Photoemission and x-ray-absorption study of misfit-layered (Bi,Pb)-Sr-Co-O compounds: Electronic structure of a hole-doped Co-O triangular lattice

We present a photoemission and x-ray-absorption study of the misfit-layered (Bi,Pb)-Sr-Co-O compounds which have a Co-O triangular lattice with a mixed valence of Co³⁺ and Co⁴⁺. The valence-band photoemission as well as the O 1s and Co 2p x-ray absorption spectra indicate that Co³⁺ and Co⁴⁺ have the low-spin t_2g⁶ and t_2g⁵ configurations, respectively. The angle-resolved photoemission spectra show that the dispersion of the t_2g feature is very small compared to its width at each angle, suggesting that the electron-lattice coupling energy is much larger than the kinetic energy of the t_2g electrons and that the carriers in the Co-O triangular lattice are essentially polarons formed by Co⁴⁺ in the nonmagnetic Co³⁺ background.