Anomalous supercurrent in Josephson devices with topological insulator barriers

Abstract:
There has been intense interest in realizing excitations of Majorana fermions (MFs) in solid-state systems. Circuits of Josephson junctions (JJs) made of closely spaced conven- tional superconductors on 3D topological insulators have been proposed to host Andreev bound states (ABSs) which can include MFs. In this talk, I will present signatures of an anomalous supercurrent carried by such topologically non-trivial low energy ABSs in various SQUIDs made of Nb/Bi2Se3/Nb junctions. An electrostatic top gate placed on the JJs allows strong modulation of the supercurrent despite a high bulk contribution to the normal state conductance. In response to a magnetic field threading flux within the superconducting loop, we find unconventional SQUID oscillations enclosed by an envelope associated with a clear Fraunhofer-like diffraction pattern, indicating spatially uniform and symmetric JJs. At a critical gate voltage, when the trivial 2DEG at the surface is nearly depleted, we observe a sharp drop in the critical current, signaling a topological phase transition in which the spatial location of the supercurrent-carrying states in the junction is changed. This transition is accompanied by qualitative changes in the SQUID oscillations, single-junction diffraction patterns, and temperature dependence of the critical current.

Acknowledgements. This work is supported by Microsoft Station-Q and NSF.