Spin-dependent tunneling between individual superconducting bound states

TL;DR

This study investigates the spin-dependent tunneling between Yu-Shiba-Rusinov states in superconductors, revealing freely rotating spins at very low temperatures and implications for Majorana bound states.

Contribution

It introduces a method to experimentally determine the relative spin orientation between YSR states using tunneling spectroscopy, advancing understanding of spin dynamics in superconductors.

Findings

Spins are freely rotating down to 7mK.

The relative spin orientation can be directly extracted from spectral features.

Non-collinear spin alignment is observed and is crucial for Majorana states.

Abstract

Magnetic impurities on superconductors induce discrete bound levels inside the superconducting gap, known as Yu-Shiba-Rusinov (YSR) states. YSR levels are fully spin-polarized such that the tunneling between YSR states depends on their relative spin orientation. Here, we use scanning tunneling spectroscopy to resolve the spin dynamics in the tunneling process between two YSR states by experimentally extracting the angle between the spins. To this end, we exploit the ratio of thermally activated and direct spectral features in the measurement to directly extract the relative spin orientation between the two YSR states. We find freely rotating spins down to 7mK, indicating a purely paramagnetic nature of the impurities. Such a non-collinear spin alignment is essential not only for producing Majorana bound states but also as an outlook manipulating and moving the Majorana state onto the tip.