Electronic structure of one-dimensional cuprates

Abstract

We have investigated the electronic structures of one-dimensional antiferromagnetic insulators Ca₂CuO₃ and Sr₂CuO₃ combining electron spectroscopic measurements and various calculations. While calculations based on a local-spin-density approach for the real magnetic structures fail to yield an insulating state, from our experiments we estimate the intrinsic band gaps in these materials to be about 1.7 eV (Ca₂CuO₃) and 1.5 eV (Sr₂CuO₃). Analysis of the core-level and the valence-band spectra in terms of model many-body Hamiltonians show that the charge-transfer energy Δ for these one-dimensional systems is significantly smaller than other cuprates, such as the high-T_c oxides (two-dimensional) and CuO (three-dimensional). Such a small Δ suggests the presence of the bare upper Hubbard band within the oxygen p bandwidth and thus provides an example of a correlated covalent insulator.