Response coefficient for the bulk photovoltaic effect in ferroelectrics

The response coefficient for the bulk photovoltaic effect is derived from second order response theory for free electrons as well as for diffusive carrier motion in low mobility systems.     

A viscoelastic model of polarization switching in polymer ferroelectrics

The theoretical aspects of the viscoelastic model for polarization switching in ??soft?? organic ferroelectrics have been considered. The model describes the amplitude-frequency dependences of the hysteresis loops obtained upon polarization switching in thin films of the ferroelectric copolymer poly(vinylidene fluoride-trifluoroethylene).     

The possible mechanism for the bulk photovoltaic effect and optical damage in ferroelectrics

The bulk photovoltaic effect and the photorefractive effect in some oxygen octahedra ferroelectrics (e.g., LiNbO₃, KNbO₃, (Sr, Ba) Nb₂O₆, BaTiO₃) are interpreted on the base of the fluctuations model. The kinetics of the photovoltaic effect and its temperature and lux-ampere characteristic are investigated.     

Pseudo-spin model of compositionally graded ferroelectrics

Recently, much attention has been focused on the study of compositionally graded ferroelectric films (CGFF). We devote this article to the microscopic theory of CGFF. Our theory is based on the transverse Ising pseudo-spin model. However the Hamiltonian used in recent papers does not contain the linear term in pseudo-spins. We show that it is necessary to modify the Ising Hamiltonian for homogenous material by addition of linear term in pseudo-spin. This term corresponds to the energy of the equivalent external field. Due to this term polarization is also included in the paraelectric phase.     

Tunability response in exponentially graded ferroelectrics: A TIM model approach

Relative dielectric function response associate to a non-homogeneous layered ferroelectric system is calculated in the framework of the Mean Field Approximation (MFA) for the Transverse Ising Model (TIM). Analytical self-consistent expressions for the average polarization, dielectric susceptibility, and tunability percentage are outlined and solved for different configurations and sizes. It is found that exponentially graded ferroelectrics magnify the tunability response for stronger interlayer coupling and it reaches its saturation value for smaller intensities of the applied electric field.     

A new formulation for the existence and calculation of nonlinear normal modes

A new formulation is presented here for the existence and calculation of nonlinear normal modes in undamped nonlinear autonomous mechanical systems. As in the linear case an expression is developed for the mode in terms of the amplitude, mode shape and frequency, with the distinctive feature that the last two quantities are amplitude and total phase dependent. The dynamic of the periodic response is defined by a one-dimensional nonlinear differential equation governing the total phase motion. The period of the oscillations, depending only on the amplitude, is easily deduced. It is established that the frequency and the mode shape provide the solution to a 2π-periodic nonlinear eigenvalue problem, from which a numerical Galerkin procedure is developed for approximating the nonlinear modes. The procedure is applied to various mechanical systems with two degrees of freedom.     

A model calculation in a new approach to the three-body problem

A method to calculate the binding energy of a system of three identical particles is proposed. The kernel of a previously derived integral equation in two variables is replaced by an approximation of finite rank. It turns out that a one term approximation is already sufficient to determine the binding energy very well.     

Dipolar interaction in the transverse Ising model of H-bonded ferroelectrics

Numerical solutions are presented for the set of approximated kinetic equations previously obtained to describe the dynamical behaviour of the pseudospin correlation functions of the transverse Ising model in H-bonded ferroelectrics. Stability of the soft mode and spectral characteristics of the correlation functions as T → ∞ are discussed in terms of an interaction made of both nearest-neighbour and dipole-dipole contributions. This infinite-temperature calculation shows how shift (towards zero frequency) and width of the soft mode vary with the interdipolar strength of the interaction. The asymmetry of the imaginary part of susceptibility _X″zz(ω) exhibited by KDP in its paraelectric phase is also shown to be due to the dipolar character of the interaction.     

With the alpha-cluster model to explain the change of separating energy

It was supposed that, the nucleus was composed of α-cluster, pn-pair, and nn-pair. The reciprocity of the α-cluster, pn-pair, and nn-pair caused the regular change of the separating energy to separate the nn-pair in the exotic nuclei. The regular change was that the separating energy was high behind low to separate the nn-pair in the light and exotic nuclei. This phenomenon must had more profound physical meaning.     

With the alpha-cluster model to explain the change of separating energy

It was supposed that, the nucleus was composed of α-cluster, pn-pair, and nn-pair. The reciprocity of the α-cluster, pn-pair, and nn-pair caused the regular change of the separating energy to separate the nn-pair in the exotic nuclei. The regular change was that the separating energy was high behind low to separate the nn-pair in the light and exotic nuclei. This phenomenon must had more profound physical meaning.     

A full numerical calculation of the Franz--Keldysh effect on magnetoexcitons in a bulk semiconductor

We have performed a full numerical calculation of the Franz--Keldysh (FK) effect on magnetoexcitons in a bulk GaAs semiconductor. By employing an initial value method in combination with the application of a perfect matched layer, the numerical effort and storage size are dramatically reduced due to a significant reduction in both computed domain and number of base functions. In the absence of an electric field, the higher magnetoexcitonic peaks show distinct Fano lineshape due to the degeneracy with continuum states of the lower Landau levels. The magnetoexcitons that belong to the zeroth Landau level remain in bound states and lead to Lorentzian lineshape, because they are not degenerated with continuum states. In the presence of an electric field, the FK effect on each magnetoexcitonic resonance can be identified for high magnetic fields. However, for low magnetic fields, the FK oscillations dominate the spectrum structure in the vicinity of the bandgap edge and the magnetoexcitonic resonances dominate the spectrum structure of higher energies. In the moderate electric fields, the interplay of FK effect and magnetoexcitonic resonance leads to a complex and rich structure in the absorption spectrum.     

铁电体中新畴成核经典模型的改进

新畴成核是外加低场下铁电体中铁电畴反转的一个重要的过程.首先介绍了新畴成核的经典模型,采用该模型研究了铁电畴反转的新畴成核过程,发现理论计算的成核速率与外场关系和实验观测结果不一致.在Tagantsev模型的基础上,选取不同的成核形状对新畴成核的经典模型进行了改进,并获得了和实验观测相符的理论计算结果. 关键词： 铁电体 铁电畴 成核速率     

Temperature dependence of effective charge in dipole model of ferroelectrics

Combining the concept of the effective charge in the dipole model with the statistical theory of ferroelectricity proposed previously, the temperature dependence of the effective charge is evaluated numerically for various values of dipole interaction.     

Extended potentials in the CPA: A model calculation

We show how the usual Coherent Potential Approximation for disordered substitutional alloys may be generalized to situations where the atomic potentials are not confined to one site. We present numerical results for the simplest one-dimensional case, and discuss the density of states and the localization of electrons in the impurity band.     

A proof of the existence of phase transitions in the anisotropic Heisenberg model

It is rigorously proved that the anisotropic Heisenberg spin model, in two or more dimensions, exhibits a first order phase transition at low temperatures and for large anisotropy (strong coupling of the third components of the spins).     

Multiclass first-order simulation model to explain non-linear traffic phenomena

With the first-order traffic model of Lighthill, Whitham and Richard (LWR), many simple traffic problems can be represented analytically such as a shock formation. However, the LWR model has some deficiencies. For example, among the other things, it fails to replicate interesting non-linear phenomena such as hysteresis and capacity drop as well as the dispersion of traffic platoon when there exists a distribution of desired speeds in heterogeneous traffic. To this end, in this paper, we propose a novel multiclass first-order simulation model based on an approximation of Riemann solver. In the developed model, each vehicle class is only characterized by their desired speeds in a free-flow traffic state where overtaking is allowed. However, when traffic is congested, all vehicle classes must travel at the same congested speed and overtaking is not possible. Numerical results show that the proposed model is not only more accurate and reliable than the existing models but also able to explain non-linear traffic phenomena on freeways.