Evidence for an energy scale for quasiparticle dispersion in Bi2Sr2CaCu2O8

Quasiparticle dispersion in Bi2Sr2CaCu2O8 is investigated with improved ang ular resolution as a function of temperature and doping. Unlike the linear dispersion predicted by the band calculation, the data show a sharp break i n dispersion at 50 +/- 15 meV binding energy where the velocity changes by a factor of 2 or more. This change provides an energy scale in the quasipar ticle self-energy. This break in dispersion is evident at and away from the cl-wave node line, but the magnitude of the dispersion change decreases wi th temperature and with increasing doping.