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 ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}.$ The valence-band photoemission as well as the $\mathrm{O}1s$ and $\mathrm{Co}2p$ x-ray absorption spectra indicate that ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ have the low-spin $t_{2g}^6$ and $t_{2g}^5$ 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 ${\mathrm{Co}}^{4+}$ in the nonmagnetic ${\mathrm{Co}}^{3+}$ background.