Doping dependence of the mass enhancement in (Pb,Bi)₂ Sr₂ Ca Cu₂ O₈ at the antinodal point in the superconducting and normal state

Angle-resolved photoemission spectroscopy (ARPES) is used to study the mass renormalization of the charge carriers in the high-T_c superconductor (Pb,Bi)_2Sr_2CaCu_2O_8 in the vicinity of the (pi,0) point in the superconducting and the normal state. Using matrix element effects at different photon energies and due to a high momentum and energy resolution the bonding and the antibonding bands could be separated in the whole dopant range. A huge anisotropic coupling to a bosonic collective mode is observed below T_c for both bands in particular for the underdoped case. Above T_c, the more isotropic coupling to a continuum or a mode at much higher energy is significantly weaker.