Polarization of thick polyvinylidene fluoride/trifluoroethylene copolymer films

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Theory of the anomalous Hall effect from the Kubo formula and the Dirac equation

A model to treat the anomalous Hall effect is developed. Based on the Kubo formalism and on the Dirac equation, this model allows the simultaneous calculation of the skew-scattering and side-jump contributions to the anomalous Hall conductivity. The continuity and the consistency with the weak-relativistic limit described by the Pauli Hamiltonian is shown. For both approaches, Dirac and Pauli, the Feynman diagrams, which lead to the skew-scattering and the side-jump contributions, are underlined. In order to illustrate this method, we apply it to a particular case: a ferromagnetic bulk compound in the limit of weak-scattering and free-electrons approximation. Explicit expressions for the anomalous Hall conductivity for both skew-scattering and side-jump mechanisms are obtained. Within this model, the recently predicted ''spin Hall effect'' appears naturally

Localization corrections to the anomalous Hall effect in a ferromagnet

We calculate the localization corrections to the anomalous Hall conductivity related to the contribution of spin-orbit scattering into the current vertex (side-jump mechanism). We show that in contrast to the ordinary Hall effect, there exists a nonvanishing localization correction to the anomalous Hall resistivity. The correction to the anomalous Hall conductivity vanishes in the case of side-jump mechanism, but is nonzero for the skew scattering. The total correction to the nondiagonal conductivity related to both mechanisms, does not compensate the correction to the diagonal conductivity.Comment: 7 pages with 7 figure

Fast extraction of neuron morphologies from large-scale SBFSEM image stacks

Neuron morphology is frequently used to classify cell-types in the mammalian cortex. Apart from the shape of the soma and the axonal projections, morphological classification is largely defined by the dendrites of a neuron and their subcellular compartments, referred to as dendritic spines. The dimensions of a neuron’s dendritic compartment, including its spines, is also a major determinant of the passive and active electrical excitability of dendrites. Furthermore, the dimensions of dendritic branches and spines change during postnatal development and, possibly, following some types of neuronal activity patterns, changes depending on the activity of a neuron. Due to their small size, accurate quantitation of spine number and structure is difficult to achieve (Larkman, J Comp Neurol 306:332, 1991). Here we follow an analysis approach using high-resolution EM techniques. Serial block-face scanning electron microscopy (SBFSEM) enables automated imaging of large specimen volumes at high resolution. The large data sets generated by this technique make manual reconstruction of neuronal structure laborious. Here we present NeuroStruct, a reconstruction environment developed for fast and automated analysis of large SBFSEM data sets containing individual stained neurons using optimized algorithms for CPU and GPU hardware. NeuroStruct is based on 3D operators and integrates image information from image stacks of individual neurons filled with biocytin and stained with osmium tetroxide. The focus of the presented work is the reconstruction of dendritic branches with detailed representation of spines. NeuroStruct delivers both a 3D surface model of the reconstructed structures and a 1D geometrical model corresponding to the skeleton of the reconstructed structures. Both representations are a prerequisite for analysis of morphological characteristics and simulation signalling within a neuron that capture the influence of spines

Novel Methods for Analysing Bacterial Tracks Reveal Persistence in Rhodobacter sphaeroides

Tracking bacteria using video microscopy is a powerful experimental approach to probe their motile behaviour. The trajectories obtained contain much information relating to the complex patterns of bacterial motility. However, methods for the quantitative analysis of such data are limited. Most swimming bacteria move in approximately straight lines, interspersed with random reorientation phases. It is therefore necessary to segment observed tracks into swimming and reorientation phases to extract useful statistics. We present novel robust analysis tools to discern these two phases in tracks. Our methods comprise a simple and effective protocol for removing spurious tracks from tracking datasets, followed by analysis based on a two-state hidden Markov model, taking advantage of the availability of mutant strains that exhibit swimming-only or reorientating-only motion to generate an empirical prior distribution. Using simulated tracks with varying levels of added noise, we validate our methods and compare them with an existing heuristic method. To our knowledge this is the first example of a systematic assessment of analysis methods in this field. The new methods are substantially more robust to noise and introduce less systematic bias than the heuristic method. We apply our methods to tracks obtained from the bacterial species Rhodobacter sphaeroides and Escherichia coli. Our results demonstrate that R. sphaeroides exhibits persistence over the course of a tumbling event, which is a novel result with important implications in the study of this and similar species

A simple theory of the anomalous hall effect in semiconductors

The anomalous Hall effect in a two band semiconductor is treated by an elementary theory based on the use of an effective Hamiltonian in the conduction band. Simple equations of motion for the position and momentum provide an obvious interpretation of the results. The theory is worked out for arbitrary frequency and d.c. magnetic field, and for a spin magnetization which is either stationary or resonant. The physical nature of the various terms that contribute to the anomalous current is discussed in detail, and it is compared to the various existing models (most of which prove to be incomplete).L'effet Hall anormal dans un semiconducteur à deux bandes est décrit dans le cadre d'une théorie élémentaire fondée sur un Hamiltonien effectif dans la bande de conduction. Des équations de mouvement simples pour la position et la quantité de mouvement fournissent une interprétation évidente des résultats. La théorie est développée pour une fréquence ou un champ magnétique quelconque, pour une aimantation de spin stationnaire ou résonnante. La nature physique des différents termes contribuant aux courant anormal est discutée en détail, et confrontée aux divers modèles existants (dont la plupart s'avèrent incomplets)

A new method for studying piezoelectric materials

In the preparation of piezoelectric materials, it is generally necessary to apply to an insulating material a high poling electric field. During this operation various processes can take place which can strongly influence the performances of the obtained transducer. Until now experience and know how have been the leading factors for the determination of the poling parameters. This may change due to a new experimental technique which can bring informations on what is happening in the bulk of the material. This technique uses the effect of a pressure wave propagation (PWP) in the sample. In the present paper we will analyze the various processes which can occur while applying an electric field to insulating materials such as ceramics or polymers. We will then describe the basic principles of the PWP method and show how it can lead to crucial informations. A typical experimental set up will be presented and preliminary data obtained on such transducer materials as quartz, polymer or ceramics will be described and discussed.L'élaboration de matériaux piézoélectriques nécessite généralement l'application d'un champ électrique intense à un matériau isolant, afin d'y créer une polarisation permanente. Pendant cette opération, divers effets peuvent se produire et influencer considérablement les performances du transducteur obtenu. Jusqu'à présent les conditions de polarisation n'étaient déterminées que par l'expérience et le savoir-faire. La mise au point d'une technique expérimentale nouvelle qui apporte des informations directes sur la distribution de charges ou de polarisation à l'intérieur du matériau pourrait radicalement changer cette situation. Cette technique utilise l'effet de la propagation d'une onde de pression (méthode PWP) dans le matériau à tester. Nous analyserons les divers processus qui peuvent se produire lorsqu'un champ électrique est appliqué à des matériaux isolants tels que des céramiques ou des polymères. Nous exposerons ensuite le principe de la méthode PWP et montrerons qu'elle peut conduire à des informations cruciales. Un dispositif expérimental sera décrit et des résultats préliminaires obtenus sur des matériaux pour transducteurs tels que des monocristaux de quartz, des films de polymères et des céramiques seront présentés et discutés