Spin-filtered measurements of Andreev bound states

TL;DR

This study uses spin-polarized measurements to directly probe the spin states of Andreev bound states in semiconductor-superconductor nanowires, revealing fully spin-polarized excitations even with strong spin-orbit coupling.

Contribution

It introduces a novel spin-filtered measurement technique using a quantum dot to directly determine the spin of Andreev bound states in hybrid nanowire devices.

Findings

Observed fully spin-polarized Andreev bound states despite strong spin-orbit interaction.

Demonstrated current blockade when ABS is trapped in an excited state.

Proposed spin-polarized spectroscopy as a tool for detecting topological superconductivity.

Abstract

A semiconductor nanowire brought in proximity to a superconductor can form discrete, particle-hole symmetric states, known as Andreev bound states (ABSs). An ABS can be found in its ground or excited states of different spin and parity, such as a spin-zero singlet state with an even number of electrons or a spin-1/2 doublet state with an odd number of electrons. Considering the difference between spin of the even and odd states, spin-filtered measurements have the potential to reveal the underlying ground state. To directly measure the spin of single-electron excitations, we probe an ABS using a spin-polarized quantum dot that acts as a bipolar spin filter, in combination with a non-polarized tunnel junction in a three-terminal circuit. We observe a spin-polarized excitation spectrum of the ABS, which in some cases is fully spin-polarized, despite the presence of strong spin-orbit interaction in the InSb nanowires. In addition, decoupling the hybrid from the normal lead blocks the ABS relaxation resulting in a current blockade where the ABS is trapped in an excited state. Spin-polarized spectroscopy of hybrid nanowire devices, as demonstrated here, is proposed as an experimental tool to support the observation of topological superconductivity.