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.
This paper is the first of two in which the coupled cluster method (CCM) or exp(S) formalism is applied to two-component Fermi systems, the aim being to describe real metals and superconductivity. In this paper the authors concentrate on exact results and restrict ourselves to a ring approximation, applicable essentially to a high-density regime. They show that in the ground-state formalism the random phase approximation (RPA) can be formulated as a system of coupled, bilinear integral equations satisfied by functions associated with the so-called four-point functions of the system which provide a measure of the two-particle two-hole components in the true ground-state wavefunction. These equations are analysed in the dimensionless parameter formed by the ratio of the species masses and exact analytic solutions obtained. For Coulombic potentials (V 11 V 22 =V 2 12 ) they show that the exact analytic solution is unique and obtain an expression for the correlation energy. For non-Coulombic potentials (V 11 V 22 not=V 2 12 ) they indicate how to obtain a possible analytic solution. An RPA-like treatment of the one- and two-body equations in the excited-state formalism is provided for completeness.
There are several situations in which the collective motion of a superfluid Fermi gas is coupled to quasiparticle excitations. Quasiparticle excitations are, for example, present at finite temperature, but also at zero temperature in the case of a rotating trap. In this paper, I present a semiclassical description of such systems, where the superfluid component is described by hydrodynamics while the normal component, formed by the quasiparticles, is described by a transport equation similar to the Vlasov equation. The coupling between the two components generates a strong damping of the collective modes. As applications of the semiclassical theory, the case of a rotating trap and several collective modes (quadrupole and scissors modes) of the trapped gas are discussed.