Mass-renormalized electronic excitations at(π,0)in the superconducting state ofBi2Sr2CaCu2O8+δ

Using high-resolution angle-resolved photoemission spectroscopy on ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}},$ we observe a new mass renormalization or ``kink'' in the E vs $\stackrel{\ensuremath{\rightarrow}}{k}$ dispersion relations localized near $(\ensuremath{\pi},0).$ 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 directi