Heavy C doping of MgB2 was studied by first principles electronic structure studies of 2 types, an ordered super-cell, Mg(B1−xCx)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 σ-bands that were the basis of the high temperature superconductivity were split by 60meV (i.e. 7meV/%C) and the σ Fermi cylinders contain 0.070 holes/cell, compared to 0.11 for MgB2. A virtual crystal treatment tends to overestimate the rate at which σ holes were filled by substitutional C. The coherent potential approximation (CPA) calculations gave the same rate of band filling as the super-cell method. The occupied local density of states of C was almost identical to that of B in the upper 2eV of the valence bands, but in the range −8 to −2eV, C had a considerably larger density of states. The calculations indicated that the σ Fermi surface cylinders pinch off at the zone center only above the maximum C concentration x ≈ 0.10. These results indicated that Mg(B1−xCx)2 as well as Mg1−xAlxB2 was a good system in which to study the evolution of the unusual electron–phonon coupling character and strength as the crucial σ hole states were filled.

On Heavy Carbon Doping of MgB2. D.Kasinathan, K.W.Lee, W.E.Pickett: Physica C, 2005, 424[3-4], 116-24