Nonequilibrium transport via spin-induced sub-gap states in superconductor/quantum dot/normal metal cotunnel junctions

TL;DR

This paper investigates nonequilibrium transport in a superconductor/quantum dot/normal metal junction, revealing spin-induced sub-gap states and their impact on conductance, with a focus on cotunneling processes and Andreev reflections.

Contribution

It introduces a theoretical model for nonequilibrium cotunneling in quantum dot junctions, highlighting the role of spin in forming sub-gap resonances.

Findings

Spin states induce sub-gap resonances in conductance.

Odd occupation leads to peak-dip structures in differential conductance.

The model aligns with recent experimental observations.

Abstract

We study low-temperature transport through a Coulomb blockaded quantum dot (QD) contacted by a normal (N), and a superconducting (S) electrode. Within an effective cotunneling model the conduction electron self energy is calculated to leading order in the cotunneling amplitudes and subsequently resummed to obtain the nonequilibrium T-matrix, from which we obtain the nonlinear cotunneling conductance. For even occupied dots the system can be conceived as an effective S/N-cotunnel junction with subgap transport mediated by Andreev reflections. The net spin of an odd occupied dot, however, leads to the formation of sub-gap resonances inside the superconducting gap which gives rise to a characteristic peak-dip structure in the differential conductance, as observed in recent experiments.