The unitary Fermi gas describes a two-component system of fermions with contact interaction in the limit of infinite scattering length.  A many-body pairing pseudogap regime has been proposed to exist above the superfluid critical temperature Tc and below a pairing temperature of T* > Tc, but its existence and extent have been debated in recent years both theoretically and experimentally [1].  To help address this debate, we have used finite-temperature auxiliary-field quantum Monte Carlo (AFMC) methods on a lattice to study thermodynamic observables of the unitary Fermi gas in the canonical ensemble [2].  In contrast to previous AFMC calculations, our single-particle model space consists of the complete first Brillouin zone of the lattice.  We present results for the heat capacity, model-independent energy-staggering pairing gap, condensate fraction, and static spin susceptibility.  We have applied finite-size scaling of the condensate fraction data to estimate Tc for the fixed filling factor n=0.06 used in our simulations.  We have found that for temperatures above Tc the pairing gap essentially vanishes and the spin susceptibility shows only a moderate suppression.  We also present AFMC results for the temperature dependence of Tan’s contact which has been the subject of recent theoretical and experimental work.  There are substantial discrepancies between different numerical approaches, in particular at high-temperature T~0.4 TF.  Our results for the contact differ qualitatively from previous AFMC simulations.

[1] For a recent review see S. Jensen, C. N. Gilbreth, and Y. Alhassid, arXiv:1807.03913

[2] S. Jensen, C. N. Gilbreth, and Y. Alhassid, arXiv:1801.06163