Probing the thermal atoms of a Bose gas through Raman transition

TL;DR

This paper investigates the finite-temperature many-body physics of a Bose condensate using Raman scattering, revealing a new gapped excitation branch influenced by exchange interactions and superfluid screening effects.

Contribution

It predicts a novel gapped thermal excitation in a Bose gas via Raman transition, distinct from phonon-like modes, and explains superfluid screening of external potentials.

Findings

Identification of a gapped, parabolic excitation branch in Raman scattering

Demonstration of superfluid screening preventing incoherent scattering

Resonance in scattering rate at the excitation gap energy

Abstract

We explore the many body physics of a Bose condensed atom gas at finite temperature through the Raman transition between two hyperfine levels. Unlike the Bragg scattering where the phonon-like nature of the collective excitations has been observed, a different branch of thermal atom excitation is found theoretically in the Raman scattering. This excitation is predicted in the generalized random phase approximation (GRPA) and has a gapped and parabolic dispersion relation. The gap energy results from the exchange interaction and is released during the Raman transition. The scattering rate is determined versus the transition frequency $\omega$ and the transferred momentum $\vc{q}$ and shows the corresponding resonance around this gap. Nevertheless, the Raman scattering process is attenuated by the superfluid part of the gas. The macroscopic wave function of the condensate deforms its shape in order to screen locally the external potential displayed by the Raman light beams. This screening is total for a condensed atom transition in order to prevent the condensate from incoherent scattering. The experimental observation of this result would explain some of the reasons why a superfluid condensate moves coherently without any friction with its surrounding.