  A d-wave, Eliashberg analysis of break junction and STM tunneling spectra on
BiSrCaCuO (Bi2212) reveals that a spectral dip feature is directly linked to
strong electronic coupling to a narrow boson spectrum, evidenced by a large
peak in the boson spectral weight. The tunneling dip feature remains robust in
the overdoped regime of Bi2212 with bulk Tc values of 56 K-62 K. This is
contrary to recent optical conductivity measurements of the self-energy that
suggest the narrow boson spectrum disappears in overdoped Bi2212 and therefore
cannot be essential for the pairing mechanism. The discrepancy is resolved by
considering the way each technique probes the electron self-energy, in
particular, the unique sensitivity of tunneling to the off-diagonal or pairing
part of the self-energy.
