Estimates for low Steklov eigenvalues of surfaces with several boundary components

In this article, we give computable lower bounds for the first non-zero Steklov eigenvalue $\sigma_1$ of a compact connected 2-dimensional Riemannian manifold $M$ with several cylindrical boundary components. These estimates show how the geometry of $M$ away from the boundary affects this eigenvalue. They involve geometric quantities specific to manifolds with boundary such as the extrinsic diameter of the boundary. In a second part, we give lower and upper estimates for the low Steklov eigenvalues of a hyperbolic surface with a geodesic boundary in terms of the length of some families of geodesics. This result is similar to a well known result of Schoen, Wolpert and Yau for Laplace eigenvalues on a closed hyperbolic surface.Comment: 25 pages, 4 figure

Three Dimensional Raman Cooling using Velocity Selective Rapid Adiabatic Passage

We present a new and efficient implementation of Raman cooling of trapped atoms. It uses Raman pulses with an appropriate frequency chirp to realize a velocity selective excitation through a rapid adiabatic passage. This method allows to address in a single pulse a large number of non zero atomic velocity classes and it produces a nearly unity transfer efficiency. We demonstrate this cooling method using cesium atoms in a far-detuned crossed dipole trap. Three-dimensional cooling of $1 \times 10^{5}$ atoms down to $2 \mu$K is performed in 100 ms. In this preliminary experiment the final atomic density is $1.3\times 10^{12}$ at/cm$^3$ (within a factor of 2) and the phase-space density increase over the uncooled sample is 20. Numerical simulations indicate that temperatures below the single photon recoil temperature should be achievable with this method.Comment: OSA TOPS on Ultracold Atoms and BEC 7 (1997) 5

Breakdown of scale invariance in a quasi-two-dimensional Bose gas due to the presence of the third dimension

In this Rapid Communication, we describe how the presence of the third dimension may break the scale invariance in a two-dimensional Bose gas in a pancake-shaped trap. From the two-dimensional perspective, the possibility of a weak spilling of the atomic density beyond the ground-state of the confinement alters the two-dimensional chemical potential; in turn, this correction no longer supports scale invariance. We compare experimental data with numerical and analytic perturbative results and find a good agreement.Comment: 4 pages, 1 figure, published in PRA Rapid Com

A Bose-Einstein condensate bouncing off a rough mirror

We present experimental results and theoretical analysis of the diffuse reflection of a Bose-Einstein condensate from a rough mirror. The mirror is produced by a blue-detuned evanescent wave supported by a dielectric substrate. The results are carefully analysed via a comparison with a numerical simulation. The scattering is clearly anisotropic, more pronounced in the direction of the evanescent wave surface propagation, as predicted theoretically

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

Two new genera of Nanophyidae with six desmomeres (Coleoptera, Curculionoidea)

A new genus Lyalia is described in Nanophyidae and three species are included in it: Lyalia curvata sp. n. (Vietnam), L. robusta (Pic, 1921), comb. n. (from Nanophyes) (Java, Bali, Laos) and L. albolineata (Pajni & Bhateja, 1982), comb. n. (from Ctenomerus) (India: Assam). Ctenomerus lagerstroemiae G. A. K. Marshall, 1923 is a syn. n. of L. robusta. Thus, the genus Ctenomerus Schoenherr, 1843 is restricted to the Afrotropical Realm. Kantohia gen. n. is erected for Kantohia taiwana (Kantoh & Kojima, 2009) (from Shiva) (Taiwan). A key to the Nanophyinae genera with six desmomeres is presented.Studies in the MNHN (Paris) by the senior author were facilitated by the attribution of a grant of Invited Professor during January-February 2010, for which he thanks the authorities of the Museum. We also thank Mr Maxwell V. L. Barclay and Dr Christopher H. C. Lyal (NHM, London) for the loan of specimens, Dr Wang Zhiliang (IZCAS, Beijing) for his help with the graphic material and Dr Christian Salcedo (IZCAS, Beijing) for the linguistic review. This research has been supported by project grant CGL2010-15786 (Ministerio de Ciencia e Innovación, Spain).Peer Reviewe

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

Diffraction of a Bose-Einstein Condensate in the Time Domain

We have observed the diffraction of a Bose-Einstein condensate of rubidium atoms on a vibrating mirror potential. The matter wave packet bounces back at normal incidence on a blue-detuned evanescent light field after a 3.6 mm free fall. The mirror vibrates at a frequency of 500 kHz with an amplitude of 3.0 nm. The atomic carrier and sidebands are directly imaged during their ballistic expansion. The locations and the relative weights of the diffracted atomic wave packets are in very good agreement with the theoretical prediction of Carsten Henkel et al. [1].Comment: submitted to Phys. Rev.

Application of non destructive testing to the detection of aeronautical defects in composite structures

A study of two Non-destructive Testing methods (NDT) was carried out in specimens with different kinds of simulated defects. Ultrasonics test (US) and Infrared Thermography (IRT) were applied with the aim to evaluate the detectability and the accuracy of each method.These techniques have acquired great importance in the aeronautics industry because they allow to control the aerostructures without intervening in their physical and mechanical integrity. In the second part of the study, a comparison of both techniques was achieved in order toanalyse their limits and advantages. It appeared that detectability of defects was much better in a sample with flat-bottomed holes defects in the case of Ultrasonic Test. However it was found that Infrared Thermography is much more limited to the thickness of the specimen than the ultrasonic waves. On the other hand, defects were all revealed with IRT in a sandwich composite including Teflon inserts, which was not the case for US