Gap and screening in Raman scattering of a Bose condensed gas

TL;DR

This paper proposes spectroscopic methods to explore thermal excitations in a trapped Bose condensed gas, contrasting theoretical spectra from different approximations and suggesting experiments to observe excitation gaps and screening effects.

Contribution

It introduces novel spectroscopic techniques to analyze excitation spectra and distinguishes between theoretical models, highlighting the role of dynamical screening in Bose gases.

Findings

Both approximations predict Bogoliubov excitations.

GRPA shows a gapped, parabolic dispersion for collective excitations.

Proposed experiments aim to observe the excitation gap and screening effects.

Abstract

We propose different spectroscopic methods to explore the nature of the thermal excitations of a trapped Bose condensed gas: 1) a four photon process to probe the uniform region in the trap center: 2) a stimulated Raman process in order to analyze the influence of a momentum transfer in the resulting scattered atom momentum distribution. We apply these methods to address specifically the energy spectrum and the scattering amplitude of these excitations in a transition between two hyperfine levels of the gas atoms. In particular, we exemplify the potential offered by these proposed techniques by contrasting the spectrum expected, from the {\it non conserving} Bogoliubov approximation valid for weak depletion, to the spectrum of the finite temperature extensions like the {\it conserving} generalized random phase approximation (GRPA). Both predict the existence of the Bogoliubov collective excitations but the GRPA approximation distinguishes them from the single atom excitations with a gapped and parabolic dispersion relation and accounts for the dynamical screening of any external perturbation applied to the gas. We propose two feasible experiments, one concerns the observation of the gap associated to this second branch of excitations and the other deals with this screening effect.