分段线性混沌电路的非光滑分岔分析

讨论了分段线性的电容混沌电路的动力学行为.由数值模拟得到了对称的周期解和混沌吸引子.通过引入广义Jacobian矩阵,以周期解为例,从理论上分析了系统由电容电量的分段线性而引起的非光滑分岔,并合理解释了系统动力学行为产生的机理及其演化规律,其结论与数值计算的结果大致符合.     

Inelastic light scattering studies of diffuse phase transition in ferroelectric Sr1.9Ca0.1NaNb5O15 thin films

The dynamic behavior of highly oriented ferroelectric Sr_1.9Ca_0.1NaNb₅O₁₅ thin films has been investigated by Brillouin light scattering over a wide temperature range between 25 and 450 °C. The temperature variations of the full‐width at half‐maximum (FWHM) of the central peak (CP) are found to exhibit marked changes around the Curie temperature (T_c = 246 °C). The CP reveals a relaxational mode behavior, which is attributed to thermally activated fast relaxation of polar nanoregions (PNRs). The width of the CP decreases steadily upon cooling from ∼400 °C to T_c and shows little subsequent change within the ferroelectric state, indicating an increase of the number of PNRs and a slowing down of their dynamics. By using a modified superparaelectric model, the activation energy (H₀) of the relaxation processes and the Burns temperature (T_B) are determined to be 2939 and 385 °C, respectively. The evidence for this diffuse phase transition provided by Brillouin scattering is in agreement with results obtained from the temperature‐dependent dielectric permittivity, refractive index, and Raman spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

负折射率媒质雷达散射特性分析

 从负折射率媒质的本构关系出发,应用分段线性递推卷积(PLRC)方法推导出负折射率媒质中的时域有限差分（FDTD）计算公式,建立了基于分段线性递推卷积的FDTD方法(PLRC-FDTD)。并应用该方法计算了负折射率媒质覆盖导体柱的雷达散射截面,分析了负折射率媒质参数对其雷达散射截面的影响。结果表明,恰当地选择负折射率媒质参数能有效减小目标的雷达散射截面。     

Raman study of phase transition in ferroelectric Ba0.95Ca0.05TiO3

Raman scattering investigation of phase transition in the ferroelectric Ba_0.95Ca_0.05TiO₃ is reported. The results suggest onset of significant dynamic disorder at 105°C. This corroborates findings of recent structural study regarding large positional disorder associated with Ti and O1 atoms well below the tetragonal to cubic transition temperature (∼150°C).     

Exactly solvable model of a structural phase transition

An exactly solvable d-dimensional model for the structural phase transition with long-range anharmonic interaction is considered. Classical as well as purely quantum regimes are discussed within the framework of the method of approximating Hamiltonians.     

BeO高压相变和声子谱的第一性原理计算

采用第一性原理方法计算了BeO在零温时的高压相变和三种结构在零温零压时的声子谱.相变的计算表明,在122GPa左右的压力下BeO会发生从纤锌矿(B4)结构到氯化钠(B1)结构的相变,而闪锌矿(B3)结构在零温零压下是一种可能的亚稳态结构.采用冷声子方法计算了这三种结构的BeO在零温零压下的声子谱.计算结果表明:B1结构在零温零压下是一种不稳定的结构;尽管B4结构和B3结构具有明显的相似性,仍然可以通过声子谱来很好的区分.最后根据准简谐近似理论计算得到了BeO的高温高压相图.     

Characterizing quantum phase transition by teleportation

In this paper we provide a novel way to explore the relation between quantum teleportation and quantum phase transition. We construct a quantum channel with a mixed state which is made from one dimensional quantum Ising chain with infinite length, and then consider the teleportation with the use of entangled Werner states as input qubits. The fidelity as a figure of merit to measure how well the quantum state is transferred is studied numerically. Remarkably we find the first-order derivative of the fidelity with respect to the parameter in quantum Ising chain exhibits a logarithmic divergence at the quantum critical point. The implications of this phenomenon and possible applications are also briefly discussed.     

负折射率媒质雷达散射特性分析

从负折射率媒质的本构关系出发,应用分段线性递推卷积(PLRC)方法推导出负折射率媒质中的时域有限差分（FDTD）计算公式,建立了基于分段线性递推卷积的FDTD方法(PLRC-FDTD)。并应用该方法计算了负折射率媒质覆盖导体柱的雷达散射截面,分析了负折射率媒质参数对其雷达散射截面的影响。结果表明,恰当地选择负折射率媒质参数能有效减小目标的雷达散射截面。     

The S-D transition and high-pressure phase stability of transition metals

Abstract

The recent discovery of a solid-solid phase change in shock-compressed Mo and the theoretical interpretation suggest valence d-electron density as a major influence on structural stability. The relationship of this experimental result to the transition metal structures and alloy phase diagrams will be discussed. Specific predictions will be presented for the locations and slopes of transition metal and transition metal alloy phase boundaries.     

Photo-induced ionic-to-neutral phase transition in tetrathiafulvalen-p-chloranil crystals

Recent experimental results are reviewed of the transformation between ionic and neutral phases in tetrathiafulvalen-p-chloranil crystals, induced by exciting the electronic states of the crystal. Spectroscopic studies have revealed that N phase domains are formed in the ionic-host crystal only above threshold-excitation intensity in the case of charge-transfer excitation; the threshold intensity depends strongly on the wavelength. On the other hand, N-phase domains can be formed without threshold in the excitation intensity in the case of intra-molecular excitation. Also, femtosecond time-resolved study has shown the intensity-dependent dynamics of the N-phase domain formation, and revealed the presence of short-lived intermediate states prior forming the N-phase domain. The mechanism of the photo-induced phase transition in this crystal is discussed based on these results.     

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.     

Structural phase transition and elastic properties of ZnSe at high pressure

We have evolved an effective interionic interaction potential to investigate the pressure-induced phase transitions from zinc blende (B3) to rock salt (B1) structure in II-VI [ZnSe] semiconductors. The elastic constants, including the long-range Coulomb and van der Waals (vdW) interactions and the short-range repulsive interaction of up to second-neighbor ions within the Hafemeister and Flygare approach, are deduced. Keeping in mind that both of the ions are polarisable, we employed the Slater-Kirkwood variational method to estimate the vdW coefficients. The estimated value of the phase transition pressure (P _t ) is higher than in the reported data, and the magnitude of the discontinuity in volume at the transition pressure is consistent with that data. The major volume discontinuity in the pressure-volume phase diagram identifies the structural phase transition from zinc blende to rock salt structure.

The variation of second-order elastic constants with pressure resembles that observed in some binary semiconductors. It is inferred that the vdW interaction is effective in obtaining the thermodynamic parameters such as the Debye temperature, the Gruneisen parameter, the thermal expansion coefficient and the compressibility. However, the inconsistency between the thermodynamic parameters as obtained from present model calculations and their experimental values is attributed to the fact that we have derived our expressions by assuming the overlap repulsion to be significant only up to the nearest second-neighbor ions, as well as neglecting thermal effects. It is thus argued that full analysis of the many physical interactions that are essential to binary semiconductors will lead to a consistent explanation of the structural and elastic properties of II–VI semiconductors.     

Identification of the pressure-induced phase transition of ZnSe with the positron annihilation method

This paper studies the pressure-induced phase transition between zincblende (B3) and NaCl (B1) structure ZnSe by using the hydrostatic pressure first-principles pseudopotential plane wave method. The energy-volume and enthalpy-pressure curves are employed to estimate the transition pressure. It is found that ZnSe undergoes a first-order phase transition from the B3 structure to the B1 structure at approximately 15 GPa derived from the energy-volume relation and 14 GPa based on deduction from enthalpy-pressure data. The pressure-related positron bulk lifetimes of the two ZnSe structures are calculated with the atomic superposition approximation method. In comparison with the 13.4% reduction in volume of ZnSe at the transition pressure, the positron bulk lifetime decreases more significantly and the relative value declines up to 22.3%. The results show that positron annihilation is an effective technique to identify and characterize the first-order phase transition and can give valuable information about changes in micro-scale, such as volume shrinkage and compressibility.     

A first-order level-2 phase transition in thermodynamic formalism

We show the existence of a phase transition at the level of measures for the generalized dimension of the maximal entropy measure in a model that was considered by F. Hofbauer and which is related to a model of M. Fisher. The model presented here is related to the one-dimensional Ising model in which a wall effect is assumed. In this situation, the problem has to be considered in the one-dimensional lattice . In general there is no first-order transition for the Ising model in the lattice , but under our assumptions such transitions can occur. The Ising model has the purpose of explaining the magnetization of ferromagnetic systems at low temperatures. The main difference of our result from a previous result of F. Hofbauer is that the transition is analyzed in the setting of the generalized dimension. This setting is more closely related to the observables. The main purpose of this paper is to explain another mathematical model for phase transition using the mathematical results obtained by F. Hofbauer. We also use results of the thermodynamic formalism in an essential way.     

晶体相场法研究预变形对熔点附近六角相/正方相相变的影响

应用双模晶体相场模型计算二维相图,并模拟了在熔点附近预变形和保温温度对六角相晶界演化以及六角相/正方相相变的影响.研究发现:在相变初期,当预变形为零、保温温度离熔点很近时在晶界发生缺陷诱发预熔;增大预变形,变形与缺陷的交互作用在熔点附近诱发预熔;随着预变形的进一步增大,变形在畸变处同时诱发液相和正方相,且预变形越大、保温温度越接近熔点,液相生长越明显,反之正方相生长明显.持续保温使得畸变能释放,晶粒最终完全转变为平衡正方相.模拟结果表明:预变形六角相在熔点附近保温时,由于晶界固有缺陷和预变形双重作用使得原子无序度增加,从而在晶界或其他缺陷处产生液相,待能量释放后晶粒再转变成平衡正方相,进而延缓了六角相/正方相相变时间.     

格子Boltzmann方法在相变过程中的应用

提出了一种新的描述气液相变过程的单组分格子Boltzmann模型,利用该模型模拟水以及氨分别在R-K,RKS和P-R状态方程控制下的相变过程,发现相对于R-K和RKS状态方程,水以及氨在P-R状态方程控制下模拟结果均与实验值更接近;特别地,P-R状态方程更适合描述氨.为验证该模型处理两相问题的能力,利用该模型模拟不同温度下水以及氨在P-R状态方程控制下的界面密度梯度,所得的结果与经典的界面理论相符.为此,进一步探讨了气泡(液滴)与周围液体(气体)处于力平衡和热平衡时,气泡(液滴)内外压力差在不同温度下与其半径之间的关系,所得的结果满足Laplace定律,并得到了不同温度下水以及氨的表面张力,发现均与实验值符合得很好,且与表面张力临界理论甚为相符.     

Hydrostatic pressure and temperature dependence study of d-camphor and dl-borneole by neutron scattering

Abstract

Neutron scattering investigations have been performed down to helium temperatures under different hydrostatic pressures up to 330 MPa. Neutron diffraction (ND) and incoherent inelastic neutron scattering (IINS) spectra showed clearly the existence of a phase transition in d-camphor, which was not observed in dl-borneole. The ND spectra point to a possible cubic structure of the dl-borneole crystal down to helium temperatures. Pressure and temperature dependence of the IINS spectra assumed a glass-like transition from the dynamical to static orientational disorder of molecules in this crystal.