Interplay of Kondo and superconducting correlations in the nonequilibrium Andreev transport through a quantum dot

TL;DR

This paper theoretically investigates how Kondo and superconducting correlations influence nonequilibrium Andreev transport in a quantum dot, revealing resonance peaks and energy level effects consistent with recent experiments.

Contribution

It introduces a modified perturbation theory approach to analyze the interplay of Kondo and superconducting effects in nonequilibrium transport through a quantum dot.

Findings

Kondo resonance enhances Andreev transport at finite bias.

Energy levels of Andreev bound states cause additional conductance peaks.

Interplay induces pinning of Andreev resonances to the Fermi level.

Abstract

Using the modified perturbation theory, we theoretically study the nonequilibrium Andreev transport through a quantum dot coupled to normal and superconducting leads (N-QD-S), which is strongly influenced by the Kondo and superconducting correlations. From the numerical calculation, we find that the renormalized couplings between the leads and the dot in the equilibrium states characterize the peak formation in the nonequilibrium differential conductance. In particular, in the Kondo regime, the enhancement of the Andreev transport via a Kondo resonance occurs in the differential conductance at a finite bias voltage, leading to an anomalous peak whose position is given by the renormalized parameters. In addition to the peak, we show that the energy levels of the Andreev bound states give rise to other peaks in the differential conductance in the strongly correlated N-QD-S system. All these features of the nonequilibrium transport are consistent with those in the recent experimental results [R. S. Deacon {\it et al.}, Phys. Rev. Lett. {\bf 104}, 076805 (2010); Phys. Rev. B {\bf 81}, 12308 (2010)]. We also find that the interplay of the Kondo and superconducting correlations induces an intriguing pinning effect of the Andreev resonances to the Fermi level and its counter position.