Anisotropy of the superconducting state properties and phase diagram of MgB 2 by torque magnetometry on single crystals

Abstract

The angular and temperature dependence of the upper critical field H c2 in MgB 2 was determined from torque magnetometry measurements on single crystals. The H c2 anisotropy γ H was found to decrease with increasing temperature, in disagreement with the anisotropic Ginzburg–Landau theory, which predicts that γ H is temperature independent. This behaviour can be explained by the two band nature of superconductivity in MgB 2. An analysis of measurements of the reversible torque in the mixed state yields a field dependent effective anisotropy γ eff, which can be at least partially explained by different anisotropies of the penetration depth and the upper critical field. It is shown that a peak effect in fields of about 0.85 H c2 is a manifestation of an order–disorder phase transition of vortex matter. The H– T phase diagram of MgB 2 for H∥ c correlates with the intermediate strength of thermal fluctuations in MgB 2, as compared to those in high and low T c superconductors.