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 composed of overtones and combinations of the LO and ordinary Raman-allowed phonons. The forbidden LO scattering weakens with increasing metallicity of the samples due to the vanishing of the CT gap and the screening of the Frölich interaction. We examine the dependence on incident energy of both the lineshape and center position of the highest frequency two-phonon feature, which can be unambiguously attributed to a pure overtone of an infrared LO vibration. Due to the two-phonon nature of the scattering process the crystal momentum, Q, is no longer restricted to small wavevectors and one can expect to observe effects of phonon dispersion. We find, indeed, changes in the lineshape as the gap is approached, accompanied by a decrease in the center frequency to a value comparable to twice that of the Q∼0 one-phonon LO mode.