On heavy carbon doping of MgB2

Heavy carbon doping of MgB2 is studied by first principles electronic structure studies of two types, an ordered supercell (Mg(B(1-x)Cx()2), x = 0.0833) and also the coherent potential approximation method that incorporates effects of B-C disorder. For the ordered model, the twofold degenerate sigma-bands that are the basis of the high temperature superconductivity are split by 60 meV (i.e. 7 meV/% C) and the sigma Fermi cylinders contain 0.070 holes/cell, compared to 0.11 for MgB2. A virtual crystal treatment tends to overestimate the rate at which sigma holes are filled by substitutional carbon. The coherent potential approximation (CPA) calculations give the same rate of band filling as the supercell method. The occupied local density of states of C is almost identical to that of B in the upper 2 eV of the valence bands, but in the range -8 eV to -2 eV, C has a considerably larger density of states. The calculations indicate that the a Fermi surface cylinders pinch off at the zone center only above the maximum C concentration x approximate to 0.10. These results indicate that Mg(B1-xCx)(2) as well as Mg1-xAlxB2 is a good system in which to study the evolution of the unusual electron-phonon coupling character and strength as the crucial sigma hole states are filled. (c) 2005 Elsevier B.V. All rights reserved.