Evidence for an anomalous current phase relation in topological insulator Josephson junctions

TL;DR

This study investigates unconventional current phase relations in topological insulator Josephson junctions, revealing evidence of anomalous supercurrent contributions potentially linked to Majorana bound states through interferometry measurements.

Contribution

It provides experimental evidence of non-sinusoidal current phase relations and node lifting in topological insulator Josephson junctions, suggesting the presence of Majorana states.

Findings

Nodes in diffraction patterns are lifted and independent of chemical potential.

At high temperatures, SQUID oscillations become conventional.

Finite nodal currents suggest an anomalous supercurrent component.

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 $\pi$. 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, coexisting 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.