Microscopic dielectric response functions in semiconductor quantum dots

We calculate and model the microscopic dielectric response function for quantum dots using first principle methods. We find that the response is bulklike inside the quantum dots, and the reduction of the macroscopic dielectric constants is a surface effect. We present a model for the microscopic dielectric function which reproduces well the directly calculated results and can be used to solve the Poisson equation in a nanosystem.     

Decomposition of the total momentum in a linear dielectric into field and matter components

The long-standing resolution of the Abraham–Minkowski electromagnetic momentum controversy is predicated on a decomposition of the total momentum of a closed continuum electrodynamic system into separate field and matter components. Using a microscopic model of a simple linear dielectric, we derive Lagrangian equations of motion for the electric dipoles and show that the dielectric can be treated as a collection of stationary simple harmonic oscillators that are driven by the electric field and produce a polarization field in response. The macroscopic energy and momentum are defined in terms of the electric, magnetic, and polarization fields that travel through the dielectric together as a pulse of electromagnetic radiation. We conclude that both the macroscopic total energy and the macroscopic total momentum are entirely electromagnetic in nature for a simple linear dielectric in the absence of significant reflections.     

Microscopic calculation of the constitutive relations

Homogenization theory is used to calculate the macroscopic dielectric constant from the quantum microscopic dielectric function in a periodic medium. The method can be used to calculate any macroscopic constitutive relation, but it is illustrated here for the case of electrodynamics of matter. The so-called cell problem of homogenization theory is solved and an explicit expression is given for the macroscopic dielectric constant in a form akin to the Clausius-Mossotti or Lorentz-Lorenz relation. The validity of this expression is checked by showing that the standard formula is recovered for cubic materials and that the average of the microscopic energy density is the macroscopic one. Finally, the general expression is applied to Bloch eigenstates.     

Microscopic theory of modified spontaneous emission in a dielectric

The modification of the radiative decay rate of a source atom embedded in a uniform, isotropic dielectric is calculated to first order in the density of the dielectric atoms using a microscopic approach. In contrast to the recent results of Crenshaw and Bowden [Phys. Rev. Lett. 85, 1851 (2000)]], the decay rate is found to be consistent with macroscopic theories based on quantization of the field in the dielectric.     

Dielectric properties of pyridine N-oxide aqueous solution under the static electric field and microwave field

Molecular dynamics simulations of the pyridine N-oxide aqueous solution have been performed in the canonical ensemble macroscopic canonical ensemble (NVT) both in the absence and presence of an external electromagnetic field. It extracts the radial distribution function for each concentration solution, dielectric constant and other information on dielectric properties. Analysing the microscopic dielectric information of the aqueous solution under the static electric field (0–3×10⁹ V/m) and microwave frequencies (2.45G, 0–3×10⁹ V/m), and comparing the dielectric information between the different concentrations and different field strengths, we can get the dielectric properties of two kinds of polar aqueous solution under microwave irradiation. Thus, this project can provide the data of the sample to other correlation studies.     

The elastic dielectric

Macroscopic field equations, boundary conditions and equations of state are derived for the non-linear, macroscopic elastic and dielectric response of an insulator. A centrosymmetric polynomial representation of order four is introduced for the energy density; the equations of state for the electric field and stress tensor are then deduced as polynomials of degree three in the displacement gradients and electric displacement field. The results are applied to the special case of m3m material symmetry.

A finite, point-charge model of a centrosymmetric ionic crystal is introduced and used to determine 0°K microscopic expressions for the electric field and stress tensor equation of state coefficients introduced in the macroscopic analysis. The results are used to calculate the full set of second and third-order non-linear coefficients for NaI, based on a Born-Mayer potential and the 4·2°K elastic stiffness data of Claytor and Marshall.     

The method of local fields: A bridge between molecular modelling and dielectric theory

At the nanoscale, the charge distribution in a cluster of several atoms or molecules can be calculated ab initio, i.e. without free parameters. Molecular modelling is limited to a relatively small number of atoms compared to macroscopic materials with myriads of atoms. On the other hand, dielectric and ferroelectric properties of macroscopic matter are described by classical theory using mean-field approximations, e. g. the formula of Clausius–Mossotti for dielectrics and the Landau–Ginzburg–Devonshire theory or rather the molecular field theory by Weiss for ferroelectrics. In the context of multiscale simulations we present a microscopic model for dielectrics and ferroelectrics consisting of discrete atoms and / or dipoles. Parameters calculated from molecular modelling can be used here as input to our simulations in order to calculate bigger systems now. All electrostatic interactions are considered and the electrodes are taken into account using the method of images. Based on thermally activated processes, permanent dipoles fluctuate in double well potentials according to the Boltzmann statistics. Neutral atoms are modelled by induced dipoles having dipole moments proportional to the locally prevailing field. The numerical calculations are based on deterministic local field computations and on weighted probabilistic dynamic Monte Carlo steps.     

Multiferroism in the dielectric function of CuO

The multiferroic properties of bulk CuO are manifested in the dielectric function which can be triggered by an external magnetic field h and by the temperature T. Within a microscopic model and a Green's function technique we have calculated the dielectric function \varepsilon ({\bf k};T,\;h). At the magnetic phase transition temperature T_{{\rm N}2} the dielectric function offers a pronounced anomaly. This kink disappears when the magnetic field is enhanced and \varepsilon ({\bf k};T,\;h) decreases with increasing h ‐field. Both properties are indications for a strong magnetoelectric coupling in this material. The observation of multiferroism in CuO within an analytical approach is achieved by considering frustration and a linear magnetoelectric coupling. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microscopic Origin of Universality in Casimir Forces

The microscopic mechanisms for universality of Casimir forces between macroscopic conductors are displayed in a model of classical charged fluids. The model consists of two slabs in empty space at distance d containing classical charged particles in thermal equilibrium (plasma, electrolyte). A direct computation of the average force per unit surface yields, at large distance, the usual form of the Casimir force in the classical limit (up to a factor 2 due to the fact that the model does not incorporate the magnetic part of the force). Universality originates from perfect screening sum rules obeyed by the microscopic charge correlations in conductors. If one of the slabs is replaced by a macroscopic dielectric medium, the result of Lifshitz theory for the force is retrieved. The techniques used are Mayer expansions and integral equations for charged fluids.     

Domain structure of triglycine sulfate family crystals according to the data of piezoresponse force microscopy and their macroscopic dielectric properties

The domain structure of triglycine sulfate (TGS) crystals, specifically nominally pure crystals and those with impurities (LADTGS+ADP, DTSG, and TGS:Cr), is studied by piezoresponse force microscopy (PFM). The measured macroscopic dielectric characteristics are compared with microscopic data on the domain structure of these crystals. The values of the spontaneous polarization, bias voltage, and dielectric permeability as a function of temperature ?(T) are shown to be in agreement with the PFM data. The anomalous behavior of the dependence ?(T) was observed for LADTGS+ADP crystals.     

Interaction of electrons with polar optical phonons in semiconductor superlattices

On the basis of the microscopic theory of lattice dynamics, simulation of the electric potentials created by optical phonons in semiconductor superlattices is performed. It is shown that the spatial distribution of the amplitudes of electric potentials differs from that in a dielectric continuum predicted by the conventional macroscopic model without dispersion. A modified macroscopic continuum theory is proposed that takes into account the dispersion of short-range interatomic forces and allows one to obtain analytical expressions for the potentials of electron-phonon interaction.     

Effect of lateral fluorination in antiferroelectric and ferroelectric mesophases: synchrotron X-ray diffraction,dielectric spectroscopy and electro-optic study

Effect of lateral fluorination in the rigid core on several macroscopic and microscopic properties of a terphenyl based mesogenic chiral ester has been studied by synchrotron X-ray, dielectric and electro-optic techniques. Correlation lengths across the smectic planes, in para-, ferro- and antiferroelectric phases, are found to be significantly less in the fluorinated compound. Para to ferroelectric transition is found to be tricritical in nature in both the compounds. Fluorination resulted in slower response under a square pulse. Collective mode relaxation behaviour, with and without bias field, in all the phases are also found to be different in the fluorinated compound.     

Electron-phonon interaction in polar solids with application to the tungsten bronzes

We investigate the interaction between electrons and lattice vibrations in polar solids taking into account their microscopic dielectric properties and constructing in a semi-phenomenological manner the nonlocal dielectric function, ?_GG'(q). The results of earlier, phenomenological theories, such as the Clausius-Mossotti formula for the macroscopic screening constant and Fröhlich's electron-phonon coupling parameter α are obtained from the RPA dielectric function when the dipole approximation is used to calculate the screening by the strongly localized valence electrons. For polar metals we also consider the screening by the free conduction electrons. We present and discuss the results of the screened electron-phonon interaction as a function of the phonon wavevector q for the sodium tungsten bronzes.     

Electric fields and surface charge in a thin-film ferroelectric-dielectric switchable structure that were determined with a spectral probe

A new method is proposed to study the macroscopic ac fields in the elements of thin-film ferroelectric-dielectric heterostructures during their electric switching. For this purpose, the local field surrounding dielectric molecules is measured. It is important that the dielectric has a characteristic very narrow molecular absorption band, which is used as an electric-field probe. The heterostructure under study consists of a glass substrate, a transparent electrode, a 170-nm-thick layer of polymer ferroelectric, a 40-nm-thick dielectric layer, and a semitransparent electrode. Both functional layers are grown by the Langmuir-Blodgett method. An ac electric field is applied to the electrodes, and the local field having appeared in the dielectric is measured by the electroabsorption method. With allowance for the Lorentz factor, the local field is easily converted into the macroscopic field in the dielectric layer and, then, in the ferroelectric layer. The classical Sawyer-Tower scheme is used as an additional tool to determine the surface charge to be switched at the dielectric-ferroelectric interface.     

One-dimensional semiconductor in a polar solvent: Solvation and low-frequency dynamics of an excess charge carrier

Due to solvation, excess charge carriers on 1d semiconductor nanostructures immersed in polar solvents undergo self-localization into polaronic states. Using a simplified theoretical model for small-diameter structures, we study low-frequency dynamical properties of resulting 1d adiabatic polarons. The combined microscopic dynamics of the electronic charge density and the solvent leads to macroscopic Langevin dynamics of a polaron and to the appearance of local dielectric relaxation modes. Polaron mobility is evaluated as a function of system parameters. Numerical estimates indicate that the solvated carriers can have mobilities orders of magnitude lower than the intrinsic values.     

Multiscale analysis of a spatially heterogeneous microscopic traffic model

The microscopic Optimal Velocity (OV) model is posed on an inhomogeneous ring-road, consisting of two spatial regimes which differ by a scaled OV function. Parameters are chosen throughout for which all uniform flows are linearly stable. The large time behaviour of this discrete system is stationary and exhibits three types of macroscopic traffic pattern, each consisting of plateaus joined together by sharp interfaces. At a coarse level, these patterns are determined by simple flow and density balances, which in some cases have non-unique solutions. The theory of characteristics for the classical Lighthill-Whitham PDE model is then applied to explain which pattern the OV model selects. A global analysis of a second-order PDE model is then performed in an attempt to explain some qualitative details of interface structure. Finally, the full microscopic model is analysed at the linear level to explain features which cannot be described by the present macroscopic approaches.     

Spin models as microfoundation of macroscopic market models

Macroscopic price evolution models are commonly used for investment strategies. There are first promising achievements in defining microscopic agent based models for the same purpose. Microscopic models allow a deeper understanding of mechanisms in the market than the purely phenomenological macroscopic models, and thus bear the chance for better models for market regulation. However microscopic models and macroscopic models are commonly studied separately. Here, we exemplify a unified view of a microscopic and a macroscopic market model in a case study, deducing a macroscopic Langevin equation from a microscopic spin market model closely related to the Ising model. The interplay of the microscopic and the macroscopic view allows for a better understanding and adjustment of the microscopic model, as well, and may guide the construction of agent based market models as basis of macroscopic models.     

脱氧核糖核酸柔性的分子动力学模拟:Amber bsc1和bsc0力场的对比研究

脱氧核糖核酸(DNA)的结构柔性对DNA生物功能的实现具有重要作用,全原子分子动力学模拟是一种研究DNA结构柔性的重要方法.DNA的分子动力学力场在Amber bsc0基础上有了进一步的发展,即Amber bsc1.本文采用基于最新bsc1力场和先前bsc0力场的分子动力学模拟对DNA的宏观柔性和微观柔性进行对比研究,发现力场的改进对DNA宏观柔性参量的预测有一定改善,即所预测的拉伸模量和扭转-伸缩耦合比与实验值更为接近,而弯曲持久长度和扭转持久长度两种力场结果皆与实验值一致.微观分析发现,除了滑移量稍变大,bsc1力场得到的微观结构参量如扭转角和倾斜角与实验值更为接近,且新力场下DNA宏观柔性的改善与DNA的微观结构参量及其涨落紧密相关.     

Towards a theory of molecular forces between deformed media

A macroscopic theory for the molecular or Casimir interaction of dielectric materials with arbitrarily shaped surfaces is developed. The interaction is generated by the quantum and thermal fluctuations of the electromagnetic field which depend on the dielectric function of the materials. Using a path integral approach for the electromagnetic gauge field, we derive an effective Gaussian action which can be used to compute the force between the objects. No assumptions about the independence of the shape and material dependent contributions to the interaction are made. In the limiting case of flat surfaces our approach yields a simple and compact derivation of the Lifshitz theory for molecular forces [Sov. Phys. JETP 2 (1956) 73]. For ideal metals with arbitrarily deformed surfaces the effective action can be calculated explicitly. For the general case of deformed dielectric materials the applicability of perturbation theory and numerical techniques to the evaluation of the force from the effective action is discussed.