Numerical Renormalization Group Approach to a Quantum Dot Coupled to Normal and Superconducting Leads

TL;DR

This paper investigates transport in a quantum dot connected to normal and superconducting leads using the numerical renormalization group, revealing a crossover between superconducting and Kondo singlet states through conductance analysis.

Contribution

It demonstrates that low-energy properties follow Fermi liquid theory despite superconducting correlations and characterizes the conductance crossover between two singlet states.

Findings

Conductance maximum indicates crossover between superconducting and Kondo singlet states.

Low-energy properties are described by local Fermi liquid theory.

Gate-voltage dependence varies between the two singlet regions.

Abstract

We study transport through a quantum dot coupled to normal and superconducting leads using the numerical renormalization group method. We show that the low-energy properties of the system are described by the local Fermi liquid theory despite of the superconducting correlations penetrated into the dot due to a proximity effect. We calculate the linear conductance due to the Andreev reflection in the presence of the Coulomb interaction. It is demonstrated that the maximum structure appearing in the conductance clearly characterizes a crossover between two distinct spin-singlet ground states, i.e. the superconducting singlet state and the Kondo singlet state. It is further elucidated that the gate-voltage dependence of the conductance shows different behavior in the superconducting singlet region from that in the Kondo singlet region.