Using high-resolution angle-resolved photoemission spectroscopy on ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta },$ we observe a new mass renormalization or “kink” in the E vs $\overrightarrow{k}$ dispersion relations localized near $(\pi ,0).\mathrm{}$ The resolution of bilayer splitting allowed the first direct measurements of this interaction effect. The kink is clearly stronger than the kink observed along the nodal direction, appears at a lower energy (near 40 meV for overdoped samples), and is only apparent in the superconducting state. The kink energy scale defines a cutoff below which well-defined quasiparticle excitations occur. The most likely origin of this effect is coupling to the magnetic-resonance mode observed in inelastic neutron scattering.

• Received 8 April 2003

DOI:https://doi.org/10.1103/PhysRevB.68.174520