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Abstract

In the seminal proposal by Fu and Kane, the superconducting proximity effect was predicted to transform the surface state of a topological insulator (TI) into a topological superconduc- tor, forming a nonchiral 1D Majorana state within a linear Josephson junction on the TI surface. The hallmark of this 1D Majorana state is a robust gap closing as a function of the superconducting phase difference φ across the junction, which alternates in and out of the topological phase. These topological phase-transitions occur at φ = (2n + 1)π with integer n, leading to a 4π-periodicity of the ground state. While the 4π-periodicity has been indirectly inferred in the AC Josephson effect, the direct observation of the 1D Majorana state in a TI Josephson junction has remained contentious. Here, we report the direct observation of topological phase-transitions in a TI Josephson junction, where the local density of states is probed via tunnel contacts and φ is controlled by a flux loop. The observed transitions are independent of the chemical potential, reinforcing their topological origin. Under an applied perpendicular magnetic field, Josephson vortices form, making φ position-dependent. In this case, the gap closing occurs locally at the Josephson vortex cores where φ = (2n + 1)π, which we also observe. Our findings provide direct confirmation of the Fu-Kane proposal and ro- bust evidence for the emergence of topological superconductivity in a TI Josephson junction.