Affiliation
University of Cologne
Talk Title
Protected gap closing and reopening in topological insulator Josephson junctions
Abstract
In the seminal proposal by Fu and Kane to use the superconducting proximity effect to turn the surface state of a topological insulator (TI) into a topological superconductor, they predicted that a linear Josephson junction made on the TI surface form a nonchiral 1D Majorana state [1]. The key characteristics of this 1D Majorana state is its dependence on the phase difference φ across the junction; namely, it presents periodic topological phase transitions and the associated gap closing at φ = (2n + 1)π with integer n, leading to a 4π-periodicity. Although the 4π-periodicity has been indirectly inferred in the ac Josephson effect in the past, the existence of the 1D Majorana state in a TI Josephson junction remained controversial. In this contribution we report the direct observation of the protected gap closing and reopening occurring at φ = (2n + 1)π in a TI Josephson junction [2] in which the local density of states is probed by tunnel contacts and φ is controlled by a flux loop, similar to the experiments performed on graphene [3] and HgTe [4], where nontopological Andreev bound states were observed. The gap closing and reopening occurring in our TI Josephson junction is confirmed to be essentially independent of the chemical potential, which supports its intrinsic origin. When a perpendicular magnetic field is applied to the junction such that a couple of Josephson vortices nucleate and φ becomes position dependent, the gap closing is found to be spatially confined to the Josephson vortex core where φ is locally (2n + 1)π. This result provides a direct confirmation of the Fu-Kane proposal and gives evidence for the appearance of topological superconductivity in a TI Josephson junction.
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