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

The photoemission line shape near \(π,0\) in Bi₂Sr₂CaCu₂O_8+δ below T_c 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 T_c 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.