Mixed-Valence Transition on a Quantum-Dot Coupled to Superconducting and Spin-Polarized Leads

TL;DR

This paper investigates a quantum dot system coupled to superconducting and spin-polarized leads, revealing a true quantum phase transition driven by the interplay of Kondo effect, superconductivity, and ferromagnetism, with distinct measurable signatures.

Contribution

It uncovers a mixed-valence quantum phase transition in such systems, showing a true transition rather than a crossover, and characterizes different states with proposed measurement methods.

Findings

Identification of a true quantum phase transition between spin doublet and singlet states.

Connection of the singlet phase to charge Kondo and superconducting states.

Proposed measurement techniques to distinguish different quantum states.

Abstract

We consider a quantum dot coupled to both superconducting and spin-polarized electrodes, and study the triad interplay of the Kondo effect, superconductivity, and ferromagnetism, any pair of which compete with and suppress each other. We find that the interplay leads to a mixed-valence quantum phase transition, which for other typical sysmstems is merely a crossover rather than a true transition. At the transition, the system changes from the spin doublet to singlet state. The singlet phase is adiabatically connected (through crossovers) to the so-called 'charge Kondo state' and to the superconducting state. We analyze in detail the physical characteristics of different states and propose that the measurement of the cross-current correlation and the charge relaxation resistance can clearly distinguish between them.