Phonon-induced hole-hole effective interactions in the cuprates - art. no.214517

We have studied the phonon-induced effective interactions between charge ca rriers (holes) in a three-band model of the CuO2 plane of high-temperature superconductors. For the phononic part we have included two oxygen phonon m odes, breathing and buckling, and two types of electron-phonon interactions : namely, the ionic coupling, for which the displaced oxygens influence the energy level on copper, and the covalent coupling, for which the Cu-O hopp ing is modified by the displaced oxygens. Using a cell-perturbation method we have calculated the effective interaction between holes, V-hh, as induce d by the electron-phonon couplings. We have found that the attractive inter action between holes, due to the breathing mode, is maximum in the presence of finite electron correlation. Furthermore, the presence of the buckling mode enhances the attractive interactions. We show that the calculated valu es of V-hh are of the same order as the values of the electronic pseudogap Delta, found in the normal state of the cuprates. We demonstrate that the i sotope effect, related to the substitution O-16 --> O-18, changes the calcu lated values of V-hh in a way consistent with the experimentally found isot ope effect of Delta. Finally, we have evaluated the superconducting transit ion temperature T-c and found a qualitative agreement with the experimental phase diagrams of the cuprates.