The response of Bose-Einstein condensates to external perturbations at finite temperature

TL;DR

This paper develops a finite-temperature, number-conserving linear response theory for Bose-Einstein condensates, explaining experimental observations of excitations and decay rates with a comprehensive, gapless formalism that includes thermal cloud dynamics.

Contribution

It introduces a novel, gapless, mean-field response theory for Bose-Einstein condensates at finite temperature, explicitly incorporating thermal cloud driving and finite size effects.

Findings

Successfully explains JILA measurements of condensate excitations.

Provides a consistent, gapless formalism aligned with the generalized Kohn theorem.

Includes thermal cloud dynamics driven by external perturbations.

Abstract

We present a theory of the linear response of a Bose-condensed gas to external perturbations at finite temperature. The theory developed here is the basis of a recent quantitative explanation of the measurements of condensate excitations and decay rates made at JILA [D. S. Jin et al., Phys. Rev. Lett. 78, 764 (1997)]. The formalism is based on a dynamic, number-conserving, mean-field scheme and is valid in the collisionless limit of well-defined quasiparticles. The theory is gapless, consistent with the generalized Kohn theorem for the dipole modes, and includes the time-dependent normal and anomalous averages, Beliaev and Landau processes, and all relevant finite size effects. The important physical process where the thermal cloud is driven directly by the external perturbation is explicitly included. This is required for consistency with the dipole modes and is also needed to explain the JILA observations.