Midinfrared optical excitations in undoped lamellar copper oxides

The weakly electric-dipole-allowed midinfrared excitations are studied in insulating single crystals of La₂CuO₄, Sr₂CuO₂Cl₂, and Nd₂CuO₄. These intrinsic excitations of the undoped CuO₂ 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 La₂CuO₄ are also presented. Recent theoretical work and optical-absorption experiments on La₂NiO₄, as well as the copper oxides, provide strong evidence that a sharp absorption peak seen in all the copper oxides at photon energy {~}0.4 eV and a related peak near 0.25 eV in La₂NiO₄ arise from phonon-assisted creation of a quasibound two-magnon state. A comparison between the intrinsic absorption in La₂NiO₄ 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_x²-y²→d_3z²-r² crystal-field exciton at {~}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.