Mass-renormalized electronic excitations at ($\pi$, 0) in the superconducting state of $B{i}_{2}S{r}_{2}CaC{u}_2{O}_{8+\delta }$

Using high-resolution angle-resolved photoemission spectroscopy on $B{i}_{2}S{r}_{2}CaC{u}_2{O}_{8+\delta }$, we have made the first observation of a mass renormalization or “kink” in the E vs. $\vec{k}$ dispersion relation localized near $(\pi ,0)$. Compared to the kink observed along the nodal direction, this new effect is clearly stronger, appears at a lower energy near 40 meV, and is only present in the superconducting state. The kink energy scale defines a cutoff below which well-defined quasiparticle excitations occur. This effect is likely due to coupling to a bosonic excitation, with the most plausible candidate being the magnetic resonance mode observed in inelastic neutron scattering.