Pair-Breaking Collective Branch in BCS Superconductors and Superfluid Fermi Gases

TL;DR

This paper identifies and characterizes a new collective excitation branch in the pair-breaking continuum of superfluid Fermi gases and BCS superconductors, providing analytical dispersion relations across interaction regimes.

Contribution

It analytically continues the collective mode energy through the continuum branch cut, revealing a complex dispersion relation and effective mass for the branch in various coupling regimes.

Findings

Existence of a collective excitation branch in the pair-breaking continuum.

Analytical expression for the complex dispersion relation.

Effective mass of the branch varies quadratically with wave number.

Abstract

We demonstrate the existence of a collective excitation branch in the pair-breaking continuum of superfluid Fermi gases and BCS superconductors. At zero temperature, we analytically continue the equation on the collective mode energy in Anderson's Random Phase Approximation or Gaussian fluctuations through its branch cut associated with the continuum, and obtain the full complex dispersion relation, including in the strong coupling regime. The branch exists as long as the chemical potential $\mu$ is positive and the wave number below $\sqrt{2m\mu}/\hbar$ (with m the fermion mass). In the long wavelength limit, the branch varies quadratically with the wave number, with a complex effective mass that we compute analytically for an arbitrary interaction strength.