Significance of dressed molecules in a quasi-two-dimensional polarized Fermi gas

TL;DR

This paper studies how excited axial states and dressed molecules influence the properties and phase stability of a spin-orbit coupled quasi-two-dimensional polarized Fermi gas, with implications for realizing topological superfluids.

Contribution

It introduces an effective two-dimensional model including dressed molecules with axial excitations, revealing their significant impact on phase stability and density profiles.

Findings

Excited axial states become significant near unitarity due to enhanced binding energy.

Dressed molecules alter density distribution and phase structure in traps.

Stability region of topological superfluid phase is increased by axial excitations.

Abstract

We investigate the properties of a spin-orbit coupled quasi-two-dimensional Fermi gas with tunable s-wave interaction between the two spin species. By analyzing the two-body bound state, we find that the population of the excited states in the tightly-confined axial direction can be significant when the two-body binding energy becomes comparable or exceeds the axial confinement. Since the Rashba spin-orbit coupling that we study here tends to enhance the two-body binding energy, this effect can become prominent at unitarity or even on the BCS side of the Feshbach resonance. To study the impact of these excited modes along the third dimension, we adopt an effective two-dimensional Hamiltonian in the form of a two-channel model, where the dressed molecules in the closed channel consist of the conventional Feshbach molecules as well as the excited states occupation in the axial direction. With properly renormalized interactions between atoms and dressed molecules, we find that both the density distribution and the phase structure in the trap can be significantly modified near a wide Feshbach resonance. In particular, the stability region of the topological superfluid phase is increased. Our findings are helpful for the experimental search for the topological superfluid phase in ultra-cold Fermi gases, and have interesting implications for quasi-low-dimensional polarized Fermi gases in general.