一维扩展离子Hubbard模型的相图研究

应用密度矩阵重整化群方法, 研究了存在交错离子势Δ时一维半满扩展Hubbard模型的相图. 通过计算关联函数、结构因子、位置算符等方法, 描绘了从Mott绝缘体-键有序绝缘体-Band 绝缘体的特性并给出了精确的相边界. 研究发现: 中间的键有序绝缘体相在相图中占据了很小的一部分区域, 当存在离子势Δ的情况下, 这个区域将会有所增大; 而当相互作用足够强时, 这个中间相消失. 给出了离子Hubbard模型(最近邻电子-电子相互作用V=0)的相图.     

环形燃烧室内气相燃烧及两相流动计算

采用TTM法与型线定点法相结合的方式生成带涡流器环形燃烧室贴体网格,在任意曲线坐标系中对其三维两相反应流场进行数值模拟,采用的紊流、燃烧以及辐射模型分别为:标准k-ε方程,EBU-Arrhenius公式,六通量热辐射模型等。在同位网格系下气相采用SIMPLE算法求解,气液两相流动用PSIC法模拟。通过对两种燃烧室的计算结果与实验值比较表明本计算方法合理,所得的计算结果可为环形燃烧室的优化设计提供指导数据。     

多轨道Hubbard模型的隶玻色子数值算法研究

通过综合模式搜索法、广义Lagrange乘子法、以及转轴法等多种数值方法, 建立了一套针对多轨道Hubbard模型隶玻色子解法的数值优化方法. 该数值方法能够在考虑晶场劈裂、轨道间跳跃以及真实能带结构基础上, 利用隶玻色子方法计算实际关联电子材料的性质. 首先利用该方法计算了两轨道体系的Mott金属-绝缘体转变性质, 得到了与目前已有工作一致的结果; 然后利用该方法讨论了Coulomb关联对三轨道体系Na_xCoO₂的影响. 结果表明: 在中间关联情况下由e_g^'轨道形成的六个小Fermi面消失, 原因是由于电子关联导致该轨道上的空穴数随U减少. 这些结果也证实了算法的正确性和有效性. 关键词： 多轨道Hubbard模型 隶玻色子 Mott转变 xCoO₂')" href="#">Na_xCoO₂     

海森伯铁磁系统的总能量

对于海森伯铁磁系统,利用多体格林函数方法,在无规相近似下,并且考虑到关联函数〈S⁺S^-〉时,得到对于任意自旋S普遍适用的总能量的表达式.对于三维和二维的情况给出了计算结果.得到的总能量的数值低于没有考虑关联函数时的数值. 关键词： 海森伯模型 铁磁系统 总能量 关联函数     

定态双向环形气体激光中的饱和效应

从理论上分析了双向环形气体激光中的全饱和效应，导出了定态激光强度分布函数的解析形式，算出了定态双模强度的自相关和相相关函数。与激光的三次模型 实验测相比较，激光的全饱和模型与实验测量符合更，而三次模型的结果偏差较大。     

电子相互作用和高聚物中的极化子

各种电致发光聚合物具有非简并基态.当电子和空穴注入后,形成的载流子是极化子.极化子的大小和能量决定于电子相互作用.目前大家采用Hubbard model来描述聚合物中的电子相互作用,所得到的结果与实验不符.Hubbard model和extended Hubbard model对窄能带体系是好的近似,但聚合物是宽能带,反映价键关联的非对角相互作用（在extended Hubbard model中被略去）有重要贡献,特别是在强屏蔽情况下,价键关联与Hubbard关联相互竞争,可纠正现有理论的缺陷. 关键词：     

环形Josephson结中的回波效应与第一零场台阶的下限偏置电流

本文用微扰理论下环形Josephson结中圆对称孤子点粒子模型所得的结果,首先分析了环形结中各种耗散情形下的孤子回波效应,部分结果与数值模拟计算结果进行了比较;然后讨论了孤子为避免回波效应而共振地在结中往返传播,环形结必须具备的最小偏置电流,分析表明,这种最小偏置电流与结的内、外半径和结中的耗散情形有关,给出了一些理论计算结果,并对此进行了讨论。     

一维元胞自动机随机交通流模型的研究

依据概率论的定义,通过对稳态时关联函数的解耦,给出一维元胞自动机单速随机交通流模型中的关联函数与转入、转出及刹车概率之间的关系,继而进一步得出车辆的密度、速度、流量等物理量随这三个概率变化的理论值,理论结果与实验模拟相一致. 关键词： 随机交通流模型 空间关联函数 解耦近似     

巨正则量子Monte Carlo方法和二维Hubbard模型的研究

用巨正则量子Monte Carlo方法,计算了二维单带Hubbard模型的局域磁矩、磁化率、交错磁化率和内能等物理量.结果表明,用巨正则量子Monte Carlo方法能够统一地研究Hubbard模型的关联强度从弱至强区域的性质,它是处理强关联多体系统的有效方法.     

基于格子玻尔兹曼方法的一类FitzHugh-Nagumo系统仿真研究

格子玻尔兹曼方法在复杂的流体系统中得到了广泛的应用.本文针对在高于阈值常电流刺激下神经元动作电位周期性振荡的FitzHugh-Nagumo系统,构造了一类带源项和修正项的仿真格子玻尔兹曼模型.通过合理选择适当的局部平衡态分布函数和修正函数,再应用Chapman-Enskog多尺度分析,可以正确恢复出一类宏观非线性方程.通过积分法得到了修正函数的构造方法,并分析了格子玻尔兹曼模型L~∞稳定的充分条件.利用网格相关性分析,本文所构造的模型具有二阶空间精度.应用本文所提出的模型,仿真模拟了几个具有解析解的初边值系统,并与传统的改进有限差分格式(MFDM)进行了对比,结果表明本文模型所得的数值解与解析解吻合,其模拟误差小于MFDM.此外,还针对不具有解析解的初边值系统进行了数值仿真,并与MFDM进行了对比.数值结果表明,两种计算格式的数值解比较吻合,进一步证明了本文所构造模型的有效性和稳定性.     

Circulating-current phase in the three-band model for two-leg CuO ladders

We calculate circulating-current (CC), charge-density-wave, and d-wave-like pairing (d-SC) correlation functions in the three-band Hubbard model for two-leg CuO ladders using the density-matrix renormalization group method and detect a dominant fluctuation in a wide range of parameter values and hole-doping rates. We find that, for model parameters leading to a realistic ground state in the undoped ladder, the CC fluctuations decay faster than the d-SC correlations at least up to a hole doping of 10%. It means that no phase with CC order or dominant CC fluctuations occur at low doping.     

Spinon and <Emphasis Type="Italic">η</Emphasis>-spinon correlation functions of the Hubbard chain

We calculate real-space static correlation functions of spin and charge degrees of freedom of the one-dimensional Hubbard model that are described by operators related to singly occupied sites with spin up or spin down (spinons) and unoccupied or doubly occupied sites (η-spinons). The spatial decay of their correlation functions is determined using density matrix renormalization group results. The nature and spatial extent of the correlations between two sites on the Hubbard chain is studied using the eigenstates and eigenvalues of the two-site reduced density matrix. The results show that the spinon-spinon correlation functions decay algebraically and the η-spinon correlation functions decay exponentially, both in the half-filling and metallic phases. The results provide evidence that these degrees of freedom are organized in boundstates in the interacting system.     

No evidence for spontaneous orbital currents in numerical studies of three-band models for the CuO planes of high temperature superconductors

We have numerically evaluated the current-current correlations for three-band models of the CuO planes in high-T(c) superconductors at hole doping x = 1/8. The results show no evidence for the orbital current patterns proposed by Varma. If such patterns exist, the associated energy is estimated to be smaller than 5 meV per link even if [formula: see text]. Assuming that the three-band models are adequate, quantum critical fluctuations of such patterns hence cannot be responsible for phenomena occurring at significantly higher energies, such as superconductivity or the anomalous properties of the strange metal phase.     

Temperature correlators in the one-dimensional Hubbard model in the strong coupling limit

We consider the one-dimensional Hubbard model with infinitely strong repulsion. The two-point dynamical temperature correlation functions are calculated. They are represented as Fredholm determinants of linear integrable integral operators.     

Functional Integrals and Excitation Energy in Three-Band Hubbard Model

The normal and anomalous Green＇s functions of antiferromagnetie state in three-band Hubbard model are studied by using functional integrals and temperature Green＇s function method. The equations of energy spectrum are derived. In addition, excitation energy of Fermi fields are calculated under long wave approximation.     

Band structure and antiferromagnetism of a single CuO2 layer

We have solved a set of self-consistency equations for the three-band model describing electrons coupled strongly by antiferromagnetic correlations in a single CuO₂ layer. Strong but finite Hubbard correlations are taken into account by using a random phase approximation for the electronic propagators which contain the combined effect of both the Hubbard correlation and the hybridization of copper and nearest neighbor oxygen states. From the Green functions the band structure is determined, which depends strongly on the doping fraction and the antiferromagnetic order parameter 〈s_Q〉. The main impact of doping is to decrease the magnitude of antiferromagnetic fluctuations, although the decrease appears to be quite slow when compared with experimental data.     

Exact duality relations in correlated electron systems

Using gauge transformations on electron bond operators, we derive exact duality relations between various order parameters for correlated electron systems. Applying these transformations, we find two duality relations in the generalized two-leg Hubbard ladder at arbitrary filling. The relations show that unconventional density-wave orders such as staggered flux or circulating spin current are dual to conventional density-wave orders and there are direct mappings between dual phases. Several exact results on the phase diagram are also concluded.     

A Monte Carlo Study of the Half-Filled Two-Dimensional Hubbard Model

Using the path integral representation,bulk thermodynamic properties of the two-dimensional Hubbard model on a square lattice are studied numerically using the hybrid Monte Carlo simulation algorithm.Results for averaged energy,spin-spin correlation functions and Cooper pair correlation functions are presented.All these results suggest that the system exhibits an anti-ferromagnetic correlation at half-filling for a wide range of temperature.     

A Monte Carlo Study of the Half-Filled Two-Dimensional Hubbard Model

Using the path integral representation,bulk thermodynamic properties of the two-dimensional Hubbard model on a square lattice are studied numerically using the hybrid Monte Carlo simulation algorithm.Results for averaged energy,spin-spin correlation functions and Cooper pair correlation functions are presented.All these results suggest that the system exhibits an anti-ferromagnetic correlation at half-filling for a wide range of temperature.``     

Segregation in the Asymmetric Hubbard Model

We study the “asymmetric” Hubbard model, where hoppings of electrons depend on their spin. For strong interactions and sufficiently asymmetric hoppings, it is proved that the ground state displays phase separation away from half-filling. This extends a recent result obtained with Freericks and Lieb for the Falicov–Kimball model. It is based on estimates for the sum of lowest eigenvalues of the discrete Laplacian in arbitrary domains.