Mass-renormalized electronic excitations at (π,0) in the superconducting state of Bi₂Sr₂CaCu₂O_8+δ

Using high-resolution angle-resolved photoemission spectroscopy on Bi₂Sr₂CaCu₂O_8+δ, we observe a new mass renormalization or “kink” in the E vs k⃗ dispersion relations localized near (π,0). 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.