Josephson currents induced by the Witten effect

TL;DR

This paper predicts a novel topological Josephson effect involving topological insulators and superconductors, where vortex lines induce fractional charges and an AC Josephson effect without external voltage, due to the Witten effect.

Contribution

It introduces a new topological Josephson effect mediated by the Witten effect in topological insulator-superconductor junctions, with specific experimental predictions.

Findings

Vortex lines induce fractional charge e/4 at the interface.

An AC Josephson effect occurs without external voltage under magnetic fields.

Potential experimental setups for observing the effect are proposed.

Abstract

We reveal the existence of a new type of topological Josephson effect involving type II superconductors and three-dimensional topological insulators as tunnel junctions. We predict that vortex lines induce a variant of the Witten effect that is the consequence of the axion electromagnetic response of the topological insulator: at the interface of the junction each flux quantum attains a fractional electrical charge of e/4. As a consequence, if an external magnetic field is applied perpendicular to the junction, the Witten effect induces an AC Josephson effect in absence of any external voltage. We derive a number of further experimental consequences and propose potential setups where these {quantized, flux induced, Witten} effects may be observed.