Further discussion is given on our proposal that cooperation of superexchange and polaron effects causes the high-Tc superconductivity in some Cu oxides. The key observation has been that the polaron effect reduces the transfer energy, but does not reduce the superexchange. It is pointed out that the wave-function renormalization makes the pairing interaction much smaller than the estimate by a mean-field theory. In the present model the isotope effect shows various characteristics depending on the strength of the polaron effect, and for a particular strength giving the highest Tc, there is no isotope effect.
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