Kondo overscreening in the presence of superconductivity

TL;DR

This paper investigates the complex interplay between overscreened Kondo effects and superconductivity in a quantum impurity system, revealing multiple phases including stable overscreened Kondo, midgap Yu-Shiba-Rusinov states, and unscreened impurities.

Contribution

It introduces a model combining overscreened Kondo effect with superconductivity, uncovering novel phase structures and boundary states in this competing system.

Findings

Stable overscreened Kondo phase with a dynamically generated Kondo scale.

Emergence of midgap Yu-Shiba-Rusinov states in certain regimes.

Existence of a phase where the impurity remains unscreened due to strong superconductivity.

Abstract

We consider a model describing a system where the superconductivity competes with the overscreened Kondo effect. The model consists of a single spin$-\frac{1}{2}$ quantum impurity at the edge of a quantum wire where spin$-1$ bulk fermions interact attractively, generating a (superconducting) mass gap. The competition between the Kondo screening and the superconductivity leads to a rich phase structure. We find that for strong Kondo coupling, there is a regime of phase space where the Kondo phase is stable with the impurity \textit{overscreened} by a multiparticle Kondo effect, and a Kondo scale is dynamically generated. When the bulk and boundary interaction strength are comparable, we find that a midgap state appears in the spectrum and screens the impurity, while in the ground state, the impurity is unscreened. This midgap state is akin to the Yu-Shiba-Rusinov (YSR) states that exist in the entire phase space in the BCS superconductor. Moreover, when the bulk superconducting interaction strength is stronger than the boundary Kondo interaction strength, the impurity can no longer be screened. Further, between the Kondo and YSR phases, we find a novel phase where, while the Kondo cloud overscreens the impurity, a boundary excitation exists that has vanishing energy in the thermodynamic limit. Similar phase diagrams that result from competition between different mechanisms were found for other models, too: the dissipative Kondo system, where dissipation competes with screening; the Kondo impurity coupled to spin-1/2 attractively interacting fermions where condensation competes with screening; and the XXX-Kondo model, where the lattice cutoff and the bulk spin interaction compete with screening.