Cu doping effects inMgB2

Abstract

We report a detailed study of the structural and electronic properties of ${\mathrm{MgB}}_{2}$ alloyed with Cu, and analyze the effect of metal substitutions at different concentrations on the superconducting properties of ${\mathrm{MgB}}_{2}.$ In particular, we calculate heats of formation, electronic properties, and phonon frequencies as a function of Cu concentration. We studied several different systems including pure ${\mathrm{CuB}}_{2},$ as well as ${\mathrm{MgCu}}_{2}$---a very stable metallic compound---and the pure ${\mathrm{MgB}}_{2}$ surface with a Cu monolayer coverage. Our results confirm the experimental findings regarding the high energy cost of the Cu-Mg exchange, mainly due to the different metal sizes which hinder stabilization of mixed $\mathrm{Mg}\ensuremath{-}{\mathrm{CuB}}_{2}$ structures at equilibrium conditions. We demonstrate that Cu does not act as a single hole dopant, due to the effect of $p\ensuremath{-}d$ hybridization on the states relevant for superconductivity. We also show that enhanced superconducting properties could be achieved by means of Cu substitutions; however, the synthesis of these compounds should be obtained in out-of-equilibrium conditions.