Abstract
Polarization-dependent x-ray-absorption spectroscopy on the O 1s and Cu 2p edges using the non-surface-sensitive fluorescence method has been performed on single crystals of , (R=Nd and Sm), and the insulating compound . The experimental results support the picture of a doped charge-transfer insulator. The symmetry of hole states on O and Cu sites has been determined. These have predominantly in-plane, i.e., O 2y and Cu 3- character. In p-type doped cuprates for 0<x<0.15 about 8% of the total number of hole states on O sites have O 2 character, which probably originates from apical O sites. At higher dopant concentration, this number increases. In the n-type doped system 4–11 % of the O 2p states in the energy range of the upper Hubbard band have O 2 character. In nearly all the systems investigated in this work the fraction of unoccupied Cu 3- states with respect to the total number of Cu holes in the upper Hubbard band is 3±3 %. At higher energies a high spectral weight with Cu 3- character (about 40% of the total amount of unoccupied Cu 3d states in the upper Hubbard band) is observed, which is probably caused by a hybridization with Cu 4s and 4p states. The implication of holes in nonplanar orbitals on high- superconductivity in cuprates is discussed.
- Received 19 October 1992
DOI:https://doi.org/10.1103/PhysRevB.47.3354
©1993 American Physical Society