Forbidden Frölich-interaction-induced 2-LO Raman scattering in Pb2Sr2RCu3O8 (R=rare earth element)

The one- and two-phonon Raman scattering spectrum of single crystal Pb2Sr2RCu3O8, where R is a rare earth element, is examined under resonance conditions. Strong features appear in the one-phonon spectrum at the positions of longitudinal optical (LO) infrared modes as the laser energy approaches that of the charge transfer (CT) gap. These modes, which we interpret as Fröhlich-interaction-induced, are accompanied by a second order spectrum …