Superconducting energy gaps, low temperature specific heat, and quasiparticle spectra of MgB2

We report first-principles calculations of the k- and T-dependent superconducting gap Delta(k,T) in MgB2 and its manifestation in various measured quantities. Because the Fermi surface has disconnected sheets with different electron-phonon coupling strengths, our calculations show that near T=0, the values of Delta(k) cluster into two groups: large values (about 6.5 to 7.5 meV) on the strongly coupled sheets and small values (about 1 to 3 meV) on the weakly coupled sheets. The calculated gap, quasiparticle density of states, and specific heat and their temperature dependences are in agreement with the recent measurements which conclude that MgB2 is a multiple gap superconductor. In fact, theory predicts four prominent values for the gap at low T.