Effects of copper doping in mgb2 superconductor

Starting from CuB24, Mg1-xCuxB2 compounds with x ranging from 0.003 to 0.03 were synthesised. The lattice parameters of the AlB2-type phase systematically changes with increasing starting CuB24 fraction, indicating that a partial Cu substitution takes place by using this method. With increasing the copper content, Tc decreases (dTc/dx = -18 K for x > 0.01) and the Hall coefficient becomes fully negative while Hc2, does not change. The excess of electronic contribution to the specific heat observed at ~Tc/4 in undoped MgB2 is not observed in Mg0.97Cu0.03B2. The results are not compatible with the hole superconductivity model but can be interpreted within a BCS multi-band gap framework.