Overscreened Kondo effect, (color) superconductivity and Shiba states in Dirac metals and quark matter

TL;DR

This paper explores the complex interplay between the overscreened Kondo effect, (color) superconductivity, and Shiba states in Dirac metals and quark matter, revealing a rich phase diagram and multiple quantum phase transitions.

Contribution

It provides a nonperturbative analysis of the phase diagram showing how superconductivity suppresses the overscreened Kondo phase and predicts multiple quantum phase transitions in these systems.

Findings

Overscreened Kondo effect exists in the normal state.

Superconducting gap suppresses the Kondo-screened phase.

Multiple quantum phase transitions occur with increasing impurity coupling.

Abstract

We study the interplay between the Kondo effect and (color) superconductivity in doped Dirac metals with magnetic impurities and in quark matter with colorful impurities. We first point out that the overscreened Kondo effect arises in the normal state of these systems. Next the (color) superconducting gap is incorporated as a mean field and the phase diagram for a varying gap and temperature is constructed nonperturbatively. A rich phase structure emerges from a competition of effects unique to a multichannel system. The Kondo-screened phase is shown to disappear for a sufficiently large gap. Peculiarity of quark matter due to the confining property of non-Abelian gauge fields is noted. We also investigate the spectrum of sub-gap excited states, called Shiba states. Based on a model calculation and physical reasoning we predict that, as the coupling of the impurity to the bulk is increased, there will be more than one quantum phase transition due to level crossing among overscreened states.