Thermal solitons as revealed by static structure factor

TL;DR

This paper investigates the static structure factor of a weakly interacting Bose gas at thermal equilibrium, revealing thermal phonon distributions and the presence of spontaneous dark solitons in elongated systems.

Contribution

It demonstrates the existence of thermal solitons in elongated Bose gases and explains discrepancies in phonon distributions using the classical fields approximation.

Findings

Thermal phonon distribution follows the Planck distribution in 3D gases.

Discrepancies in phonon distributions in elongated systems are due to spontaneous dark solitons.

Spontaneous dark solitons are present at thermal equilibrium in elongated Bose gases.

Abstract

We study, within a framework of the classical fields approximation, the static structure factor of a weakly interacting Bose gas at thermal equilibrium. As in a recent experiment (R. Schley et al., Phys. Rev. Lett. 111, 055301 (2013)), we find that the thermal distribution of phonons in a three-dimensional Bose gas follows the Planck distribution. On the other hand we find a disagreement between the Planck and phonon (calculated just like for the bulk gas) distributions in the case of elongated quasi-one-dimensional systems. We attribute this discrepancy to the existence of spontaneous dark solitons (i.e., thermal solitons as reported in T. Karpiuk et al., Phys. Rev. Lett. 109, 205302 (2012)) in an elongated Bose gas at thermal equilibrium.