Reopening of the superconducting gap in a planar Josephson junction*

The reopening of the superconducting gap is a long-sought after hallmark of topological superconductivity. Here we study a one-dimensional superconductor formed in a planar Josephson junction that can undergo a topological phase transition in the presence of an in-plane magnetic field. As the external magnetic field is increased, we observe the closing and reopening of the superconducting gap, followed immediately by the appearance of a zero-bias mode, suggesting the formation of a topological state. We test the topological origin of the gap reopening by studying its dependence on superconducting phase difference, chemical potential and magnetic field direction.