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Abstract:
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(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 Co4+ in the nonmagnetic Co3+ background.
