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

Abstract

The specific heat C of a sintered polycrystalline sample of MgB 2 with a bulk superconducting transition temperature T c=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 γ=0.89±0.05 mJ/K 2/gat (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 c. Several moments of the 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 upper critical field, (d B c2/d T) T c =0.56 T/K; the Ginzburg–Landau parameter, κ=38; the coherence length, ξ≅5 nm; the lower critical field, B c1≅0.018 T; the London penetration depth, λ(0)≅180 nm. These results characterize MgB 2 as a type-II superconductor. The nearly quadratic dependence of C( T) versus T at T≪ T c, its non-linear field dependence, and the discrepancy between the electron–phonon coupling constant λ ep as determined by the renormalization of the electron density of states ( λ ep≅0.6) and by McMillan's equation for isotropic superconductors ( λ ep≅1.1), are inconsistent with a single isotropic gap. In addition to high phonon frequencies, anisotropy or two-band gap structure may explain why the critical temperature of this superconductor is high in spite of its low condensation energy, which does not exceed 1/16 of that of YBa 2Cu 3O 7 and 1/4 of that of Nb 3Sn.