The excitonic-enhancement model (EEM) is based on the simultaneous excitonic and ‘‘generalized’’ Cooper-pair condensation of a charge-unbalanced system that is described by a two-band model consisting of a valence and a conduction band separated by a band gap G. A detailed numerical analysis of the consequences of the EEM with respect to shift of ${\mathit{T}}_{\mathit{c}}$ due to the oxygen isotope effect on the La-doped Y-Ba-Cu-O system is carried out. The results are found to be in excellent agreement with the recent experimental data of Bornemann and Morris, covering a ${\mathit{T}}_{\mathit{c}}$ range of 38.3–92.3 K.

• Received 13 November 1991

DOI:https://doi.org/10.1103/PhysRevB.45.13017