Superconductivity in boron-doped crystalline and amorphous carbon*

We investigate the superconducting properties of boron-doped carbon materials composed of sp³ hybridized atoms: cubic diamond, hexagonal diamond, and body centered tetragonal C₄ (bct C₄). We find that a high density of states (DOS) at the Fermi energy result in a high superconducting transition temperature (T_c) in all three materials. The high DOS is particularly realized when boron dopants are not placed at nearest neighbor sites. A T_c above 60 K can be obtained for cubic diamond, although other crystalline allotropes can exhibit a T_c of 40-50 K at 25 % boron-doping. We also consider superconductivity in boron-doped amorphous carbon.