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.

We observe the formation of shock waves in a Bose-Einstein condensate containing a large number of sodium atoms. The shock wave is initiated with a repulsive blue-detuned light barrier, intersecting the BoseEinstein condensate, after which two shock fronts appear. We observe breaking of these waves when the size of these waves approaches the healing length of the condensate. At this time, the wave front splits into two parts and clear fringes appear. The experiment is modeled using an effective one-dimensional Gross-Pitaevskiilike equation and gives excellent quantitative agreement with the experiment, even though matter waves with wavelengths two orders of magnitude smaller than the healing length are present. In these experiments, no significant heating or particle loss is observed.

Phase contrast imaging is used to observe Bose-Einstein condensates at finite temperature in situ. The imaging technique is used to accurately derive the absolute phase shift of a probe laser beam due to both the condensate and the thermal cloud. The accuracy of the method is enhanced by using the periodicity of the intensity signal as a function of the accumulated phase. The measured density profiles can be described using a two-relevant-parameter fit, in which only the chemical potential and the temperature are to be determined. This allows us to directly compare the measured density profiles to different mean-field models in which the interaction between the condensed and the thermal atoms is taken into account to various degrees.

We describe the setup to create a large Bose-Einstein condensate containing more than 120·10 6 atoms. In the experiment a thermal beam is slowed by a Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). A typical dark-spot MOT in our experiments contains 2.0·10 10 atoms with a temperature of 320 µK and a density of about 1.0·10 11 atoms/cm 3 . The sample is spin polarized in a high magnetic field, before the atoms are loaded in the magnetic trap. Spin polarizing in a high magnetic field results in an increase in the transfer efficiency by a factor of 2 compared to experiments without spin polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by evaporative cooling. To suppress the 3-body losses at the end of the evaporation the magnetic trap is decompressed in the axial direction.

One of the principal signatures of superfluidity is the frictionless flow of a superfluid through another substance. Here, we study the flow of a Bose-Einstein condensate through a thermal cloud and study its damping for different harmonic confinements and temperatures. The damping rates close to the collisionless regime are found to be in good agreement with Landau damping and become smaller for more homogeneous systems. In the hydrodynamic regime, we observe additional damping due to collisions, and we discuss the implications of these findings for superfluidity in this system. DOI: 10.1103/PhysRevLett.103.265301 PACS numbers: 67.85.De, 03.75.Kk, 47.37.+q, 67.90.+z In 1938, Kapitza, and independently Allen and Misener, discovered that liquid 4 He below the -point can flow almost frictionless. Kapitza named this behavior superfluidity [1,2]. Many of the properties of superfluid helium also appear in dilute Bose-Einstein condensation (BEC). The most striking signatures of superfluidity in BEC are quantized vortices [3,4], second sound [5], Josephson oscillations [6], and persistent flow [7]. In contrast to liquid helium, where the interatomic interaction is too strong to investigate the microscopic properties of superfluidity, the interactions in BEC are much weaker. The study of superfluid flow in dilute BECs can therefore deepen our understanding of superfluidity.In this Letter, the flow of a BEC (the superfluid) through a thermal cloud is studied in a harmonic potential by exciting a dipole oscillation of the BEC, whereas the thermal cloud initially remains at rest. In the hydrodynamic regime, this out-of-phase mode of the trapped Bose gas is the analog of the usual second sound mode in bulk superfluid helium [8]. For this second sound dipole mode, we study for the first time its frequency and damping rate from the collisionless to the hydrodynamic regime. In contrast to liquid helium, our analysis allows for a direct measurement of the position of the superfluid component (condensed atoms) with respect to the normal fluid (thermal atoms), which allows for an unequivocal determination of the second sound dipole mode.The thermal cloud can be tuned from the hydrodynamic regime into the collisionless regime, in which the meanfree path of the thermal atoms is larger than the axial size of the cloud. As we will show, the damping of the second sound dipole mode in a collisionless, partly condensed BEC is primarily caused by Landau damping; i.e., meanfield interactions mediate the transfer of energy from the condensate to the thermal cloud, leading to the damping of collective modes. Landau damping was first discussed by Landau in the context of the damping of plasma oscillations and plays a key role in a broad variety of fields, for instance the damping of phonons in metals, the damping of quarks and gluons in quark-gluon plasmas, and the anomalous skin effect in metals.Previously, some experiments have been performed in which the BEC and the thermal cloud move with respect to each other [9,10]. In the latter e...