Electronic structure of one-dimensional cuprates

We have investigated the electronic structures of one-dimensional antiferromagnetic insulators (Formula presented) and (Formula presented) 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 (Formula presented) and 1.5 eV (Formula presented). Analysis of the core-level and the valence-band spectra in terms of model many-body Hamiltonians show that the charge-transfer energy (Formula presented) for these one-dimensional systems is significantly smaller than other cuprates, such as the high-(Formula presented) oxides (two-dimensional) and CuO (three-dimensional). Such a small (Formula presented) suggests the presence of the bare upper Hubbard band within the oxygen (Formula presented) bandwidth and thus provides an example of a correlated covalent insulator.