Formation of magnetic impurities and pair-breaking effect in a superfluid Fermi gas

TL;DR

This paper explores how nonmagnetic impurities in a population-imbalanced superfluid Fermi gas can become magnetized, leading to pair-breaking effects and localized excitations, using mean-field theory to analyze local superfluid properties.

Contribution

It demonstrates that nonmagnetic impurities can induce magnetization and pair-breaking effects in a superfluid Fermi gas with population imbalance, a novel insight into impurity effects in such systems.

Findings

Impurities become magnetized in imbalanced superfluids.

Localized states form below the superfluid gap due to pair-breaking.

Superfluid order parameter is suppressed around impurities.

Abstract

We theoretically investigate a possible idea to introduce magnetic impurities to a superfluid Fermi gas. In the presence of population imbalance ($N_\uparrow>N_\downarrow$, where $N_\sigma$ is the number of Fermi atoms with pseudospin $\sigma=\uparrow,\downarrow$), we show that nonmagnetic potential scatterers embedded in the system are magnetized in the sense that some of excess $\uparrow$-spin atoms are localized around them. They destroy the superfluid order parameter around them, as in the case of magnetic impurity effect discussed in the superconductivity literature. This pair-breaking effect naturally leads to localized excited states below the superfluid excitation gap. To confirm our idea in a simply manner, we treat an attractive Fermi Hubbard model within the mean-field theory at T=0. We self-consistently determine superfluid properties around a nonmagnetic impurity, such as the superfluid order parameter, local population imbalance, as well as single-particle density of states, in the presence of population imbalance. Since the competition between superconductivity and magnetism is one of the most fundamental problems in condensed matter physics, our results would be useful for the study of this important issue in cold Fermi gases.