Management control – International Observatory 2014

International audienc

Fine structure of excitons in Cu2_2O

Three experimental observations on 1s-excitons in Cu$_2$O are not consistent with the picture of the exciton as a simple hydrogenic bound state: the energies of the 1s-excitons deviate from the Rydberg formula, the total exciton mass exceeds the sum of the electron and hole effective masses, and the triplet-state excitons lie above the singlet. Incorporating the band structure of the material, we calculate the corrections to this simple picture arising from the fact that the exciton Bohr radius is comparable to the lattice constant. By means of a self-consistent variational calculation of the total exciton mass as well as the ground-state energy of the singlet and the triplet-state excitons, we find excellent agreement with experiment.Comment: Revised abstract; 10 pages, revtex, 3 figures available from G. Kavoulakis, Physics Department, University of Illinois, Urban

Quantum saturation and condensation of excitons in Cu2_2O: a theoretical study

Recent experiments on high density excitons in Cu$_2$O provide evidence for degenerate quantum statistics and Bose-Einstein condensation of this nearly ideal gas. We model the time dependence of this bosonic system including exciton decay mechanisms, energy exchange with phonons, and interconversion between ortho (triplet-state) and para (singlet-state) excitons, using parameters for the excitonic decay, the coupling to acoustic and low-lying optical phonons, Auger recombination, and ortho-para interconversion derived from experiment. The single adjustable parameter in our model is the optical-phonon cooling rate for Auger and laser-produced hot excitons. We show that the orthoexcitons move along the phase boundary without crossing it (i.e., exhibit a ``quantum saturation''), as a consequence of the balance of entropy changes due to cooling of excitons by phonons and heating by the non-radiative Auger two-exciton recombination process. The Auger annihilation rate for para-para collisions is much smaller than that for ortho-para and ortho-ortho collisions, explaining why, under the given experimental conditions, the paraexcitons condense while the orthoexcitons fail to do so.Comment: Revised to improve clarity and physical content 18 pages, revtex, figures available from G. Kavoulakis, Physics Department, University of Illinois, Urban

Propagation d'ondes électromagnétiques de basse fréquence dans un métal compensé : l'étain

We first study the different electromagnetic modes that can propagate at low temperature (4.2 °K) and high magnetic fields (5 to 55 kgauss), in pure single crystals. We then verify experimentally by surface impedance measurements the existence of such modes in tin.Nous étudions les divers modes électromagnétiques susceptibles de se propager à basse température (4,2 °K), à haut champ magnétique (de 5 à 55 kgauss), dans des échantillons métalliques monocristallins et purs. Nous vérifions expérimentalement par la méthode de l'impédance de surface l'existence de ces modes dans l'étain

HYDRODYNAMIC INSTABILITIES IN THE ROTATING COUETTE FLOW OF SUPERFLUID HELIUM

Nous présentons une étude expérimentale des différents régimes d'écoulement de l'hélium superfluide en géométrie de Couette. Nous avons mesuré l'atténuation de résonances de 2ème son pour différentes valeurs de températures et d'espacements entre cylindres en fonction de la vitesse de rotation. Nous avons ainsi successivement identifié trois différents régimes conduisant à l'écoulement turbulent.We present an experimental study of the different flow regimes of superfluid helium in the Couette geometry. We have measured the attenuation of second sound resonances with the rotation velocity for different temperature values and spacings between cylinders. Three flow regimes leading to turbulent flow have been successively identified

Multiple scale structure of non wetting fluid invasion fronts in 3D model porous media

We report experimental measurements on the structure of invasion fronts obtained by slow vertical injection of liquid, non wetting, Wood metal into non consolidated crushed glass. The geometry of plane cuts of the invasion front both parallel and perpendicular to the flow direction is analysed. The structure of perpendicular cuts is self similar in a length scale range of 1 to 20 with a fractal dimension 1.65. Saturation profiles measured from parallel cuts are in qualitative agreement with invasion percolation models. Possible theoretical predictions for the geometry of horizontal cuts are discussed.Nous présentons des résultats de mesures expérimèntales de la structure de fronts d'invasion obtenus par une injection verticale lente de métal de Wood non mouillant dans un massif de verre pilé non consolidé. Nous analysons la géométrie de coupes du front d'invasion perpendiculaires et parallèles à la direction de l'écoulement. La structure des coupes perpendiculaires est autohomothétique dans une gamme d'échelles de longueur de 1 à 20 avec une dimension fractale de 1,65. Les profils de saturation obtenus à partir de coupes parallèles à l'écoulement sont qualitativement en accord avec les modèles de percolation. Nous discutons différentes prédictions théoriques sur la géométrie des coupes perpendiculaires à l'écoulement

Experimental analysis of fracture rugosity in granular and compact rocks

We compare high resolution roughness measurements on fractured granite, basalt and sandstone samples. For granite and basalt, the fractures display over the full experimental length scale range a self-affine geometry with a characteristic Hurst exponent consistent with the value 0.8 suggested to be universal by several authors. For sandstone, the rugosity spectrum reMayns independent on the fracturation velocity but displays self-affine characteristics only for characteristic length scales roughly larger than the grain size: furthermore, the corresponding Hurst exponent takes a value 0.47 ± 0.05 distinctly smaller than the previous one. At characteristic length scales below the grain size, a clear cutoff is observed in the spectrum. This cut-off is shown through numerical simulations to be largely explainable by the faceted shape of the sand grains and to a minor degree by the finite size of the profilometer sensor tip. None of these factors seems to influence the low frequency part of the spectrum from which Hurst's exponent is determined. The differences between the values of Hurst's exponent may reflect the intergranular nature of fracture in the sandstone samples while no effect of structural heterogeneities on fracture propagation is observed in granite