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

TL;DR

This study explores how first and second sound modes in a 2D Bose gas change with temperature and interaction strength, highlighting a qualitative shift from weak to strong interactions and the impact of the BKT transition.

Contribution

It provides a detailed analysis of sound mode behavior across interaction regimes in 2D Bose gases using Landau's hydrodynamics, emphasizing the BKT transition effects.

Findings

Deep qualitative change in sound behavior from weak to strong interactions.

Discontinuity of superfluid density causes jumps at BKT transition.

Second sound excitation weakens as interaction strength increases.

Abstract

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.