Normal State of Attractive Fermi Gases from Coupled-Cluster Theory

We apply coupled-cluster (CC) theory to numerically study the normal state of two-component dilute Fermi gases with attractive short-range interactions at zero temperature. We focus on CC theory with double excitations (CCD) and discuss its close relationship with-and improvement upon-the t-matrix approximation, i.e., the resummation of ladder diagrams via a random-phase approximation. We further discuss its relationship with Chevy's variational wave-function ansatz for the Fermi polaron and argue that CCD is its natural extension to nonzero minority species concentrations. Studying normal-state energetics for a range of interaction strengths below and above unitarity, we find that CCD yields good agreement with fixed-node diffusion Monte Carlo. We find that CCD does not converge for small polarizations and large interaction strengths, which we speculatively attribute to the nascent instability to a superfluid state.