Evidence for anisotropic excitonlike enhancement of the Raman scattering from La2CuO4

Polarized Raman studies on oriented single crystals of La2Cu04 yield a ,,spectra with narrow lines at 429 and 228 cm'that are identi6ed as the two Rig modes expected for the tetragonal K2NiF4 structure: a ,,, spectra with one line at 228 cm'that has Eg symmetry; and a spectra with numerous peaksthat are due to normally forbiddenphonon excitations. The a, spectra also show strong second-orderfeatures, suggesting a highly anisotropic, excitonlike enhancement of the Raman scattering.