Absence of fermionic quasi-particles in the superfluid state of the attractive Fermi gas

TL;DR

This paper demonstrates that in the superfluid state of an attractive Fermi gas, fermionic quasi-particles are absent due to singular interactions mediated by gapless modes, especially in dimensions three or less.

Contribution

It provides a detailed calculation showing the suppression of quasi-particles caused by order parameter fluctuations in superfluid Fermi gases.

Findings

Quasi-particles are not well-defined in D  3 due to singular interactions.

In the BEC regime, the spectral function shows logarithmic suppression of quasi-particle residue.

Order parameter fluctuations significantly affect fermionic excitations in superfluid Fermi gases.

Abstract

We calculate the effect of order parameter fluctuations on the fermionic single-particle excitations in the superfluid state of neutral fermions interacting with short range attractive forces. We show that in dimensions D \leq 3 the singular effective interaction between the fermions mediated by the gapless Bogoliubov-Anderson mode prohibits the existence of well-defined quasi-particles. We explicitly calculate the single-particle spectral function in the BEC regime in D=3 and show that in this case the quasi-particle residue and the density of states are logarithmically suppressed.