Fröhlich-interaction-induced multiphonon Raman scattering in ${\mathrm{SrCuO}}_2$sand ${\mathrm{Sr}}_{0.5}$ ${\mathrm{Ca}}_{0.5}$ ${\mathrm{CuO}}_2$s

Polarized Raman spectra of ${\mathrm{Sr}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ca}}_{\mathrm{x}}$ ${\mathrm{CuO}}_2$ (x=0, 0.5) microcrystals were studied experimentally. In addition to the Raman-allowed modes, we observed numerous lines with ${\mathrm{A}}_{\mathrm{g}}$ symmetry and frequencies up to 2400 ${\mathrm{cm}}^{\mathrm{-}1}$, which appear only for polarizations parallel to the chains. We show that this additional Raman scattering originates from the three infrared-active-only ${\mathrm{B}}_{1\mathrm{u}}$ LO modes and their overtones and combinations, becoming Raman active via Fröhlich interaction.