Electronic Spectra and Their Relation to the (π, π) Collective Mode in High-Tc Superconductors

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The photoemission line shape near (π,0) in Bi2Sr2CaCu2O8+δ below Tc is characterized by a sharp peak, followed at higher energy by a dip and hump. We study the evolution of this line shape as a function of momentum, temperature, and doping. We find the hump scales with the peak and persists above Tc in the pseudogap state. We present strong evidence that the peak-dip-hump structure arises from the interaction of electrons with a collective mode of wave vector (π,π). The inferred mode energy and its doping dependence agree well with a magnetic resonance observed by neutron scattering.