Polarized O K alpha and Cu L alpha x-ray emission spectra of Bi-Sr-Ca-Cu-O single crystals

Polarized soft-x-ray emission measurements of the high-temperature superconducting Bi{sub 2.2}Sr{sub 1.8}Ca{sub 0.7}Cu{sub 1.9}O{sub {ital x}} compound are presented. The O {ital K}{alpha} spectrum shows a strong anisotropy and three main features can be identified. When the {ital c} axis of the crystal is nearly perpendicular to the polarization vector {bold e}({ital c}{perpendicular}{bold e}) of x-ray radiation, the O {ital K}{alpha} spectrum shows the main peak at 3.8 eV from the Fermi level and a shoulder at about 1.4 eV, correspondingly. It is assumed that this shoulder can be assigned to the {ital p}{sigma}{sub {ital x},}{ital y} states in the CuO{sub 2} plane which form a {sigma} bond with the Cu 3{ital d}{sub {ital x}}{sup 2-}{ital y2} states. In the case of {ital c} axis almost parallel to the polarization vector ({ital c}{parallel}{bold e}) the spectrum consists of the main peak at 3.4 eV and a feature at about 6.0 eV from the Fermi level. The possible explanation for this 6.0-eV feature is the {ital p}{sigma}{sub {ital z}} component associated with the Cu 3{ital d}{sub 3{ital z}}{sup 2}{minus}{ital r}{sup 2} orbital. It should be mentioned that this notation also contains the in-plane {ital p}{sigma}{sub {ital x},}{ital y} orbitals. The most striking property of the Cu {ital L}{alpha} spectrum is the energy shift between the main peaks in the case of the two different polarization directions. It turns out that when {ital c}{perpendicular}{bold e} the peak energy is larger than that for {ital c}{parallel}{bold e}. This is in accordance with the suggestion that the largely {pi}-bonded Cu 3{ital d}{sub {ital x}{ital y}} states should have a smaller binding energy than that of the corresponding Cu 3{ital d}{sub {ital x}{ital z},}{ital yz} states.