Cu doping effects inMgB2

<p>We report a detailed study of the structural and electronic properties of MgB2 alloyed with Cu, and analyze the effect of metal substitutions at different concentrations on the superconducting properties of MgB2. 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 CuB2, as well as MgCu2—a very stable metallic compound—and the pure MgB2 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 Mg?CuB2 structures at equilibrium conditions. We demonstrate that Cu does not act as a single hole dopant, due to the effect of p?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.</p>