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-site negative-U centers, Hg2+2. 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 Hg2+2 centers are oriented in the x and y directions and can have phase differences compatible with the d-wave symmetry of the CuO2 planes.