Unusual Dispersion and Line Shape of the Superconducting State Spectra ofBi2Sr2CaCu2O8+δ

Photoemission spectra of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ below $T_c$ show two features near the $(\pi ,0\mathrm{})$ point of the zone: a sharp peak at low energy and a higher binding energy hump. We find that the sharp peak persists at low energy even as one moves towards $(0,0)$, while the broad hump shows significant dispersion which correlates well with the normal state dispersion. We argue that these features are naturally explained by the interaction of electrons with a sharp mode which appears only below $T_c$, and speculate that the latter may be related to the resonance seen in recent neutron data.