Electronic structure of superconducting MgB 2 and related binary and ternary borides

First-principles full potential linear muffin-tin orbital--generalized gradient approximation electronic structure calculations of the new medium-T{sub C} superconductor (MTSC) MgB{sub 2} and related diborides indicate that superconductivity in these compounds is related to the existence of p{sub x,y}-band holes at the {Gamma} point. Based on these calculations, we explain the absence of medium-T{sub C} superconductivity for BeB{sub 2}, AlB{sub 2}, ScB{sub 2}, and YB{sub 2}. The simulation of a number of MgB{sub 2}-based ternary systems using a supercell approach demonstrates that (i) the electron doping of MgB{sub 2} (i.e., MgB{sub 2-y}X{sub y} with X=Be, C, N, O) and the creation of defects in the boron sublattice (nonstoichiometric MgB{sub 2-y}) are not favorable for superconductivity, and (ii) a possible way of searching for similar or higher MTSC should be via hole doping of MgB{sub 2} (CaB{sub 2}) or isoelectronic substitution of Mg (i.e., Mg{sub 1-x}M{sub x}B{sub 2} with M=Be, Ca, Li, Na, Cu, Zn) or creating layered superstructures of the MgB{sub 2}/CaB{sub 2} type.