Two new Hg---Sr cuprate superconductors

Abstract

Two new series of Hg---Sr cuprate superconductors: (a) (Hg, Mo) Sr₂CuO_4+δ and (b) (Hg,Bi)Sr₂(Ca,Y)Cu₂O_7-δ have been synthesized. Hg---Sr superconductors, which are otherwise difficult to make, have been stabilized by partial substitution of higher-valent cations for Hg. T_c, however, is substantially reduced as compared to the corresponding unsubstituted Hg---Ba cuprate superconductor. The best T_c in series (a) is 78 K for (Hg_0.85Mo_0.15)Sr₂CuO_4+δ and that in series (b) is 92 Kfor(Hg_0.67Bi_0.33)Sr₂(Y_0.5Ca_0.5)Cu₂O_7-δ as compared to 95 K and 125 K respectively for the corresponding unsubstituted Hg---Ba cuprates. In both series, high p(O₂) annealing lowers the T_c even further suggesting that the samples are overdoped and the substitution of higher-valent cation for Hg did not decrease the hole concentration. Addition of Mo/Bi appears to be accompanied by the uptake of additional oxygen, δ , which go to the Hg-Oδ layer and the lowering of T_c results from the presence of these extra oxygens in the Hg later.