Doping dependence of the electron-phonon and electron-spin fluctuation   interactions in the high-$T_{c}$ superconductor   Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$
  Using ultrafast optical techniques, we detect two types of bosons strongly coupled to electrons in the family of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ from the underdoped to overdoped regimes. The different doping dependences of the electron-boson coupling strengths enables us to identify them as phonons and spin fluctuations: electron-phonon coupling ($\lambda_{e-ph}$) peaks at optimal doping, and electron-spin fluctuation coupling ($\lambda_{e-sf}$) decreases monotonically with doping. This observation is consistent with two facts: (1) superconductivity is in close proximity with antiferromagnetism at low dopings, and (2) a pronounced lattice renormalization effect at larger dopings. 
Elbert E. M. Chia, D. Springer, Saritha K. Nair, X. Q. Zou, S. A.   Cheong, C. Panagopoulos, T. Tamegai, H. Eisaki, S. Ishid...
cond-mat.supr-con cond-mat.str-el