$\mathbb{Z}_4$ Parafermions in an interacting quantum spin Hall Josephson junction coupled to an impurity spin

TL;DR

This paper demonstrates the emergence of $ ext{Z}_4$ parafermions in a weakly interacting quantum spin Hall Josephson junction coupled to an impurity spin, revealing fractional charge tunneling and an unusual $8 ext{pi}$ supercurrent periodicity.

Contribution

It introduces a model with weak interactions and coupling that hosts $ ext{Z}_4$ parafermions, explicitly constructs their operators, and analyzes their effect on supercurrent behavior.

Findings

Presence of $ ext{Z}_4$ parafermions at low temperatures

Fractional $e/2$ charge tunneling facilitated by parafermions

Supercurrent exhibits $8 ext{pi}$ periodicity

Abstract

$\mathbb{Z}_4$ parafermions can be realized in a strongly interacting quantum spin Hall Josephson junction or in a spin Hall Josephson junction strongly coupled to an impurity spin. In this paper we study a system that has both features, but with weak (repulsive) interactions and a weakly coupled spin. We show that for a strongly anisotropic exchange interaction, at low temperatures the system enters a strong coupling limit in which it hosts two $\mathbb{Z}_4$ parafermions, characterizing a fourfold degeneracy of the ground state. We construct the parafermion operators explicitly, and show that they facilitate fractional $e/2$ charge tunneling across the junction. The dependence of the effective low-energy spectrum on the superconducting phase difference reveals an $8\pi$ periodicity of the supercurrent.