Probing superfluidity in a quasi two-dimensional Bose gas through its local dynamics

We report direct evidence of superfluidity in a quasi two-dimensional Bose gas by observing its dynamical response to a collective excitation. Relying on a novel local correlation analysis, we are able to probe inhomogeneous clouds and reveal their local dynamics. We identify in this way the superfluid and thermal phases inside the gas and locate the boundary at which the Berezinskii--Kosterlitz--Thouless crossover occurs. This new analysis also allows to evidence the coupling of the two fluids which induces at finite temperatures damping rates larger than the usual Landau damping

Improved version of the eikonal model for absorbing spherical particles

We present a new expression of the scattering amplitude, valid for spherical absorbing objects, which leads to an improved version of the eikonal method outside the diffraction region. Limitations of this method are discussed and numerical results are presented and compared successfully with the Mie theory.Comment: 7 pages, postscript figures available on cpt.univ-mrs.fr, to appear in J. Mod. Optic

Imaging the collective excitations of an ultracold gas using statistical correlations

Advanced data analysis techniques have proved to be crucial for extracting information from noisy images. Here we show that principal component analysis can be successfully applied to ultracold gases to unveil their collective excitations. By analyzing the correlations in a series of images we are able to identify the collective modes which are excited, determine their population, image their eigenfunction, and measure their frequency. Our method allows to discriminate the relevant modes from other noise components and is robust with respect to the data sampling procedure. It can be extended to other dynamical systems including cavity polariton quantum gases or trapped ions.Comment: See also the supplementary material and the video abstrac

A case study of spin-11 Heisenberg model in a triangular lattice

We study the spin-$1$ model in a triangular lattice in presence of a uniaxial anisotropy field using a Cluster Mean-Field approach (CMF). The interplay between antiferromagnetic exchange, lattice geometry and anisotropy forces Gutzwiller mean-field approaches to fail in a certain region of the phase diagram. There, the CMF yields two supersolid (SS) phases compatible with those present in the spin-$1/2$ XXZ model onto which the spin-$1$ system maps. Between these two SS phases, the three-sublattice order is broken and the results of the CMF depend heavily on the geometry and size of the cluster. We discuss the possible presence of a spin liquid in this region.Comment: 7 pages, 4 figures, RevTeX 4. The abstract and conclusions have been modified and the manuscript has been extende

Agile low phase noise radio-frequency sine wave generator applied to experiments on ultracold atoms

We report on the frequency performance of a low cost (~500$) radio-frequency sine wave generator, using direct digital synthesis (DDS) and a field-programmable gate array (FPGA). The output frequency of the device may be changed dynamically to any arbitrary value ranging from DC to 10 MHz without any phase slip. Sampling effects are substantially reduced by a high sample rate, up to 1 MHz, and by a large memory length, more than 2.10^5 samples. By using a low noise external oscillator to clock the DDS, we demonstrate a phase noise as low as that of the master clock, that is at the level of -113 dB.rad^2/Hz at 1 Hz from the carrier for an output frequency of 3.75 MHz. The device is successfully used to confine an ultracold atomic cloud of rubidium 87 in a RF-based trap, and there is no extra heating from the RF source.Comment: 10 pages, 6 figure

Bifix codes and interval exchanges

We investigate the relation between bifix codes and interval exchange transformations. We prove that the class of natural codings of regular interval echange transformations is closed under maximal bifix decoding.Comment: arXiv admin note: substantial text overlap with arXiv:1305.0127, arXiv:1308.539

Rotational friction on small globular proteins: Combined dielectric and hydrodynamic effect

Rotational friction on proteins and macromolecules is known to derive contributions from at least two distinct sources -- hydrodynamic (due to viscosity) and dielectric friction (due to polar interactions). In the existing theoretical approaches, the effect of the latter is taken into account in an {\it ad hoc} manner, by increasing the size of the protein with the addition of a hydration layer. Here we calculate the rotational dielectric friction on a protein ($\zeta_{DF}$) by using a generalized arbitrary charge distribution model (where the charges are obtained from quantum chemical calculation) and the hydrodynamic friction with stick boundary condition, ($\zeta_{hyd}^{stick}$) by using the sophisticated theoretical technique known as tri-axial ellipsoidal method, formulated by Harding [S. E. Harding, Comp. Biol. Med. {\bf 12}, 75 (1982)]. The calculation of hydrodynamic friction is done with only the dry volume of the protein (no hydration layer). We find that the total friction obtained by summing up $\zeta_{DF}$ and $\zeta_{hyd}^{stick}$ gives reasonable agreement with the experimental results, i.e., $\zeta_{exp} \approx \zeta_{DF} + \zeta_{hyd}^{stick}$

Deelrapport: Bevraging van de aanbodzijde In het kader van de DIVERSITEITSBAROMETER HUISVESTING

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