Polarized OKα and CuLα x-ray emission spectra of Bi-Sr-Ca-Cu-O single crystals

Polarized soft-x-ray emission measurements of the high-temperature superconducting ${\mathrm{Bi}}_{2.2}$ ${\mathrm{Sr}}_{1.8}$ ${\mathrm{Ca}}_{0.7}$ ${\mathrm{Cu}}_{1.9}$ ${\mathrm{O}}_{\mathit{x}}$ compound are presented. The O Kα spectrum shows a strong anisotropy and three main features can be identified. When the c axis of the crystal is nearly perpendicular to the polarization vector e(c⊥e) of x-ray radiation, the O Kα 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 p${\mathrm{\sigma }}_{\mathit{x},}$y states in the ${\mathrm{CuO}}_2$ plane which form a σ bond with the Cu 3${\mathit{d}}_{\mathit{x}}^{2\mathrm{-}}$ ${\mathit{y}}^2$ states. In the case of c axis almost parallel to the polarization vector (c∥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 p${\mathrm{\sigma }}_{\mathit{z}}$ component associated with the Cu 3${\mathit{d}}_{3\mathit{z}}^2$-${\mathit{r}}^2$ orbital. It should be mentioned that this notation also contains the in-plane p${\mathrm{\sigma }}_{\mathit{x},}$y orbitals. The most striking property of the Cu Lα spectrum is the energy shift between the main peaks in the case of the two different polarization directions. It turns out that when c⊥e the peak energy is larger than that for c∥e. This is in accordance with the suggestion that the largely π-bonded Cu 3${\mathit{d}}_{\mathit{x}\mathit{y}}$ states should have a smaller binding energy than that of the corresponding Cu 3${\mathit{d}}_{\mathit{x}\mathit{z},}$yz states.