Universality in the BCS - BEC Crossover in Cold Fermion Gases

TL;DR

This paper investigates the universal properties of the phase diagram in ultracold fermionic gases across Feshbach resonances, emphasizing symmetry considerations and the effects of narrow versus broad resonances.

Contribution

It introduces a symmetry-based framework to analyze the phase diagram and thermodynamics of fermionic gases, including exact solutions for narrow resonances and assessments of deviations from universality.

Findings

Exact solution for narrow resonance above critical temperature

Identification of universal behavior across broad and narrow resonances

Assessment of experimental deviations from universality

Abstract

We address the finite temperature phase diagram of ultracold fermionic atoms across a Feshbach resonance based on a functional integral for an atom-molecule model. This allows to fully exploit the presence of the global symmetry of phase rotations, U(1). Both the equation of state and the classification of the thermodynamic phases are obtained from a symmetry consideration. We focus on the universal aspects associated to narrow and broad resonances, in turn connected by an additional form of crossover. The narrow resonance limit can be solved exactly above the critical temperature. We also assess deviations from the broad resonance universality which have recently been probed experimentally.