Abstract
Josephson junctions with topological insulator weak links can host low-energy Andreev-bound states giving rise to a current-phase relation that deviates from sinusoidal behaviour. Of particular interest are zero-energy Majorana-bound states that form at a phase difference of π. Here we report on interferometry studies of Josephson junctions and superconducting quantum interference devices (SQUIDs) incorporating topological insulator weak links. We find that the nodes in single-junction diffraction patterns and SQUID oscillations are lifted and independent of chemical potential. At high temperatures, the SQUID oscillations revert to conventional behaviour, ruling out asymmetry. The node-lifting of the SQUID oscillations is consistent with low-energy Andreev-bound states exhibiting a nonsinusoidal current-phase relation, co-existing with states possessing a conventional sinusoidal current-phase relation. However, the finite nodal currents in the single-junction diffraction pattern suggest an anomalous contribution to the supercurrent possibly carried by Majorana-bound states, although we also consider the possibility of inhomogeneity.
Highlights
Josephson junctions with topological insulator weak links can host low-energy Andreevbound states giving rise to a current–phase relation that deviates from sinusoidal behaviour
We focus on one particular tri-junction superconducting quantum interference devices (SQUIDs), with similar results having been observed in many other devices
We focus first on the SQUID nodes because there are a number of well-known phenomena that can lift the nodes of SQUID oscillations, finite inductance of the SQUID loop, parallel conductance mechanisms, asymmetry in junctions and a nonsinusoidal current–phase relation (CPR)
Summary
Josephson junctions with topological insulator weak links can host low-energy Andreevbound states giving rise to a current–phase relation that deviates from sinusoidal behaviour. By coupling a TI with two superconducting leads, one can generate a Josephson junction with anomalous current–phase relation (CPR) because of the presence of lowenergy Andreev-bound states (ABSs)[2,5,6] Among these states, there is a special pair with 4p periodicity that crosses zero energy when a phase difference of p is introduced across the junction. In a Josephson junction in the small junction limit with a uniform current density and a sinusoidal CPR, this results in a Fraunhofer diffraction pattern, characterized by vanishing of the critical current from destructive interference whenever an integer number of flux quanta are enclosed by the junction These nodes remain zero even for nonsinusoidal CPRs that are 2p-periodic. While such states might include MBSs, further work is likely required to firmly exclude other effects
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