RESEARCH INTO A NEW PRECURSOR FOR SUPERCONDUCTING MERCURY CUPRATES

By studying the influence of Hg stoichiometry on the Tc of the superconducting compound HgBa 2 CuO 4-δ (Hg-1201), we have started to re-examine the composition of the only intermediate phase reported in the Cu-Hg diagram. The oxidation of this phase could lead to a very unstable Cu-Hg oxide to employ with BaO 2 in the synthesis of superconducting mercury cuprates or could be used as a source of mercury in the reaction with the precursor Ba 2 CuO 3.

Reservoir Layers in High Tc Mercury Cuprates

We put forward the hypothesis that cations with 6s electrons (Hg,Tl,Pb,Bi) in the charge reservoir layers of high Tc cuprate superconductors actively participate in the pairing interaction as negative-U centers. We further argue that the Hg-cuprates are outstanding superconductors (Tc > 160 K) because they can exist as two-ion negative-U centers, . Their electrons are less localized than in single-site centers (negative-U or bipolaron) and can have a strong pairing interaction with a smaller increase in effective mass. The centers are oriented in the x and y directions and can have phase differences compatible with the d-wave symmetry of the CuO2 planes.