Magnetic-Field-Induced Transition in a Quantum Dot Coupled to a Superconductor

TL;DR

This paper investigates how magnetic fields influence the transition between screened and unscreened magnetic moments in a quantum dot coupled to superconductors, revealing a re-entrant phase boundary and thermal effects through high-resolution spectroscopy.

Contribution

It demonstrates the magnetic field as a control parameter for the screening transition in S-QD-S junctions, providing new insights into the interplay of Zeeman energy and superconducting gap reduction.

Findings

Re-entrant phase boundary driven by magnetic field.

Thermal replicas of subgap states observed.

Transition evolution with temperature characterized.

Abstract

The magnetic moment of a quantum dot can be screened by its coupling to a superconducting reservoir, depending on the hierarchy of the superconducting gap and the relevant Kondo scale. This screening-unscreening transition can be driven by electrostatic gating, tunnel coupling, and, as we demonstrate here, magnetic field. We perform high-resolution spectroscopy of subgap excitations near the screening-unscreening transition of asymmetric superconductor - quantum dot - superconductor (S-QD-S) junctions formed by the electromigration technique. Our measurements reveal a re-entrant phase boundary determined by the competition between Zeeman energy and gap reduction with magnetic field. We further track the evolution of the phase transition with increasing temperature, which is also evidenced by thermal replicas of subgap states.