The Landau two-fluid model provides a phenomenological description of a low-temperature atomic fluid, by modeling the system as composed by a normal (viscous) part and by a superfluid (non-viscous) part. This general formalism, which is independent on the spatial dimension and on the bosonic or fermionic statistics, applies both to superfluid Helium and to weakly-interacting atomic gases. I will review the two-fluid model predictions concerning the long-wavelength
hydrodynamic excitations near the equilibrium configuration, whose
physics conjugates the static thermodynamic properties, enclosed in
the Bogoliubov theory, and the transport ones, related to superfluidity. In particular, I will analyze the propagation of sound in weakly-interacting two-dimensional bosonic and fermionic superfluids, showing how the vanishing of the superfluid density at the Berezinskii-Kosterlitz-Thouless (BKT) temperature leads to a discontinuity of the sound velocities. This non-monotonic behavior is also expected to occur in bubble-shaped superfluids, and I will discuss and connect the theoretical results for all these systems with past and ongoing experiments.
References:
A. Tononi, A. Cappellaro, G. Bighin, and L. Salasnich, Phys. Rev. A
103, L061303 (2021).
K. Furutani, A. Tononi, and L. Salasnich, New J. of Phys. 23, 043043
(2021).
A. Tononi, A. Pelster and L. Salasnich, arXiv:2104.04585
Prof. Luca Salasnich
