Mobile magnetic impurities in a Fermi superfluid: a route to designer molecules

TL;DR

This paper explores how mobile magnetic impurities in a fermionic superfluid form tunable midgap molecules with unique dispersion, opening avenues for new many-body quantum states and experimental detection methods.

Contribution

It introduces the concept of mobile impurity-induced midgap molecules with tunable dispersion in fermionic superfluids, a novel theoretical framework.

Findings

Identification of a Mexican-hat dispersion in impurity molecules

Mapping of impurity dispersion into atomic and molecular branches

Proposal for experimental realization using lithium-cesium mixtures

Abstract

A magnetic impurity in a fermionic superfluid hosts bound quasiparticle states known as Yu-Shiba-Rusinov (YSR) states. We argue here that, if the impurity is mobile (i.e., has a finite mass), the impurity and its bound YSR quasiparticle move together as a midgap molecule, which has an unusual "Mexican-hat" dispersion that is tunable via the fermion density. We map out the impurity dispersion, which consists of an "atomic" branch (in which the impurity is dressed by quasiparticle pairs) and a "molecular" branch (in which the impurity binds a quasiparticle). We discuss the experimental realization and detection of midgap Shiba molecules, focusing on lithium-cesium mixtures, and comment on the prospects they offer for realizing exotic many-body states.