Transport across a junction of topological insulators and a superconductor

TL;DR

This paper investigates charge and spin transport across a junction between topological insulators and a superconductor, revealing unique conductance features and the potential to identify the superconductor's spin state.

Contribution

It introduces a formalism for analyzing the effects of a general time-reversal invariant barrier and explores how topological insulators influence conductance and spin properties in superconductor junctions.

Findings

Presence of Dirac and Schrödinger-like features in conductance.

Identification of bound state effects on conductance.

Detection of a satellite peak in spin conductance for p-wave superconductors.

Abstract

We study transport across a line junction lying between two orthogonal topological insulator surfaces and a superconductor which can have either s-wave (spin-singlet) or p-wave (spin-triplet) pairing symmetry. We present a formalism for studying the effect of a general time-reversal invariant barrier at the junction and show that such a barrier can be completely described by three arbitrary parameters. We compute the charge and the spin conductance across such a junction and study their behaviors as a function of the bias voltage applied across the junction and the three parameters used to characterize the barrier. We find that the presence of topological insulators and a superconductor leads to both Dirac and Schrodinger-like features in charge and spin conductances. We discuss the effect of bound states on the superconducting side of the barrier on the conductance; in particular, we show that for triplet p-wave superconductors such a junction may be used to determine the spin state of its Cooper pairs. Our study reveals that there is a non-zero spin conductance for some particular spin states of the triplet Cooper pairs; this is an effect of the topological insulators which break the spin rotation symmetry. Finally, we find an unusual satellite peak (in addition to the usual zero bias peak) in the spin conductance for p-wave symmetry of the superconductor order parameter.