FORBIDDEN FROHLICH-INTERACTION INDUCED 2-LO RAMAN-SCATTERING IN PB(2)SR(2)RCU(3)O(8) (R EQUALS RARE-EARTH ELEMENT)

The one- and two-phonon Raman scattering spectrum of single crystal Pb (2)Sr(2)RCu(3)O(8), where R is a rare earth element, is examined under resonance conditions. Strong features appear in the one-phonon spectr um at the positions of longitudinal optical (LO) infrared modes as the laser energy approaches that of the charge transfer (CT) gap. These m odes, which we interpret as Frohlich-interaction-induced, are accompan ied by a second order spectrum composed of overtones and combinations of the LO and ordinary Raman-allowed phonons. The forbidden LO scatter ing weakens with increasing metallicity of the samples due to the vani shing of the CT gap and the screening of the Frohlich interaction. We examine the dependence on incident energy of both the lineshape and ce nter position of the highest frequency two-phonon feature, which can b e unambiguously attributed to a pure overtone of an infrared LO vibrat ion. Due to the two-phonon nature of the scattering process the crysta l momentum, Q, is no longer restricted to small wavevectors and one ca n expect to observe effects of phonon dispersion. We find, indeed, cha nges in the lineshape as the gap is approached, accompanied by a decre ase in the center frequency to a value comparable to twice that of the and Q approximate to 0 one-phonon LO mode.