Carbon Doping in MgB2 : Role of Boron and Carbon px(y) Bands

We have studied the changes in the electronic structure and the superconducting transition temperature Tc of Mg(B1−xCx)2 alloys as a function of x with 0 ≤ x ≤ 0.3. Our densityfunctional-based approach uses coherent-potential approximation to describe the effects of disorder, Gaspari-Gyorffy formalism to estimate the electron-phonon matrix elements and Allen-Dynes equation to calculate Tc in these alloys. We find that the changes in the electronic structure of Mg(B1−xCx)2 alloys , especially near the Fermi energy EF , come mainly from the outward movement of EF with increasing x, and the effects of disorder in the B plane are small. In particular, our results show a sharp decline in both B and C px(y) states for 0.2 ≤ x ≤ 0.3. Our calculated variation in Tc of Mg(B1−xCx)2 alloys is in qualitative agreement with the experiments.