Quasiparticle Dispersions and Lifetimes in the Normal State of the BCS-BEC Crossover

TL;DR

This paper analyzes the spectral density and quasiparticle properties of a Fermi gas across the BCS-BEC crossover, revealing the breakdown of Fermi liquid behavior near unitarity through theoretical calculations.

Contribution

It introduces a T-matrix approximation to compute spectral densities and quasiparticle dispersions, providing new insights into the pseudogap and quasiparticle lifetimes in the normal state.

Findings

Effective Fermi-wavevector vanishes beyond a threshold interaction.

Quasiparticle lifetimes near unitarity are comparable to inverse Fermi-energy.

Results support the breakdown of Fermi liquid theory in this regime.

Abstract

We compute the spectral density in the normal phase of an interacting homogenous Fermi gas using a T-matrix approximation. We fit the quasiparticle peaks of the spectral density to BCS-like dispersion relations, and extract estimates of a "pseudo-gap" energy scale and an effective Fermi-wavevector as a function of interaction strength. We find that the effective Fermi-wavevector of the quasiparticles vanishes when the inverse scattering length exceeds some positive threshold. We also find that near unitarity the quasiparticle lifetimes, estimated from the widths of the peaks in the spectral density, approach values on the order of the inverse Fermi-energy. These results are consistent with the "breakdown of Fermi liquid theory" observed in recent experiments.