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

The two-gap structure in the superconducting state of MgB2 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 T-c.pi congruent to 13 K, and a critical temperature T-c = T-c.sigma congruent to 38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field mu(0)H(c2.pi) congruent to 0.35 T, and an anisotropic upper critical field with extreme values mu(0)H(c2.pisigma.c) congruent to 3.5 T and mu(0)H(c2.sigma.ab) congruent to 19 T, where the indexes pi and sigma refer to the three-dimensional and two-dimensional 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 similar to28 T when T-c congruent to 30 K. (C) 2002 Elsevier Science B.V. All rights reserved.