Highly Polarized Fermi Gases across a Narrow Feshbach Resonance

TL;DR

This paper investigates the phase behavior of highly polarized Fermi gases near a narrow Feshbach resonance, revealing a polaron to pairing transition on the BCS side and potential coexistence of magnetism with superfluidity.

Contribution

It provides the first analysis of polaron and pairing states across a narrow Feshbach resonance, highlighting a transition on the BCS side and conditions for coexistence of magnetism and superfluidity.

Findings

Polaron to pairing transition occurs on the BCS side of the resonance.

Critical repulsive interaction strength prevents phase separation.

Magnetism can coexist with s-wave superfluidity at large Zeeman fields.

Abstract

We address the phase of a highly polarized Fermi gas across a narrow Feshbach resonance starting from the problem of a single down spin fermion immersed in a Fermi sea of up spins. Both polaron and pairing states are considered using the variational wave function approach, and we find that the polaron to pairing transition will take place at the BCS side of the resonance, strongly in contrast to a wide resonance where the transition is located at the BEC side. For pairing phase, we find out the critical strength of repulsive interaction between pairs above which the mixture of pairs and fermions will not phase separate. Therefore, nearby a narrow resonance, it is quite likely that magnetism can coexist with s-wave BCS superfluidity at large Zeeman field, which is a remarkable property absent in conventional BCS superconductors (or fermion pair superfluids).