Theoretical Study on the Anomalous Electronic States of Mixed-Valent Vanadium Oxides with Trellis-Lattice Structure

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Transition-metal oxides, materials typical of strong electron correlation, include a class of compounds with mixed valent state. The systems often show anomalous behaviors in their charge degrees of freedom and thus have attracted much attention from both experimental and theoretical viewpoints. In our project, we have paid particular attention in vanadium-oxide materials such as α'-NaV_2O_5 with trellis-lattice structure, β-Ag_<0.33>V_2O_5 (A=Li, Na, Ag, Ca, Sr) and β'-Cu_xV_2O_5 (x=0.65) with two-dimensional structure, and a new vanadate material Bi_xV_8gO_<16> with a one-dimensional framework and have studied microscopic origin of their charge ordering (CO), metal-insulator transition (MIT) with CO, pressure-induced superconductivity, and MIT with both CO and orbital ordering (OO). In the following, we list up our representative results obtained in our project. 1. We studied electronic states of the trellis-lattice t-J model at quarter-filling, and from its optical conductivity calc … More ulation for α'-NaV_2O_5, we showed the presence of anomalous charge fluctuation above the transition temperature. Furthermore, to clarify magnetic response of the spin system in the presence of large charge fluctuation, we developed a new computer code of the Quantum Monte Carlo method. 2. For β-phase vanadate bronzes, we made studies of MIT accompanied by CO and related anomalous electronic properties. In particular, we propose a mechanism that the MIT in this system be caused by the self-doping. Moreover, we present the simplest model "Acentric Hubbard ladder model at quarter filling", in which this mechanism is realized, and we carefully examined the electronic state of the model by using mean-field calculation and density-matrix renormalization group method. 3. As for related materials, we took some organic conductors and CuO double chains in PrBa_2Cu_4O_8, and clarified the origin of CO and anomalous optical spectra. Also for a new material Bi_xV_8O_<16>, which is recently recognized as a system of MIT with CO and OO, we constructed a basic model and proposed its possible CO and OO spatial pattern. Less