Landau phonon-roton theory revisited for superfluid helium 4 and Fermi gases

TL;DR

This paper revisits Landau's theory for superfluid helium-4 and Fermi gases, deriving universal interaction expressions and analyzing phonon damping, which aids experimental understanding of superfluid excitations.

Contribution

It extends Landau and Khalatnikov's theory to include universal multi-body couplings in superfluid helium and Fermi gases, providing new analytical tools.

Findings

Derived universal three- and four-body coupling expressions.

Calculated phonon damping rates at low temperatures.

Compared damping in helium and Fermi gases, guiding experiments.

Abstract

Liquid helium and spin-1/2 cold-atom Fermi gases both exhibit in their superfluid phase two distinct types of excitations, gapless phonons and gapped rotons or fermionic pair-breaking excitations. In the long wavelength limit, revising and extending Landau and Khalatnikov's theory initially developed for helium [ZhETF 19, 637 (1949)], we obtain universal expressions for three- and four-body couplings among these two types of excitations. We calculate the corresponding phonon damping rates at low temperature and compare them to those of a pure phononic origin in high-pressure liquid helium and in strongly interacting Fermi gases, paving the way to experimental observations.