Anomalous Enhancement of the Coupling to the Magnetic Resonance Mode in Underdoped Pb-Bi2212

High-resolution angle-resolved photoemission with variable excitation energies is used to disentangle bilayer splitting effects and intrinsic (self-energy) effects in the electronic spectral function near the (π,0) point of differently doped (Pb,Bi)₂Sr₂CaCu₂O_8+δ. In contrast to overdoped samples, where intrinsic effects at the (π,0) point are virtually absent, we find in underdoped samples intrinsic effects in the superconducting-state (π,0) spectra of the antibonding band. This intrinsic effect is present only below the critical temperature and weakens considerably with doping. Our results give strong support for models which involve a strong coupling of electronic excitations with the resonance mode seen in inelastic neutron scattering experiments.