ORBITAL CHARACTER OF STATES AT THE FERMI LEVEL IN LA2-XSRXCUO4 AND R2-XCEXCUO4 (R = ND,SM)

Polarization-dependent x-ray-absorption spectroscopy on the O 1s and Cu 2p edges using the nonsurface-sensitive fluorescence method has been performed on single crystals of La2-xSrxCuO4+delta, R2-xCexCuO4-delta (R = Nd and Sm), and the insulating compound Ca0.86Sr0.14CuO2. 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 2p(x,y) and Cu 3d(x2-y2) 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 2p(z) 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 2p(z) character. In nearly all the systems investigated in this work the fraction of unoccupied Cu 3d3z2-r2 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 3d3z2-r2 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-T(c) superconductivity in cuprates is discussed.