A New Class of High Frequency Excitations in Cuprate Superconductors.

Analysis of Raman spectra obtained in Ba sub 2 YCu sub 3 O sub (6,7) and Bi sub 2 Sr sub 2 CaCu sub 2 O sub 8 demonstrates the existence of two sets of high frequency excitations (extending well beyond 1000 cm sup (-1)). The first arises from spin fluctuations as in related insulators and exhibits no rapid change near T sub c. The second is characterized by a very flat spectrum extending beyond 0.5 eV, and exhibits a sudden reduction in intensity at T sub c. The microscopic cause of this behavior remains mysterious, but we point out interpretations of recent NMR, IR, photoemission, and specific heat experiments which are compatible with the interpretation that the carriers are scattered strongly by a class of excitations with a self energy which is itself strongly modified by the onset of superconductivity.