Phonon contribution to high-Tc superconductivity:: Tunneling study of slightly overdoped Bi2Sr2CaCu2O8

Reproducibility of phonon structures in the tunneling conductance is studied by using mechanical Bi2Sr2CaCu2O9/Au contacts. The structures are much weaker than a previous clear phonon structure for a Bi2Sr2CaCu2O8/GaAs junction, and a smearing of the structures is discussed. The step-like increase in a critical temperature T-c with a number of CuO2 layer is explained as a natural consequence of the phonon mechanism in a layered lattice. For explaining an extraordinarily large relative gap, 2Delta(0)/k(B) T-c, and a T-c dependent isotope effect, an Eliashberg-magnetic pair break scenario is proposed for underdoped oxides.