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We present soft-x-ray absorption spectra (XAS) of LaCo03 taken at different temperatures (80 -630 K). The shape of the multiplets in the Co 2p XAS spectra conveys information on the symmetry and spin of the ground state. The 0 1s XAS spectra are related to unoccupied metal bands through covalent mixing. The changes in the spectra taken at different temperatures provide information on the spin-state transition in this compound. At low temperature, 80 and 300 K, the material is in a highly covalent low-spin state. The main contribution to the ground state in this case is given by t2g(' A l ) with an occupancy of 0.56. At higher temperature, 550 and 630 K, the results indicate a gradual transition to a mixed-spin state. The main contribution to the high-spin part of the mixture is given by t2geg('T2 ) with an occupancy of 0.71. There is no evidence of charge disproportionation in the temperature range 80 -630 K. The 0 1s XAS spectra reAect important changes in the unoccupied Co 3d bands across the spin-state transition.

We found direct experimental evidence for an orbital switching in the V 3d states across the metal-insulator transition in VO2. We have used soft-x-ray absorption spectroscopy at the V L2,3 edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.

Polarization dependent x-ray-absorption measurements were performed on pure and Cr-doped V 2 O 3 single crystals in the antiferromagnetic insulating, paramagnetic insulating, and metallic phases. The orbital occupation of the V 3d 2 ion is found to depend appreciably on the phase, but always with the Sϭ1 character, requiring an explanation which is beyond the elegant simplicity of the pure one-band Hubbard model or of models in which the a 1g orbital is projected out by means of a simple dimerization. The results reveal the critical role of the spin and orbital dependence of the on-site 3d-3d Coulomb energy, and a mechanism is proposed to explain the closing or opening of the band gaps which are of much higher energy scale than the transition temperatures.

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