双轴分子构成向列型液晶中向列序的计算

通过格胞理论研究双轴分子构成的向列型液晶。发展新的数值方法液晶中的向列序。不用任何函数展开,通过迭代方法求解平衡态方程得到精确的取向分布函数。计算了序参数和单分子内能对温度的依赖关系。给出了有关物理量在向列相一各向同性相相变点的值。数值结果较分子场理论更接近于Ｍｏｎｔｅ Ｃａｒｌｏ模拟的结果。     

双轴性向列相液晶的相变理论

假设一双轴性向列相液晶分子简单相互作用模型，利用平均场理论，得到从各向同性相至单轴向列相、单轴向列相至双轴向列相的相变. 结果表明，在一定温度范围内，分子形状的非轴对称性可导致系统双轴向列相的产生. 在核磁共振实验中，导出双轴向列相序参数与谱线分裂频率间的一般关系，并讨论序参数测量性的可能性. 关键词： 向列相液晶 相变     

螺状相液晶旋光特性理论模拟与实验研究

利用旋光理论模拟了一种用于螺状相液晶显示器(CH-LCD)的螺状相液晶的旋光率与入射波长、螺距的关系,并从理论上给予定量分析;用磁光调制倍频法测试了这种螺状相液晶旋光率随温度变化的规律,并利用所测的旋光率的值计算得出这种螺状相液晶的螺距、折射率、及双折射率随温度变化的规律;对模拟结果给予了实验验证。     

变温红外光谱研究表面双稳态液晶分子的相变过程

运用变温红外和样本-样本相关光谱对40～150 ℃温度区间内的表面双稳态液晶分子MHOCPOOB的相变过程中的分子构象、排布及相互作用的变化进行研究。结果表明：室温时,分子烷基链中同时存在Zigzag和Gauche两种构象。随温度升高,其中有序的Zigzag构象转化为无序的Gauche 构象,链的扭曲程度增加。刚性核部分,羰基与相邻的苯环形成共轭体系,苯环之间相互倾斜排列,在相变过程中羰基与苯环的共平面作用逐渐被打破,且在相变点苯环间的二面角明显增大。由于表面稳定化的作用,使得在液晶盒表面上的一层膜,其结构并不随温度、相结构的变化而变化,因而在液晶盒的光谱中观察到的相变点较少。通过二维光谱作者发现,在122 ℃时分子出现细微结构调整。     

非拓扑孤子模型的袋常数与退禁闭相变

用有限温度场论方法,通过计算非拓扑孤子袋模型中与温度和密度相关的有效势,进而得到了袋常数随温度和化学势的变化关系,相应地讨论了退禁闭相变的性质.     

利用核磁共振氢谱的哈勒分析获得分子序列取向参数

本文通过全氢液晶材料的核磁共振氢谱（¹H NMR）的哈勒关联研究推测了分子序列取向参数,包括随温度变化的双峰距离及全谱图的半高峰宽.通过哈勒拟合并外推所得到的4'-正戊基-4'-氰基联苯（5CB）和混合液晶E7分子取向参数,与其它方法获得的取值一致.     

磁性多层膜磁特性的表面效应

利用Monte-Carlo方法和转移矩阵法研究了具有不同表面交换耦合J_s和薄膜厚度 磁性多层 膜的表面和尺寸对磁相变的影响.模拟结果表明，系统的相变温度随薄膜层数的变化取决于J_s/J(J为体内交换耦合)，当J_s/J大于某一临界值时，由于表面磁 有序先于体内磁有序 ，系统的相变温度随薄膜层数的增多而降低，反之，表面磁无序可与体内磁有序共存，系统 的相变温度随薄膜层数的增多而升高；当J_s/J较小时，随J_s增大 ，系统的居里温度缓慢 升高，趋近于体内相变温度，而当J_s/J较大时，随J_s增大，系统的 居里温度 呈线性升高.模拟结果与用转移矩阵法推导出的结果相当符合，且很好地解释了实验事实. 关键词： 磁星多层膜 交换耦合 Monte-Carlo模拟 转移矩阵法     

液晶的转动粘质系数的测试原理与方法(英文)

液晶的转动粘质度是液晶用于显示和通讯的重要物理参量.本文设计开发了基于微机的液晶参数测试光学系统,在该系统上研究了液晶的转动粘质系数的测试原理与方法:通过测试液晶盒在偏置电压下相位驰预时间,推导出转动粘质系数的值.测试Merck公司的E7液晶的转动粘质系数随温度变化的曲线,进行了理论拟合,测试结果与理论符合得很好.通过分析另一系统对液晶介质常数随温度变化的曲线的测试结果,验证了该测试原理与方法的正确性.该测试方法具有简单、自动化的特点.     

用时间分辨简并四波混频研究液晶相变前行为

我们采用时间分辨简并四波混频研究了MBBA液晶相变前分子取向弛豫时间随温度的变化。实验结果与Landau-de Gennes模型的理论预测基本相符。 关键词：     

序参量的自洽方程与手征对称性恢复的相变

本文用解析方法研究了有限温度和密度下的手征相变.把零温情形下序参量的自洽方程加以推广,建立了有限温度情形下的相应方程.由此得到了序参量随温度的变化关系,证实了手征对称性恢复的相变的存在,求出了理论的相图.     

CHIRAL PHASE TRANSITION IN A MODEL WITH DYNAMICAL SPONTANEOUS-SYMMETRY-BREAKING AT FINITE TEMPERATURE AND DENSITY

The chiral-symmetry-restoring phase transition in a model with dynamical spontaneous-symmetry-breaking is discussed qualitatively,making use of an approximation method.The selfconsistency equation of the model is established.The condensation and mass of fermions as well as the temperature or density dependence of energy density and specific heat are obtained.It turns out that,in this approximation,the chiral-phase-transition is second order at zero chemical potential and finite temperature; and the transition is first order for both cases at finite temperature and density and at zero temperature and finite density,this moment.the transition temperature or density from broken phase to normal phase differs from that from normal phase to broken phase.     

Calorimetric investigation on the paramagnetic–antiferromagnetic phase transition in CoO

The paramagnetic–antiferromagnetic phase transition of a single crystal of CoO, whose first- or second-order character is controversial, has been studied using a high sensitive calorimetric technique. Although both specific heat and differential thermal analysis (DTA) trace obtained at very low temperature rate (0.1 K h⁻¹) show strong anomalies at the Néel temperature T_N, the DTA trace and that calculated from the specific heat anomaly coincides indicating a continuous phase transition. In agreement with the theoretical predictions, the specific heat follows the 3D Ising model in a temperature range of 3 K below T_N. Fisher relation for antiferromagnets is also obeyed in the same temperature range.     

Itinerant antiferromagnetism of α-Titanium

The present study reports magnetic susceptibility, electrical resistivity and specific heat measurements on α-titanium. The experimental results indicate the existence of an anomaly in the temperature dependence of these parameters. The temperature dependence of the specific heat indicates that the anomaly observed in these parameters is a second order phase transition. The similarity between the temperature dependence of the magnetic susceptibility of α-titanium and of chromium suggests that α-titanium is an itinerant-electron antiferromagnet, such as chromium, with the Néel temperature at 276 ± 4 K.     

磁性薄膜自旋重取向行为的Monte Carlo模拟

利用Monte Carlo方法模拟了二维简单立方结构磁性薄膜的自旋重取向行为，重点研究了各向异性和偶极相互作用对系统自旋取向的影响．通过计算，获得了系统的相图以及系统组态、磁分量、比热等随偶极相互作用和温度的变化规律．模拟结果表明，在一定的参数范围内，随着温度的升高，系统的自旋取向将由垂直向平行方向转变。     

Investigation of the phase transitions in a model of metal magnetic Fe/V superlattice

A model of iron/vanadium superlattice has been proposed to study the phase transition in multilayers. The temperature dependences of the magnetization, susceptibility, and specific heat have been obtained. Two specific heat peaks are revealed at small ratios between the interlayer and intralayer exchanges.     

Microcanonical Phase Diagram of the BEG and Ising Models

The density of states of long-range Blume-Emery-Griffiths (BEG) and short-range Ising models are obtained by using Wang-Landau sampling with adaptive windows in energy and magnetization space. With accurate density of states, we are able to calculate the microcanonical specific heat of fixed magnetization introduced by Kastner et al. in the regions of positive and negative temperature. The microcanonical phase diagram of the Ising model shows a continuous phase transition at a negative temperature in energy and magnetization plane. However the phase diagram of the long-range model constructed by peaks of the microcanonical specific heat looks obviously different from the Ising chart.     

Microcanonical Phase Diagram of the BEG and Ising Models

The density of states of long-range Blume-Emery-Griffiths(BEG) and short-range Ising models are obtained by using Wang-Landau sampling with adaptive windows in energy and magnetization space.With accurate density of states,we are able to calculate the microcanonical specific heat of fixed magnetization introduced by Kastner et al.in the regions of positive and negative temperature.The microcanonical phase diagram of the Ising model shows a continuous phase transition at a negative temperature in energy and magnetization plane.However the phase diagram of the long-range model constructed by peaks of the microcanonical specific heat looks obviously different from the Ising chart.     

Monte Carlo study of 2D generalized XY-models

In this paper we study a recent generalization of the XY-model in two dimensions by using Monte Carlo method. The vortex density, specific heat, energy and critical temperature are obtained. Some results are compared with approximated analytical calculations. The nature of the phase transition as the generalization parameter varies is discussed.     

Melting temperature and enthalpy variations of phase change materials (PCMs): a differential scanning calorimetry (DSC) analysis

Differential scanning calorimetry (DSC) analysis is a standard thermal analysis technique used to determine the phase transition temperature, enthalpy, heat of fusion, specific heat and activation energy of phase change materials (PCMs). To determine the appropriate heating rate and sample mass, various DSC measurements were carried out using two kinds of PCMs, namely N-octadecane paraffin and calcium chloride hexahydrate. The variations in phase transition temperature, enthalpy, heat of fusion, specific heat and activation energy were observed within applicable heating rates and sample masses. It was found that the phase transition temperature range increased with increasing heating rate and sample mass; while the heat of fusion varied without any established pattern. The specific heat decreased with the increase of heating rate and sample mass. For accuracy purpose, it is recommended that for PCMs with high thermal conductivity (e.g. hydrated salt) the focus will be on heating rate rather than sample mass.     

The thermoelastic properties of [N(CH3)4]2ZnBr4 and [N(CH3)4]2MnBr4

The ferrodistortive phase transition in the bis-tetramethylammonium tetrabromide crystals below room temperature is studied within the framework of the Landau theory. The specific heats of [N(CH₃)₄]₂MnBr₄ and [N(CH₃)₄]₂ZnBr₄ are correctly described down to 40°C below the transition temperature. The phenomenological parameters are determined from calorimetric results, elastic constants and thermal expansion data. Using these coefficients, the monoclinic angle in the ferrodistortive phases is obtained. The anharmonic quantities, such as the isothermal compressibility, calculated from the specific heat data, are in good agreement with the values derived from the elastic measurements.