Specific Heat of Mg11B2 : Evidence for a Second Energy Gap

Measurements of the specific heat of Mg11B 2 from 1 to 50K, in magnetic fields to 9T, give the Debye temperature, Θ=1050 K , the coefficient of the normal-state electron contribution, γn=2.6 mJ mol-1 K-2 , and a discontinuity in the zero-field specific heat of 133 mJmol -1 K-1 at Tc=38.7 K . The estimated value of the electron-phonon coupling parameter, λ=0.62 , could account for the observed Tc only if the important phonon frequencies are unusually high relative to Θ . 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