Unusual effects of anisotropy on the specific heat of ceramic and single crystal MgB2

TL;DR

This study investigates the unusual thermodynamic effects caused by anisotropy in MgB2's two-gap superconducting state, revealing complex temperature and field dependencies in specific heat measurements.

Contribution

It provides detailed experimental data on the specific heat of MgB2, highlighting the effects of anisotropy and interband scattering on its superconducting properties.

Findings

Identification of two characteristic temperatures in zero field.

Observation of three characteristic fields in the mixed state.

Irradiation increases the upper critical field up to ~28 T.

Abstract

The two-gap structure in the superconducting state of MgB_2 gives rise to unusual thermodynamic properties which depart markedly from the isotropic single-band BCS model, both in their temperature- and field dependence. We report and discuss measurements of the specific heat up to 16 T on ceramic, and up to 14 T on single crystal samples, which demonstrate these effects in the bulk. The behavior in zero field is described in terms of two characteristic temperatures, a crossover temperature Tc_pi ~ 13 K, and a critical temperature Tc = Tc_sigma ~ 38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field Hc2_pi ~ 0.35 T, and an anisotropic upper critical field with extreme values Hc2_sigma_c ~ 3.5 T and Hc2_sigma_ab ~ 19 T, where the indexes \pi and \sigma refer to the 3D and 2D sheets of the Fermi surface. Irradiation-induced interband scattering tends to move the gaps toward a common value, and increases the upper critical field up to ~ 28 T when Tc = 30 K.