Second sound in a two-dimensional Bose gas: From the weakly to the strongly interacting regime

Ota, Miki; Stringari, S.

doi:10.1103/physreva.97.033604

Cited by 31 publications

(39 citation statements)

References 45 publications

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“…They all support experimental observation that below the BKT transition the generated sound waves propagate with velocities close to the speed of second sound, predicted by two-fluid hydrodynamic model. They also predict that sound waves can propagate above the BKT transition, in agreement with experiment and in opposition to what is predicted by two-fluid hydrodynamic model with respect to second sound 15 , 16 .…”

Section: Introductionsupporting

confidence: 70%

Berezinskii–Kosterlitz–Thouless phase induced by dissipating quasisolitons

Gawryluk

Brewczyk

2021

Sci Rep

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We theoretically study the sound propagation in a two-dimensional weakly interacting uniform Bose gas. Using the classical fields approximation we analyze in detail the properties of density waves generated both in a weak and strong perturbation regimes. While in the former case density excitations can be described in terms of hydrodynamic or collisionless sound, the strong disturbance of the system results in a qualitatively different response. We identify observed structures as quasisolitons and uncover their internal complexity for strong perturbation case. For this regime quasisolitons break into vortex pairs as time progresses, eventually reaching an equilibrium state. We find this state, characterized by only fluctuating in time averaged number of pairs of opposite charge vortices and by appearance of a quasi-long-range order, as the Berezinskii–Kosterlitz–Thouless (BKT) phase.

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Section: Introductionsupporting

confidence: 70%

Berezinskii–Kosterlitz–Thouless phase induced by dissipating quasisolitons

Gawryluk

Brewczyk

2021

Sci Rep

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“…The recent developments in creating quasi-uniform box traps [18][19][20][21] have led to intriguing new possibilities. These traps provide a textbook setting for the study of shortwavelength excitations [22], but they also raise new questions on the nature of long-wavelength (system-size) collective modes, as highlighted by recent studies of sound propagation in 3D Bose [23] and Fermi [24] gases, and 2D Bose gases [25] (see also [26][27][28]). Due to the hard-wall boundary conditions the dynamics depend only on the interplay between kinetic and interaction energy; this is in stark contrast to harmonically trapped gases, where the lowest mode frequency is independent of interaction strength [29].…”

Section: Introductionmentioning

confidence: 99%

From single-particle excitations to sound waves in a box-trapped atomic Bose-Einstein condensate

et al. 2019

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We experimentally and theoretically investigate the lowest-lying axial excitation of an atomic Bose-Einstein condensate in a cylindrical box trap. By tuning the atomic density, we observe how the nature of the mode changes from a single-particle excitation (in the low-density limit) to a sound wave (in the high-density limit). Throughout this crossover the measured mode frequency agrees with Bogoliubov theory. Using approximate low-energy models we show that the evolution of the mode frequency is directly related to the interactioninduced shape changes of the condensate and the excitation. Finally, if we create a large-amplitude excitation, and then let the system evolve freely, we observe that the mode amplitude decays non-exponentially in time; this nonlinear behaviour is indicative of interactions between the elementary excitations, but remains to be quantitatively understood.

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“…In weakly interacting Bose-Einstein condensates (BECs), one still expects two branches of sound with speeds c (1) > c (2) but the nature of first and second sound is strongly modified because of their large compressibility [5]. While at zero temperature density perturbations propagate as Bogoliubov sound waves, at finite temperature we expect them to couple mostly to second sound -a behavior contrasting with the case of liquid heliumwith a sound speed proportional to the square root of the superfluid fraction [5,6]. Sound waves in an elongated three-dimensional (3D) BEC were observed in Refs.…”

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confidence: 99%

Sound Propagation in a Uniform Superfluid Two-Dimensional Bose Gas

et al. 2018

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In superfluid systems several sound modes can be excited, as for example first and second sound in liquid helium. Here, we excite propagating and standing waves in a uniform two-dimensional Bose gas and we characterize the propagation of sound in both the superfluid and normal regime. In the superfluid phase, the measured speed of sound is well described by a two-fluid hydrodynamic model, and the weak damping rate is well explained by the scattering with thermal excitations. In the normal phase the sound becomes strongly damped due to a departure from hydrodynamic behavior.Propagation of sound waves is at the heart of our understanding of quantum fluids. In liquid helium, the celebrated two-fluid model was confirmed by the observation of first and second sound modes [1]. There, first sound stands for the usual sound appellation, namely a density wave for which normal and superfluid fractions oscillate in phase. Second sound corresponds to a pure entropy wave with no perturbation in density (normal and superfluid components oscillating out of phase), and is generally considered as a smoking gun of superfluidity.Sound wave propagation is also central to the study of dilute quantum gases, providing information on thermodynamic properties, relaxation mechanisms and superfluid behavior. In ultracold strongly interacting Fermi gases, the existence of first and second sound modes in the superfluid phase was predicted [2] and observed in experiments [3,4], with a behavior similar to liquid helium. In weakly interacting Bose-Einstein condensates (BECs), one still expects two branches of sound with speeds c (1) > c (2) but the nature of first and second sound is strongly modified because of their large compressibility [5]. While at zero temperature density perturbations propagate as Bogoliubov sound waves, at finite temperature we expect them to couple mostly to second sound -a behavior contrasting with the case of liquid heliumwith a sound speed proportional to the square root of the superfluid fraction [5,6]. Sound waves in an elongated three-dimensional (3D) BEC were observed in Refs. [7,8] in a regime where the sound speed remains close to the Bogoliubov sound speed.Propagation of sound in weakly interacting twodimensional (2D) Bose gases was recently discussed in Ref.[9] using a hydrodynamic two-fluid model, predicting the existence of first and second sound modes of associated speeds c (1) HD and c(2) HD , respectively. In 2D Bose gases, superfluidity occurs via the Berezinskii-Kosterlitz-Thouless (BKT) mechanism [10]. The superfluid to normal transition is associated with a jump of the superfluid density that cannot be revealed from the thermodynamic properties of the gas. As the second sound speed is related to the superfluid fraction, one expects c (2) HD to remain non-zero just below the critical point of the super-FIG. 1. Experimental protocol and observation of propagating waves. (a) Absorption image of the cloud perturbed by a local additional potential. The excitation is delimited by the horizontal dashed lin...

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Second sound in a two-dimensional Bose gas: From the weakly to the strongly interacting regime

Cited by 31 publications

References 45 publications

Berezinskii–Kosterlitz–Thouless phase induced by dissipating quasisolitons

Berezinskii–Kosterlitz–Thouless phase induced by dissipating quasisolitons

From single-particle excitations to sound waves in a box-trapped atomic Bose-Einstein condensate

Sound Propagation in a Uniform Superfluid Two-Dimensional Bose Gas

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