Tunnel spectroscopy of Majorana bound states in topological superconductor-quantum dot Josephson junctions

TL;DR

This paper models electronic transport in a quantum dot-superconductor nanowire junction, revealing how Majorana bound states alter conductance patterns, and offers a new intuitive picture of the transport processes involved.

Contribution

It introduces a theoretical framework for analyzing Majorana bound states in quantum dot-superconductor junctions using nonequilibrium Green's functions and a novel pictorial transport description.

Findings

Majorana bound states cause qualitative changes in stability diagrams.

The transport process can be understood via a multiple Andreev reflection picture.

The model is exact without Coulomb interactions on the quantum dot.

Abstract

We theoretically investigate electronic transport through a junction where a quantum dot (QD) is tunnel coupled on both sides to semiconductor nanowires with strong spin-orbit interaction and proximity-induced superconductivity. The results are presented as stability diagrams, i.e., the differential conductance as a function of the bias voltage applied across the junction and the gate voltage used to control the electrostatic potential on the QD. A small applied magnetic field splits and modifies the resonances due to the Zeeman splitting of the QD level. Above a critical field strength, Majorana bound states (MBS) appear at the interfaces between the two superconducting nanowires and the QD, resulting in a qualitative change of the entire stability diagram, suggesting this setup as a promising platform to identify MBS. Our calculations are based on a nonequilibrium Green's function description and is exact when Coulomb interactions on the QD can be neglected. In addition, we develop a simple pictorial view of the involved transport processes, which is equivalent to a description in terms of multiple Andreev reflections, but provides an alternative way to understand the role of the QD level in enhancing transport for certain gate and bias voltages. We believe that this description will be useful in future studies of interacting QDs coupled to superconducting leads (with or without MBS), where it can be used to develop a perturbation expansion in the tunnel coupling.