铁电体中极化长程涨落的光子关联谱实验研究

从极化团簇的随机涨落出发, 利用维纳过程模型, 推导了铁电体中极化长程涨落的弛豫规律以及光强自相关函数所可能的表现形式. 阐述了微观极化团簇的弛豫过程与宏观测量弛豫规律之间的联系. 通过对原有氦氖激光光子关联谱实验装置进行改进, 观测了BaTiO₃和0.71Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃单晶中极化团簇长程涨落在居里点和立方到四方相变点附近的弛豫过程. 在BaTiO₃中发现极化团簇长程涨落在居里点之上4 K存在双弛豫现象, 此现象与其有序无序相变机理相联系. 在Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃中发现极化团簇长程涨落在相变点两侧都存在双弛豫现象. 利用推导的理论结果很好地拟合了实验结果并提取了极化团簇长程涨落的弛豫时间. 两种样品中极化团簇长程涨落的弛豫时间都在相变点出现突变, 并呈现临界慢化现象.     

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     

广义Suzuki公式及其在液晶中的应用

本文推广了Suzuki处理经典系统和Ising模型的关联函数的严格公式,并应用在描述液晶相变具有转动不变性的Maier-Saupe哈密顿量上。结果表明,作者之一最近有关关联函数和磁双折射系数的微观处理,在所述近似范围内是正确的。文中并改进了Vertogen和Van der Meer有关液晶向列相的无规相近似的结果,提出了超越平均场近似描述液晶热力学性质的一个方案。 关键词：     

由部分相干场驱动的吸收型光学系统的统计性质

本文讨论部分相干场驱动的吸收型光学系统的统计性质.指出当驱动场的相干分量较弱时,在一定条件下输出光的二阶关联函数有一个独立于双稳阈峰的涨落峰,它是由输入光的位相涨落引起的,其峰值远小于双稳阈峰.对于噪声很小的极限情况,利用高斯近似导出输出光的二阶关联函数双稳阈峰的位置及高度与输入场和腔的参数之间简单定量关系.在噪声很大的极限条件下,我们采用一定的近似方法得到了关于输出光二阶关联函数的简单解析表达式.最后给出输入场的相干分量与非相干分量及系统的协作参数一般取值的情况下输出场二阶关联函数的双稳阈峰的数值计算结果.     

实虚部关联的量子噪声和泵噪声对单模激光动力学性质的影响

采用具有实虚部关联的量子噪声和泵噪声驱动的单模激光损失模型，用线性化近似方法研究了反映激光动力学性质的光强关联函数，讨论了光强关联函数随时间的演化；并对线性化近似方法的适用范围进行了详细分析，分别讨论了量子噪声强度、泵噪声强度、量子噪声实虚部关联系数对光强相对涨落的影响，得出在小噪声、远离阈值时，线性化近似方法适用范围扩大；小噪声、远离阈值且当量子噪声实虚部无关联时，线性化近似方法适用范围最大的结论. 关键词： 单模激光 光强关联函数 光强相对涨落     

非高斯噪声激励下FHN神经元系统的定态概率密度与平均首次穿越时间

研究了非高斯噪声激励下的FHN神经元系统,应用路径积分法和统一色噪声近似得到系统的定态概率密度函数和平均首次穿越时间表达式.发现了加性噪声强度Q能够诱导非平衡相变的产生,乘性噪声强度D、偏离参数p及关联时间τ₀均不能诱导非平衡相变发生;非高斯噪声的存在缩短了细胞神经元系统静息态和激发态之间的转化时间,有利于神经元信息的传递. 关键词： FHN神经元系统 非高斯噪声 定态概率密度 平均首次穿越时间     

偏置信号调制下色关联噪声驱动的单模激光的光强相对涨落

采用线性化近似，计算了由具有色关联的受偏置信号调制的色泵噪声和色量子噪声驱动 的单模激光增益模型的光强相对涨落，发现相对涨落随噪声强度的变化曲线中存在极小值，分析了偏置信号的强度、周期信号频率、噪声间的互关联强度和互关联时间对曲线的影响； 对单模激光增益模型在偏置信号调制和直接信号调制两种方式下的输出光强相对涨落进行了比较，发现光强涨落与偏置信号的强度密切相关. 关键词： 偏置信号调制 光强相对涨落 线性化近似     

各向异性超导体电阻转变的修正Kosterlitz-Thouless相变模型

从二维系统的Kosterlitz-Thouless (KT) 相变理论出发，在关联长度中引入热激活能和平均钉扎高度，提出了修正的KT相变模型.该模型与库伦气体标度理论和Halperin-Nelson关系具有一致性.应用修正的KT相变模型研究磁场下Tl₂Ba₂CaCu₂O_x(Tl-2212)薄膜电阻转变的标度行为，发现由电阻转变计算得到的平均钉扎高度与温度具有线性依赖关系，实验结果支持修正的KT相变模型. 关键词： 标度行为 各向异性超导体 电阻转变     

晶格量子涨落对spin-Peierls系统低能量激发态的影响

在非绝热近似下，研究了一维spin-Peierls系统中晶格量子涨落 对系统性质的影响，讨论了系统的二聚化相变、单粒子激发和双粒子束缚态. 结果表明，量子晶格涨落会抑制晶格的二聚化，破坏系统的spin-Peierls基态稳定性.在临界点，系统发生从二聚化spin-Peierls态向无能隙态的相变. 自旋声子耦合强度对束缚态能隙的影响比单粒子激发谱能隙显著. 关键词： sin-Peierls系统 非绝热近似 玻色化 相图     

非线性漂移的Fokker-Planck方程的近似非定态解

研究了由高斯白噪声和色噪声作用下的非线性动力学系统的不稳定态演化问题. 在弱噪声极限下, 运用本征值本征矢理论得到了非定态解ρ(x, t)的近似表达式; 分析了色噪声自关联时间τ, 强度α对ρ(x, t)以及对一、二阶矩的影响. 数值模拟发现: 1)t在一定范围内, ρ(x, t)是变量x和t的单调函数, 且随τ的增大而增大, 反之, 随α的增大而减小; 2)一阶矩是τ和α的单调函数, 但二阶矩却是非单调函数, 在参数影响下发生了相变现象. 关键词： 奥恩斯坦-乌伦贝克过程 本征值 本征矢 非定态解     

Liquids in equilibrium: Beyond the hypernetted chain

Density functional techniques are used to derive a charging expression for the non-uniform density of a molecular liquid. In the atomic limit the equation reduces to an exact form due to Fixman. The theory is simplified greatly via a physical approximation that accounts for three-body correlations beyond those included in the hypernetted chain (HNC) closure of the Ornstein-Zernike (OZ) equation. The radial distribution function is obtained as a special case. The theory is tested by examining the phase behavior of two fundamental complex fluids: the homopolymer blend and diblock copolymer melts. For the former it is found, contrary to HNC theory and its molecular generalizations, that a critical temperature T_c is predicted from the structure route. This T_c scales linearly with degree of polymerization N in agreement with Flory theory. The simplest form of the theory can be considered as a way to incorporate attractive interactions within a formalism that is very similar to that of the OZ or reference interaction site model (RISM). The relevance of the theory to charged liquids is also discussed.     

Phase Separation of Penetrable Core Mixtures

A two-component system of penetrable particles interacting via a gaussian core potential is considered, which may serve as a crude model for binary polymer solutions. The pair structure and thermodynamic properties are calculated within the random phase approximation (RPA) and the hypernetted chain (HNC) integral equation. The analytical RPA predictions are in semi-quantitative agreement with the numerical solutions of the HNC approximation, which itself is very accurate for gaussian core systems. A fluid-fluid phase separation is predicted to occur for a broad range of potential parameters. The pair structure exhibits a nontrivial clustering behaviour of the minority component. Similiar conclusions hold for the related model of parabolic core mixtures, which is frequently used in dissipative particle dynamics (DPD) simulations.     

Theoretical analysis of trends in hydrogen bonding involving halogen acceptors (F−At) covalently bonded to a group 14 atom (C−Pb)

In this work, extensive quantum-chemical calculations have been carried out to identify and elucidate trends in the hydrogen-bonding (HB) interaction involving halogen acceptors covalently bonded to a group 14 atom. A series of 25 heterodimers composed of MH₃X (where M = C?Pb and X = F?At) and HNC molecules have been selected as model complexes stabilised by the HB interaction occurring between the X atom of MH₃X and the H atom of HNC. The interaction energy (E_int) between MH₃X and HNC in the MH₃X···HNC complexes falls in the range from ?2.7 to ?10.8 kcal/mol, indicating weak or medium strength of HB in these complexes. The strength of HB in the complexes remains consistent with the well-known HB weakening as X gets heavier. Regarding the effect of M on E_int, the gradual strengthening of HB is observed while descending group 14, but only from M = Si to M = Pb. The trends in E_int are compared with various HB-related parameters obtained from vibrational analysis, the natural bond orbital (NBO) method, the symmetry-adapted perturbation theory (SAPT) and the quantum theory of atoms in molecules (QTAIM). The parameters that present clear (possibly linear) relationships with E_int have been selected to characterise the effect of M and X on the HB interaction.     

Phase transitions and dynamic effects in crystals characterized by single-cell potentials with a multi-well excited state

A study has been made of phase transitions and phase states in crystals whose unit cell potentials have a multi-well excited state. New phase states compared to the conventional order-disorder-type phase-transition models have been revealed. A phase diagram has been constructed. The applicability criteria of the mean field approximation employed are analyzed. A region of parameter variation where the system is close to the tricritical point has been found. It is shown that microdomains of the new phase can efficiently transfer to the original phase within this region, and vice versa, by resonant tunneling. This tunneling has a relaxational nature. Interaction of such a relaxor with an oscillator (the soft mode) creates in this system an efficient mechanism of formation of the central peak. Besides, this model includes a possibility of coexistence of the order-disorder-type and displacive behavior. This coexistence manifests itself, in particular, in an induced phase transition associated with interaction of the order-disorder-type soft mode with the displacive mode for oscillations in the same potential wells. This induced phase transition may serve as a microscopic model of the improper ferroelastic phase transition in the Hg₂Cl₂ model system. This transition may produce a long-range incommensurate phase involving formation of the corresponding domain system, which is likewise in agreement with the case of Hg₂Cl₂. The model developed here can be used also in describing phase transitions in oxygen-octahedron perovskites, where the relative low-symmetry minima of the single-cell potentials can be related to the charge-transfer vibronic excitons. Fiz. Tverd. Tela (St. Petersburg) 39, 547–556 (March 1997)     

Zero field particle correlations in a ferrofluid using Monte Carlo simulations and the hypernetted chain approximation

A comparison is made between a 3-dimensional model of a ferrofluid using Monte Carlo (MC) and Integral Equation techniques in the absence of an applied field. The MC simulations were performed using standard Metropolis sampling. Pair distribution functions were obtained and compared with those determined using the hypernetted chain (HNC) approximation which for systems with nonspherical potentials entails the expansion of the correlation functions in terms of rotational invariants. At densities applicable to real fluids agreement between the two methods is found to be excellent, and this justifies the use of the HNC approximation to determine thermodynamic data which are free statistical errors in this regime.     

Superconductivity of triplet bipolarons

Superconductivity of triplet bipolarons has been investigated in a phase analogous to the superfluid A-phase of ³He by use of the broken-symmetry Hartree approximation. The transition temperature T_c, the chemical potential and the order-parameters have been obtained, and the specific heat and the thermodynamic critical field have also been calculated. It has been found that there exist two energy gaps in the superconducting state. In the normal phase there also exists a gap, whose magnitude is consistent with the recent experimental evidence of the heavy-fermion superconductor (HFS), UPt₃. Temperature dependence of the critical field of another HFS UBe₁₃, which is not yet understood, can also be qualitatively accounted for by this model. These show the possibility that triplet bipolarons might be formed in the HFSs.     

Anisotropic magnetic exchange in orthorhombic RCu2 compounds (R = rare earth)

The magnetic excitations in the field induced ferromagnetic phase F3 of a NdCu₂ single crystal were investigated by means of inelastic neutron scattering experiments. A mean field model using the random phase approximation in connection with anisotropic magnetic bilinear R-R (R denotes a rare earth) exchange interactions is proposed to account for the observed dispersion. The relevance of this model to the analysis of the magnetic ordering process in other RCu₂ compounds is discussed. Received 21 April 1999     

Highly Asymmetric Electrolytes in the Associative Mean-Spherical Approximation

The associate mean-spherical approximation (AMSA) is used to derive the closed-form expressions for the thermodynamic properties of an (n+m)-component mixture of sticky charged hard spheres, with m components representing polyions and n components representing counterions. The present version of the AMSA explicitly takes into account association effects due to the high asymmetry in charge and size of the ions, assuming that counterions bind to only one polyion, while the polyions can bind to an arbitrary number of counterions. Within this formalism an extension of the Ebeling–Grigo choice for the association constant is proposed. The derived equations apply to an arbitrary number of components; however, the numerical results for thermodynamic properties presented here are obtained for a system containing one counterion and one macroion (1+1 component) species only. In our calculation the ions are pictured as charged spheres of different sizes (primitive model) embedded in a dielectric continuum. Asymmetries in charge of –10:+1, –10:+2, –20:+1, and –20:+2 and asymmetries in diameter of 2:0.4nm and 3:0.4nm are studied. Monte Carlo simulations are performed for the same model solution. By comparison with new and existing computer simulations it is demonstrated that the present version of the AMSA provides semiquantitative or better predictions for the excess internal energy and osmotic coefficient in the range of parameters where the regular hypernetted chain (HNC) and improved (associative) HNC do not yield convergent solutions. The AMSA liquid–gas phase diagram in the limit of complete association (infinitely strong sticky interaction) is calculated for models with different degrees of asymmetry.     

Chemical potential of electrolytes adsorbed in porous media with charged obstacles: application of the continuum replica methodology

A theoretical study is reported of a quenched-annealed system where both components were modelled as size symmetric +1: ?1 primitive model electrolytes. The partly quenched system was studied by using the replica Ornstein-Zernike (ROZ) integral equation theory in the hypernetted chain (HNC) approximation and grand canonical Monte Carlo (GCMC) simulations. The primary interest was the excess Gibbs free energy (logarithm of the mean activity coefficient) of the adsorbed electrolyte and an expression for this quantity, valid within the ROZ/HNC formalism, was derived. The effects of the concentration of matrix ions, pre-quenching conditions, and the electrolyte and solvent conditions (concentration, temperature, dielectric constant) on the structure and thermodynamics of the adsorbed electrolyte were examined. The numerical results indicated that the mean activity of the adsorbed electrolyte differs substantially from the corresponding quantity for the bulk electrolyte. The excess chemical potential depends strongly on the concentration of charged obstacles and matrix preparation, and also on the temperature and dielectric constant of the annealed electrolyte solution. Newly generated computer simulation results for the structural and thermodynamic parameters, obtained by the grand canonical Monte Carlo method, were used to assess the validity of the ROZ/HNC approximation. It was shown that the ROZ/HNC theory yields good agreement with the computer simulations.