Description
An interacting two-dimensional (2D) Bose gas becomes superfluid below a critical temperature through the BKT mechanism. In this phase, under hydrodynamic conditions where collisions keep the gas in local thermodynamic equilibrium, Landau's two-fluid model predicts the existence of two distinct sound-like excitations. These, so-called first and second sounds, have recently been observed in 2D [1], verifying the validity of the two-fluid model for a 2D superfluid. In the opposite limit of infrequent collisions, a distinct sound wave originating from the mean-field interactions of the quasicondensate is predicted to prevail even in the absence of the superfluid [2]; this ‘collisionless’ sound has also been observed in experiments [3].
Here, aiming to bridge the gap between the above observations, we extend our previous measurements [1] to explore the sound excitations of the interacting 2D $^{39}$K Bose gas throughout the crossover between the hydrodynamic and the collisionless limits. To control the nature of the induced sound waves, we vary the wavelength of sound excitations by directly exciting higher-wavevector modes using a digital micromirror device (DMD). This allows us to study the crossover from hydrodynamic to collisionless sound under the same conditions of the gas, including the different damping mechanisms of these sound modes. Moreover, we also probe sound waves at various temperatures, to connect the hydrodynamic sound modes with the well-known zero-temperature (collisionless) Bogoliubov excitations.
[1] Christodoulou, Panagiotis, et al. "Observation of first and second sound in a BKT superfluid." Nature 594.7862 (2021): 191-194.
[2] Ota, Miki, et al. "Collisionless sound in a uniform two-dimensional Bose gas." Physical Review Letters 121.14 (2018): 145302.
[3] Ville, J. L., et al. "Sound propagation in a uniform superfluid two-dimensional Bose gas." Physical Review Letters 121.14 (2018): 145301.
| Presenter name | Martin Gazo |
|---|---|
| How will you attend ICAP-27? | I am planning on in-person attendance |
