Signatures of a topological phase transition in a planar Josephson junction

A growing body of work suggests that planar Josephson junctions fabricated using superconducting hybrid materials provide a highly controllable route toward one-dimensional topological superconductivity. Among the experimental controls are the in-plane magnetic field, phase difference across the junction, and carrier density set by electrostatic gate voltages. Here, we investigate wide planar Josephson junctions based on an epitaxial InAs/Al heterostructure, embedded in a superconducting loop, probed with integrated quantum point contacts (QPCs) at both ends of the junction. For a large range of gate voltages, a closing and reopening of the superconducting gap is observed that appears strongly correlated at the two ends of the junction. The reopening occurred roughly at in-plane magnetic field B11 ' 0.2 T, was uniform across multiple devices, and tunable with phase bias. A narrower range of the junction gate voltage, covering roughly 10% of the operable gate space, supports a zero-bias conductance peak (ZBCP) that appears upon reopening of the gap. However, the height, shape, and even presence of ZBCPs typically differed between the ends, suggesting a weak end-to-end correlation. Theoretical modeling suggests an orbital-effect driven mechanism for the gap-reopening. Within this model, the gap reopening represents a topological phase transition when associated with a zero-energy state. While several features of the experimental data are captured by our model, the lack of end-to-end correlation of ZBCPs in the experiment suggests the absence of an uninterrupted topological phase along the junction, presumably due to disorder. Deliberately tuned local end dots show distinctly different phenomenology, particularly with respect to phase bias, providing a check on possible topological and nontopological interpretations.