Incommensurability and transfer mechanism in bismuth high Tc superconductors

The structure of bismuth high Tc superconductors is investigated. A correlation is found between incommensurate modulations and transport properties: Bi2Sr2CuO6 is not a good metal because the transfer of electrons from the Cu-O2 layers into the Bi-O bi-layers induces an incommensurate and monoclinic distortion, responsible for the localization of carriers. Substitution of Sr2+ by La3+ reduces the transfer degree and the distortion; a superconductive transition is therefore observed with the remaining carriers after a monoclinic-orthorhombic phase transition. Further substitution tending to a complete compensation of transfer leads to a compensated insulator. It is suggested that the c* component of the modulation is linked to the lattice distortion and the b* component to the charge transfer degree. Similar changes are observed in Bi2Sr2(Ca1-zYz)Cu2O8 except that the monoclinic distortion is never present. (orig.)