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2007 2003 2000

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@phdthesis\{-unpub-1,
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"
}