Theoretical study of anomalous electronic states of vanadium oxides with pyroclore lattice structures

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We have made theoretical and computational studies of strongly correlated electron systems, aiming in particular at elucidation of the mechanisms of the anomalous electronic properties of transition-metal oxides and molecular conductors. We have used the density-matrix renormalization group(DMRG) method for calculating the electronic states of pseudo-one-dimensional models in high accuracy. The main results obtained are the following.1.We have clarified the mechanism of the charge and orbital ordering of the triangular-lattice Hollandite compound Bi_xV_8O_<16>.2.We have clarified the mechanism of the orbital ordering and spin-singlet formation in pyroxene compound NaTiSi_2O_6.3.We have clarified the nature of the anomalous spin excitations that appear in the states of large charge fluctuation.4.We have elucidated the anomalous charge dynamics in the CuO double-chain systems.5.We have proposed the novel mechanism of formation of the spin-gap liquid state that appears when the quantum melting of charge-ordered state occurs in the CuO double-chain systems.6.We have calculated the precise electronic phase diagram of the ground state of the one-dimensional extended Hubbard model.7.We have proposed the novel mechanism of the spin-triplet superconductivity that appears in the two-chain zigzag-bond Hubbard model.