Abstract
Motivated by recent experiments in Phys. Rev. Lett. 121, 145301 (2018), we study sound propagation in a two-dimensional (2D) Bose gas across the superfluid-thermal transition using classical field dynamics. Below the transition temperature we find a Bogoliubov and a non-Bogoliubov mode, above it we find the normal sound mode and the diffusive mode, as we determine from the dynamical structure factor. Our simulations of the experimental procedure agree with the measured velocities, and show that below the transition temperature the measurements detect the Bogoliubov mode. Above the transition, they either detect the normal sound mode for low densities or weak interactions, or the diffusive mode for high densities or strong interactions. As a key observation, we discuss the weak coupling regime in which the non-Bogoliubov mode has a higher velocity than the Bogoliubov mode, in contrast to a hydrodynamic scenario. We propose to detect this regime via step-pulse density perturbation, which simultaneously detects both sound modes
Highlights
While the studies of superfluid helium were of crucial importance for understanding quantum liquids, the creation of Bose-Einstein condensates of dilute gases strongly expanded the scope of these studies
We have studied the propagation of sound in a 2D quasicondensate of 87Rb atoms across the superfluid-thermal transition using c-field dynamics
The first two methods are inspired by Ref. [33]: We excite running and standing waves with a weak Gaussian potential, from which we obtain a single velocity
Summary
Controlled excitation of quantum liquids has created insight into collective modes [1,2,3,4,5,6,7,8], superfluidity [7,9,10,11,12,13,14,15,16,17,18], excitation properties [19], and sound diffusion [20]. The temperature dependence of the measured sound velocity shows no discernible jump in the crossover regime and a nonzero velocity above the transition Theoretical studies of this measurement were reported in Refs. We investigate sound mode dynamics of a uniform 2D Bose gas of 87Rb atoms across the superfluid-thermal transition using classical-field simulations, see Ref. Near and above Tc, c displays a temperature dependence that depends on the density in a qualitative manner: c increases and decreases for low and high densities, respectively This is reflected in the dynamic structure factor, showing the density-dependent interplay between two sound modes that we refer to as the Bogoliubov and the non-Bogoliubov mode below Tc, and the diffusive and the normal sound mode above Tc. The results of c show a breaking of the universal scale invariance at nonzero temperature due to Landau damping.
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