MIDINFRARED OPTICAL EXCITATIONS IN UNDOPED LAMELLAR COPPER OXIDES

The weakly electric-dipole-allowed midinfrared excitations are studied in insulating single crystals of ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4},$ ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2},$ and ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}.$ These intrinsic excitations of the undoped ${\mathrm{CuO}}_{2}$ layers are lower in energy than the charge-transfer excitation. Temperature-dependent optical-absorption measurements are presented from 10 to 450 K. Photoinduced absorption measurements on single-crystal ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ are also presented. Recent theoretical work and optical-absorption experiments on ${\mathrm{La}}_{2}{\mathrm{NiO}}_{4},$ as well as the copper oxides, provide strong evidence that a sharp absorption peak seen in all the copper oxides at photon energy \ensuremath{\sim}0.4 eV and a related peak near 0.25 eV in ${\mathrm{La}}_{2}{\mathrm{NiO}}_{4}$ arise from phonon-assisted creation of a quasibound two-magnon state. A comparison between the intrinsic absorption in ${\mathrm{La}}_{2}{\mathrm{NiO}}_{4}$ and that in the copper oxides suggests that the broad midinfrared absorption bands observed between 0.4 and 1.2 eV in the undoped copper oxides have a different electronic origin. We discuss our measurements in regards to two proposed origins (phonon-multimagnon and exciton sidebands) for these broad higher-energy bands. We find that multimagnon and phonon sidebands associated with a Cu ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}\ensuremath{\rightarrow}{d}_{{3z}^{2}\ensuremath{-}{r}^{2}}$ crystal-field exciton at \ensuremath{\sim}0.5 eV plausibly explain the structure, strength, and polarization dependence of these broadbands. Direct observation of this exciton would unambiguously confirm or refute this model.