CEA Saclay Nuclear Research Center
CEA Saclay Nuclear Research Center
Le Bihan D.,CEA Saclay Nuclear Research Center |
Le Bihan D.,Kyoto University
NeuroImage | Year: 2012
Diffusion MRI has been introduced in 1985 and has had a very successful life on its own. While it has become a standard for imaging stroke and white matter disorders, the borders between diffusion MRI and the general field of fMRI have always remained fuzzy. First, diffusion MRI has been used to obtain images of brain function, based on the idea that diffusion MRI could also be made sensitive to blood flow, through the intravoxel incoherent motion (IVIM) concept. Second, the IVIM concept helped better understand the contribution from different vasculature components to the BOLD fMRI signal. Third, it has been shown recently that a genuine fMRI signal can be obtained with diffusion MRI. This "DfMRI" signal is notably different from the BOLD fMRI signal, especially for its much faster response to brain activation both at onset and offset, which points out to structural changes in the neural tissues, perhaps such as cell swelling, occurring in activated neural tissue. This short article reviews the major steps which have paved the way for this exciting development, underlying how technical progress with MRI equipment has each time been instrumental to expand the horizon of diffusion MRI toward the field of fMRI. © 2011 Elsevier Inc.
Zdeborova L.,CEA Saclay Nuclear Research Center |
Krzakala F.,University Pierre and Marie Curie
Advances in Physics | Year: 2016
Many questions of fundamental interest in today's science can be formulated as inference problems: some partial, or noisy, observations are performed over a set of variables and the goal is to recover, or infer, the values of the variables based on the indirect information contained in the measurements. For such problems, the central scientific questions are: Under what conditions is the information contained in the measurements sufficient for a satisfactory inference to be possible? What are the most efficient algorithms for this task? A growing body of work has shown that often we can understand and locate these fundamental barriers by thinking of them as phase transitions in the sense of statistical physics. Moreover, it turned out that we can use the gained physical insight to develop new promising algorithms. The connection between inference and statistical physics is currently witnessing an impressive renaissance and we review here the current state-of-the-art, with a pedagogical focus on the Ising model which, formulated as an inference problem, we call the planted spin glass. In terms of applications we review two classes of problems: (i) inference of clusters on graphs and networks, with community detection as a special case and (ii) estimating a signal from its noisy linear measurements, with compressed sensing as a case of sparse estimation. Our goal is to provide a pedagogical review for researchers in physics and other fields interested in this fascinating topic. © 2016 Informa UK Limited, trading as Taylor & Francis Group.
Le Bihan D.,CEA Saclay Nuclear Research Center |
Johansen-Berg H.,John Radcliffe Hospital
NeuroImage | Year: 2012
Diffusion MRI (or dMRI) came into existence in the mid-1980s. During the last 25. years, diffusion MRI has been extraordinarily successful (with more than 300,000 entries on Google Scholar for . diffusion MRI). Its main clinical domain of application has been neurological disorders, especially for the management of patients with acute stroke. It is also rapidly becoming a standard for white matter disorders, as diffusion tensor imaging (DTI) can reveal abnormalities in white matter fiber structure and provide outstanding maps of brain connectivity. The ability to visualize anatomical connections between different parts of the brain, non-invasively and on an individual basis, has emerged as a major breakthrough for neurosciences. The driving force of dMRI is to monitor microscopic, natural displacements of water molecules that occur in brain tissues as part of the physical diffusion process. Water molecules are thus used as a probe that can reveal microscopic details about tissue architecture, either normal or in a diseased state. © 2011 Elsevier Inc.
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.
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.