Doping dependence of the electron-phonon and electron-spin fluctuation interactions in the high-T<inf>c</inf> superconductor Bi<inf>2</inf>Sr <inf>2</inf>CaCu<inf>2</inf>O<inf>8</inf>+δ

Using ultrafast optical techniques, we detect two types of bosons strongly coupled to electrons in the family of Bi2Sr2CaCu 2O8+δ (Bi-2212) from the underdoped to overdoped regimes. The different doping dependences of the electron-boson coupling strengths enable us to identify them as phonons and spin fluctuations: electron-phonon coupling (λe-ph) peaks at optimal doping, while electron-spin fluctuation coupling (λe-sf) decreases monotonically with doping. This observation is consistent with two facts: (i) superconductivity is in close proximity with antiferromagnetism at low dopings and (ii) a pronounced lattice renormalization effect at larger dopings. © IOP Publishing and Deutsche Physikalische Gesellschaft.