Cooper pairing above the critical temperature in a unitary Fermi gas

TL;DR

This study uses quantum Monte Carlo methods to analyze spin responses in a unitary Fermi gas, revealing suppression of spin susceptibility and conductivity due to Cooper pairs above the superfluid transition, with smooth behavior across T_c.

Contribution

First ab initio calculation of spin responses in a unitary Fermi gas across the superfluid transition using quantum Monte Carlo and linear response theory.

Findings

Spin susceptibility and conductivity are suppressed above T_c due to Cooper pairs.

Spin diffusion coefficient drops significantly in the superfluid phase.

Spin observables change smoothly across the critical temperature.

Abstract

We present an ab initio determination of spin responses of the unitary Fermi gas, based on finite temperature quantum Monte Carlo calculations and the Kubo linear-response formalism. We determine the temperature dependence of the spin susceptibility and the spin conductivity. We show that both quantities exhibit suppression above the critical temperature of the superfluid to normal phase transition due to presence of the Cooper pairs. The spin diffusion transport coefficient does not display the existence of a minimum in the vicinity of the critical temperature and it drops to very low values D_s approx 0.8hbar/m in the superfluid phase. All these spin observables show a smooth and monotonic behavior with temperature when crossing the critical temperature T_c, until the Fermi liquid regime is attained at the temperature T*, where the pseudogap regime disappears.