Machine learning for many-body physics: The case of the Anderson impurity model

Machine learning methods are applied to finding the Green's function of the Anderson impurity model, a basic model system of quantum many-body condensed-matter physics. Different methods of parametrizing the Green's function are investigated; a representation in terms of Legendre polynomials is found to be superior due to its limited number of coefficients and its applicability to state of the art methods of solution. The dependence of the errors on the size of the training set is determined. The results indicate that a machine learning approach to dynamical mean-field theory may be feasible.Comment: 18 pages, 11 figures. Sections II. A and B have been modified and an appendix was adde

Les macroiinvertébrés dulçaquicoles de Nuku-Hiva (Marquises) : compte-rendu d'une mission en saison des pluies et proposition d'un protocole de surveillance

Dans le cadre du programme de lutte contre #Simulium buissoni, les macroinvertébrés des cours d'eau de l'île de Nuku-Hiva (Marquises) ont été échantillonnés. La faune apparaît pauvre et peu diversifiée, résultat probable des dimensions réduites et de la localisation de l'île, ainsi que des conditions hydrologiques particulièrement difficiles dans ces cours d'eau. Les Mollusques Gastéropodes et les Crustacés Décapodes occupent une place très importante dans les peuplements invertébrés et méritent d'être particulièrement étudiés. Les chevrettes (#Macrobrachium spp.) représentent le seul groupe exploité parmi ces invertébrés et exigent, à ce titre, d'être plus spécialement observées. Un protocole d'étude et de surveillance des peuplements invertébrés, en relation avec les traitements insecticides et constitué de quatre phases chronologiques, est ensuite proposé. (Résumé d'auteur

Half-integer Shapiro steps at the 0-pi crossover of a ferromagnetic Josephson junction

We investigate the current-phase relation of S/F/S junctions near the crossover between the 0 and the pi ground states. We use Nb/CuNi/Nb junctions where this crossover is driven both by thickness and temperature. For a certain thickness a non-zero minimum of critical current is observed at the crossover temperature. We analyze this residual supercurrent by applying a high frequency excitation and observe the formation of half-integer Shapiro steps. We attribute these fractional steps to a doubling of the Josephson frequency due to a sin(2*phi) current-phase relation. This phase dependence is explained by the splitting of the energy levels in the ferromagnetic exchange field.Comment: 4 pages, 5 figures, accepted for publication in Phys. Rev. Let

Travelling and Standing Waves in a Spatially Forced 2D Convection Experiment

International audienceRayleigh-Bénard convection is studied in a rectangular geometry with a spatial forcing induced in one direction by electric wires. When using fluids of relatively large Prandtl numbers, this forcing allows the existence of a perfect one-dimensional pattern until the onset of bimodal convection. The transition to bidimensional convection is studied for increasing Rayleigh number and reveals the existence of different spatio-temporal regimes depending on the value of the forcing. At the onset of the transition, a stationary pattern is observed for weak forcing, while travelling waves are evidenced for strong forcing. Both behaviours give place to collective oscillations at higher Rayleigh number

Wave chaos in rapidly rotating stars

Effects of rapid stellar rotation on acoustic oscillation modes are poorly understood. We study the dynamics of acoustic rays in rotating polytropic stars and show using quantum chaos concepts that the eigenfrequency spectrum is a superposition of regular frequency patterns and an irregular frequency subset respectively associated with near-integrable and chaotic phase space regions. This opens new perspectives for rapidly rotating star seismology and also provides a new and potentially observable manifestation of wave chaos in a large scale natural system.Comment: 5 pages, 3 figures; accepted for publication in Phys. Rev.

Another derivation of the geometrical KPZ relations

We give a physicist's derivation of the geometrical (in the spirit of Duplantier-Sheffield) KPZ relations, via heat kernel methods. It gives a covariant way to define neighborhoods of fractals in 2d quantum gravity, and shows that these relations are in the realm of conformal field theory

Sodium recycling at Europa: what do we learn from the sodium cloud variability ?

International audienceWe study the ejection of sodium atoms from Europa's surface by both magnetospheric ion and electron sputtering and desorption stimulated by UV solar photons. The depletion of the surface by ejection and its enrichment by redeposition of sodium atoms are described. The redistribution of sodium atoms at the surface induced by photo-stimulated desorption from the dayside and by sputtering ejection from the trailing hemisphere cannot explain the observed variation of the Na emission brightness. However, a transient increase of the sputtering rate due to a plasma injection may explain such an increase. The relationship between the sodium surface content and the sodium exosphere are also discussed