The Anderson transition in conjugated polymers

We study the electronic transport properties of mutually connected wires with loops in a “spaghetti”—like morphology by transfer matrix scaling methods. We obtain the conductance of the system and show the existence of an Anderson metal-insulator transition. Our results are discussed in connection with recent experiments in conjugated polymers.     

Pressure effect on phase transition in ferroelectic polymers

Pressure effects of phase transition behaviour in two kinds of ferroelectric polymers of poly(vinylidene fluoride), PVDF, and copolymers of vinylidene fluoride and trifluoroethylene, (VDF/TrFE), are discussed. In the case of PVDF, several high-pressure treatments including a high-pressure annealing and a uniaxial compression were shown to induce a crystal transformation from a non-polar Form II crystal to a polar Form I crystal, which has ferroelectric characteristics and high piezoelectric activity. In addition, substantial pressure effects on ferroelectric phase transition points as well as crystal structures were observed for (VDF/TrFE) copolymers with different VDF contents. The most significant pressure effects were observed for copolymer samples with unstable ferroelectric structures at atmospheric pressure. From high-pressure X-ray and Raman scattering studies, these pressure effects were suggested to originate from the pressure-induced conformational transition from gauche to trans in the molecular chains.     

Influence of boundary conditions on the ordering of elastic dipoles

We consider elastic dipoles in several two-dimensional geometries. Using Cauchy integral techniques and the image method, the state of elastic equilibrium under different boundary conditions can be determined. The results are used to find the ground state of systems of anisotropic dipoles via the simulated annealing method. Only in the case of fixed boundaries the ordering depends on the boundary condition.     

Formation and stability of coreless vortex dipoles in phase-separated binary condensates

We study the motion of the Gaussian obstacle potential created by a blue detuned laser beam through a phase-separated binary condensate in the pancake-shaped traps. We show that phase-separated binary condensates like ⁸⁵Rb–⁸⁷Rb, with appropriate interaction parameters, can be used experimentally to create obstacle assisted droplet and coreless vortex dipoles. We theoretically analyze the energetic stability of condensates with normal and coreless vortices. We confirm our analytic and semi-analytic results by numerical solutions of coupled Gross–Pitaevskii equations.     

拉伸分子高次谐波产生的阿秒控制

通过数值求解一维含时薛定谔方程,本文研究了高频阿秒脉冲对拉伸双原子分子模型与低频飞秒激光脉冲相互作用产生高次谐波谱的影响.研究表明,若阿秒脉冲在低频脉冲的第二个光学周期截止处注入时,只第二个平台截止位置IP 5.6UP(IP为电离势,UP为有质动力能)附近谐波的效率得到显著提高;若阿秒脉冲的包络峰值处于低频脉冲第三个光学周期T/4(为飞秒激光脉冲的光学周期)附近时,则谐波谱出现了明显的整体抬高现象.对此,本文根据拉伸分子谐波谱的四种产生机制及阿秒脉冲在飞秒激光不同相位注入时的电子电离曲线,对这些谐波谱的结构给出了合理解释.     

Observation of ferroelectricity induced by defect dipoles in the strain-free epitaxial CaTiO3 thin film

CaTiO₃ is a well-known incipient ferroelectric material that does not undergo a ferroelectric phase transition in spite of the intriguing dielectric constant behavior. Especially, unlike a prototypical incipient ferroelectric SrTiO₃, the paraelectric state of CaTiO₃ cannot be easily destroyed by small perturbations, including cation doping and epitaxial strain. We present that a nearly strain-free epitaxial CaTiO₃ film grown at a low oxygen partial pressure exhibits polarization–voltage hysteresis loops and the distinct difference of piezoresponse force microscopy phase signals, implying that a ferroelectric phase is induced. Such results are shown even at room temperature. We suggest that the observed ferroelectric behavior in CaTiO₃ film comes from the defect dipoles composed of vacancies inside the film. Using electron-probe microanalysis and optical absorption spectra measurements, we found that CaTiO₃ film has considerable Ca and O vacancies, forming the localized defect state in electronic structure. This work highlights the importance of vacancies and their clusters, such as defect dipoles, in understanding the electronic properties of perovskite oxide thin films, including ferroelectricity.     

真实头模型中的多电流偶极子脑磁源定位

给出了真实头模型下多电流偶极子定位的通用公式,并对多种源的情况进行了计算机模拟,逆问题的求解采用全局优化与局域优化相结合的优化方法,在较强的噪声背景下得到了较好的定位结果.其中,在偶极子源较少时,采用了球对称导体模型与真实头模型相结合的混合模型,进一步提高了求解速度 关键词： 脑磁技术 真实头模型 多偶极子脑磁源 优化方法     

Switching of partially switched KNbO3 single crystals containing cooperatively ordered impurity dipoles

Switching studies have been carried out in partially switched KNbO₃ single crystals by observing switching transients and hysteresis loops. The crystals used contained ordered impurity dipoles that are active in nucleating domains around them. Partial initial switching was obtained by applying known compressive stress to the crystal by means of a spring. The partially switched nature was determined by recording the photograph of the crystal surface. The changed domain structure on the surface gave a clear idea of the extent of partial switching. As the compressive stress was gradually increased, the crystal showed increased initial mechanical switching through the mechanism of evaporation of domain walls associated with ordered impurity dipoles. The dipoles then switch systematically converting 90° domains with polar axes in the plane of plate into 60° domains with polar axes in the perpendicular pseudocubic {001} planes. The initial switching condition changes the switching characteristics as determined by hysteresis loops and switching transients. The results are interpreted in terms of domains in the crystal. If the dipole density is quite high, the effect of the dipoles becomes negligible, and the switching behaviour approximates that of a normal ferroelectric. The switching transients and the hysteresis loops in the crystals containing cooperatively ordered dipoles are basically different from the ones observed in normal ferroelectrics. The anomalous behaviour is detrimental to the use of material in device applications. Hence, it is shown that the switching transients and hysteresis loops provide a ready means of detecting the presence of these ordered impurity dipoles.     

The structure of strongly dipolar hard sphere fluids with extended dipoles by Monte Carlo simulations

We have investigated the structural change of dipolar hard sphere fluid while we change the dipole from an idealised point dipole (pDHS fluid) to a physically more realistic extended dipole (eDHS fluid) by increasing the distance d of the two point charges ±q while keeping the dipole moment μ = qd fixed. We discuss our results on the basis of the first- and second-rank orientational order parameters, angular distribution functions, chain-length distributions, and snapshots. At a low density, we have found chain formation with longer chains as the distance d is increased. At a high density, we have found phase transition from an orientationally ordered ferroelectric nematic phase (at low d) into an isotropic liquid containing chains (at large d).     

Self-diffusion of multiarm star polymers in solution far from and near the ordering transition

The self-diffusion of three 128-arm polybutadiene star solutions in toluene was investigated over a broad concentration range from dilute to the ordering region with pulsed field gradient NMR (PFG-NMR). The strong concentration dependence of the self-diffusion coefficient in the fluid state is distinguished clearly from that of linear chains and can be described by a non-Arrhenius VFT-like equation with the concentration playing the role of inverse temperature. In the concentrated regime, the observation of two dynamic phases reflects the coexistence of crystalline and liquid phases over a limited concentration region. The concentration dependences of both the ordinary (fluid-like) diffusion and the completely restricted in-cage motion of these hyper stars are in agreement with the behavior of concentrated colloidal suspensions.     

Structural phase transition in ferroelectric fluorine polymers: X-ray diffraction and infrared/Raman spectroscopic study

The structural phase transition has been investigated by X-ray diffraction and infrared and Raman spectroscopic measurements for ferroelectric fluorine polymers, including poly(vinylidene fluoride) and its copolymers with trifluoroethylene or tetrafluoroethylene. One of the most characteristic features of this ferroelectric transition is the large conformational change of the molecular chains between the trans and gauche rotational isomers, quite different from the structural change observed generally in the usual ionic ferroelectric materials. The crystallization and transition behaviors depend sensitively on the monomer composition in the copolymers as well as on the sample preparation conditions. The roles of the optic and acoustic phonons in the ferroelectric phase transition have been discussed based on the temperature dependences of the far-infrared spectra and the ultrasonic velocity.     

利用脑磁图-多重信号分类算法求解真实头模型中磁源定位问题

脑磁图-多重信号分类(MEG-MUSIC)算法与常见的利用时域空域数据定位多偶极子源的全局优化方法相比，具有求解速度快、独立确定各偶极子源的位置、求解难度与偶极子数量无关等优点，在偶极子数量较多的情况下，MEG-MUSIC算法的优势显得更为突出.利用MEG-MUSIC算法求解了真实头模型中脑磁源定位问题. 关键词： 脑磁图-多重信号分类算法 真实头模型 多偶极子     

The localized near-field enhancement of metallic periodic bowtie structure: An oscillating dipoles picture

The metallic periodic bowtie structure is systematically investigated using a three-dimensional finite difference time domain (FDTD) method. The oscillating dipoles picture is proposed to explain the novel enhancement of localized near-field. It is indicated that the shift and the intensity change of notable resonant dips in the transmission spectrum result from the variation of localized surface plasmon resonance (LSPR) conditions due to the different geometrical sizes of the structure and the array periodicity. The results are helpful for guiding the bowtie structures to the application of the near-field imaging and sensing in the subwavelength optics.     

Isotropic-to-nematic transition in melt of polydisperse semi-flexible homopolymers

The isotropic-to-nematic phase transition in a melt of semi-flexible homopolymers with length polydispersity have been considered within the Landau–de Gennes approach. The number of monomer units in chain is assumed to be a random variable distributed by the Schulz–Zimm distribution; the stiffness of macromolecules has been taken into account within discrete worm-like chain model. It was found that increase of polydispersity yields the increase of the temperature of the isotropic–nematic transition.     

Adsorbate/absorbate interactions with organic ferroelectric polymers

We discuss the interactions of adsorbates with the organic ferroelectric copolymer poly(vinylidene fluoride (PVDF)–trifluoroethylene (TrFE)). Range of molecular adsorbates is discussed from the smaller polar molecules like water, which is small enough to both adsorb and absorb, to the larger macrocyclic metal–organic metal phthalocyanines. The changes in local dipole orientation may affect the strength of the coupling between adsorbate or absorbate and the copolymer poly(vinylidene fluoride–trifluoroethylene). The interface dipole interactions may also affect device properties. The dipole interactions are implicated at the interface between copper phthalocyanine and poly(vinylidene fluoride with trifluoroethylene) affecting the band offsets and the diode properties.     

Possible stretched exponential parametrization for humidity absorption in polymers

Polymer thin films have irregular transient current characteristics under constant voltage. In hydrophilic and hydrophobic polymers, the irregularity is also known to depend on the humidity absorbed by the polymer sample. Different stretched exponential models are studied and it is shown that the absorption of humidity as a function of time can be adequately modelled by a class of these stretched exponential absorption models.     

引张线技术在加速器准直测量中应用概述

为研究引张线技术在加速器准直测量中的应用和发展,首先简介了引张线技术原理,然后回顾了引张线准直法在加速器工程中的发展历史,接着介绍了引张线在加速器准直测量中的最新发展动态。最后讨论了各种引张线准直法的优缺点及其适宜环境,比较了国内外引张线准直法的发展与区别。指出引张线准直法的发展方向,给出了加速器准直测量的努力方向。     

Quantum phase transition in a two-dimensional system of dipoles

The ground-state phase diagram of a two-dimensional Bose system with dipole-dipole interactions is studied by means of a quantum Monte Carlo technique. Our calculation predicts a quantum phase transition from a gas to a solid phase when the density increases. In the gas phase, the condensate fraction is calculated as a function of the density. Using the Feynman approximation, the collective excitation branch is studied and the appearance of a roton minimum is observed. The results of the static structure factor at both sides of the gas-solid phase are also presented. The Lindemann ratio at the transition point becomes gamma=0.230(6). The condensate fraction in the gas phase is estimated as a function of the density.