Electron density variation at copper sites at the superconducting transition in cuprates

Abstract

The temperature dependence of the center of gravity S of the Mossbauer spectrum produced by 67Zn2+ impurity ions occupying copper and yttrium sites in YBa2Cu3O6.6, YBa2Cu3O6.9, YBa2Cu4O8, Bi2Sr2CaCu2O8, HgBa2CuO4, and HgBa2CaCu2O6 at temperatures above the superconducting transition point T c was shown to be dominated by the second-order Doppler shift. For T<T c, the quantity S is affected by the band mechanism associated with the formation of Cooper pairs and their Bose condensation. The variation of electron density at a metal site of the crystal was found to be related to the superconducting transition temperature. The variation of electron density created by the Cooper pair Bose condensate in compounds with two structurally inequivalent copper positions was shown to be different for these copper sites. The experimental dependence of the fraction of superconducting electrons on temperature agrees with the analogous dependence following from BCS theory for all the sites studied.