A link between the formation of defect bipolarons (U-centers or U-bipolarons) and superconductivity in high-Tc superconductors was considered. It was shown that the formation of k-space U-bipolarons and their subsequent attractive single particle and pair condensation led to a decreased superconductivity, due to a large mass of such U-bipolarons. It was suggested that the coexistence of k-space lattice bipolarons and r-space U-bipolarons led to a shift, in the maximum of the concentration dependence, to higher carrier concentrations.

S.Dzhumanov, B.Yavidov, N.A.Makhmudov: Superlattices and Microstructures, 1997, 21[3], 325-8