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 density-functional-based approach uses the coherent-potential approximation to describe the effects of disorder, the Gaspari–Gyorffy formalism to estimate the electron–phonon matrix elements and the 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.