| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 2005: ¥600,000 (Direct Cost: ¥600,000)
Fiscal Year 2004: ¥1,400,000 (Direct Cost: ¥1,400,000)
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| Research Abstract |
The purpose of this research is to reconsider electronic states and metal-insulator transitions in typical strongly correlated electron systems aided by information obtained from newly developed experimental technique such as high-energy spectroscopy. We have investigated 3d transition-metal oxides which exhibit metal-insulator transitions V_2O_3,VO_2,Ti_2O_3,Fe_3O_4 and also those with the perovskite structure. The followings are the outline of the results.
1)Charge and orbital ordering and the Verwey transition in magnetite (Fe_3O_4) have been discussed with a spinless three-band Hubbard model Complex-orbital orders with noncollinear orbital moments, where the occupied t_<2g> orbitals with minority spin are described by a linear combination of the yz, zx and xy orbitals with complex number coefficients, are found to be stabilized. From the results, the transition is expected to be caused by a strong electron-lattice coupling and the resultant modification of the complex- orbital order
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2)Recently, resonant soft x-ray diffraction at the NiL_<2,3> absorption edge is observed in La_<1.8>Sr_<0.2>NiO_4. From the analysis with a configuration-interaction cluster model, we found that both the antiferromagnetic order and the charge-order superstructure resides within the NiO_2 layers ; the holes are mainly located on in-plane oxygens surrounding a divalent Ni ion site with the spins coupled antiparallel in dose analogy to Zhang-Rice singlets in the cuprates.
3)The metal-insulator transitions (MIT) in VO_2 and Ti_2O_3 have been investigated in connection with the on-site exchange interaction and the lattice distortion on the basis of the many-body theory. The mechanism on the MIT can be understood from a unified viewpoint, where the transitions are caused by a change in the electron configuration of doubly occupied sites from an S=1 Hund's coupling state to a low-spin S=0 state driven by an increase in the level splitting among the t_<2g> orbitals originating from the lattice distortion. Less
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