Imprint of topological degeneracy in quasi-one-dimensional fractional quantum Hall states

TL;DR

This paper investigates how topological degeneracy in fractional quantum Hall states manifests in a quasi-one-dimensional Josephson junction, revealing unique Josephson effects and protected degeneracy crossings.

Contribution

It demonstrates the persistence of topological degeneracy effects in quasi-one-dimensional limits and explores their implications for Josephson phenomena and protected degeneracy crossings.

Findings

Josephson effect exhibits a $2 extpi d$-periodicity due to topological degeneracy.

Protected crossing points occur at specific phase values, maintaining degeneracy.

Results extend to fractional topological insulators with time-reversal symmetry.

Abstract

We consider an annular superconductor-insulator-superconductor Josephson-junction, with the insulator being a double layer of electron and holes at Abelian fractional quantum Hall states of identical fillings. When the two superconductors gap out the edge modes, the system has a topological ground state degeneracy in the thermodynamic limit akin to the fractional quantum Hall degeneracy on a torus. In the quasi-one-dimensional limit, where the width of the insulator becomes small, the ground state energies are split. We discuss several implications of the topological degeneracy that survive the crossover to the quasi-one-dimensional limit. In particular, the Josephson effect shows a $2\pi d$-periodicity, where $d$ is the ground state degeneracy in the 2 dimensional limit. We find that at special values of the relative phase between the two superconductors there are protected crossing points in which the degeneracy is not completely lifted. These features occur also if the insulator is a time-reversal-invariant fractional topological insulator. We describe the latter using a construction based on coupled wires. Furthermore, when the superconductors are replaced by systems with an appropriate magnetic order that gap the edges via a spin-flipping backscattering, the Josephson effect is replaced by a spin Josephson effect.