Specific heat of Mg11B2:: Evidence for a second energy cap -: art. no. 047001

Measurements of the specific heat of (MgB2)-B-11 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.