论文标题:高温超导体中环形电流相的量子蒙特卡罗模拟
Preparation of SiO_2 Microspheres and Metal Chelate Microspheres and Application in Immobilized Papain
论文作者 任志儒
论文导师 韩福祥,论文学位 硕士,论文专业 等离子体物理
论文单位 大连理工大学,点击次数 719,论文页数 59页File Size2315k
2004-05-29论文网 http://www.lw23.com/lunwen_172512/ 高温超导体,约束路径量子蒙特卡罗方法,环形电流相,三带Hubbard模型
high-temperature superconductors, constrained path Monte Carlo method, circulating current phases, three-band Hubbard model
自从高温超导体发现以来，其微观机制一直被人们所关注，特别是对于正常态的一些反常性质的解释更具有挑战性。 本文首先回顾了人们为高温超导体而提出的一些模型及理论，主要介绍了单带、两带、三带与扩展Hubbard模型，以及唯象边缘费米液体理论。然后详细叙述了用来研究相互作用费米子系统基态性质的约束路径量子蒙特卡罗方法，通过二维单带Hubbard模型中电子系统的基态能量以及各种关联函数的计算，检验了这种算法的可行性与正确性。 在本文中我们运用约束路径量子蒙特卡罗方法，对二维三带Hubbard模型中的环形电流相存在的可能性进行了探讨。为了解析地表示环形电流相，针对电流的不同流动形式，我们定义了相应的二次量子化形式的算符及其关联函数。在具体模拟中，我们对所有可能自旋组合的关联函数进行了测量。 模拟结果表明，在适当的Hamiltonian量参数范围内，对应于闭合回路算符的关联函数在相同距离上比其他所有算符对应的关联函数大一个数量级，因此证明了在三带Hubbard模型中存在环形电流相。
Since the discovery of the high temperature superconductors, much attention has been paid to the search of the microscopic superconducting mechanism. Especially, the explanation of the anomalous normal-state properties is more challenging.Firstly, the models and theories proposed for the high temperature superconductors are reviewed, including the one-, two-, three-band and the extended Hubbard models as well as the phenomenological marginal-Fermi-Liquid theory. Secondly, the constrained path quantum Monte Carlo method to compute the ground-state properties of interacting fermion systems is described in detail. The validity of the algorithm has been verified through the calculation of the ground-state energy and the various correlation functions of the electronic system in the two-dimensional one-band Hubbard model.The possibility of the existence of the circulating current phases in the two dimensional three-band Hubbard model has been studied numerically by applying the constrained-path quantum Monte Carlo method. In order to characterize the circulating current phases, various operators in the second quantization form are introduced and their correlation functions of different spin configurations are numerically computed in our simulations.The simulation results demonstrate that, within the physical ranges of the model parameters, the correlation functions of hole hoppings giving rise to closed current flow patterns in the circulating current phases are larger by one order of magnitude than those of hole hoppings corresponding to open current flows. This provides the evidence for the existence of the circulating current phases in the three-band Hubbard model.
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