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 Tc,π congruent with 13 K, and a critical temperature Tc=Tc,σ congruent with 38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field μ0Hc2,π congruent with 0.35 T, and an anisotropic upper critical field with extreme values μ0Hc2,σ,c congruent with 3.5 T and μ0Hc2,σ,ab congruent with 19 T, where the indexes π and σ 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 ∼28 T when Tc congruent with 30 K