Spin-state transition and high-spin polarons in LaCoO3

The spin gap energy (≈ 30 meV) associated with the low-spin ($S=0\mathrm{}$) to high-spin ($S=2\mathrm{}$) transition in LaCo${\mathrm{O}}_3$ was proved to be considerably smaller than the charge gap energy (≈ 0.1 eV) which was estimated by optical spectroscopy. Hole doping in the low-spin ground state of LaCo${\mathrm{O}}_3$ leads to formation of localized magnetic polarons with unusually high spin number ($S=10-16\mathrm{}$), which can be viewed as a precursor of the doping-induced ferromagnetic metallic state.