Luzum M.,CEA Saclay Nuclear Research Center
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2011
Making use of recently released data on dihadron correlations by the STAR Collaboration, I analyze the long-range ("ridge-like") part of these data and show that the dependence on both transverse momentum as well as orientation with respect to the event plane are consistent with correlations expected from only collective flow. In combination with previously analyzed centrality-dependent data, they provide strong evidence that only collective flow effects are present at large relative pseudorapidity. In contrast, by analyzing a "background subtracted" signal, the authors presenting the new data concluded that the ridge-like part of the measured correlation could not in fact be entirely generated from collective flow of the medium. I explain the discrepancy and illustrate some pitfalls of using the ZYAM prescription to remove flow background. © 2011 Elsevier B.V.
Luzum M.,CEA Saclay Nuclear Research Center
Physical Review C - Nuclear Physics | Year: 2011
I compare the first viscous hydrodynamic prediction for integrated elliptic flow in Pb-Pb collisions at the Large Hadron Collider with the first data released by the ALICE Collaboration. These new data are found to be consistent with hydrodynamic extrapolations of the Relativistic Heavy-Ion Collider data with no change in medium parameters (e.g., average viscosity). I also discuss how, in general, a precise comparison of data to theoretical calculations requires an understanding of some subtleties of the measurement-most notably the cut on transverse momentum of the particles used and the differing sensitivities to flow fluctuations and nonflow effects of the various measurement methods. © 2011 American Physical Society.
Varoquaux E.,CEA Saclay Nuclear Research Center
Reviews of Modern Physics | Year: 2015
Nearly five decades have elapsed since the seminal 1966 paper of P.W. Anderson on the flow of superfluid helium, He4 at that time. Some of his "considerations" - the role of the quantum phase as a dynamical variable, the interplay between the motion of quantized vortices and potential superflow, its incidence on dissipation in the superfluid and the appearance of critical velocities, the quest for the hydrodynamic analogs of the Josephson effects in helium - and the way they have evolved over the past half century are recounted in this review. But it is due to key advances on the experimental front that phase slippage could be harnessed in the laboratory, leading to a deeper understanding of superflow, vortex nucleation, the various intrinsic and extrinsic dissipation mechanisms in superfluids, macroscopic quantum effects, and the superfluid analog of both ac and dc Josephson effects - pivotal concepts in superfluid physics - have been performed. Some of the experiments that have shed light on the more intimate effect of quantum mechanics on the hydrodynamics of the dense heliums are surveyed, including the nucleation of quantized vortices both by Arrhenius processes and by macroscopic quantum tunneling, the setting up of vortex mills, and superfluid interferometry. © 2015 American Physical Society.
Crocombette J.-P.,CEA Saclay Nuclear Research Center
Physical Review B - Condensed Matter and Materials Physics | Year: 2012
The variation of formation energies of point defects and clusters in uranium dioxide (UO 2) as a function of their charge states is studied by density functional theory (DFT). Di- and trivacancies are considered as well as various assemblies of oxygen interstitials, namely the cuboctahedral defect (either empty or filled) and the split-di-interstitial. The energies of formation of these defects for various possible charge states are calculated using the DFT+U approach. The occurrence of multiple minima is circumvented by the use of the U-ramping technique. One finds that point defects and vacancy clusters bear their formal charges, deduced from the ionic picture of bonding in UO 2. Conversely, clusters of oxygen interstitials are much less charged than this fully ionic limit. The energy gain upon clustering is vastly modified when the possible charge of defects is taken into account. Vacancy clusters prove only marginally stable compared to their isolated counterparts. Clusters of oxygen interstitials are found energetically unstable with respect to isolated interstitials in the stoichiometric compound. © 2012 American Physical Society.
Ephritikhine M.,CEA Saclay Nuclear Research Center
Organometallics | Year: 2013
The ubiquity of the cyclopentadienyl ligand permits us to use its complexes as representative examples for the description of recent highlights in organometallic and more generally in coordination chemistry of the actinides. Uranium(III) complexes exhibit a remarkable reactivity, especially in the activation of small molecules, and are valuable precursors of higher valent derivatives. Using redox-active ligands led to the design of reactive complexes which have been considered as "synthons" of AnII and AnIII (An = Th, U). Studies of low-valent compounds gave a better insight into lanthanide(III)/actinide(III) differentiation. Organoactinide(IV) complexes with the bis-Cp* platform play a major role in the synthesis of a variety of compounds containing single and double metal-ligand bonds, revealing novel structures and reactions. The bis(cyclopentadienyl) uranium(IV) and thorium(IV) complexes were also found to be quite efficient in catalytic processes. Cyclopentadienyl complexes afford systems in which actinide ions potentially engage in magnetic exchange interactions. Organoactinide complexes in the +5 and +6 oxidation states remain relatively rare, and most of these are cyclopentadienyl derivatives with oxo and imido ligands. © 2013 American Chemical Society.