Abstract
Layered cobalt oxide Bi2Sr2CoO6+δ have attracted much interest because it is isostructural to the high-Tc superconductor Bi2Sr2CuO6+δ. Furthermore, Bi2Sr2CoO6+δ exhibit a series of surprising electronic and magnetic properties with various oxygen contents. We have carried out polarization dependent X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) measurements to study the orbital symmetry and the energies of ligand to metal charge transfer excitations. The XAS measurement reveals that because the centrosymmetry is broken from D4h symmetry due to strong local distortion, there exists a large mixing among 4p states or hybridization between 4p and 3d states. This distortion of CoO6 octahedron with δ > 0.25 is more severe than δ ~ 0.25 due to extra oxygen insertion In the RIXS measurement, spectra of the Tpeak~280 K sample exhibit three clear inelastic features around 2.8 eV, 4.6 eV and 7.0 eV. Similarly in the Tpeak~150 K sample three peaks appear around 3.0 eV, 4.6 eV and 7.7 eV. The lack of dispersion suggests the highly localized characteristics of these excitations. These features may be referred to charge transfer mechanism from a 2p electron of O to a 3d hole of Co in the CoO2 plane. Particularly the energy loss feature at 4.5 eV can be clearly identified as a charge transfer excitation because it resonates at the absorption transition of 1s→4px’, 4py’ with ligand to metal charge transfer (LMCT) from O 2p→Co 3dx’2- y’2.