First and Second Sound in a Compressible 3D Bose Fluid

Area of Science:

• Atomic, Molecular, and Optical Physics
• Condensed Matter Physics
• Quantum Gases

Background:

• The two-fluid model describes superfluidity, successfully explaining first and second sound (density and entropy waves) in nearly incompressible liquids like helium.
• These phenomena are crucial for understanding quantum fluids and their unique wave behaviors.

Purpose of the Study:

• To investigate the behavior of first and second sound in a highly compressible fluid regime.
• To explore the applicability of the two-fluid model beyond the incompressible limit using ultracold atomic gases.

Main Methods:

• Utilized an ultracold ^{39}K Bose gas in a three-dimensional box trap.
• Excited the longest-wavelength mode to observe resonant oscillations.
• Performed microscopic mode-structure analysis and varied interaction strength.

Main Results:

• Observed two distinct resonant oscillations below the critical temperature, with one persisting above it.
• Found agreement between experimental observations and hydrodynamic theory predictions.
• Demonstrated that first and second sound involve density oscillations dominated by thermal and condensed atoms, respectively.

Conclusions:

• The two-fluid model successfully describes first and second sound in a highly compressible Bose gas.
• The study provides insights into the crossover from hydrodynamic to collisionless behavior in normal gases.
• This work extends the understanding of quantum fluid dynamics to new regimes.