Near-band gap electronic structure of the tetragonal rare-earth cupratesR2CuO4and the bismuth cuprateBi2CuO4

Abstract

Complex optical dielectric function in the tetragonal rare-earth cuprates ${R}_{2}{\text{CuO}}_{4}$ ($R=\text{La}$, Pr, Nd, and Sm) and in the tetragonal bismuth cuprate ${\text{Bi}}_{2}{\text{CuO}}_{4}$ is studied in the spectral range of 0.6--5.4 eV using a method of optical ellipsometry. The dielectric spectra are studied for the two main polarizations and analyzed in terms of a cluster model for ${\text{CuO}}_{4}^{6\ensuremath{-}}$ complexes taking into account intracenter $p\text{\ensuremath{-}}d$ and intercenter $d\text{\ensuremath{-}}d$ charge-transfer (CT) transitions. The band gap in the rare-earth cuprates is defined by an electric-dipole-allowed CT transitions centered at 1.54--1.59 eV in Pr, Nd, and Sm cuprates, and 2.1 eV in La cuprate. Optical response of ${\text{Bi}}_{2}{\text{CuO}}_{4}$ strongly differs from the rare-earth cuprates which we relate with strong covalency of Bi-O bonding and strong ionicity of $\text{Cu}(3d)\text{-O}(2p)$ bonding. These features are manifested in suppression of low-energy intense intracenter $p\text{\ensuremath{-}}d$ and intercenter $d\text{\ensuremath{-}}d$ CT transitions, and by appearance of strong intense absorption bands near 5 eV. Regardless the strong distinctions of optical response, on one hand, of La, Pr, Nd, and Sm cuprates, and on the other hand, of the Bi cuprate, the dielectric gap in these compounds shows comparable values defined by a superposition of intracenter $p\text{\ensuremath{-}}d$ CT transitions and two-center $d\text{\ensuremath{-}}d$ CT transitions. Thus these cuprates should be classified as compounds intermediate between CT and Mott-Hubbard insulators.