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2007 2003 2000

4 publications found . Showing records 1 to 4 | html | bibtex

title      = "An interaction model on random networks: 
              from social to airports networks",
author     = "Philippe Curty",
abstract   = "Social movements, neurons in the brain 
              or even industrial suppliers are best described by agents 
              evolving on networks with basic interaction rules. In these 
              real systems, the connectivity between agents corresponds to the 
              a critical state 
              of the system related to the 
              of the system. The new idea is that 
              connectivity adjusts itself because of two opposite tendencies: on 
              the one hand informations percolation is better when the 
              network connectivity is small but all agents have rapidely 
              the same state and the dynamics stops. On the 
              other hand, when agents have a large connectivity, the 
              state of a node (opinion of a person, 
              of a neuron, ...) tends to freeze: agents find 
              always a minority among their neighbours to support their 
              state. The model introduced here captures this essential feature 
              showing a clear transition between the two tendencies at 
              some critical connectivity. Depending on the noise, the dynamics 
              of the system can only take place at a 
              precise critical connectivity since, away from this critical point, 
              the system remains in a static phase. When the 
              noise is very small, the critical connectivity becomes very 
              large, and highly connected networks are obtained like the 
              airports network and the Internet. This model may be 
              used as a starting point for understanding the evolution 
              of agents living on networks.",
status     = "preprint",
year       = "2007"
title      = "Amplitude and Phase Fluctuations in High Temperature Superconductors",
author     = "Philippe Curty",
abstract   = "We begin this work by an 
              overview of selected topics of phase transitions and condensed 
              matter physics, then we have the following chapters: Chapter 
              3 is devoted to the study the reciprocal influence 
              between the phase  and the amplitude |Psi| of 
              the complex field Psi=|Psi| exp(i Phi) in the Ginzburg-Landau 
              (GL) functional. This functional contains two parts: the amplitude 
              part, involving only the amplitude |Psi| and a coupling 
              constant coming from the phase part, and the phase 
              part, XY like, with a coupling 
              constant coming from 
              the amplitude part. The essential result of this chapter 
              is a new approach for solving the GL functional 
              integral by separating amplitude and phase Phi. One important 
              consequence is the possibility of a first order transition 
              (that is a jump of the order parameter) at 
              the transition temperature. The aim of the chapter 4 
              is to focus on the problem of the pseudogap 
              phase of underdoped high temperature superconductors. The starting point 
              will be a pairing hamiltonian for fermions like in 
              BCS theory [4]. Using the Hubbard-Stratonovich transformation with a 
              complex pairing field, the main goal will be to 
              take into account both amplitude and phase influence on 
              the electronic properties. One of the results is that 
              the mean amplitude of the pairing field remains large 
              at high temperature: it is never zero because of 
              fluctuations especially in the underdoped regime where the charge 
              carrier density is low. Phase fluctuations are still correlated 
              above Tc until some crossover temperature T which is 
              typically 30% above Tc. Comparison with measured specific heat 
              on underdoped YBCO reproduces the double peak structure: a 
              sharp peak at Tc coming from phase fluctuations and 
              a wide hump above Tc rounded by amplitude fluctuations. 
              The spin susceptibility, related to the amplitude, recovers its 
              normal behaviour near the temperature T whereas the orbital 
              magnetic susceptibility, related to the phases, disappears near T. 
              All these findings provide additional evidence for the fact 
              that superconductivity and pseudogap have the same origin. The 
              former is primarily related to phases of the pairing 
              field, which order below the transition temperature and whose 
              correlations survive over a limited temperature region 
              above Tc 
              until T. The pseudogap regime of underdoped materials then 
              extends to much higher temperatures thanks to the persisting 
              amplitude fluctuations of the pairing field.",
journal    = "RERO",
status     = "published",
pages      = "1784",
year       = "2003"
title      = "Thermodynamics and phase diagram of high temperature superconductors.",
author     = "Philippe Curty and  Hans Beck",
abstract   = "Thermodynamic quantities are derived for superconducting 
              and pseudogap regimes by taking into account both amplitude 
              and phase fluctuations of the pairing field. In the 
              normal (pseudogap) state of the underdoped cuprates, two domains 
              have to be distinguished: near the superconducting region, phase 
              correlations are important up to temperature T(phi). Above T(phi), 
              the pseudogap region is determined only by amplitudes, and 
              phases are uncorrelated. Our calculations show excellent quantitative agreement 
              with specific heat and magnetic susceptibility experiments on cuprates. 
              We find that the mean field temperature T0 has 
              a similar doping dependence as the pseudogap temperature T(*), 
              whereas the pseudogap energy scale is given by the 
              average amplitude above T(c).",
journal    = "Physical review letters",
status     = "published",
volume     = "91",
number     = "25",
pages      = "257002",
year       = "2003"
title      = "First Order Transition in the Ginzburg-Landau Model",
author     = "Philippe Curty and  Hans Beck",
abstract   = "The d-dimensional complex Ginzburg-Landau (GL) model 
              is solved according to a variational method by separating 
              phase and amplitude. The GL transition becomes first order 
              for high superfluid density because of effects of phase 
              fluctuations. We discuss its origin with various arguments showing 
              that, in particular for d = 3, the validity 
              of our approach lies precisely in the first order domain.",
journal    = "Phys. Rev. Lett.",
status     = "published",
volume     = "85",
pages      = "796",
year       = "2000"