Finite-Temperature Pairing Gap of a Unitary Fermi Gas by Quantum Monte Carlo Calculations

We calculate the one-body temperature Green’s (Matsubara) function of the unitary Fermi gas via quantum Monte Carlo, and extract the spectral weight function A(p,ω) using the methods of maximum entropy and singular value decomposition. From A(p,ω) we determine the quasiparticle spectrum, which can be accurately parametrized by three functions of temperature: an effective mass m^*, a mean-field potential U, and a gap Δ. Below the critical temperature T_c=0.15ε_F the results for m^*, U, and Δ can be accurately reproduced using an independent quasiparticle model. We find evidence of a pseudogap in the fermionic excitation spectrum for temperatures up to T^*≈0.20ε_F>T_c.