11,687 research outputs found
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- Heisenberg model in a triangular lattice
We study the spin- 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- XXZ model onto which the spin- 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 () 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,
() 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 and
gives reasonable agreement with the experimental results,
i.e.,
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