The electronic structure of the doped one-dimensional transition metal oxide Y1-xCaxBaNiO5 studied using x-ray absorption

A strong anisotropic distribution of the holes in Ni 3d and O 2p orbitals is observed in the polarization dependent O1s and Ni2p3/2 x-ray absorption spectroscopy of the linear-chain nickelate Y1-xCaxBaNiO5 (x = 0, 0.05, 0.1, 0.2), which demonstrates the one-dimensional nature of the electronic state in these compounds. Furthermore, the additional holes introduced by Ca-doping occupy both O 2p and Ni 3d orbitals along the NiO5 chains. By comparing the experimental Ni 2p3/2 absorption spectra of Y1-xCaxBaNiO5 to those from charge transfer multiplet calculations we can derive the orbital character of the additional holes to be of ca. 60% O2p and ca. 40% Ni 3d.Comment: pdf only. Submitted to PR

Unoccupied electronic structure of Sr 2 CuO 2 Cl 2 and Ba 2 Cu 3 O 4 Cl 2 : Experiment and theory

The unoccupied electronic structure of the layered cuprates Sr 2 CuO 2 Cl 2 and Ba 2 Cu 3 O 4 Cl 2 has been studied using polarization-dependent x-ray absorption spectroscopy at the O 1s and Cu 2p 3/2 edges and band-structure calculations within the local-density approximation. In contrast to almost all high-temperature superconductors, there are no oxygen atoms outside the Cu-O planes in these oxychlorides. Our results represent therefore direct experimental information regarding the oxygen-derived unoccupied electronic structure of undoped Cu-O planes in square-planar coordination. The O 1s and Cu 2p 3/2 x-ray absorption spectra of Sr 2 CuO 2 Cl 2 and Ba 2 Cu 3 O 4 Cl 2 are quite similar with an essentially two-dimensional upper Hubbard band and extra oxygen and copper related absorption fine structure at higher energies. From its polarization dependence and a comparison to the results of the band-structure calculations, it is concluded that this extra fine structure is related to transitions into O 2 p and Cu 3d 3z 2 Ϫr 2 orbitals which have become partly unoccupied due to hybridization with Sr 4d/Ba 5d/Cu 4 p z and Cu 4s orbitals, respectively. Differences between these two compounds are the larger width of the upper Hubbard band and the higher relative Cu 3d 3z 2 Ϫr 2 hole occupation above the upper Hubbard band in Ba 2 Cu 3 O 4 Cl 2 . These differences can be related to the extra Cu B atoms in the Cu 3 O 4 planes of Ba 2 Cu 3 O 4 Cl 2 with respect to the CuO 2 planes of Sr 2 CuO 2 Cl 2 . ͓S0163-1829͑98͒03706-0

High-energy electron-energy-loss study of sodium-tungsten bronzes.

Single-crystal metallic cubic sodium-tungsten bronzes NaxWO3 (x0.25) and NaxTayW1-yO3 (x-y=0.42) and monoclinic reduced WO3-δ have been investigated by high-energy electron-energy-loss spectroscopy (EELS) in transmission. For all electron densities the volume plasmon dispersion appears to be positive quadratic in momentum transfer q. The dispersion coefficient is much smaller than that predicted from the random-phase approximation for one isotropic parabolic band. This deviation can be reduced by recognizing the threefold degeneracy of the conduction-band t2g states in an octahedral field and narrowing of these bands with increasing sodium content. Anisotropy of the dispersion between the (100) and (110) direction is not observed. Optical effective masses m*(x) of the conduction electrons and background dielectric constants ε(x) have been determined and compare well with data from optical spectroscopy and EELS in reflection, but not with photoemission results. This discrepancy is a result of the photoemission-data evaluation in which the conduction-band degeneracy was neglected. Na 2p core-level excitation energies argue against an admixture of sodium orbitals to the conduction band near the metal-nonmetal transition at x∼0.2. Na 3s states admixed to O 2p states are observed at about 10-11 eV above the Fermi level in O 1s absorption edges. The x dependence of m* and of the width of the O 1s absorption edge of NaxWO3 supports a model of conduction-band narrowing with increasing Na concentration. © 1995 The American Physical Society