Enhancement of the Kondo effect in a quantum dot formed in a full-shell nanowire

TL;DR

This paper explains the enhancement of the Kondo effect in a quantum dot within full-shell nanowires due to geometric factors and proposes this setup as a versatile platform to study the interplay between superconductivity and Kondo physics, avoiding complex gating.

Contribution

It demonstrates that the observed Kondo enhancement is caused by geometry and magnetic flux effects, not Majorana states, and introduces a new experimental platform for studying quantum phase transitions.

Findings

Kondo effect is enhanced by magnetic flux in full-shell nanowires.

The setup allows control of the Kondo-superconductivity competition via magnetic flux.

The system exhibits a quantum phase transition between doublet and Kondo singlet states.

Abstract

We analyze results of a recent experiment [D. Razmadze et al., Phys. Rev. Lett., 125, 116803 (2020)] on transport through a quantum dot between two full-shell nanowires and show that the observed effects are caused by the Kondo effect enhancement due to a nontrivial geometry (magnetic flux in a full-shell nanowire) rather than the presence of Majorana bound states. Moreover, we propose that such a setup presents a unique and convenient system to study the competition between superconductivity and the Kondo effect and has significant advantages in comparison to other known approaches, as the important parameter is controlled by the magnetic flux through the full-shell nanowire, which can be significantly varied with small changes of magnetic field, and does not require additional gates. This competition is of fundamental interest as it results in a quantum phase transition between an unscreened doublet and a many-body Kondo singlet ground states of the system.