取向玻璃体系中分子之间取向关联的模型化及其模拟与分析

提出了一个描述取向玻璃系统中分子间取向关联的有效键模型.基于该模型对该系统中分子之间取向关联的拓扑结构进行了计算机模拟.结果表明当成键概率较小时,成键分子大部分以分子串的形式出现,并基于概率分析对此现象进行了解释.深入地,用分子串模型对模拟结果进行了分析,并且基于分析结果讨论了分子串模型的合理性和可行性. 关键词： 玻璃化转变 取向玻璃 拓扑结构 分子串模型     

Low-temperature specific heat of orientational glasses

This review summarizes specific heat data measured at low temperatures (T<1 K) on orientational glasses. Three species of mixed molecular crystals exhibiting orientational disorder are considered, namely (KBr)_1–x (KCN) _x , (NaCN)_1–x (KCN) _x (Rb)_1–x (NH₄) _x H₂PO₄. For intermediate concentrations of the anisotropic components, glass-like excitations have been observed. It is demonstrated that with respect to thermal properties, orientational disorder leads to the same universal behaviours than for structural disorder, i.e. a specific heat which varies below 1 K and for times 10^–4 s–10 s as:C _p(T,t)T ¹×ln(t). The variation of the glass-like anomaly with compositional disorder is also discussed. It is concluded that the low-energy excitations are related to orientational degrees of freedom which are frozen at low temperatures but still perform reorientations via quantum tunneling motions.     

Bond orientational order in atomic clusters

Bond orientational order parameters for the global minimum energy configurations of a number of Lennard-Jones and Morse clusters are calculated. The results demonstrate that the bond orientational order parameters, particularly the third-order invariants, provide a fairly discriminating set of symmetry markers for different cluster geometries. Since they are relatively easy to calculate from simulation data, they may be useful in studies of local order in a wide variety of contexts, including studies of melting, nucleation and the glass transition     

Long-time relaxation behavior in orientational glasses

The long-time relaxation observed in the orientational glass phase of solid ortho-parahydrogen mixtures is related to the low-temperature specific heat using a simple unified model. The use of stimulated spin echoes to study slow molecular motions is discussed in detail. The analysis is based on the domination of low-energy large-scale cluster excitations in the relaxation processes as introduced in Fisher and Huse's recent picture of the low-temperature behavior of short-range spin glasses.     

Landau theory of long-range orientational order

The most general quartic Landau free energies are constructed for orientational order parameters which transform like L = 2, 4, and 6 irreducible representations of SO(3). In each case a complete phase diagram is determined using group theoretical techniques in conjunction with the linear programming method. The accessible low-symmetry phases for L = 6 representation are D_∞, Y, O, and D₆. For L = 4 they are D_∞, O, and D₄. D_∞ is accessible for L = 2. In the cases L = 6 and 4 it is found that the phase diagram is dominated by the icosahedral and octahedral symmetries, respectively. The bond densities in the low-symmetry phases are also analyzed.     

A model for two dimensional orientational order

We use a generalization of Kikuchi's cluster variational method (CVM), in the pair approximation, to study anisotropic plane rotors on a square lattice as a simple model for two dimensional nematics. The system was shown to exhibit nematic ordered states at low temperature, disordering, for all anisotropies, through a continuous transition. The orientational distribution functions, order parameter, internal energy and specific heat were calculated within the CVM and compared with the results of a recent computer simulation. Estimates for the non-classical exponents were obtained using a mean field renormalization group analysis. A spin wave argument was also used to investigate the role of long range fluctuations near the isotropic fixed point. The accuracy of these different techniques is discussed within the context of our model.     

On the orientational dependence of giant magnetoresistance

The functional dependence of the giant magnetoresistance (GMR) with respect to the relative angle between the orientations of the magnetization in the magnetic slabs of a trilayer system is calculated by using the Kubo-Greenwood formula for electrical transport together with the fully-relativistic spin-polarized screened Korringa-Kohn-Rostoker method for semi-infinite systems and the coherent potential approximation. It is found that the functional dependence of the GMR is essentially of the form . Received 30 November 1998     

Local orientational order in the stockmayer model

A new method based on the local analysis of the orientational order has been presented for analysis of the phase behavior of the Stockmayer fluid. A quantity has been introduced to quantitatively describe the ordering degree of particles at small distances. It has been used to analyze the phase diagram of the model under consideration.     

Shear thinning and orientational ordering of wormlike micelles

Shear thinning and orientation of cylindrical surfactant and block copolymer micelles was investigated by rheo-SANS (small-angle neutron scattering) experiments. Shear thinning and orientation occur for shear rates (.)gamma tau(dis)>1, where tau(dis) is the disentanglement time of the micelles. Micelles align in the flow direction with an orientational distribution that can be well described by an Onsager-type distribution function. Over nearly the whole range of concentrations and for all cylindrical micelles investigated, the shear viscosity eta follows a simple eta approximately e(-aS) behavior as a function of the orientational order parameter S with the same prefactor a.     

Dipole orientational order at liquid/vapor surfaces

We present ellipsometric observations of the orientational order alpha(2) of highly polar molecules at the noncritical liquid/vapor surface of critical polar+nonpolar mixtures. The dipoles, which are repelled from the interface via interactions with their image dipoles, are preferentially oriented with their axes parallel to the surface and possess an orientational order which is well described by alpha(2) approximately -t(2beta)D+(z/xi), where t=[T-T(c)]/T(c) is the reduced temperature, beta=0.328 is a critical exponent, and D+ is a universal function of the dimensionless depth z/xi with surface correlation length xi.     

Scaling properties of dynamic response in orientational glasses

The freezing transition in dipolar and quadrupolar glasses is characterized by the presence of local random electric and strain fields generated by substitutional disorder. The dynamic response in the ergodic phase above the freezing temperature T_F is studied in terms of Langevin dynamics applied to the recently formulated symmetry-adapted random-bond-random-field (SARBRF) model of orientational glasses. Following the theory of spin glasses it is assumed that for T≥T_F the response can be written in a dynamic scaling form by introducing a scaling exponent v and a frequency scaling variable. The value of v(T) is explicitly evaluated for the quadrupolar (100) SARBRF model, and its relation to the experimentally observed effective exponent u_eFF(T) in dipolar and quadrupolar glasses is discussed.