ON THE OCCURRENCE OF PLATEAUS IN THE DEPENDENCE OF CRITICAL-TEMPERATURES ON OXYGEN-CONTENT IN HTSC CUPRATES - QUANTITATIVE-ANALYSIS FOR BI2SR2CACU2O8-EXCHANGE PAIRING(X ON THE BASIS OF INDIRECT)

Plateaus observed, with several cuprates, in the dependence of critica l temperatures on oxygen content are analyzed on the basis of indirect -exchange Cooper-pair formation via oxygen anions (oxygen-mediated sup erconductivity), with emphasis on recent experimental results by Li et al. (1994) for Bi2Sr2CaCu2O8+x. It is shown that these results can be quantitatively explained in the framework of the indirect-exchange fo rmalism, The plateau structure is found to arise as a compensating eff ect on T-c from hole doping and a change in density of oxygen anions i n (or near) the CuO2 layers. The observed steep decrease of T-c in und erdoped samples is ascribed to a rapid loss of macroscopic phase coher ence with decreasing oxygen content. Extrapolating the analysis to the double-plateau (''chair'') structure of T-c(x) in YBa2Cu3O7-x, the or igin of the first (60 K) plateau is similar to that in the Bi-compound , whereas the increase to T-c approximate to 90 K is due to participat ion by the basal (chain) plane in superconductivity.