Existence et effet régularisant pour des équations de Schrödinger non linéaires généralisées : le cas de l'intéraction quadratique

We improve the result obtained by C. E. Kenig, G. Ponce and L. Vega, 1998, Inventionnes Math., on generalized nonlinear Schrödinger equations in the case where the interactions are quadratic. We establish the well-posedness of the Cauchy problem for such equations in weighted Sobolev spaces H s,σ (R n) for s > n/2 + 2 and σ > 1.Nous améliorons le resultat obtenu par C. E. Kenig, G. Ponce and L. Vega, 1998, Inventionnes Math., sur les équations de Schrödinger non linéaires généralisées dans le cas où les interactions sont quadratiques. Nous démontrons le caractère bien posé de telles équations dans des espaces de Sobolev à poids H s,σ (R n) avec s > n/2 + 2 et σ > 1

Creation and counting of defects in a temperature quenched Bose-Einstein Condensate

We study the spontaneous formation of defects in the order parameter of a trapped ultracold bosonic gas while crossing the critical temperature for Bose-Einstein Condensation (BEC) at different rates. The system has the shape of an elongated ellipsoid, whose transverse width can be varied to explore dimensionality effects. For slow enough temperature quenches we find a power-law scaling of the average defect number with the quench rate, as predicted by the Kibble-Zurek mechanism. A breakdown of such a scaling is found for fast quenches, leading to a saturation of the average defect number. We suggest an explanation for this saturation in terms of the mutual interactions among defects.Comment: 9 pages, 10 figure

Eric Trudel, La Terreur à l’œuvre, théorie, poétique et éthique chez Jean Paulhan

L’argomento così enunciato nel titolo pone in evidenza il concetto-cardine della riflessione di Jean Paulhan sull’interdipendenza, equivalenza e identità di pensiero e linguaggio, ovvero la denuncia di una sorta di Terrorismo che minaccia la Letteratura. In Les Fleurs de Tarbes (1941), un’opera alla quale si è dedicato a più riprese per trent’anni, J. Paulhan considera che, in fatto di Letteratura, vi sia stato un “divorzio” tra il pensiero e la parola. I Romantici, i Surrealisti e infine l’i..

Emergence of coherence in a uniform quasi-two-dimensional Bose gas

Phase transitions are ubiquitous in our three-dimensional world. By contrast most conventional transitions do not occur in infinite uniform two-dimensional systems because of the increased role of thermal fluctuations. Here we explore the dimensional crossover of Bose-Einstein condensation (BEC) for a weakly interacting atomic gas confined in a novel quasi-two-dimensional geometry, with a flat in-plane trap bottom. We detect the onset of an extended phase coherence, using velocity distribution measurements and matter-wave interferometry. We relate this coherence to the transverse condensation phenomenon, in which a significant fraction of atoms accumulate in the ground state of the motion perpendicular to the atom plane. We also investigate the dynamical aspects of the transition through the detection of topological defects that are nucleated in a quench cooling of the gas, and we compare our results to the predictions of the Kibble-Zurek theory for the conventional BEC second-order phase transition.Comment: main text = 24 pages, 6 figures + supplementary material = 10 pages, 5 figure

The walk and jump of Equisetum spores

International audienceEquisetum plants (horsetails) reproduce by producing tiny spherical spores that are typically 50 micrometers in diameter. The spores have four elaters, which are flexible ribbon-like appendages that are initially wrapped around the main spore body, and that deploy upon drying or fold back in humid air. If elaters are believed to help dispersal, the exact mechanism for spore motion remains unclear in the literature. In this manuscript we present observations of the 'walks' and 'jumps' of Equisetum spores, which are novel types of spore locomotion mechanisms compared to the ones of other spores. Walks are driven by humidity cycles, each cycle inducing a small step in a random direction. The dispersal range from the walk is limited, but the walk provides key steps to either exit the sporangium or to reorient and refold. Jumps occur when the spores suddenly thrust themselves after being tightly folded. They result in a very efficient dispersal: even spores jumping from the ground can catch again the wind, while non-jumping spores stay on ground. The understanding of these movements, which are solely driven by humidity variations, conveys biomimetic inspiration for a new class of self-propelled objects

Compensation of Beer-Lambert attenuation using non-diffracting Bessel beams

We report on a versatile method to compensate the linear attenuation in a medium, independently of its microscopic origin. The method exploits diffraction-limited Bessel beams and tailored on-axis intensity profiles which are generated using a phase-only spatial light modulator. This technique for compensating one of the most fundamental limiting processes in linear optics is shown to be efficient for a wide range of experimental conditions (modifying the refractive index and the attenuation coefficient). Finally, we explain how this method can be advantageously exploited in applications ranging from bio-imaging light sheet microscopy to quantum memories for future quantum communication networks

Il sogno e la scrittura automatica nella «Révolution Surréaliste» (1925)

Immaginazione e Realtà Corre un filo resistentissimo tra i Campi Magnetici (1920), la seconda serie della Rivista «Littérature» (1922-1923) e il Manifesto, preceduto da Poisson soluble, entrambi della fine del 1924, quando inizia il momento più luminoso del surrealismo allo stato nascente. La poesia, sempre percorsa da nuove folgorazioni, attinge ad insospettate riserve di energia. I surrealisti acquisiscono un linguaggio trasgressivo, ellittico, allusivo, accogliendo i segnali manifesti del ..