.3 when a parallel magnetic field B ‖ is applied. The sign and the magnitude of η are tunable not only by B ‖ but also by the back-gate voltage. This diode effect can be understood by modeling the system as a nano–superconducting quantum interference device (SQUID), in which the top and bottom surfaces of the TI nanowire each form a line junction and B ‖ creates a magnetic flux to thread the SQUID loop. This model further shows that the observed diode effect marks the emergence of topological superconductivity in TI nanowire–based Josephson junction.