First and second sound in a compressible 3D Bose fluid

TL;DR

This study investigates first and second sound in a highly compressible 3D Bose gas, revealing their behavior and crossover from hydrodynamic to collisionless regimes through experimental and theoretical analysis.

Contribution

It demonstrates the existence and characteristics of first and second sound in a highly compressible Bose gas, extending the understanding beyond the nearly-incompressible regime.

Findings

Observation of two distinct resonant oscillations below the critical temperature.

Agreement between experimental results and hydrodynamic theory.

Exploration of the crossover from hydrodynamic to collisionless behavior.

Abstract

The two-fluid model is fundamental for the description of superfluidity. In the nearly-incompressible-liquid regime, it successfully describes first and second sound, corresponding, respectively, to density and entropy waves, in both liquid helium and unitary Fermi gases. Here, we study the two sounds in the opposite regime of a highly compressible fluid, using an ultracold $^{39}$K Bose gas in a three-dimensional box trap. We excite the longest-wavelength mode of our homogeneous gas, and observe two distinct resonant oscillations below the critical temperature, of which only one persists above it. In a microscopic mode-structure analysis, we find agreement with the hydrodynamic theory, where first and second sound involve density oscillations dominated by, respectively, thermal and condensed atoms. Varying the interaction strength, we explore the crossover from hydrodynamic to collisionless behavior in a normal gas.