Yu-Shiba-Rusinov screening of spins in double quantum dots

TL;DR

This paper investigates Yu-Shiba-Rusinov states in InAs nanowire double quantum dots coupled to a superconductor, demonstrating spin screening effects and their dependence on charge states and coupling strength.

Contribution

It provides experimental evidence of YSR screening in double quantum dots and compares results with theoretical models, advancing understanding of impurity spins in superconducting nanostructures.

Findings

YSR states replace normal conductance patterns in double dots.

Distinct spectral features indicate spin screening by YSR states.

Results agree with theoretical models like NRG and zero-bandwidth approximation.

Abstract

A magnetic impurity coupled to a superconductor gives rise to a Yu-Shiba-Rusinov (YSR) state inside the superconducting energy gap. With increasing exchange coupling the excitation energy of this state eventually crosses zero and the system switches to a YSR groundstate with bound quasiparticles screening the impurity spin by $\hbar/2$. Here we explore InAs nanowire double quantum dots tunnel coupled to a superconductor and demonstrate YSR screening of spin-1/2 and spin-1 states. Gating the double dot through 9 different charge states, we show that the honeycomb pattern of zero-bias conductance peaks, archetypal of double dots coupled to normal leads, is replaced by lines of zero-energy YSR states. These enclose regions of YSR-screened dot spins displaying distinctive spectral features, and their characteristic shape and topology change markedly with tunnel coupling strengths. We find excellent agreement with a simple zero-bandwidth approximation, and with numerical renormalization group calculations for the two-orbital Anderson model.