Unusual dispersion and line shape of the superconducting state spectra of Bi₂Sr₂CaCu₂O_{8 + δ}

Photoemission spectra of Bi₂Sr₂CaCu₂O_{8 + δ} below T_c show two features near the (π, 0) 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.