The effect of second neighbour repulsion on fcc binary alloy phase diagrams: A Monte Carlo study

We report the results of the Monte Carlo simulation of the phase diagram of fcc binary alloys using a 3-D Ising model with nearest and next-nearest neighbour repulsive interactions. Calculations are carried out for a ratio of second- to first-neighbour interaction energies of 0.4. The resulting phase diagram contains three superstructures A₂B₂, A₂B and A₅B, each separated by a disordered fcc phase. There was no evidence for the formation of an A₃B phase. These results are in qualitative agreement with CVM results.     

Design and properties of binary axilens

A novel optical element, axilens, proposed by Davidson et al., for achieving extended focal length is introduced. We propose a method to design the binary axilens which has high diffraction efficiency. Using computer numerical simulation, we designed 4 axilenses each has different parameters. In this paper we present our results and make comparison with that in ref. [1]. The properties of binary axilens such as the focal depth, the lateral resolution and the intensity distribution along axis is discussed.     

基于二元光学设计的高能激光窗口的初步探讨

对非均匀近场强度分布影响强激光远场聚焦效果的原因进行了分析。对高能激光窗口进行了二元光学设计。提高了光束强度分布的均匀性,改善了光束质量。介绍了二元光学设计的基本原理,采用随机并行扰动爬山法对窗口的位相分布、变换后的光束强度分布、衍射效率和光束均匀性进行了计算。计算结果表明,基于二元光学设计的激光窗口能有效地改善光束强度分布的均匀性,提高了发射光束的质量。     

T1 and T2 effects during radio-frequency pulses in spoiled gradient echo sequences

Finite pulse durations in diverse pulse schemes lead to the reduction of the magnitude of the magnetization vector due to T₁ and T₂ effects during the radio-frequency pulses. This paper presents an analysis of the steady state signal in the presence of relaxation effects during radio-frequency pulses in MRI spoiled gradient echo sequences. It is shown that minor attenuations of the magnetization vector can have dramatic consequences on the measured signal, and may thus entail a loss in SNR benefits at high static magnetic fields if a careful analysis is not performed. It is emphasized that it is the time-integrated magnetization vector trajectory that matters for these effects and not only the pulse duration. Some experimental results obtained on a phantom at 3 T verify this analysis.     

静止水滴真空闪蒸模型及实验研究

本文建立了描述真空制取二元冰过程中静止水滴的传热传质模型,研究冰晶形成过程的结晶成核与生长现象及其影响因素。设计并建立了一种可视化的真空制冰系统,清晰地展示了静态水滴成核结冰时液态蒸发、伴随气泡生长的蒸发、稳态蒸发结冰、伴随气泡生长的结冰、外部结冰内部气泡逸出最终爆裂的五种形态;利用所设计的二元冰真空制备装置进行冰晶的生成实验研究,分析讨论各种因素对二元冰真空制备的影响,并通过实验对照比较来证明模型的可靠性。     

Regular Binary Thermal Lattice-Gases

We analyze the power spectrum of a regular binary thermal lattice gas in two dimensions and derive a Landau–Placzek formula, describing the power spectrum in the long-wavelength, low frequency domain, for both the full mixture and a single component in the binary mixture. The theoretical results are compared with simulations performed on this model and show a perfect agreement. The power spectrums are found to be similar in structure as the ones obtained for the continuous theory, in which the central peak is a complicated superposition of entropy and concentration contributions, due to the coupling of the fluctuations in these quantities. Spectra based on the relative difference between both components have in general additional Brillouin peaks as a consequence of the equipartition failure.     

Low dimensional chaos in the AT and GC skew profiles of DNA sequences

This paper investigates the existence of low-dimensional deterministic chaos in the AT and GC skew profiles of DNA sequences. It has taken DNA sequences from eight organisms as samples. The skew profiles are analysed using continuous wavelet transform and then nonlinear time series methods. The invariant measures of correlation dimension and the largest Lyapunov exponent are calculated. It is demonstrated that the AT and GC skew profiles of these DNA sequences all exhibit low dimensional chaotic behaviour. It suggests that chaotic properties may be ubiquitous in the DNA sequences of all organisms.     

The formation of biaxial nematic phases in binary mixtures of thermotropic liquid-crystals composed of uniaxial molecules

ABSTRACT

Monte Carlo simulations in the isothermal-isobaric ensemble are used to investigate the formation of an ordered, biaxial nematic phase in a binary mixture of thermotropic liquid crystals. The orientational dependence of the interaction between molecules of each pure component is the same as in the well-known Maier-Saupe model; each pure component of the mixture is therefore capable of forming a uniaxial nematic phase. For the interaction between molecules of different components, we use the same Maier-Saupe model but change the sign of the coupling constant. As a consequence a T-shaped arrangement of these molecules is energetically favoured. The formation of the biaxial phase occurs in two steps. At higher temperatures T, one of the components forms a uniaxial nematic phase whereas the other is in a quasi two-dimensional restricted isotropic liquid state. We develop a simple theoretical model to understand the high degree of (ostensible) nematic order in the latter. At lower T, the second component becomes nematic and then the entire mixture of the two compounds has biaxial symmetry. The biaxial nematic phase does not demix into domains rich in molecules of one or the other species.