Specific heat in the superconducting and normal state (2-300 K, 0-16 T), and magnetic susceptibility of the 38 K superconductor MgB2: evidence for a multicomponent gap

The specific heat C of a sintered polycrystalline sample of MgB2 with a bul k superconducting transition temperature T-c = 36.7 K is measured as a func tion of the temperature (2-300 K) and magnetic field (0-16 T), together wit h magnetic properties (normal-state susceptibility, superconducting-state m agnetization, etc.). The Sommerfeld constant gamma = 0.89 +/- 0.05 mJ/K-2/g at (2.7 mJ/K-2/mol) is determined in the normal state above H-c2 The normal - and superconducting-state entropies are equal at T,. Several moments of t he PDOS are obtained from the lattice specific heat. We report bulk values for: the thermodynamic critical field, B-c(0) = 0.26 T; the slope of the up per critical field, (dB(c2)/ dT)(Tc) = 0.56 T/K; the Ginzburg-Landau parame ter, kappa = 38; the coherence length, xi approximate to 5 nm; the lower cr itical field, B-c1 approximate to 0.018 T; the London penetration depth, la mbda (0) approximate to 180 nm. These results characterize MgB2 as a type-I I superconductor. The nearly quadratic dependence of C(T) versus T at T muc h less than T-c, its non-linear field dependence, and the discrepancy betwe en the electron-phonon coupling constant lambda (ep) as determined by the r enormalization of the electron density of states (lambda (ep) approximate t o 0.6) and by McMillan's equation for isotropic superconductors (lambda (ep ) approximate to 1.1), are inconsistent with a single isotropic gap. In add ition to high phonon frequencies, anisotropy or two-band gap structure may explain why the critical temperature of this superconductor is high in spit e of its low condensation energy, which does not exceed 1/16 of that of YBa 2Cu3O7 and 1/4 of that of Nb3Sn. (C) 2001 Elsevier Science B.V. All rights reserved.