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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., Comment: 31 pages, 9 figures. Accepted in the Physica C special issue on MgB2

The specific heat C of a sintered polycrystalline sample of MgB2 with a bulk superconducting transition temperature Tc=36.7 K is measured as a function of the temperature (2-300 K) and magnetic field (0-16 T), together with magnetic properties (normal-state susceptibility, superconducting state magnetization, etc.). The Sommerfeld constant gamma=0.89+-0.05 mJ/K^2/gat (2.7 mJ/K^2/mol) is determined in the normal state above Hc2. The normal- and superconducting state entropies are equal at Tc. Several moments of the phonon density of states are obtained from the lattice specific heat. We report bulk values for: the thermodynamic critical field, the slope of the upper critical field, the Ginzburg-Landau parameter, the coherence length, the lower critical field, the London penetration depth. These results characterize MgB2 as a type-II superconductor. The nearly quadratic dependence of C(T) versus T at T<, Comment: 17 pages, 11 figures, pdf

We performed specific heat measurements to study the superconducting properties of the ∼40 K superconductor MgB 2 , up to 16 T, using polycrystal and single crystal samples. Our results establish the validity of the two-gap model. We tested the effect of disorder by irradiating our sample. This procedure decreased T c down to ∼26 K, but did not suppress completely the smaller gap, at variance with theoretical expectations. A positive effect of the irradiation was the increase of H c2 up to almost 30 T. Our results on the single crystal allow the anisotropy of each band to be determined independently, and show the existence of a cross-over field well below H c2 characterizing the physics of the small-gapped band. We also present preliminary results on Nb 3 Sn, showing similar, but weaker effects.

The two-gap structure of MgB 2 gives rise to unusual thermodynamic properties which depart markedly from the single-gap BCS model, both in their temperature- and field-dependence. We report measurements of the specific heat up to 16 T on ceramic and single crystal samples, which demonstrate these effects in bulk. The low-temperature mixed-state specific heat reveals a field-dependent anisotropy, and points to the existence of unusually large vortices, in agreement with local density-of-states measurements by scanning tunneling spectroscopy. It is finally shown that a suitable irradiation process nearly doubles H c2 in the bulk.

We have measured the low temperature specific heat of a single crystal of Yba 2 Cu 3 O 7 grown in BaZrO 3 which contains a very small amount of paramagnetic defects. The presence of line nodes in the gap is directly seen in the difference C(B,T)—C(0,T) as a function of the magnetic field, which behaves as expected from the d-wave symmetry. Comparison with theory provides new information about the low energy excitation spectrum near the nodes.