Sound Can Travel In A 2D Bose Gas Even When Atoms Don’t Collide

  • Researchers demonstrate that sound waves can travel in a thin sheet of bosons of rubidium atoms, even in extremely low density where atoms do not collide with each other. 
  • They performed numerical simulations to prove this phenomenon. 

Usually, sound propagates through a medium when atoms collide with each other. The propagation of sound waves is one of the most important aspects of studying dilute quantum gases, which offers insights on thermodynamic behavior, superfluid behavior, and relaxation mechanisms.

In hydrodynamics, sound represents a density wave that propagates via collisions between atoms. However, in superfluids, the situation is bit complex: there could be two modes of sounds if collisions are powerful enough to ensure local thermalization.

Sometimes, these collisions are not required to propagate a sound wave through a medium. Recently, two independent group of researchers demonstrated that the sound can travel in a thin sheet of bosons — two-dimensional Bose gas — despite the lack of atoms’ collision in low density.

Strange Behavior of Sound Waves in 2D Bose Gas of Rubidium Atoms

The phenomenon of collisionless sound wave propagation has been reported in several systems such as liquid helium and Bose-Einstein condensates near zero Kelvin temperature. However, it hasn’t been observed yet in supercold two-dimensional dilute gas.

Now, researchers have shown that sound waves can travel in a thin layer of bosons of rubidium atoms. They calculated the amplitude and speed of sound waves traveling through this medium as they increased the temperature from 50 to 300 Nanokelvin.

In this temperature range, the state rubidium changes from superfluid to gas. At 200 Nanokelvin (temperature at which superfluid nature disappears), researchers anticipated a rapid decline in both speed and amplitude, but it didn’t happen.

Reference: Physical Review Letters | doi:10.1103/PhysRevLett.121.145301 

In the superfluid state, the sound speed matches the estimation of a two-fluid hydrodynamic model, and the weak damping is justified by the scattering with thermal excitation. The team observed a stronger damping in the normal state, which attributes to a departure from hydrodynamic nature.

Simulating Sound Waves In Collisionless 2D Bose Gas

Sound Can Travel In A 2D Bose Gas

The second team of researchers came with a theory and carried out numerical simulations (with Gross-Pitaevskii equation) of sound traveling in a low-density 2D Bose gas. The results indicate that weak repulsive interactions among atoms are responsible for moving the sound waves. Some previous studies had detected the same interactions driving the movement of waves in superfluids.

Reference: Physical Review Letters | doi:10.1103/PhysRevLett.121.145302

Overall, their work demonstrates that collisionless sound can travel both above and below the superfluid Berezinskii-Kosterlitz-Thouless phase transition, as a result of interactions between particles.

Read: New Printing Technology Uses Sound Waves To Control Liquid Droplets

Both teams have mentioned that their outcomes can be extended to three-dimensional Bose gases and other ultracold fluids. In the future, we can use sound as a tool to study the characteristics of these systems.

Written by
Varun Kumar

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