Bound states of a localized magnetic impurity in a superfluid of paired ultracold fermions

TL;DR

This paper models a localized impurity in a superfluid of paired ultracold fermions, revealing the formation of Shiba bound states and deeply bound states, using an effective scattering length approach.

Contribution

It introduces an effective scattering length framework to describe impurity-fermion interactions and demonstrates the formation of bound states within the superfluid gap.

Findings

Formation of Shiba bound states inside the gap.

Existence of deeply bound states below the Fermi sea.

Impurity acts as a magnetic impurity in the superfluid.

Abstract

We consider a localized impurity atom that interacts with a cloud of fermions in the paired state. We develop an effective scattering length description of the interaction between an impurity and a fermionic atom using their vacuum scattering length. Treating the pairing of fermions at the mean-field level, we show that the impurity atom acts like a magnetic impurity in the condensed matter context, and leads to the formation of a pair of Shiba bound states inside the superconducting gap. In addition, the impurity atom can lead to the formation of deeply bound states below the Fermi sea.