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Phonon structures in the tunneling conductance must be properly normalized to evaluate the intensity of the electron-phonon interaction. However, this is very difficult because of the absence of the exact normal-state conductivity. Therefore we propose another method to judge whether or not the intensity becomes stronger with a critical temperature T c. This method compares the intensity of anharmonic structures, which are normalized to the single-phonon structures, between high- and low-T c substances. Our data for Bi-cuprates strongly suggest that the electron-phonon interaction is stronger in higher-T c substances.

Phonon structures appeared in the tunneling conductance of a Bi2-x Pb x Sr2Ca2Cu3O10-y -GaAs junction. They are similar to the previous structures obtained for SnO2 junctions. There is no large structure other than that of the phonon in a single phonon frequency range. Above that range, tiny peaks appeared up to 180 meV, which may be anharmonic phonon structures. From the phonon positions, 2Δ is determined to be 60±4 meV and from the temperature of disappearance, T c is estimated to be 95±6 K. Thus, 2Δ/k B T c is 7.3±1.0.