Blaizot J.-P.,CEA Saclay Nuclear Research Center
Reports on Progress in Physics | Year: 2017
The early stages of heavy ion collisions are dominated by high density systems of gluons that carry each a small fraction x of the momenta of the colliding nucleons. A distinguishing feature of such systems is the phenomenon of 'saturation' which tames the expected growth of the gluon density as the energy of the collision increases. The onset of saturation occurs at a particular transverse momentum scale, the 'saturation momentum', that emerges dynamically and that marks the onset of non-linear gluon interactions. At high energy, and for large nuclei, the saturation momentum is large compared to the typical hadronic scale, making high density gluons amenable to a description with weak coupling techniques. This paper reviews some of the challenges faced in the study of such dense systems of small x gluons, and of the progress made in addressing them. The focus is on conceptual issues, and the presentation is both pedagogical, and critical. Examples where high gluon density could play a visible role in heavy ion collisions are briefly discussed at the end, for illustration purpose. © 2017 IOP Publishing Ltd.
Berthier L.,CNRS Charles Coulomb Laboratory |
Biroli G.,CEA Saclay Nuclear Research Center |
Biroli G.,French National Center for Scientific Research
Reviews of Modern Physics | Year: 2011
A theoretical perspective is provided on the glass transition in molecular liquids at thermal equilibrium, on the spatially heterogeneous and aging dynamics of disordered materials, and on the rheology of soft glassy materials. We start with a broad introduction to the field and emphasize its connections with other subjects and its relevance. The important role played by computer simulations in studying and understanding the dynamics of systems close to the glass transition at the molecular level is given. The recent progress on the subject of the spatially heterogeneous dynamics that characterizes structural relaxation in materials with slow dynamics is reviewed. The main theoretical approaches are presented describing the glass transition in supercooled liquids, focusing on theories that have a microscopic, statistical mechanics basis. We describe both successes and failures and critically assess the current status of each of these approaches. The physics of aging dynamics in disordered materials and the rheology of soft glassy materials are then discussed, and recent theoretical progress is described. For each section, an extensive overview is given of the most recent advances, but we also describe in some detail the important open problems that will occupy a central place in this field in the coming years. © 2011 American Physical Society.
Barthelemy M.,CEA Saclay Nuclear Research Center |
Barthelemy M.,French School for Advanced Studies in the Social Sciences
Physics Reports | Year: 2011
Complex systems are very often organized under the form of networks where nodes and edges are embedded in space. Transportation and mobility networks, Internet, mobile phone networks, power grids, social and contact networks, and neural networks, are all examples where space is relevant and where topology alone does not contain all the information. Characterizing and understanding the structure and the evolution of spatial networks is thus crucial for many different fields, ranging from urbanism to epidemiology. An important consequence of space on networks is that there is a cost associated with the length of edges which in turn has dramatic effects on the topological structure of these networks. We will thoroughly explain the current state of our understanding of how the spatial constraints affect the structure and properties of these networks. We will review the most recent empirical observations and the most important models of spatial networks. We will also discuss various processes which take place on these spatial networks, such as phase transitions, random walks, synchronization, navigation, resilience, and disease spread. © 2010 Elsevier B.V.
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.
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.
Bruneval F.,CEA Saclay Nuclear Research Center
Physical Review Letters | Year: 2012
The random-phase approximation (RPA) is a promising approximation to the exchange-correlation energy of density functional theory, since it contains the van der Waals (vdW) interaction and yields a potential with the correct band gap. However, its calculation is computationally very demanding. We apply a range-separation concept to RPA and demonstrate how it drastically speeds up the calculations without loss of accuracy. The scheme is then successfully applied to a layered system subjected to weak vdW attraction and is used to address the controversy of the self-diffusion in silicon. We calculate the formation and migration energies of self-interstitials and vacancies taking into account atomic relaxations. The obtained activation energies deviate significantly from the earlier calculations and challenge some of the experimental interpretations: the diffusion of vacancies and interstitials has almost the same activation energy. © 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.
Thuery P.,CEA Saclay Nuclear Research Center
Inorganic Chemistry | Year: 2013
The reaction of uranyl nitrate hexahydrate with 2-sulfobenzoate (SB 2-) in the presence of various amines gave the series of complexes [UO2(SB)(H2O)] (1), [UO2(SB)(H 2O)]2·pyz (2), [2,2′-bipyH] 2[UO2(SB)2(H2O)]·4H 2O (3), [4,4′-bipyH2]2[UO 2(SB)2]2 (4), [4,4′-bipyH] 2[(UO2)2(SB)3(H2O)] ·4H2O (5), [NMe4]2[(UO2) 2(SB)3(H2O)1.15]·1.35H 2O (6), [NMe4]2[(UO2) 3(SB)2O2] (7), and [H2DABCO] 2[(UO2)5(SB)4O2(OH) 2]·4H2O (8), where pyz = pyrazine, bipy = bipyridine, and DABCO = 1,4-diazabicyclo[2.2.2]octane, with all compounds but 5 having been obtained under hydrothermal conditions. The crystal structures of these complexes display a common motif in which uranyl is chelated by the carboxylate and sulfonate groups of SB, giving a seven-membered ring. Structure-directing effects due to the amine and the presence in 7 and 8 of additional μ3-oxo or μ2-hydroxo bridges result in much structural variety, with different bridging by the carboxylate and sulfonate groups giving rise to zero- (3, 4), one- (1, 5-8), or two-dimensional (2) assemblies. Some unusual uranyl secondary building units are observed, such as the pentanuclear [(UO2)5O2(OH)2] discrete motif. Addition of 3d-block metal cations (Cu2+, Ni 2+) in the presence of nitrogen donors gave the heterometallic molecular complex [UO2Cu(SB)2(2,2′-bipy) 2]2·2H2O (9), the heterogeneous compound [Cu(4,4′-bipy)(H2O)3]2[UO 2(SB)2]2·2H2O (10), in which molecular uranyl dimers are encompassing copper-containing chains, and the heterometallic one-dimensional polymers [(UO2)2Cu 2(SB)4(bipym)(H2O)4] (11) and [UO2Ni(SB)2(bipym)(H2O)2] ·3H2O (12), where bipym = bipyrimidine. The latter two complexes display two different arrangements: in 11, bipym bridges two [UO 2Cu(SB)2] chains to give a ladderlike assembly, while the uranyl cations are merely decorating species in 12. In contrast to those of phosphonates, the actinide complexes of sulfonates in the solid state have been little investigated up to now. The present results show that sulfocarboxylates such as 2-sulfobenzoate, in which sulfonate coordination is promoted by chelate effects, are of interest in the synthesis of uranyl-organic coordination polymers. © 2012 American Chemical Society.
Bonamy D.,CEA Saclay Nuclear Research Center |
Bouchaud E.,CEA Saclay Nuclear Research Center
Physics Reports | Year: 2011
While there exists a unified theoretical framework-Linear Elastic Fracture Mechanics-to describe the failure of homogeneous materials, understanding and modeling the mechanical properties of heterogeneous media continue to raise significant fundamental challenges. Stress enhancement in the vicinity of cracks indeed makes classical homogenization methods irrelevant to predict the toughness and lifetime of heterogeneous materials. "Mean field" approaches have been proposed to estimate these quantities, but they remain limited to dilute damage. Numerical simulations do not suffer from such limitations, and disorder can be tuned continuously. Molecular Dynamics simulations allow one to characterize damage and fracture in amorphous materials at the nanoscale, i.e. at the scale of their inhomogeneities. However, these simulations are up to now limited to dynamic fracture, which confers further complexity to the observed mechanisms. A "minimalist" approach consists in exploiting the analogy between scalar mode III elasticity and electricity through the study of random fuse networks breakdown. In two dimensions, powerful algorithms can compute the exact stress field in an elastic medium containing cracks of arbitrary shapes. However, although these tools have been useful in solving some classical problems (e.g. size dependence of materials strength), a clear predictive unified theoretical framework is still missing. An efficient theory should be able to predict, a minima, the morphology of fracture surfaces, which encodes the interaction between the propagating crack front and the surrounding microstructure. We provide a review of recent quantitative fractography experiments. The most striking observations in this field is the existence of universal morphological features, independent of both the material and the loading conditions, reminiscent of interface growth problems. In this context, we analyze models which describe the crack front as an elastic line that propagates in a random potential. In these models, the onset of crack propagation is interpreted as a dynamic phase transition, while sub-critical crack growth is assimilated to thermally assisted depinning. © 2010 Elsevier B.V.