Amplitude and Phase Fluctuations in High Temperature Superconductors
| [Thèse de doctorat - Université de Neuchâtel, 2003 ; nº1784] [PDF source (1.69MB)]
|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 . 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.