FLUCTUATION EFFECTS NEAR THE UPPER CRITICAL MAGNETIC FIELD IN HIGH-TcSUPERCONDUCTORS

Abstract

The effect of fluctuations on the thermodynamics of high-Tc superconductors near the upper critical magnetic field is studied. Lattice effects and, in particular, Harper broadening and splitting of the Landau levels are shown to reduce the problem in consideration to some renormalizable field theory with q-component, a complex order parameter. The number q is determined by the relation Φ/Φ0=p/q, where Φ is the magnetic flux through the plaquette, p and q are integral numbers, Φ0=hc/e is the Onsager flux quantum. A critical behaviour of the model for small values q=1, 2, 3, 4 is analysed and the phase transition is shown to be of the second order. A crossover from one- to three-dimensional fluctuation regime is analysed. The case of q→∞ is qualitatively discussed too.