Interaction effects in superconductor/quantum spin Hall devices: universal transport signatures and fractional Coulomb blockade

TL;DR

This paper investigates universal transport signatures and fractional Coulomb blockade phenomena in superconductor/quantum spin Hall devices, revealing how interactions and magnetic barriers influence topological superconductivity and charge transport.

Contribution

It introduces a renormalization group analysis of transport in superconductor/quantum spin Hall heterostructures, highlighting the effects of interactions and magnetic barriers on conductance and Coulomb blockade.

Findings

Universal conductance scaling depends on interaction strength and device size.

Predicted e-periodic Coulomb peaks with fractional charge tunability.

Transition from e to e/2 periodicity when magnetic barriers spontaneously break time-reversal symmetry.

Abstract

Interfacing s-wave superconductors and quantum spin Hall edges produces time-reversal-invariant topological superconductivity of a type that can not arise in strictly 1D systems. With the aim of establishing sharp fingerprints of this novel phase, we use renormalization group methods to extract universal transport characteristics of superconductor/quantum spin Hall heterostructures where the native edge states serve as leads. We determine scaling forms for the conductance through a grounded superconductor and show that the results depend sensitively on the interaction strength in the leads, the size of the superconducting region, and the presence or absence of time-reversal-breaking perturbations. We also study transport across a floating superconducting island isolated by magnetic barriers. Here we predict e-periodic Coulomb-blockade peaks, as recently observed in nanowire devices [Albrecht et al., Nature 531, 206 (2016)], with the added feature that the island can support fractional charge tunable via the relative orientation of the barrier magnetizations. As an interesting corollary, when the magnetic barriers arise from strong interactions at the edge that spontaneously break time-reversal symmetry, the Coulomb-blockade periodicity changes from e to e/2. These findings suggest several future experiments that probe unique characteristics of topological superconductivity at the quantum spin Hall edge.