Common optical absorption features are observed near 0.5 eV in four undoped single-crystal copper oxides: La2CuC>4, Nd2CuC>4, Pr2Cu04, and S^CuC^Ch. These absorption bands are shown to be weakly electric dipole allowed excitations of the Cu02 layers. The features are ascribed to an exciton near 0.4 eV, probably of crystal-field origin, with strong multimagnon sidebands. Our observations provide an important basis for understanding the doped materials.
Polarized electrore6ectance spectra are reported for single-crystal La2Cu04+y Depending on the electrode structure, the primary effect of modulating the electrode potential is addition of charge carriers or variation of the electric field in the space-charge region. By adding charge carriers we observe changes in the optical spectra similar to those caused by chemical doping. For modulation of the electric field the perturbed absorption spectrum is very different, corresponding to electric-field-induced crystal-field transitions. Bands near 1.4 and 1.6 eV respond to the electric field in ways consistent with d 2 2~d y and d 2 2~d dy transitions, respectively. Our observations are consistent with Perkins Zy Z et al. [Phys. Rev. Lett. 71, 1621] who ascribe transitions near 0.4 and 1.5 eV to the d z 2~d 2 2 and d 2 2~d"~e xcitons. z p 3z Zy 2.0
Measurements are reported of the transmission of single crystals of La2Ni04 containing no excess oxygen. Several weak absorption bands are observed in the range 0.15 -2 eV arising from magnetic and crystal-field excitations. The lowest-energy band, at 0.25 eV, is quantitatively described by a recent theory for phononassisted creation of a virtual bound state of two magnons. However, unlike La2Cu04, no higher-energy sidebands are detected, suggesting that the broad, structured absorption seen clearly in La2Cu04 between -0.4 and 1 eV results from sidebands to crystal-field excitons as previously proposed.LazNi04 is an intrinsically interesting material, as evinced, for example, by the recent work of Tranquada et al. , which reveals a rich variety of structural and magnetic phases as a function of oxygen doping. However,
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