Second sound in 2D Bose gas: from the weakly interacting to the strongly interacting regime

Using Landau's theory of two-fluid hydrodynamics, we investigate first and second sound propagating in a two-dimensional Bose gas. We study the temperature and interaction dependence of both sound modes and show that their behaviour exhibits a deep qualitative change as the gas evolves from the weakly interacting to the strongly interacting regime. Special emphasis is given to the jump of both sounds at the Berezinskii-Kosterlitz-Thouless transition, caused by the discontinuity of the superfluid density. We find that the excitation of second sound through a density perturbation becomes weaker and weaker as the interaction strength increases as a consequence of the decrease of the thermal expansion coefficient. Our results can be relevant for future experiments on the propagation of sound in the BEC side of the BCS-BEC crossover of a 2D superfluid Fermi gas.