Frequency and damping of hydrodynamic modes in a trapped Bose-condensed gas

TL;DR

This paper analyzes the collective modes of a trapped Bose gas at finite temperatures using two-fluid hydrodynamics, deriving expressions for frequency and damping, and compares theoretical predictions with recent experimental data.

Contribution

It introduces frequency-dependent transport coefficients into hydrodynamic equations and discusses the crossover from collisionless to hydrodynamic regimes in trapped Bose gases.

Findings

Derived variational formulas for mode frequency and damping.

Demonstrated the cutoff effect of frequency-dependent transport coefficients.

Compared theoretical damping predictions with experimental data on metastable helium.

Abstract

Recently it was shown that the Landau-Khalatnikov two-fluid hydrodynamics describes the collision-dominated region of a trapped Bose condensate interacting with a thermal cloud. We use these equations to discuss the low frequency hydrodynamic collective modes in a trapped Bose gas at finite temperatures. We derive a variational expressions based on these equations for both the frequency and damping of collective modes. A new feature is our use of frequency-dependent transport coefficients, which produce a natural cutoff by eliminating the collisionless low-density tail of the thermal cloud. Above the superfluid transition, our expression for the damping in trapped inhomogeneous gases is analogous to the result first obtained by Landau and Lifshitz for uniform classical fluids. We also use the moment method to discuss the crossover from the collisionless to the hydrodynamic region. Recent data for the monopole-quadrupole mode in the hydrodynamic region of a trapped gas of metastable $^4$He is discussed. We also present calculations for the damping of the analogous $m=0$ monopole-quadrupole condensate mode in the superfluid phase.