Role of bosonic modes in the mechanism of high temperature   Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ superconductors using ultrafast   optical techniques
  Using ultrafast optical techniques, we probe the hole-doping dependence of the electron-boson coupling constant $\lambda$ in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$. In the overdoped region, we observe a correlation between ($\lambda$) and the superconducting transition temperature $T_{c}$. Upon performing the McMillan analysis, however, we find that $\lambda$ is too small to explain the high $T_{c}$'s, and that the Coulomb pseudopotential $\mu^{\ast}$ is negative. Our analysis therefore reveals two components in the mechanism of high-$T_{c}$ superconductivity -- a dominant pre-existing pairing interaction, together with a weaker electron-phonon interaction that fine-tunes $T_{c}$. 
Jian-Xin Zhu, Elbert E. M. Chia, T. Tamegai, H. Eisaki, Kyu-Hwan Oh,   S.-I. Lee, A. J. Taylor