Order-disorder transitions in atactic polystyrene films

The results of the electron microscopy investigation of the surface topology of the films obtained from solutions of linear atactic polystyrene in chloroform are presented. It has been shown that the distribution of density fluctuations in the films can be described using the model of a fractal percolation cluster of macromolecular coils. A decrease in the local packing density of particles upon going from θ-coils to blobs is associated with mutual penetration of the coils. An increase in density fluctuations and a decrease in the relative area and fractal surface of the cluster of the particles are associated with a decrease in the short-range order caused by the formation of the percolation cluster, which reflects portions of the chains not involved in the blobs.     

Order-disorder transitions in alloys near Au3Cu

We applied diffractometry to the order-disorder transition in Cu-72.5 at.% Au alloy with a Ll₂ superlattice; the long-range order parameter and the lattice parameter have been measured in relation to annealing temperature. Disordering occurs through a two-phase Ll₂ + Al region; the upper bound to the phase-transition temperature is 208±3°C. A study has been made of the dimensions of the antiphase domains as defined by superlattice lines.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 70–74, March, 1980.     

磁通格子的有序-无序相变和反向熔化

考虑了无序钉扎、热涨落和磁通互作用, 用Monte Carlo分子动力学模拟方法研究二维磁通格子在无序钉扎强度和温度空间的相图, 以及由布拉格玻璃相到非晶磁通玻璃相和到磁通液体相的有序-无序相变.为了决定磁通格子的序，计算了静态结构因子和磁通格子位形的有限尺寸指数.计算结果表明，Bragg玻璃相在低温的无序磁通玻璃相和高温的磁通液体相之间 , 表现出磁通格子的反向熔化行为.分析后认为，这一反向熔化行为起因于磁通之间互作用的温度效应. 关键词： Ⅱ类超导体 磁通格子 相图 结构因子 反向熔化     

Order-disorder transitions: structural and magnetic ordering in double perovskites

Using an effective lattice gas-like model for B-B′ site ordering we study order-disorder transitions in double perovskite materials A₂BB′O₆. We motivate this effective model from a microscopic hamiltonian. Using this model, we are able to address several experimentally observed issues including nonmonotonic dependence of the degree of order on annealing temperature, and the rapid decrease of order upon overdoping with either B or B′ species. We also study ordering in the ‘ternary’ compounds A₂BB′_1−yB″_yO₆, using a variant of the Blume-Emery-Griffiths model. Several issues related to structural and magnetic ordering are discussed.     

Order-disorder transitions in a chemisorbed system: Hydrogen on Ni (111)

Atomic hydrogen chemisorbed on a Ni (111) surface forms at coverages between 0.3 and 0.6 an ordered 2 × 1-structure as observed by low energy electron diffraction (LEED). The intensity of the fractional-order LEED spots was measured at different coverages as a function of temperature. Continuous order-disorder transitions are found, the maximum transition temperature (270 K) being at θ = 0.5. The phase diagram, however, is asymmetric with respect to this coverage and can therefore presumably not be explained on the basis of (independent) pairwise interactions.     

Beam quality in a turbulent mixing medium

A criterion for evaluating beam quality is discussed.By dividing the mediumdisturbances into the ordered and random part, introducing the time-averaged variance ofphase distortion and fitting the wavefront with Zernike polynomials,a simplified formula forevaluating beam quality in a turbulent mixing medium is derived.     

Order-disorder structural phase transitions described by the three-states potts model

Orientational ordering of spontaneously distorted building blocks of a crystal is a mechanism for order-disorder structural phase transitions. We study as a model substance the case of a simple cubic array of tetragonally deformed octahedral complexes. The three equivalent tetragonal deformations along the cube axes are taken as the three states of the Potts model. We investigate the various structures which can appear depending on the type of interaction between the complexes. The phase diagram, the stability of the various phases, the type of transitions, and the temperature dependence of the order parameter are studied in mean-field approximation. We further show that piezodistortive coupling to the elastic strain may stabilize different low-temperature structures, and that it gives rise to a renormalization of elastic constants and to acoustic anomalies.Work supported by the Swiss National Science Foundation     

Resolving power of antennas in a turbulent medium

Using a linear measure of the resolving power, the resolution of an antenna with a circular aperture is calculated for the case when a flat turbulent layer is situated between the antenna and the point of observation. Formulas are obtained for the radius of the illuminated zone beyond the layer for an arbitrary shape of radiation pattern and for arbitrary fluctuation properties of the complex phase on emerging from the layer. It is assumed that the complex phase fluctuations are statistically homogeneous, and a detailed examination is made of the case when the power pattern of the antenna is Gaussian, and a turbulent model is taken for the refractive index fluctuations in the layer.Izvestiya VUZ. Radiofizika, Vol. 9, No. 5, pp. 888–899, 1966     

Absolute parametric instability in a turbulent,inhomogeneous medium

It is shown analytically that the convective saturation of parametric instabilities in an infinite inhomogeneous medium can disappear in the presence of background turbulence. Absolute instability then occurs.