Das Lorentzfeld und die Dipolkopplung

The field at the center of a Lorentz-sphere due to all dipoles within this sphere has been calculated, assuming inhomogeneous orientations of the dipoles in the sphere, as one must expect in real crystals. This field has very often been neglected. The calculation shows the existence of a coupling field which stiffens the dipole directions against localized torques due to “distortion fields”. In the case of ferromagnetism, one can calculate the coupling field of an expanded Lorentz volume. This is analogous to the quantummechanical exchange field, which is due to the excess exchange energy. In many cases the above dipole coupling field is more important than the wellknown exchange field because the “dipole coupling constants” exceed the exchange constant by orders of magnitude.     

Elastic Analysis of Physisorption-Induced Substrate Deformation

Physisorption may cause a dimple on a deformable solid surface due to adsorbat-substrate interaction. The interactive force between the adsorbate and the crystal atoms depends on their distances, which may change with substrate deformation. This feature of displacement-dependence indicates that the equilibrium problem is a force-deformation coupled nonlinear procedure. In the present study, a continuum mechanics model, in which the force is considered as a function of the displacement field of the medium, is presented to calculate the physisorption-induced deformation in a semi-infinite elastic medium. It is found that the nonlinear effect due to force-deformation coupling should be taken in consideration in the adsorbate-substrate interaction analysis.     

Effect of microscopic inhomogeneity of the medium on reaction-diffusion front velocity

A model of random walks on an inhomogeneous lattice is used to analyze the effect of microscopic inhomogeneity of the medium on reaction-diffusion front velocity. It is shown that the front velocity can be substantially higher on an inhomogeneous lattice as compared to that on a homogeneous one with the same onsite waiting time. The increase in velocity is due to a bias in the particle distribution between sites of different types toward those with shorter waiting times. The amount of bias depends on the rate of chemical reaction: the higher the rate, the stronger the bias. The front velocity also strongly depends on the reaction mechanism, more precisely, on how product particles are distributed between sites of different types.     

Particle transfer problem in a two-region inhomogeneous slab with generalized boundary condition

The problem of particle transfer in a two-region inhomogeneous slab of equal or unequal thickness has been solved with general boundary conditions. A Galerkin-iterative technique is used to solve the coupled integral equations for two-regions. Numerical results are obtained for the reflection and the transmission coefficients for the homogeneous and inhomogeneous cases with diffusely and specularly reflecting boundaries condition. Results obtained for homogeneous and inhomogeneous medium are compared well with the published calculations.     

Synchronization and oscillator death in oscillatory media with stirring

The effect of stirring in an inhomogeneous oscillatory medium is investigated. We show that the stirring rate can control the macroscopic behavior of the system producing collective oscillations (synchronization) or complete quenching of the oscillations (oscillator death). We interpret the homogenization rate due to mixing as a measure of global coupling and compare the phase diagrams of stirred oscillatory media and of populations of globally coupled oscillators.     

Adsorption of carbon monoxide on Pt{100} surfaces: dependence of the CO stretching vibrational frequency on surface coverage

We have used the ab initio cluster model approach to study the dependence of the CO stretching frequency on CO surface coverage. We have also investigated the relative importance of the various factors that can affect the position of the CO stretching band as coverage increases. Two effects can change the CO stretching frequency: the adsorbate–adsorbate dipole coupling, which is a purely physical effect, and the changes in the 2π^* CO molecular orbitals, due to the different chemical environment at higher coverages. From our vibrational analysis, we conclude that CO–CO dipole coupling is the main cause of the upward shift of the CO stretching band when the CO coverage is increased. The population of the 2π^* CO molecular orbitals does not change at any coverage within the region considered. We have also estimated the ¹²CO–¹³CO dipole coupling, which previous studies have assumed to be weak. Our results demonstrate that the ¹²CO–¹³CO dipole coupling is indeed weak compared with the ¹²CO–¹²CO dipole coupling. At a CO surface coverage of 0.5 monolayers (ML), we have calculated a band shift of 40 cm⁻¹ to higher frequency. However, we should point out that when one ¹²CO molecule is surrounded by a ¹³CO environment, the ¹²CO stretching band shifts 10 cm⁻¹ upwards. We have also computed the heat of adsorption of CO on Pt{100}-(1×1) as a function of CO coverage. The initial heat of adsorption is calculated to be about 192 kJ mol⁻¹ and then drops to 180 kJ mol⁻¹ at 0.5 ML. These results agree quite well with recent calorimetric measurements. Besides that, we have estimated that the CO–CO interaction energy at 0.5 ML is repulsive and has a value of 5 kJ mol⁻¹.     

一维对称光子晶体的简正耦合模

对一维对称光子晶体中的色散介质采用洛仑兹振子模型,通过考虑色散介质层两侧的边界条件,得到了表征色散介质层的转移矩阵.对线性层及色散δ层均采用传输矩阵的方法,研究了一维含色散介质的光子晶体微腔中的简正耦合模.由于光与色散介质的相互作用,纵腔模将分裂成简正耦合模.通过改变色散介质的相关参数,详细研究了简正耦合模频率的移动、均匀展宽效应和失谐效应.发现两个简正的耦合模的频率间距主要依赖于振子的耦合强度,与约化的振子的HWHM线宽无关.失谐效应则会使其中的一个峰降低,而另一个峰相对拉高,这一现象可以通过Fabry-Pérot腔得到很好的解释.     

Soliton lattices in the Gross–Pitaevskii equation with nonlocal and repulsive coupling

Spatially-periodic patterns are studied in nonlocally coupled Gross–Pitaevskii equation. We show first that spatially periodic patterns appear in a model with the dipole–dipole interaction. Next, we study a model with a finite-range coupling, and show that a repulsively coupled system is closely related with an attractively coupled system and its soliton solution becomes a building block of the spatially-periodic structure. That is, the spatially-periodic structure can be interpreted as a soliton lattice. An approximate form of the soliton is given by a variational method. Furthermore, the effects of the rotating harmonic potential and spin-orbit coupling are numerically studied.     

Nucleation theory for a model bistable chemical reaction

The theory of homogeneous nucleation is developed for a model nonlinear bistable chemical reaction driven far from equilibrium (trimolecular Schlögl model). The theory is restricted to the vicinity of the stable/unstable transition, where the nucleation barrier is small but nonvanishing. The nucleation rates are derived for two types of fluctuations: first, fluctuations due to a homogeneous external white noise source, and second, internal chemical fluctuations, described by a reastion-diffusion multivariate master equation. In the white noise case, a Landau-Ginzburg potential can be defined, and the standard nucleation formalism can be applied; this is not true for the internal case and a new result is used. The inhomogeneous chemical fluctuations, due to the coupling between the nonlinear reaction and diffusion, are shown to have an influence on the nucleation rate. Quantitative conditions are also given to evaluate the possibility of homogeneous nucleation in nonlinear chemical systems.     

On the interaction of strange attractors

This paper deals with the dynamics of diffusively coupled strange attractors. Such interaction tends to equalize their instantaneous states and, for large coupling constant, results in a homogeneous state that is chaotic in time. The stability of this state depends on the relation between the Lyapunov exponent and the coupling constant. Statistical properties are determined for weakly inhomogeneous disturbances near a stable homogeneous regime. The inhomogeneous state beyond the stability threshold is treated by using the mean-field approximation. We show that both cases of soft (supercritical) and hard (subcritical) excitation of the inhomogeneous state may occur.     

On the theory of multiple scattering of sound waves by spherical particles in liquid and elastic media

Two independent systems of equations are derived for describing the scalar and vector potentials of the sound field in a liquid or elastic medium containing discrete inhomogeneities. One of the systems determines the sound field as the sum of the fields scattered by the particles according to the law of scattering by a single particle with the oscillation amplitudes governed by the properties of the inhomogeneous medium. The other system determines the sound field as the sum of the scattered fields formed in the inhomogeneous medium with the oscillation amplitudes of a particle in a homogeneous medium. Expressions relating the fields that occur in a medium consisting of N particles to the fields in a medium consisting of N − 1 particles are proposed. These expressions may simplify and diversify the methods used for computer simulation of sound fields with the aim to verify the calculations. The results of the study are valid for any particle concentrations under the condition that the scattering by a single particle is determined by its monopole, dipole, and rotary oscillations.     

各向异性超导体的Ginzburg-Landau理论

本文内容主要有二个方面：1）由各各同性均匀系的BCS理论扩展到各向异性非均匀系，并给出各种异性非均匀系的完整的GL方程式，定出了各个宏微观参量间的关系式。2）结合层状结构氧化物超导体的导电层在导电层相间的特征等，用有效调制势模型和电子有效质量近似，交各向同性均匀系的BCS理论较具体的扩展到各向异性非均匀系并进行深化研究，扩展理论所给出的临界温度和能隙方程公式，主要的热力学性质和电磁性质公式等应用到YBaCuO超导体上，理论结果与实验结果均相符，并也给出各向异性非均匀系的完整的GL方程式，与上面1所给出的形式相同，也定出了各个宏微观参量间的关系，应用到YBaCuO超导体的一些主要性质上也与实验结果相符。     

各向异性超导体的Ginzburg-Landau理论

本文内容主要有二个方面 :1 )由各向同性均匀系的BCS理论扩展到各向异性非均匀系 ,并给出各向异性非均匀系的完整的GL方程式 ,定出了各个宏微观参量间的关系式。2 )结合层状结构氧化物超导体的导电层与非导电层相间的特征等 ,用有效调制势模型和电子有效质量近似 ,将各向同性均匀系的BCS理论较具体地扩展到各向异性非均匀系并进行深化研究。扩展理论所给出的临界温度和能隙方程公式 ,主要的热力学性质和电磁性质公式等应用到YBaCuO超导体上 ,理论结果与实验结果均相符 ,并也给出各向异性非均匀系的完整的GL方程式 ,与上面 1所给出的形式相同 ,也定出了各个宏微观参量间的关系 ,应用到YBaCuO超导体的一些主要性质上也与实验结果相符。     

Coulomb bare interactions in inhomogeneous 4-layer graphene structures

We investigate Coulomb bare interactions in 4-layer graphene systems, taking into account the inhomogeneity of dielectric environment. By solving Poisson equation in momentum space, 16 elements of Coulomb potential tensor have been formed with analytical expressions. The formulae illustrate that Coulomb bare interactions in inhomogeneous 4-layer graphene system differ noticeably from those in homogeneous one. Nevertheless, both intra- and interlayer Coulomb potentials become independent with dielectric constants of contacting media, but they are decided by those of dielectric layer surrounding the system in long wavelength approximation. Besides, numerical results demonstrate that Coulomb bare interactions decrease considerably with the increase in carrier density in graphene sheets as well as their separated distance. In all investigations, the inhomogeneity of dielectric background should not be neglected for improvement in the model.     

金纳米棒三聚体中的等离激元诱导透明

基于金纳米棒构成的三聚体微元结构模型,详细地研究了等离激元诱导透明(plasmon induced transparency,PIT)现象产生的物理过程.研究发现,三聚体的吸收谱线随着其耦合距离以及尺寸的变化,竖直金纳米棒所对应的偶极明模在平行双长条金纳米棒对应的暗模作用下会产生分裂.依据这一结果提出了一个新的物理解释,PIT现象的产生主要来自于竖直金纳米棒中偶极振荡的模式分裂后的相干叠加.同时,考虑到两个振子之间的耦合会伴随着一定的相位关联性,进而引入了耦合相位因子修正了洛伦兹振子耦合模型,解析地研究了耦合相位因子对吸收谱的调控作用和分裂明模之间的相干叠加效应对PIT效应的影响.这为在纳米尺寸范围设计人造原子、光开关、慢光效应等方面的应用提供了理论参考.     

Equations for the distributions of functionals of a random-walk trajectory in an inhomogeneous medium

Based on the random-trap model and using the mean-field approximation, we derive an equation that allows the distribution of a functional of the trajectory of a particle making random walks over inhomogeneous-lattice site to be calculated. The derived equation is a generalization of the Feynman-Kac equation to an inhomogeneous medium. We also derive a backward equation in which not the final position of the particle but its position at the initial time is used as an independent variable. As an example of applying the derived equations, we consider the one-dimensional problem of calculating the first-passage time distribution. We show that the average first-passage times for homogeneous and inhomogeneous media with identical diffusion coefficients coincide, but the variance of the distribution for an inhomogeneous medium can be many times larger than that for a homogeneous one.     

Angle-resolved UV photoelectron spectroscopy of ethylene and benzene on nickel

A review of results obtained by Angle-Resolved UV-Photoelectron Spectroscopy (ARUPS) using linearly polarized synchrotron radiation is presented for two model systems, ethylene/Ni and benzene/Ni. It is shown that for these systems detailed conclusions concerning adsorbate/substrate and adsorbate/adsorbate interactions can be derived from ARUPS spectra using symmetry selection rules, and in combination with model calculations. In particular, electronic structure, bonding, orientation and symmetry of the adsorbates in dilute and saturated layers will be discussed. It is shown that at high adsorbate coverages lateral interactions in the adsorbate layer play a dominant role. Steric effects in densely packed layers can lead to a reorientation of the molecules as compared to the orientation of the single molecules. The ARUPS spectra of well ordered, densely packed layers exhibit significant (up to 2 eV) dispersion of the various adsorbate bands and allow detailed conclusions on two-dimensional adsorbate band structures.     

Magnetic field studies of the frictional drag between coupled two-dimensional electronic systems – Coulomb versus phonon coupling

The coupling between systems of two spatially separated two-dimensional (2D) electron gases and between systems of a separated 2D electron gas and a 2D hole gas is studied as a function of magnetic field . The small barrier (30 nm) separated coupled electron gases showed a transition from a phonon dominated interaction at to a Coulomb dominated one at quantising fields. For large barriers (190 nm) phonons have been found to be the dominant coupling mechanism both at zero and finite fields. However, for all barriers investigated we could observe novel screening effects manifested in a suppression of the coupling at half-filled Landau levels. For the coupled electron–hole gases we have investigated samples with large barriers ( ) so that the coupling is both in zero and finite fields dominated by phonon mediated processes. The enhanced screening effects could not be observed in those samples possibly due to the less pronounced quantisation of the hole gas.     

Why spin ice obeys the ice rules

The low-temperature entropy of the spin ice compounds, such as and , is well described by the nearest-neighbor antiferromagnetic Ising model on the pyrochlore lattice, i.e., by the "ice rules." This is surprising since the dominant coupling between the spins is their long ranged dipole interaction. We show that this phenomenon can be understood rather elegantly: one can construct a model dipole interaction, by adding terms of shorter range, which yields precisely the same ground states, and hence entropy, as the nearest-neighbor interaction. A treatment of the small difference between the model and true dipole interactions reproduces the numerical work by Gingras et al. in detail. We are also led to a more general concept of projective equivalence between interactions.