Evidence for an energy scale for quasiparticle dispersion in Bi_2Sr_2CaCu_2O_8

Quasiparticle dispersion in $B{i}_{2}S{r}_{2}CaC{u}_2{O}_8$ is investigated with improved angular resolution as a function of temperature and doping. Unlike the linear dispersion predicted by the band calculation, the data show a sharp break in dispersion at $50\pm 10$ $meV$ binding energy where the velocity changes by a factor of two or more. This change provides an energy scale in the quasiparticle self-energy. This break in dispersion is evident at and away from the d-wave node line, but the magnitude of the dispersion change decreases with temperature and with increasing doping.