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 Bi2Sr2CaCu2O8/Au contacts. The structures are much weaker than a previous clear phonon structure for a BiSr2CaCu2O8/GaAs junction, and a smearing of the structures is discussed. The step-like increase in a critical temperature Tc 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, 2Δ(0)/kB Tc, and a Tc dependent isotope effect, an Eliashberg-magnetic pair break scenario is proposed for underdoped oxides.

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