Asymmetric Two-component Fermion Systems in Strong Coupling

TL;DR

This paper investigates the phase behavior of strongly interacting two-component Fermi gases, revealing a transition from phase separation to homogeneous gapless superfluid phases at high coupling, relevant for cold atom experiments.

Contribution

It provides a detailed analysis of the phase structure and excitation spectra of strongly coupled Fermi systems, highlighting the emergence of gapless superfluid phases near unitarity.

Findings

Weak coupling favors phase separation with asymmetry.

Strong coupling leads to homogeneous gapless superfluid phases.

Near unitarity, quasi-particle excitations dominate at modest polarizations.

Abstract

We study the phase structure of a dilute two-component Fermi system with attractive interactions as a function of the coupling and the polarization or number difference between the two components. In weak coupling, a finite number asymmetry results in phase separation. A mixed phase containing symmetric superfluid matter and an asymmetric normal phase is favored. With increasing coupling strength, we show that the stress on the superfluid phase to accommodate a number asymmetry increases. Near the infinite-scattering length limit, we calculate the single-particle excitation spectrum and the ground-state energy at various polarizations. A picture of weakly-interacting quasi-particles emerges for modest polarizations. In this regime near infinite scattering length, and for modest polarizations, a homogeneous phase with a finite population of excited quasi-particle states characterized by a gapless spectrum should be favored over the phase separated state. These states may be realized in cold atom experiments.