Anomalous $h/2e$ periodicity and Majorana zero modes in chiral Josephson junctions

TL;DR

This paper explains the emergence of $h/2e$ periodicity in chiral Josephson junctions through chiral Andreev edge states and links it to the presence of Majorana zero modes, providing a theoretical resolution to experimental observations.

Contribution

The work introduces a theory showing how chiral Andreev edge states cause $h/2e$ periodicity and Majorana modes in short junctions, clarifying experimental puzzles.

Findings

Chiral Andreev edge states induce $h/2e$ periodicity.

Majorana zero modes appear at junction ends.

Fraunhofer oscillation period indicates Majorana modes presence.

Abstract

Recent experiments reported that quantum Hall chiral edge state-mediated Josephson junctions (chiral Josephson junctions) could exhibit Fraunhofer oscillations with a periodicity of either $h/e$ [Vignaud \textit{et al}.,~Nature~(2023)] or $h/2e$ [Amet \textit{et al}.,~Science~\textbf{352}~966~(2016)]. While the $h/e$-periodic component of the supercurrent had been anticipated theoretically before, the emergence of the $h/2e$-periodicity is still not fully understood. In this work, we show that the chiral edge states coupled to the superconductors become chiral Andreev edge states. In short junctions, the coupling of the chiral Andreev edge states can cause the $h/2e$-magnetic flux periodicity. Our theory resolves the long-standing puzzle concerning the appearance of the $h/2e$-periodicity in chiral Josephson junctions. Furthermore, we explain that when the chiral Andreev edge state couple, a pair of localized Majorana modes appear at the ends of the Josephson junction, which are robust and independent of the phase difference between the two superconductors. As the $h/2e$-periodicity and the Majorana zero modes have the same physical origin, the Fraunhofer oscillation period can be used to identify the regime with Majorana zero modes.