A Different Theoretical Approach to the Spin Polaron Problem

Using the finite temperature (Matsubara) Green's function method, the spin polaron problem is investigated in the strong and weak coupling limits. The holon entropy and specific heat are calculated from the thermodynamic potential in the linked-cluster expansion method.     

Calculation of Entropy and Specific Heat in the Spin Polaron Formulation at Finite Temperature

Entropy and specific heat are calculated using the spin polaron formulation at finite temperature. Our theoretical approach makes use of the Matsubara Green's function method where the interaction term in the S-matrix is the spin polaron Hamiltonian, which is constructed in a representation where holes are described as spinless fermions (holons) and spins as normal bosons. In the absence of this interaction term, the normal entropy and specific heat are obtained from the free holon thermodynamic potential and are found to resemble the BCS expressions in the low temperature regime. A second cumulant expansion of the thermodynamic potential with the spin polaron interaction yields an expression for the specific heat whose dominant term in the low temperature limit and small quasiparticle energy difference, resembles the superconducting-state electronic specific heat of the BCS theory.     

The Coulomb interaction in the generalized Hartree-Fock theory of superconductivity

The matrix self-energy equations of the generalized Hartree-Fock (GHF) theory of HTSC are reevaluated and rederived, while avoiding some shortcomings of the former derivation. The density of states is given for an interacted and renormalized electronic system. The self-energy equations are essentially different in several respects from the conventional Eliashberg equations. The matrix self-energy is analyzed with respect to its dependence on (p). It is found to depend one very weakly, which results in self-energy equations which depend on a single variable,, as in the conventional theory. However, it is found that the bare Coulomb interaction contributes extra terms to the pairing self-energy and to the renormalization function. In the simplified version of the equations, the effect of these two extra terms is incorporated as a single extra term of the renormalization function.     

The generalized Hartree-Fock theory of nonmagnetic high-temperature superconductors

The Eliashberg equations are generalized to apply to high-temperature superconductors without spin correlations. The generalization assumes any general electronic density of states. Consequently, it treats the Coulomb interaction dynamically, and takes into account the averaged momentum dependence of the self-energy and of the interactions. Unlike in the conventional Eliashberg equation, the bare Coulomb interaction yields a frequency-independent term in the renormalization function. This term breaks the symmetry of the self-energy, and changes the renormalization.     

An Analytical Approach for the Pseudogap in the Spin Fluctuations Model

The electronic self-energy due to the electron–spin fluctuation interaction is calculated using the one loop approximation for a two-dimensional system and quasi-two-dimensional (anisotropic) model. We analyzed the relevance of the diffusive modes and the temperature dependence of the magnetic correlation length for a possible temperature dependence of the pseudogap.     

Study of Hole Pair Condensation based on the SU(2) Slave-Boson Approach to the t-J Hamiltonian; Temperature, Momentum and Doping Dependences of Spectral Functions

Based on the U(1) and SU(2) slave-boson approaches to the t-J Hamil-tonian, we evaluate the one electron spectral functions for the hole doped high T _c cuprates for comparison with the angle resolved photoemission spectroscopy(ARPES) data. We find that the observed quasiparticle peak in the superconducting state is correlated with the hump which exists in the normal state. We find that the spectral weight of the quasiparticle peak increases as doping rate increases, which is consistent with observation. As a conse-quence of the phase fluctuation effects of the spinon and holon pairing order parameters the spectral weight of the predicted peak obtained from the SU(2) theory is found to be smaller than the one predicted from U(1) mean field theory.     

Selective Raman Scattering Detection of the Dirac Node and the Anti-node of the Spin Density Wave Gap and Magnetic Excitations in BaFe2As2

The electronic and magnetic excitations at the spin density wave (SDW) transition are investigated by Raman scattering. The multi-orbital electronic states induce the Dirac nodes in the SDW gap. The excitations near the nodes and anti-nodes are separately detected in accordance with the two-orbital tight-binding model. The exchange interactions are found to be given by the second derivative of the total energy with respect to the angle of the moment from two-magnon scattering. The two-magnon peak has the large spectral weight above twice the maximum energy of magnon. It is interpreted by the magnetic self-energy of the electron spectral function in the localized spin model or particle-hole excitations in the itinerant spin model.     

A Functional Generalization of the Field-Theoretical Renormalization Group Approach for the Single-Impurity Anderson Model

We apply a functional implementation of the field-theoretical renormalization group (RG) method up to two loops to the single-impurity Anderson model. To achieve this, we follow a RG strategy similar to that proposed by Vojta et al. (in Phys. Rev. Lett. 85:4940, 2000), which consists of defining a soft ultraviolet regulator in the space of Matsubara frequencies for the renormalized Green’s function. Then we proceed to derive analytically and solve numerically integro-differential flow equations for the effective couplings and the quasiparticle weight of the present model, which fully treat the interplay of particle-particle and particle-hole parquet diagrams and the effect of the two-loop self-energy feedback into them. We show that our results correctly reproduce accurate numerical renormalization group data for weak to slightly moderate interactions. These results are in excellent agreement with other functional Wilsonian RG works available in the literature. Since the field-theoretical RG method turns out to be easier to implement at higher loops than the Wilsonian approach, higher-order calculations within the present approach could improve further the results for this model at stronger couplings. We argue that the present RG scheme could thus offer a possible alternative to other functional RG methods to describe electronic correlations within this model.     

大气压等离子体射流加工材料去除函数模型

应用大气压等离子体射流化学方法加工高精度无损伤的光学表面越来越受到重视,而加工过程中去除函数是提高面型精度的关键因素之一.基于大气压等离子体加工的化学反应本质,根据化学动力学原理分析了活性氟原子浓度分布和温度场分布对去除函数轮廓的影响,建立了去除函数模型.该模型将活性氟原子浓度转换为采用发射光谱技术测量得到的活性氟原子光谱强度作为输入参数.将测得的光谱强度分布和温度场与加工轮廓对比,结果表明活性氟原子和温度场分布与去除函数轮廓有很大相关性,验证了所建立模型的可行性.该模型为研究去除函数形成机理和大气压等离子发生器设计奠定了基础.     

用功率谱密度函数评价光学面形中频误差特性

采用二维ＰＳＤ功率谱密度函数来评价光学元件面形中频波前．而这部分波前误差在传统的分析中被表示为"残余量"．它们产生的散射会严重地影响光学系统的成象质量。简要叙述了ＰＳＤ的计算方法及实验结果。     

不忽略一次项损失时望小特性质量损失函数设计

讨论了在一次项损失不能忽略时,一次项和二次项损失系数的确定方法,并且对一次项损失和二次项损失的大小进行了比较分析。研究表明,二次项望小特性质量损失函数只是望小特性质量损失函数的一种特殊形式。最后通过实际问题对研究结果进行了验证。     

Spontaneously Broken Matsubara’s Time Invariance in Fermionic System: Macroscopic Quantum Ordered State of Matter

Spontaneous break down of translational invariance along the Matsubara time axis in a fermionic system is predicted and investigated using an exactly solvable model in analytical form. The broken symmetry state possesses discrete translational invariance along the Matsubara time axis and is characterized by the quantum order parameter (QOP) formed by condensed collective bosonic degrees of freedom of interacting fermions. The QOP’s Green’s function is finite and periodic along the Matsubara axis, but Wick-rotated to the axis of real frequencies it reveals a periodic “chain” of second order poles. Hence, QOP is not dissipative and, therefore, is “invisible” having a zero scattering cross section. Despite this, QOP changes measurable fermionic properties: instead of Landau quasi-particles with Fermi-velocity, there appear light-mass coherent fermionic states in the narrow vicinity of the Fermi-level, surrounded with overdamped states region (“pseudo-gap”) of a width proportional to QOP amplitude. Relevance of the picture to high-T _c “hidden order” and pseudo-gap state is discussed.     

Electron-Phonon vs. Electron-Impurity Interactions with Small Electron Bandwidths

It is a common practice to try to understand electron interactions in metals by defining a hierarchy of energy scales. Very often, the Fermi energy is considered the largest, so much so that frequently bandwidths are approximated as infinite. The reasoning is that attention should properly be focused on energy levels near the Fermi level, and details of the bands well away from the Fermi level are unimportant. However, a finite bandwidth can play an important role for low frequency properties: following a number of recent papers, we examine electron–impurity and electron–phonon interactions in bands with finite widths. In particular, we examine the behavior of the electron self-energy, spectral function, density of states, and dispersion, when the phonon spectral function is treated realistically as a broad Lorentzian function. With this phonon spectrum, impurity scattering has a significant nonlinear effect. PACS Numbers: 71.10.Ay, 71.20.-b, 63.20.Kr, 72.10.Fk Note that in the present paper we will restrict ourselves to situations with particle–hole symmetry, so that the bandwidth is twice the Fermi energy and the Fermi level lies precisely in between     

矩形高层建筑横风向风振动力反应分析计算

从湍流理论的基本方程出发,根据Taylor关于湍流的"冻结"假定,导出了湍流脉动风压谱密度函数的解析计算公式。在分析过程中考虑了压力控制方程源项中全部"紊动-剪切"项的影响。若取用合适的湍流积分尺度,则由此公式得出的横风向脉动风压谱密度函数值与足尺观测数据相吻合。在接近建筑物第一阶自振频率附近谱值函数高于由风洞实验得出的结果。因此根据湍流脉动风压谱密度函数计算得出的结构横风向风振动力反应位移值与加速度值均高于由日本规范公式等计算得出的值。     

具有间隙非线性的二元机翼的随机分岔和功率谱分析

研究了在纵向和垂向随机激励联和作用下,在俯仰方向具有间隙非线性的二元机翼系统的随机颤振。主要由随机系统的二维概率密度和最大Lyapunov指数研究了随机分岔,包括P-分岔和D-分岔,还分析了系统的功率谱密度函数。得到结果如下：当气流流速分别位于颤振前区域和颤振后区域时,随着随机扰动强度的由弱变强,双维概率密度的形状都发生了变化,证明了气流流速在这两种区域都发生了P-分岔。而由系统的最大Lyapunov指数表明不论在弱或强的随机扰动下,也不论气流流速位于哪种区域内,而D-分岔都没有发生。还得到了不同气流流速时的功率谱密度函数曲线,证实了颤振发生时,系统的能量更集中在颤振频率上。     

Thermodynamics and Single Particle Spectra of the Kondo Insulator by the Variational Self-Energy Method

The variational self-energy method is applied to a study of the half-filled periodic Anderson model. Trial-self-energies and the numerical value of the Luttinger–Ward functional are obtained by diagonalization of a single c – f dimer. The dependence of the zero-temperature single-particle gap on the c – f-hybridization is found in qualitative agreement with mean-field theory for the Kondo-lattice, the specific heat agrees well with numerical results from density-matrix-renormalization group calculations. The single particle spectral function at finite-temperature shows marked deviations from a hybridization picture which agree well with quantum Monte–Carlo results.     

Development of a New Alpha Function for the Peng–Robinson Equation of State: Comparative Study of Alpha Function Models for Pure Gases (Natural Gas Components) and Water-Gas Systems

Numerous modifications have been suggested for the temperature dependence of the attractive term of the Peng–Robinson equation of state (PR-EOS), through the alpha function. In this work, a new alpha function combining both exponential and polynomial forms is proposed. Pure-compound vapor pressures for different molecular species were fitted and compared using different alpha functions including the Mathias–Copeman and Trebble–Bishnoi alpha functions. The new alpha function allows significant improvements of pure compound vapor pressure predictions (about 1.2% absolute average percent deviations) for all the systems considered, starting from a reduced temperature of 0.4. In addition, a generalization of the classical Mathias–Copeman alpha function was proposed as a function of the acentric factor. These alpha functions were used for VLE calculations on water+various gases including gaseous hydrocarbons. A general procedure is presented to fit experimental VLE data. The corresponding thermodynamic approach is based on the Peng–Robinson equation of state with the above cited alpha functions. It includes the classical mixing rules for the vapor phase and a Henry's law approach to treat the aqueous phase.