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

过渡金属氧化物是一类具有强电子相关性的化合物，是一类混合价态的化合物。这类体系在电荷自由度上经常表现出反常行为，因而引起了实验和理论上的广泛关注。在本课题组的研究中，我们重点研究了具有格子结构的钒氧化物材料，如具有二维结构的β-Ag_V_2O_5<0.33>（A=Li，Na，Ag，Ca，Sr）和β-Cu_xV_2O_5（x=0.65），以及具有一维结构的新型钒酸盐材料Bi_xV_8gO_5，<16>并研究了它们的电荷有序（CO）、金属-绝缘体转变（MIT）、压力诱导超导以及CO和轨道有序（OO）的MIT的微观起源。在下文中，我们列出了我们在项目中获得的代表性结果。1.本文研究了四分之一填充的格子t-J模型的电子态，并从其光学电导率计算了其电子态。 ...更多信息 对α ′-NaV_2O_5的电子结构的研究表明，在高于相变温度时，存在反常的电荷涨落。此外，为了阐明自旋系统在大电荷涨落存在下的磁响应，我们开发了一个新的量子蒙特卡罗方法的计算机程序。2.对于β相钒酸盐青铜，我们研究了CO伴随的MIT及其相关的反常电子性质。特别地，我们提出了一种机制，即该系统中的MIT是由自掺杂引起的。在此基础上，我们提出了实现这一机制的最简单模型“四分之一填充的偏心Hubbard阶梯模型”，并利用平均场计算和密度矩阵重整化群方法对该模型的电子态进行了细致的研究. 3.在相关材料方面，我们在PrBa_2Cu_4O_8中取了一些有机导体和CuO双链，阐明了CO的来源和反常光谱。对于一种新材料Bi_xV_8O_2<16>（最近被认为是一个具有CO和OO的MIT体系），我们建立了一个基本模型，并提出了其可能的CO和OO空间分布。少

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