Effective-range dependence of resonant Fermi gases

Abstract

A Fermi gas of cold atoms allows precise control over the dimensionless effective range, ${k}_{\mathrm{F}}{R}_{\mathrm{eff}}$, of the Feshbach resonance. Our pseudopotential formalism allows us to create smooth potentials with effective range, $\ensuremath{-}2\ensuremath{\le}{k}_{\mathrm{F}}{R}_{\mathrm{eff}}\ensuremath{\le}2$, which we use for a variational and diffusion Monte Carlo study of the ground state of a unitary Fermi gas. We report values for the universal constants of $\ensuremath{\xi}=0.388(1)$ and $\ensuremath{\zeta}=0.087(1)$, and compute the condensate fraction, momentum distribution, and pair correlations functions. Finally, we show that a gas with ${k}_{\mathrm{F}}{R}_{\mathrm{eff}}\ensuremath{\gtrsim}1.9$ is thermodynamically unstable.