Measurements of the specific heat of Mg11B2 from 1 to 50 K, in magnetic fields to 9 T, give the Debye temperature, Theta = 1050 K, the coefficient of the normal-state electron contribution, gamma(n) = 2.6 mJ mol(-1) K-2, and a discontinuity in the zero-field specific heat of 133 mJ mol(-1) K-1 at T(c) = 38.7 K. The estimated value of the electron-phonon coupling parameter, lambda = 0.62, could account for the observed T(c) only if the important phonon frequencies are unusually high relative to Theta. At low T, there is a strongly field-dependent feature that suggests the existence of a second energy gap, about 4 times smaller than the major gap.

Abstract. -We show that the specific heat of the superconductor MgB2 in zero field, for which significant non-BCS features have been reported, can be fitted, essentially within experimental error, over the entire range of temperature to Tc by a phenomenological two-gap model. The resulting gap parameters agree with previous determinations from band-structure calculations, and from various spectroscopic experiments. The determination from specific heat, a bulk property, shows that the presence of two superconducting gaps in MgB2 is a volume effect.The discovery of superconductivity in MgB 2 [1] raised the questions of its nature and the origin of its relatively high transition temperature T c ∼ 40 K. Specific heat (C) is a powerful tool to aid in answering these questions and, more generally, to provide information on the thermodynamics of the transition. Several groups have reported such measurements on MgB 2 [2-10]. It is now established that C significantly deviates from the standard BCS behaviour. First, a large excess in C is observed in the vicinity of T c /4 [2-6]. Second, an exponential fit of C(T ) in the region T ≪ T c indicates a gap ratio 2∆ 0 /k B T c only onequarter to one-third of the isotropic BCS value [3,4,6]. This excess was interpreted as a possible sign of a second superconducting gap, whose existence is predicted by band-structure calculations [11][12][13]. The specific heat near T c is puzzling also with the jump ∆C at T c consistently smaller than the BCS weak-coupling lower bound. In this Letter, we present an empirical two-gap model that fits the experimental data over the whole range of temperature to T c . This model resolves the apparent contradiction between different analyses of the specific heat, and relevant parameters show good agreement with determinations based on independent experiments.We focus on two sets of specific-heat data obtained independently in two different laboratories. Experimental methods and results have been described elsewhere [2,3,5,6]. The unusual excess specific heat at ∼ T c /4, which denotes the presence of excitations within the c EDP Sciences