The spin gap energy (Ϸ 30 meV͒ associated with the low-spin (Sϭ0) to high-spin (Sϭ2) transition in LaCoO 3 was proved to be considerably smaller than the charge gap energy (Ϸ 0.1 eV͒ which was estimated by optical spectroscopy. Hole doping in the low-spin ground state of LaCoO 3 leads to formation of localized magnetic polarons with unusually high spin number (Sϭ10-16͒, which can be viewed as a precursor of the doping-induced ferromagnetic metallic state.
The thermally induced insulator-metal transition ͑IMT͒ has been investigated for single crystals of undoped and lightly doped LaCoO 3 by measurements of the optical-conductivity spectra, the resistivity, and the Hall coefficient up to 800 K. The IMT around 500 K, where the spin-state transition from the nonmagnetic (S ϭ0) ground state to the intermediate-spin (Sϭ1) state is nearly completed, is characterized by a large energy-scale change of the electronic structure as well as by a steep increase of carrier number up to a typical metallic value due to closing of the charge gap. These behaviors are strikingly parallel to those characteristic of the Mott transition. ͓S0163-1829͑98͒50628-5͔PRB 58
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