On the Presence of Cu1+ in the Superconducting (Hg,M)Sr2CuO4+δ; M = Cr, Mo, or Re

The Cu K-edge measurements on the optimum Tc 1201 compounds (Hg1−xM x )Sr2CuO4+δ (M = Cr, Mo, or Re; 0.10 ≤ x ≤ 0.30) show besides Cu2+ (predominant) a very weak but distinct signature of Cu1+, only in the superconducting (SC) samples. The Cu1+ feature is conspicuously absent when these compositions are non-superconducting because of different processing conditions. Our finding of monovalent Cu in SC (Hg,M)/Sr-1201 favors the presence of Cu at the Hg-site and this seems to facilitate superconductivity in these cuprates.