Enhanced Heat Flow in the Hydrodynamic Collisionless Regime

Atom Optics and Ultrafast Dynamics, Utrecht University, P.O. Box 80,000, 3508 TA Utrecht, The Netherlands.
Physical Review Letters (Impact Factor: 7.51). 08/2009; 103(9):095301. DOI: 10.1103/PhysRevLett.103.095301
Source: PubMed


We study the heat conduction of a cold, thermal cloud in a highly asymmetric
trap. The cloud is axially hydrodynamic, but due to the asymmetric trap
radially collisionless. By locally heating the cloud we excite a thermal dipole
mode and measure its oscillation frequency and damping rate. We find an
unexpectedly large heat conduction compared to the homogeneous case. The
enhanced heat conduction in this regime is partially caused by atoms with a
high angular momentum spiraling in trajectories around the core of the cloud.
Since atoms in these trajectories are almost collisionless they strongly
contribute to the heat transfer. We observe a second, oscillating hydrodynamic
mode, which we identify as a standing wave sound mode.

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    ABSTRACT: In 1995 Bose-Einstein condensation (BEC) in dilute Bose gases has been realized experimentally for the first time. Although the first condensates were created with a few million atoms or less, it has been speculated at that time that soon the number of atoms would increase considerably such that the sample becomes hydrodynamic. This would allow to enter the regime of the Landau two-fluid model for dilute Bose gases, where experiments in liquid helium below the lambda-point have been very successful. Since that time a few experiments have been carried out where the sample was close to hydrodynamic, although most of the experiment using dilute Bose gases have been in the collisionless regime. We have been carrying out experiments, where for the first time the sample is fully hydrodynamic in the axial direction. We have displaced the condensate with respect to the thermal cloud and subsequently released the condensate, such that it moves through the thermal cloud [1]. Contrary to the superfluid properties of the condensate we observe damping of the out-of-phase motion between condensate and thermal cloud. In another experiment we locally heat the sample of condensate and thermal cloud and observe the equilibration of the sample to a homogeneous temperature extending our work above Tc [2]. We observe two standing wave sound modes, where the mode in the condensate (thermal cloud) is associated with second (first) sound. In a final experiment we directly induce a wave by locally decreasing the density in the condensate and measure its propagation speed [3]. The speed of sound, which is 5-10% smaller compared to the Bogoliubov speed of sound, is compared to the speed of second sound in the Landau two-fluid hydrodynamics model. We observe excellent agreement between the model and experiment in a large range of temperatures. These experiments open the field of quantum hydrodynamics for dilute Bose gases and broadens our knowledge on second sound and superfluidity. [4pt] [1] R. Meppelink et al., Damping of superfluid flow by a thermal cloud, Phys. Rev. Lett. (accepted).[0pt] [2] R. Meppelink et al., Enhanced Heat Flow in the Hydrodynamic Collisionless Regime, Phys. Rev. Lett. 1030953012009.[0pt] [3] R. Meppelink et al., Sound propagation in a Bose-Einstein condensate at finite temperatures, Phys. Rev. A 80 043605 2009.
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    ABSTRACT: We study the second sound dipole mode in a partially Bose-Einstein condensed gas. This mode is excited by spatially separating and releasing the center-of-mass of the Bose-Einstein condensate (BEC) with respect to the thermal cloud, after which the equilibration is observed. The oscillation frequency and the damping rate of this mode is studied for different harmonic confinements and temperatures. The measured damping rates close to the collisionless regime are found to be in good agreement with Landau damping. For increasing hydrodynamicity of the cloud we observe an increase of the damping. Comment: Submitted to Phys. Rev. Lett. with 5 figures
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    ABSTRACT: We study the propagation of a density wave in a magnetically trapped Bose-Einstein condensate at finite temperatures. The thermal cloud is in the hydrodynamic regime and the system is therefore described by the two-fluid model. A phase-contrast imaging technique is used to image the cloud of atoms and allows us to observe small density excitations. The propagation of the density wave in the condensate is used to determine the speed of sound as a function of the temperature. We find the speed of sound to be in good agreement with calculations based on the Landau two-fluid model.
    Physical Review A 09/2009; 80(4). DOI:10.1103/PhysRevA.80.043605 · 2.81 Impact Factor
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