Crossover polarons in a strongly interacting Fermi superfluid

TL;DR

This paper studies the properties of a mobile impurity in a strongly interacting Fermi superfluid across the BEC-BCS crossover, revealing how pairing gaps affect polaron stability and spectral features.

Contribution

It introduces the concept of crossover polarons and analyzes their spectral properties using a many-body T-matrix approach excluding Efimov states.

Findings

Attractive polaron remains robust below 2Δ threshold.

Repulsive polaron becomes less well-defined due to pairing gap.

Predicted polaron energy, residue, and effective mass for experiments.

Abstract

We investigate the zero-temperature quasiparticle properties of a mobile impurity immersed in a strongly interacting Fermi superfluid at the crossover from a Bose-Einstein condensate (BEC) to a Bardeen--Cooper--Schrieffer (BCS) superfluid, by using a many-body $T$-matrix approach that excludes Efimov trimer bound states. Termed BEC-BCS crossover polaron, or crossover polaron in short, this quasiparticle couples to elementary excitations of a many-body background and therefore could provide a useful probe of the underlying strongly interacting Fermi superfluid. Due to the existence of a significant pairing gap $\Delta$, we find that the repulsive polaron branch becomes less well-defined. In contrast, the attractive polaron branch is protected by the pairing gap and becomes more robust at finite momentum. It remains as a delta-function peak in the impurity spectral function below a threshold $2\Delta$. Above the threshold, the attractive polaron enters the particle-hole continuum and starts to get damped. We predict the polaron energy, residue and effective mass for realistic Bose-Fermi mixtures, where the minority bosonic atoms play the role of impurity. These results are practically useful for future cold-atom experiments on crossover polarons.