Incommensurability and transfer mechanism in bismuth high superconductors

(Received 2 March 1992, revised and accepted 2 April 1992)

Abstract
The structure of bismuth high superconductors is investigated. A correlation is found between incommensurate modulations and transport properties : Bi ₂Sr ₂CuO ₆ is not a good metal because the transfer of electrons from the Cu-O ₂ layers into the Bi-O bi-layers induces an incommensurate and monoclinic distortion, responsible for the localization of carriers. Substitution of Sr ²⁺ by La ³⁺ reduces the transfer degree and the distortion : a superconductive transition is therefore observed with the remaining carriers after a monoclinic-orthorhombic phase transition. Futher substitution tending to a complete compensation of transfer leads to a compensated insulator. It is suggested that the component of the modulation is linked to the lattice distortion and the component to the charge transfer degree. Similar changes are observed in Bi ₂Sr ₂(Ca _1-zY _z)Cu ₂O ₈ except that the monoclinic distorsion is never present.