On the occurrence of plateaus in the dependence of critical temperatures on oxygen content in HTSC cuprates Quantitative analysis for Bi2Sr2CaCu2O8+x on the basis of indirect-exchange pairing

Plateaus observed, with several cuprates, in the dependence of critical temperatures on oxygen content are analyzed on the basis of indirect-exchange Cooper-pair formation via oxygen anions (oxygen-mediated superconductivity), 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 formalism. The plateau structure is found to arise as a compensating effect on Tc from hole doping and a change in density of oxygen anions in (or near) the CuO2 layers. The observed steep decrease of TC in underdoped samples is ascribed to a rapid loss of macroscopic phase coherence with decreasing oxygen content. Extrapolating the analysis to the double-plateau (“chair”) structure of TC(x) in YBa2Cu3O7−x, the origin of the first (60 K) plateau is similar to that in the Bi-compound, whereas the increase to TC ≈ 90 K is due to participation by the basal (chain) plane in superconductivity.