Fractionalization in Superconductor Josephson Junction Arrays Hinged by Quantum Spin Hall edges

TL;DR

This paper explores a novel Josephson junction array on a quantum spin Hall insulator, revealing Majorana-induced charge fractionalization, emergent Z_2 gauge structures, and new insulating phases with a unique superconductor-insulator transition.

Contribution

It introduces a new model of superconductor-ferromagnet-superconductor junctions on QSH insulators, highlighting Majorana bound states and resulting fractionalized charge phases.

Findings

Majorana bound states enable charge-e tunneling.

Emergence of Z_2 gauge structure leads to new insulating phases.

Identification of a novel superconductor-insulator transition.

Abstract

In this paper we study a novel superconductor-ferromagnet-superconductor (SC-FM-SC) Josephson junction array deposited on top of a two-dimensional quantum spin Hall (QSH) insulator. The existence of Majorana bound states at the interface between SC and FM gives rise to charge-e tunneling, in addition to the usual charge-2e Cooper pair tunneling, between neighboring superconductor islands. Moreover, because Majorana fermions encode the information of charge number parity, an exact Z_2 gauge structure naturally emerges and leads to many new insulating phases, including a deconfined phase where electrons fractionalize into charge-e bosons and topological defects. A new superconductor-insulator transition has also been found.