The normal state of attractive Fermi gases from coupled-cluster theory

TL;DR

This paper develops coupled-cluster theory for dilute attractive Fermi gases, showing it improves upon existing approximations and aligns well with Monte Carlo results, while also exploring its limitations at high interactions.

Contribution

It introduces coupled-cluster theory for Fermi gases, connecting it with existing methods and extending variational wavefunctions to finite minority concentrations.

Findings

CCD agrees with diffusion Monte Carlo for various interactions.

CCD fails to converge at high interaction strengths and polarizations.

The method suggests a potential instability towards superfluidity.

Abstract

We introduce coupled-cluster (CC) theory for the numerical study of 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 wavefunction 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.