Multiterminal transport spectroscopy of subgap states in Coulomb-blockaded superconductors

TL;DR

This paper investigates how subgap states in Coulomb-blockaded superconducting islands influence local and nonlocal transport, providing insights into their topological properties and spatial structures through advanced spectroscopy and modeling.

Contribution

It introduces a next-to-leading order master equation for multiterminal superconducting devices with Coulomb interactions and subgap states, linking transport patterns to topological features.

Findings

Nonlocal conductance reveals the spatial extent of subgap states.

Sign changes in conductance indicate energy crossings of excited states.

Transport patterns can distinguish topological from trivial states.

Abstract

Subgap states are responsible for the low-bias transport features of hybrid superconducting--semiconducting devices. Here, we analyze the local and nonlocal differential conductance of Coulomb-blockaded multiterminal superconducting islands that host subgap states with different spatial structures. The emerging patterns of their transport spectroscopy are used to characterize the possible topological nature of these devices and offer the possibility of controlling their transport properties. We develop a next-to-leading order master equation to describe the multiterminal transport in superconductors with both strong Coulomb interactions and multiple subgap states, coupled with metallic leads. We show that the nonlocal differential conductance characterizes the spatial extension of the subgap states and signals the presence of degenerate bound states with a finite support on different parts of the device. Additionally, it displays sharp sign changes as a function of the induced charge of the superconductor, signaling energy crossings among its lowest excited states.