Yu-Shiba-Rusinov states in color superconducting quark matter

TL;DR

This paper investigates impurity-induced subgap states in color superconducting quark matter, revealing multiple phase transitions driven by impurity-bulk coupling, using a surrogate model approach adapted from condensed matter physics.

Contribution

It introduces a surrogate model method to study impurity effects in color superconducting quark matter, enabling analysis of complex phase transitions with simplified effective levels.

Findings

Multiple phase transitions identified with increasing impurity-bulk coupling.

Presence of impurity-induced subgap states in color superconducting quark matter.

Method can be extended to more realistic QCD scenarios.

Abstract

The high-density region of the QCD phase diagram displays an intricate competition between color superconductivity and the QCD Kondo effect due to color exchange in quark matter containing a single heavy quark impurity. We explore the characteristic impurity-induced superconducting subgap states arising in such systems by generalizing the surrogate model solver, recently considered in the context of condensed matter physics [Phys. Rev. B 108, L220506 (2023)]. The method consists of approximating the full superconducting bulk by only a small number of effective levels whose parameters are chosen so as to best reproduce the Matsubara frequency dependence of the impurity-bulk hybridization function. We numerically solve a surrogate QCD Kondo model describing a quantum impurity color-exchange coupled to a two-color two-flavor superconducting bulk. The results directly indicate the presence of multiple phase transitions as the coupling of the impurity to the bulk is increased, due to the interplay between various overscreened states. The methods introduced here are straightforward enough to be extended to more realistic QCD scenarios.