Signatures of a topological phase transition in a planar Josephson junction

Abstract

Intense scrutiny over the last decade of experiments on a variety of platforms aiming to identify topological superconductivity has raised the stakes in this search. Because nontopological Andreev bound states at or near zero energy can resemble topological states, signatures beyond spectroscopy of zero-energy modes are called for. A key signature of a topological phase transition - namely, the closing and reopening of the superconducting gap - is mostly unobserved in previous experiments. Here, this phenomenon is reported in planar Josephson junction devices made from superconductor-semiconductor hybrid materials. Consistency between experiment and numerics, along with a number of other characteristic signatures, including dependence on phase, magnetic field, and chemical potential, support the conclusion that the observed gap closing and reopening, with associated appearance of a zero-bias conductance peaks upon reopening, are associated with topological transition.