SPECIFIC HEAT OF Mg11B2 IN MAGNETIC FIELDS: TWO ENERGY GAPS IN THE SUPERCONDUCTING STATE

Abstract

We present specific-heat measurements on Mg 11 B 2 in magnetic fields to 9 T. The anomaly at Tc is rapidly broadened and attenuated in fields, as expected for an anisotropic, randomly oriented superconductor. At low temperature there is a strongly field-dependent feature that shows the existence of a second energy gap. The Sommerfeld constant, γ, increases rapidly and non-linearly with magnetic field, which cannot be accounted for by anisotropy. It approaches γn = 2.6 mJ K -2 mol -1, the coefficient of the normal-state electron contribution, asymptotically for fields greater than 5 T. In zero magnetic field the data can be fitted with a phenomenological two-gap model, a generalization of a semi-empirical model for single-gap superconductors. Both of the gaps close at the same Tc; one is larger and one smaller than the BCS weak coupling limit, in the ratio ~ 4:1, and each accounts for ~ 50% of the normal-state electron density of states. The parameters characterizing the fit agree well with those from theory and are in approximate agreement with some spectroscopic measurements.