A formal theory of the conductivity and application to the giant magnetoresistance

The transport in a system with inhomogeneous elastic scattering is described in terms of a probabilityconserved Boltzmann equation. We demonstrate that the spatially varied current density depends only on the voltage drop between the ends of the sample. This fact enables us to develop a formal and general theory for the conductivity without determining the actual electric field inside the sample. The theory is first applied to multilayer systems and shown to recover the previous theory. By including the spin-dependent interface scattering and bulk scattering, we employ our theory to account for the giant magnetoresistance (MR) in magnetic granular systems with both spherical and cylindrical granules. The results obtained reproduce the experimental dependence of the MR on annealing temperature.     

Phenomenological theory of the giant magnetoresistance of ferromagnet-nonmagnetic metal granular media

A phenomenological model is developed for the giant magnetoresistance of granular media comprising ferromagnetic granules in a nonmagnetic metal matrix. Both volume and surface spin-dependent scattering by the ferromagnetic granules are taken into account. The internal electric fields are inhomogeneous because of the different conductivities of the granules and the matrix. The dependence of the effective conductivity of the medium on the average magnetization is calculated and used to explain the giant magnetoresistance effect. The magnetoresistance is plotted as a function of the volume concentration of ferromagnetic granules and the granule radius. Experiments on Co-Cu and Co-Ag granular films are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1871–1875 (October 1998)     

Optical properties of planar structures containing oxidized copper nanoparticles

Planar structures consisting of oxidized copper granules obtained by laser electrodispersion are studied. The samples have different packing densities of granules and different amounts of their chains and aggregates. Each granule 5.5 ± 0.5 nm in size consists of a copper core with an amorphous structure and an oxide shell of about 0.7 nm thick. Some granules are randomly charged. The spectra of coherent transmission, diffusion transmission, and reflection of the samples are measured. Using the experimental data, the absorption spectra and the effective absorption, extinction, and scattering coefficients of monolayers are calculated and the luminescence spectra are estimated. A long-wavelength shift of the plasmon resonance of the copper granules with oxide shells as compared to that of the unoxidized granules is observed. The shift depends on the thickness of the oxide layer. A similar shift of the plasmon resonance is observed for the chains of copper granules. The spectra are compared with the spectra calculated theoretically taking into account some parameters of the planar structures and the size dependence of the optical constants of copper. The luminescence observed in some cases is associated with specifics of oxidation of copper granules.     

Optical and magnetooptical properties of granular alloys with giant magnetoresistance in the IR region of the spectrum

The features of the optical and magnetooptical properties of granular alloys with giant magnetoresistance in the IR region are examined in reference to the magnetorefractive effect and the equatorial Kerr effect. Calculations are performed within the semiclassical approximation with consideration of spin-dependent scattering in the bulk of the granules and on their surfaces (interfaces). The expressions obtained for σ _xx(ω) and σ _xy(ω) are found to be sensitive to scattering on the surfaces and in the bulk of the granules, as well as to granule size, the type of impurities trapped on the interfaces, the frequency of the incident light, and the external magnetic field. For granular thin films exhibiting giant magnetoresistance, the theory predicts significant relative changes in the optical reflection and transmission coefficients when the sample is magnetized to saturation (0.02% and 20%, respectively, for giant magnetoresistance of the order of 20%), as well as Kerr and Faraday effects that are nonlinear with respect to magnetization. Zh. éksp. Teor. Fiz. 116, 1762–1769 (November 1999)     

表面特征与软X射线掠入射光学散射

介绍了短波段掠入射表面散射线性模型,并且根据这个理论,分析计算了不同的表面特征对软X射线掠入射光学散射特性的影响。实验发现:随着粗糙度的增加、自相关长度的减小,软X射线掠入射光学散射越来越严重。     

Characterization of Nanoparticles by Scattering Techniques

Basic principles and applications of different scattering techniques (including static and dynamic light scattering (SLS and DLS), small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD) and small-angle neutron scattering (SANS)) on the characterization of nanoparticles are reviewed in this paper. By choosing a suitable scattering technique or a combination of different techniques for nanoparticle characterization, the particles' molecular weight, radius of gyration, hydrodynamic radius, size distribution, shape and internal structure as well as interparticle interactions of nanoparticles, can be determined. Examples including some sophisticated colloidal systems are presented.     

用中子散射法研究凝聚态物质的新进展

在近２０年间,由于中子源和散射装置的改进,中子散射在凝聚态物质中的应用日益广泛,许多方面是其它（ｘ射线、电子）散射技术无可比拟的。本文在简单评述供散射用的中子源和散射实验技术进展之后,重点介绍中子散射在凝聚态物质研究中的应用。它们包括晶体结构和磁结构的测定、表面、界面和薄膜的表征、测定结构涨落、磁涨落的现代相变研究、畸变、无序系统（包括分形和小角散射）和高分子材料、高Ｔ＿ｃ氧化物超导体的研究,工业上应用也作了简短讨论。     

Dynamics of self-assembled systems studied by neutron scattering: Current state and perspectives

In general self-assembled systems, composed of building blocks and aggregates, exhibit rather complex dynamic properties, which so far are only poorly understood, despite their high importance both for fundamental understanding and applications. Due to their typical sizes of 2-200 nm they are in general very suited for investigations by means of neutron scattering. This has been exploited extensively for the determination of their static mesoscopic structure but is similarly feasible for the investigation of their dynamic properties. This review gives an overview about investigations done so far by means of neutron scattering with respect to time-dependent properties of self-aggregating systems. In general, two types of studies can be distinguished: dynamics under equilibrium conditions (e.g. structural fluctuations) or morphological changes as a function of time (non-equilibrium). In the first case typically inelastic neutron scattering is applied in order to obtain information about dynamic properties of the systems, such as membrane elasticity or movements in membranes. The other direction of experiments are kinetic investigations of changes of the mesoscopic structure in self-aggregating systems after changing composition, temperature or other external parameters, which are mostly followed by time-resolved SANS. These investigations have given substantial new insight into morphological changes occurring in self-assembling systems and altered our view on it. This field of ??mesodynamics?? is still an emerging research topic of colloid science but neutron scattering has already contributed substantially to it and will certainly do much more so in the future. Especially as for more complex systems neutron scattering with its flexibility in contrast allows to obtain unique structural and dynamical information. This will certainly be of central importance for developing our understanding of more complex self-assembled systems further and also lead to new approaches for bringing them to novel applications.     

Characteristics of plastic scintillator granules

Methods are described for measuring the absorption coefficient of the material, the absorption coefficient of a layer of granules, and the scintillation performance. The theory of light diffusion in a scattering medium is used and is compared with experiment.We are indebted to A. R. Daich and L. E. Pargamanik for a discussion.     

Effects of matrices on Mie scattering in the mid-infrared region

Various Mie scattering systems, each having a transparent matrix, are studied in the mid-infrared region. Our three theoretical scattering systems correspond to a lossless scatterer, an anomalous dispersive dielectric scatterer and a metal scatterer, each in a non-air usual matrix. The refractive-index effects of the matrix on scattering and extinction efficiencies in the mid-infrared region are found to be quite different in different cases. Although the non-air usual matrix reduces scattering and extinction efficiencies in the first kind of system, it may or may not help scatter and extinguish the mid-infrared radiation in the second, and it has little effect on them in the third.     

The effect of multiple scattering in disperse media on polarization characteristics of scattered light

The effect of scattering of different multiplicity on polarization characteristics of scattered light is studied by the Monte Carlo computer simulation technique. The scattering multiplicity distribution versus the direction of scattering and dimensions of the scattering system is obtained for monodisperse systems of spherical particles of different size. The angular dependences of the elements of the light-scattering matrix (LSM) are calculated. It is shown that in a system of spherical particles, specific features of the LSM structure associated with multiple scattering have much in common with similar features of the LSM in systems of nonspherical particles under conditions of single scattering. The angular dependences of the degree of depolarization of the scattered light are studied.     

Colloids as model systems for metals and alloys: a case study of crystallization

Metallic systems are widely used as materials in daily human life. Their properties depend very much on the production route. In order to improve the production process and even develop novel materials a detailed knowledge of all physical processes involved in crystallization is mandatory. Atomic systems like metals are characterized by very high relaxation rates, which make direct investigations of crystallization very difficult and in some cases impossible. In contrast, phase transitions in colloidal systems are very sluggish and colloidal suspensions are optically transparent. Therefore, colloidal systems are often discussed as model systems for metals. In the present work, we study the crystallization process of charged colloidal systems from the very beginning. Charged colloids offer the advantage that the interaction potential can be systematically tuned by a variation of the particle number density and the salt concentration. We apply light scattering and ultra-small angle x-ray scattering to investigate the formation of short-range order in the liquid state even far from equilibrium, crystal nucleation and crystal growth. The results are compared with equivalent studies on metallic systems.     

Long-time relaxation of the magnetization and tunneling magnetoresistance of granular ferromagnets

Magnetic anisotropy and orientational variance as well as shape diversity of granules largely determine the magnetic properties of granular ferromagnetic metals. The model of magnetically anisotropic ellipsoidal granules explains the glassy nature of the magnetic state of such systems. The relaxation of the magnetization and the magnetoresistance of granular ferromagnetic metals is examined on the basis of this model.     

The scattering spectra and color of disperse systems of weakly absorbing particles

The scattering spectra of weakly absorbing systems of spherical particles are calculated using various approximate and numerical methods. Comparative estimates are made for applicability of the single-scattering and mean-field approximations and the Monte Carlo computer simulation for different scattering multiplicities. The results of the calculations well agree with the experimental scattering spectra of the crystalline lens. Based on the spectral characteristics obtained, the chromaticity coordinates are calculated and regularities in variations of the color characteristics are analyzed as functions of the disperse system parameters.     

Coherent effects in crystal collimation

We present theory for coherent effects observed in crystal collimation experiments that is in good quantitative agreement with the RHIC and Tevatron data. We show that these effects are caused by a coherent scattering on the field of bent crystal atomic planes, which amplifies beam diffusion in accelerator by orders of magnitude compared to the scattering in amorphous material. This coherent scattering could replace the traditional amorphous scattering in accelerator collimation systems. We predict that for negative particles this effect is as strong as for positive ones, opening a principle way for efficient crystal steering of negative particles at accelerators. Predictions are made for high energy accelerators where crystal collimation is seen as an interesting application.     

General N-Body Theory of Nonrelativistic Quantum Scattering

 The two-Hilbert-space theory of scattering is reviewed with particular reference to its application to nonrelativistic multichannel quantum- mechanical scattering theory. In Part I the abstract assumptions of the theory are collected, transition operators (both on- and off-energy-shell) are defined, the dynamical equations that determine the off-shell transition operators are presented and their real-energy limits examined, and the convergence of sequences of approximate transition operators is established. A section on how to incorporate group symmetries into the formalism reports new work. The material of Part I is relevant to a variety of both classical and quantum scattering systems. In Part II attention is directed specifically to N-body nonrelativistic quantum scattering systems in which the particles interact via short-range pair potentials. A method of constructing approximate transition operators is presented and shown to satisfy all the abstract assumptions of Part I. The dynamical equations that determine the half-on-shell approximate transition operators are shown to be coupled one-dimensional integral equations that have compact kernels and unique solutions when considered as operators on a Hilbert space of H?lder continuous functions. Moreover, the on-shell parts of those approximate transition amplitudes are shown to converge to the exact on-shell amplitudes as the order of the approximation increases. Detailed formulas for the kernels of the integral equations are written down for systems of particles that are distinguishable and for systems containing identical particles. Finally, some important open problems are described. Received July 2, 1999; accepted in final form October 27, 1999     

Scattering and propagation of terahertz pulses in random soot aggregate systems

Scattering and propagation of terahertz pulses in random soot aggregate systems are studied by using the generalized multi-particle Mie-solution(GMM) and the pulse propagation theory. Soot aggregates are obtained by the diffusion-limited aggregation(DLA) model. For a soot aggregate in soot aggregate systems, scattering characteristics are analyzed by using the GMM. Scattering intensities versus scattering angles are given. The effects of different positions of the aggregate on the scattering intensities, scattering cross sections, extinction cross sections, and absorption cross sections are computed and compared. Based on pulse propagation in random media, the transmission of terahertz pulses in random soot aggregate systems is determined by the two-frequency mutual coherence function. Numerical simulations and analysis are given for terahertz pulses(0.7956 THz).     

Basin topology in dissipative chaotic scattering

Chaotic scattering in open Hamiltonian systems under weak dissipation is not only of fundamental interest but also important for problems of current concern such as the advection and transport of inertial particles in fluid flows. Previous work using discrete maps demonstrated that nonhyperbolic chaotic scattering is structurally unstable in the sense that the algebraic decay of scattering particles immediately becomes exponential in the presence of weak dissipation. Here we extend the result to continuous-time Hamiltonian systems by using the Henon-Heiles system as a prototype model. More importantly, we go beyond to investigate the basin structure of scattering dynamics. A surprising finding is that, in the common case where multiple destinations exist for scattering trajectories, Wada basin boundaries are common and they appear to be structurally stable under weak dissipation, even when other characteristics of the nonhyperbolic scattering dynamics are not. We provide numerical evidence and a geometric theory for the structural stability of the complex basin topology.     

Resonances in positron-atom scattering

Theoretical studies of atomic resonances involving positrons will be discussed in this talk. Investigations on resonances in positron-hydrogen scattering below various hydrogen and positronium thresholds are reviewed, as well as resonances in positronalkali and e⁺-He⁺ scattering. Resonance phenomena in other atomic systems involving positrons will also be discussed. These systems include positronium ions Ps⁻, positronium molecules Ps₂, and positronium hydride PsH.