Evidence for anisotropic excitonlike enhancement of the Raman scattering fromLa2CuO4

Abstract

Polarized Raman studies on oriented single crystals of ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$ yield ${\alpha }_{\mathrm{zz}}$ spectra with narrow lines at 429 and 228 ${\mathrm{cm}}^{-1\mathrm{}}$ that are identified as the two $A_{1g}$ modes expected for the tetragonal ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$ structure: ${\alpha }_{\mathrm{xz}}$ spectra with one line at 228 ${\mathrm{cm}}^{-1\mathrm{}}$ that has $E_g$ symmetry; and ${\alpha }_{\mathrm{xx}}$ spectra with numerous peaks that are due to normally forbidden phonon excitations. The ${\alpha }_{\mathrm{xx}}$ spectra also show strong second-order features, suggesting a highly anisotropic, excitonlike enhancement of the Raman scattering.