First-principles study on the electronic structure and optical properties of MgCNi3

The electronic and optical properties of newly discovered alloy superconductor MgCNi3 are studied using density functional calculations and generalized gradient approximation method. The calculated results exhibit that the conduction bands in this compound are derived by Ni 3d and C 2p states. The top valence states have essentially Ni 3d characters and the C 2p states occupy the region from 4.0 to 7.0 eV below Fermi energy. EF locates just at the step slope of sharply structured Ni 3dyz+zx and 3d3z2-r2 peaks. The doping carriers from Mg atoms destroy the magnetic order. The calculated optical properties show that the optical conductivity in the energy range 0-12 eV is contributed from the Ni 3d to Ni 4s and C 2p transitions. The densities of states near Fermi energy N(EF) are evaluated to be 5.50 states/eV/cell, which yields a theoretical Sommerfeld constant γcal∼4.45 mJ/mol-Ni K2. Based on BCS strong-coupling theory and first-principle calculation, the Debye frequency for MgCNi3 is estimated to be 87 cm-1 approximately. We conclude that the mechanism of superconductivity in MgCNi3 is of the type of BCS with strong electron-phonon coupling