Tunneling observation of phonon contribution to the Cooper pairing in high-Tc superconductors

Improved tunneling spectroscopy was carried out on high-T _c BSCCO(2212) single crystal, and it provided us a reflection spectrum of the quasi-particle that mediates the strong Cooper pairing. The point contact junction enabled us to observe significant fine structure spectrum beyond the gap edge up to 0.2 eV. The spectrum peaks both in positive and negative bias ranges were found to be in a good one-to-one correspondence to the phonon density-of-states peaks at energy level ℏ approximately ega _i reported by the neutron scattering experiment. The significant peaks at higher energies could be also assigned by nε _O + ℏ approximately egai (n equals 0.1.2) where ε _O is the breathing mode energy. Numerical computation in terms of the Eliashberg gap equation was carried out and the electron-phonon coupling intensity function α ²F(approximately ega) was elucidated. Most of the peak modes were attributed to the optical phonons due to oxygen oscillations in and around the (CuO ₂) _n plane, wherein the superconducting electronic state takes place. Consequently it turns out that the breathing mode has an essential role in the high-T _c superconductivity in the perovskite type-oxides.