Extensive study of the optical spectra for high-temperature superconducting oxides and their related materials from the infrared to the vacuum-ultraviolet energy region

Abstract

Optical reflectivity spectra of (La, Sr)₂CuO₄ and YBa₂Cu₃O_y 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.