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 T_c of Mg(B_1−xC_x)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 T_c in these alloys. We find that the changes in the electronic structure of Mg(B_1−xC_x)₂ alloys, especially near the Fermi energy E_F, come mainly from the outward movement of E_F 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 p_x(y) states for 0.2≤x≤0.3. Our calculated variation in T_c of Mg(B_1−xC_x)₂ alloys is in qualitative agreement with the experiments.