Low frequency elastic measurements on solid 4^{4}He in Vycor using a torsional oscillator

Torsional oscillator experiments involving solid $^{4}$He confined in the nanoscale pores of Vycor glass showed anomalous frequency changes at temperatures below 200 mK. These were initially attributed to decoupling of some of the helium's mass from the oscillator, the expected signature of a supersolid. However, these and similar anomalous effects seen with bulk $^{4}$He now appear to be artifacts arising from large shear modulus changes when mobile dislocations are pinned by $^{3}$He impurities. We have used a torsional oscillator (TO) technique to directly measure the shear modulus of the solid $^{4}$He/Vycor system at a frequency (1.2 kHz) comparable to that used in previous TO experiments. The shear modulus increases gradually as the TO is cooled from 1 K to 20 mK. We attribute the gradual modulus change to the freezing out of thermally activated relaxation processes in the solid helium. The absence of rapid changes below 200 mK is expected since mobile dislocations could not exist in pores as small as those of Vycor. Our results support the interpretation of a recent torsional oscillator experiment that showed no anomaly when elastic effects in bulk helium were eliminated by ensuring that there were no gaps around the Vycor sample.Comment: Accepted by Journal of Low Temperature Physic

Critical dislocation speed in helium-4 crystals

Our experiments show that in $^4$He crystals, the binding of $^3$He impurities to dislocations does not necessarily imply their pinning. Indeed, in these crystals, there are two different regimes of the motion of dislocations when impurities bind to them. At lowdriving strain $\epsilon$ and frequency $\omega$, where the dislocation speed is less than a critical value (45 $\mu$m/s), dislocations and impurities apparently move together. Impurities really pin the dislocations only at higher values of $\omega$. The critical speed separating the two regimes is two orders of magnitude smaller than the average speed of free $^3$He impurities in the bulk crystal lattice.We obtained this result by studying the dissipation of dislocation motion as a function of the frequency and amplitude of a driving strain applied to a crystal at low temperature. Our results solve an apparent contradiction between some experiments, which showed a frequency-dependent transition temperature from a soft to a stiff state, and other experiments or models where this temperature was assumed to be independent of frequency. The impurity pinning mechanism for dislocations appears to be more complicated than previously assumed

Dislocation networks in helium-4 crystals

The mechanical behavior of crystals is dominated by dislocation networks, their structure and their interactions with impurities or thermal phonons. However, in classical crystals, networks are usually random with impurities often forming non-equilibrium clusters when their motion freezes at low temperature. Helium provides unique advantages for the study of dislocations: crystals are free of all but isotopic impurities, the concentration of these can be reduced to the ppb level, and the impurities are mobile at all temperatures and therefore remain in equilibrium with the dislocations. We have achieved a comprehensive study of the mechanical response of 4He crystals to a driving strain as a function of temperature, frequency and strain amplitude. The quality of our fits to the complete set of data strongly supports our assumption of string-like vibrating dislocations. It leads to a precise determination of the distribution of dislocation network lengths and to detailed information about the interaction between dislocations and both thermal phonons and 3He impurities. The width of the dissipation peak associated with impurity binding is larger than predicted by a simple Debye model, and much of this broadening is due to the distribution of network lengths.Comment: accepted by Phys. Rev.

Note on the dissipation for the general Muskat problem

We consider the dissipation of the Muskat problem and we give an elementary proof of a surprising inequality of Constantin-Cordoba-Gancedo-Strain which holds in greater generality

Critical well-posedness for the 2D Peskin problem with general tension

In this paper, we study the two dimensional Peskin problem with general elasticity law. Specifically, we prove global regularity for small perturbations, in suitable critical spaces, of the circle solution, possibly containing corners. For such initial data we prove asymptotic stability in the sense that as $t\to\infty$, the solution converges to a translated and rotated disk.Comment: 33 pages, 2 figure

La trace du geste en peinture à la lumière des sciences cognitives

Jusqu’à la seconde moitié du XIXe, la présence physique de l’artiste, facture apparente, trace du geste, touche visible, ne devait pas apparaître dans l’œuvre finie, ou de manière discrète. Or à partir de la fin du XIXe, avec la révolution Impressionniste, la peinture évolue vers une présence physique de l’artiste dans son œuvre de plus en plus visible. La « touche », ces traces furtives de gestes, apportent à la représentation cet aspect fugitif, dans des œuvres qui tentaient souvent de capturer l’instant, comme chez Claude Monet et Berthe Morisot. Les Pointillistes, au contraire, cherchaient, par le point, à rendre compte d’un instant éternel, immobile. Pour Vincent Van Gogh, tout ce qui témoigne de l’œuvre en train de se faire donne à l’œuvre un résultat plus naturel qu’une image où la trace de son auteur a disparu. Francis Bacon trouve aussi qu’une peinture réalisée de manière immédiate, « touche plus violemment le système nerveux ». A la même époque, avec l’Action Painting de Jackson Pollock, le geste et sa conséquence sur la matière picturale deviennent les sujets même des peintures. De nos jours, grâce aux neurosciences, nous connaissons bien mieux le fonctionnement de notre perception. Nous savons que les éléments non représentatifs d’une image (texture, traces, touches apparentes), ne perturbent pas notre compréhension de la représentation, et nous fournissent des informations que notre cerveau interprète : la trace du geste donne des informations motrices que nous percevons comme telles. Les différentes textures nous font pénétrer au cœur du processus de fabrication matérielle de l’œuvre. Cet article mettra en parallèle ce que certains artistes, par leur recherche et leur pratique, ont pu mettre en lumière quant au rôle du geste dans la création, et la réception artistique. Les neurosciences nous apportent les connaissances physiologiques qui expliquent dans une certaine mesure ce que la trace du geste apporte à l’expérience d’une œuvre.Up to the late nineteenth century, the physical presence of the artist and/or the trace of his gestures, were not supposed to appear in his finished works. Then, the development of Impressionism favoured the visible presence of the artists in their works. The traces, the fugitive remnants of the artists’ gestures, brought to the work the transient aspect sought for by Impressionist painters, who often tried to seize the moment, such as Claude Monet or Berthe Morisot. On the contrary, Pointillists were looking to capture the eternity of a motionless moment. For Van Gogh, anything that reminded the spectator of the making of the work added a natural touch to the works. Francis Bacon argued that our nervous system was more affected by an art work in which the immediacy of the painter’s action was obvious. With Jackson Pollock and Action Painting, the gesture itself becomes the subject of the painting. Today, with the development of neuroscience, perception is better known, and scientists have shown that the non-representative elements of an image (traces, texture, etc) do not disturb our understanding of it, and may even enhance it by giving us information that the brain interprets. The traces left by gestures give us clues to the way the image was produced, taking us into the heart of the creative process. This article will try to analyze how artists interpreted the role of gestures within their creative process, and how reception confirmed or not their ideas. Neurosciences contribute to our understanding of art appreciation

Study of Coil Configuration and Local Optics Effects for the GaToroid Ion Gantry Design

GaToroid, a novel configuration for hadron therapy gantry, is based on superconducting coils that gen- erate a toroidal magnetic field to deliver the beam onto the patient. Designing the complex GaToroid coils requires careful consideration of the local beam optical effects. We present a Python-based tool for charged particle transport in complex electromagnetic fields. The code implements fast tracking in arbitrary three-dimensional field maps, and it is not limited to specific or regular reference trajectories, as is generally the case in accelerator physics. The tool was used to characterise the beam behaviour inside the GaToroid system. It automatically determines the reference trajectories in the symmetry plane and analyses three-dimensional beam dynamics around these trajectories. Beam optical parameters in the field region were compared for various magnetic configurations of GaToroid. This paper introduces the new tracker and shows the benchmarking results. Furthermore, first- order beam optics studies for different arrangements demonstrate the main code features and serve for the design optimisation

Traveling water waves — the ebb and flow of two centuries

This survey covers the mathematical theory of steady water waves with an emphasis on topics that are at the forefront of current research. These areas include: variational characterizations of traveling water waves; analytical and numerical studies of periodic waves with critical layers that may overhang; existence, nonexistence, and qualitative theory of solitary waves and fronts; traveling waves with localized vorticity or density stratification; and waves in three dimensions

Small cells lung epidermoid carcinoma in a HTLV1-infected patient: case report and literature review

The human T cell lymphotropic virus type 1 (HTLV-1) is the first human retrovirus discovered. Since then, it has spread worldwide and is mainly associated with adult T cell leukemia/lymphoma (ATLL) and HTLV1-associated myelopathy (HAM). Its relationship, however, with other types of cancer is controversial. We describe the case of a patient presenting with small cells lung epidermoid carcinoma who had recently developed HAM, and a review of the literature related to these conditions. This is the first case of this type of lung cancer, the same of the first description in the literature, associated with HAM outside Japan