Symmetry of the ${3 d}^{9}$ ligand hole induced by doping in $Y {Ba}_{2} {Cu}_{3} O_{7 - δ}$

Polarized Cu

L_{3}

-edge x-ray absorption spectra of a single crystal of Y

{Ba}_{2}

{Cu}_{3}

O_{6.8}

have been measured to determine the symmetry of the orbital of the electronic holes induced by doping. The states induced by doping in

Y {Ba}_{2} {Cu}_{3} O_{7 - δ}

(

0 < δ < \sim 0.5

) have been found to be ligand holes

L^{*}

, giving the

{3 d}^{9} L^{*}

configuration, and their weight is correlated with

T_{c}

. The polarization dependence shows that the hole

L^{*}

, induced by doping, is found to be mainly in the Cu-O orbital oriented along the

c

axis involving the

O {2 p}_{z}

orbital of the O(4) atom in the Ba plane and the

Cu {3 d}_{z^{2}}

orbitals. This result indicates the important role of BaO layers and shows that theories of high-

T_{c}

superconductivity have to take account of a second band with different symmetry, beyond the single band (the two-dimensional band derived from the

Cu {3 d}_{x^{2} - y^{2}}

and O

{2 p}_{x}

{2 p}_{y}

orbitals in the

a - b

plane of the Cu

O_{2}

layers) usually considered in models of the electronic structure of high-

T_{c}

superconductors.

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Symmetry of the 3d9 ligand hole induced by doping in YBa2Cu3O7−δ