Anisotropy of the superconducting state properties and phase diagram of MgB2 by torque magnetometry on single crystals

TL;DR

This study investigates the anisotropic superconducting properties and phase diagram of MgB2 single crystals using torque magnetometry, revealing temperature-dependent anisotropy and vortex matter phase transitions.

Contribution

It provides the first detailed analysis of the anisotropy and vortex phase diagram of MgB2, highlighting deviations from traditional theories due to its two-band superconductivity.

Findings

Hc2 anisotropy decreases with temperature

Peak effect indicates an order-disorder vortex transition

H-T phase diagram shows intermediate thermal fluctuation strength

Abstract

The angular and temperature dependence of the upper critical field Hc2 in MgB2 was determined from torque magnetometry measurements on single crystals. The Hc2 anisotropy gamma_H was found to decrease with increasing temperature, in disagreement with the anisotropic Ginzburg-Landau theory, which predicts that the gamma_H is temperature independent. This behaviour can be explained by the two band nature of superconductivity in MgB2. An analysis of measurements of the reversible torque in the mixed state yields a field dependent effective anisotropy gamma_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 Hc2 is a manifestation of an order-disorder phase transition of vortex matter. The H-T phase diagram of MgB2 for H//c correlates with the intermediate strength of thermal fluctuations in MgB2, as compared to those in high and low Tc superconductors.