Abstract

Optical reflectivity spectra of (La, Sr)2CuO4 and YBa2Cu3Oy have been measured over a wide spectral range as high as 40 eV. Although overall features of the observed spectra are explained well by the results of the available band calculations, the spectra differ significantly from the band calculation in the low-energy region below 5 eV, where the spectrum would presumably be dominated by the transitions between the states involving Cu 3d and O 2p or the charge-transfer (CT) excitation between the O 2p and Cu 3d states. In contrast to the calculation, the observed absorption that probably corresponds to the Cu–O CT process is weak and broad, even for insulating La2CuO4; thus we cannot firmly determine the CT energy that characterizes the electronic states of these cuprates. To estimate this parameter, the spectra of the cuprates are compared with those for isostructural (La, Sr)2MO4 (with M = Ni, Co, Fe) as well as with A-site-substituted Nd2CuO4. From this and also from the measurement of the anisotropy of the spectrum with respect to the Cu–O planes, we estimate the CT energy to be 2–4 eV for both cuprate superconductors. This value is larger than that predicted by the band theory, thus indicating the breakdown or great modification of the band picture in the low-energy region.

© 1989 Optical Society of America

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