Polarons in Ultracold Fermi Superfluids

TL;DR

This paper investigates a novel Fermi polaron in ultracold Fermi superfluids, revealing complex spectra, instabilities, and the limitations of mean-field theory, with implications for cold atom experiments.

Contribution

It introduces the concept of a trimer-like polaron in a three-component system and analyzes its spectral features and instabilities, advancing understanding of impurity physics in superfluids.

Findings

Multiple avoided crossings in polaron spectra.

Increased widths of avoided crossings near BCS-BEC crossover.

Mean-field theory is inadequate for small scattering lengths.

Abstract

We study a new type of Fermi polaron induced by an impurity interacting with an ultracold Fermi superfluid. Due to the three-component nature of the system, the polaron can become trimer-like with a non-universal energy spectrum. We identify multiple avoided crossings between impurity- and trimer-like solutions in both the attractive and the repulsive polaron spectra. In particular, the widths of avoided crossings gradually increase as the Fermi superfluid undergoes a crossover from the BCS side towards the BEC side, which suggests instabilities towards three-body losses. Such losses can be reduced for interaction potentials with small effective ranges. We also demonstrate, using the second-order perturbation theory, that the mean-field evaluation of the fermion-impurity interaction energy is inadequate even for small fermion-impurity scattering lengths, due to the essential effects of Fermi superfluid and short-range physics in such a system. Our results are practically useful for cold atom experiments on mixtures.