Extreme type II superconductors. Universal properties and trends

Abstract

We shall review and analyze experimental results of the specific heat, chemical potential, muon-spin (μSR)-relaxation rate and fluctuation contributions to the magnetization and DC conductivity of extreme type II superconductors in the context of critical phenomena. Our estimates of critical exponents and amplitudes, together with measured scaling behavior and the consistency with the universal relations between the critical amplitudes, provide considerable evidence for a three-dimensional xy-critical point. The associated order parameter is a complex scalar describing singlet and s-wave pairing. The estimated volume of the critical correlation length amplitudes turns out to be comparable to that in superfluid helium and is several orders of magnitude smaller than in BCS superconductors. Moreover, motivated by the Uemura plot, which relates the measured transition temperature (Tc) to the zero-temperature μSR-relaxation rate, and by the hyperuniversal relation between Tc and critical amplitudes of the London penetration depth and phase correlation length, we propose a simple ansatz, where the rescaled transition temperature and μSR-relaxation rate should fall on a single parabola. Our analysis of the μSR data reveals excellent agreement with this scaling ansatz for a large class of cuprate and Chevrel-phase superconductors. The resulting dependence of Tc on the zero-temperature condensate density is then used to explore universal trends in the pressure (α) and isotope (β) coefficients. In good agreement with experiment we find that α and β fall into common Tc-α and Tc-β regions, respectively, thus forming two branches, one for systems with positive and the other for compounds with negative pressure or isotope coefficient. The two branches merge at the maximum Tc where the coefficients vanish and the magnitude of the coefficients increases with reduced Tc. Our results provide crucial constraints on the microscopic theory. © 1993.