Study of the yttrium and zinc substitutions effects in Bi2Sr2CaCu2O8+δ compounds by transport measurements

The effects of yttrium Y and zinc Zn substitution on calcium Ca and cupper Cu sites in Bi2Sr2CaCu2O8+δ compounds have been investigated by resistivity measurements. At high temperature, after a superconducting transition, the electrical behaviour evolves from a metallic state in compounds with a weak Y concentration into a semi-conducting state (hopping conduction) in compounds with a large Y concentration. Similarly, the Zn substitution modifies the electrical behaviour from metallic to insulating with a much lower concentration comparatively to Y. In the two cases, the semi-conducting behaviour is described by hopping between localised states in the conduction band. This localization is explained by the random distribution of the doping elements which originates from the Y/Zn and Cu wave-functions overlap. The obtained results show the localization is improved in the case of Zn. Consequently, it is suggested that the direct substitution of Zn into the planes leads to a greater overlap. Moreover, the superconducting transition (critical temperature: TC) shows an irregular depression when Zn is substituted. According to an X-Ray diffraction study, this result can be interpreted by internal micro-strains in the structure.