55 research outputs found
Quantitative imaging of dielectric permittivity and tunability with a near-field scanning microwave microscope
We describe the use of a near-field scanning microwave microscope to image
the permittivity and tunability of bulk and thin film dielectric samples on a
length scale of about 1 micron. The microscope is sensitive to the linear
permittivity, as well as to nonlinear dielectric terms, which can be measured
as a function of an applied electric field. We introduce a versatile finite
element model for the system, which allows quantitative results to be obtained.
We demonstrate use of the microscope at 7.2 GHz with a 370 nm thick barium
strontium titanate thin film on a lanthanum aluminate substrate. This technique
is nondestructive and has broadband (0.1-50 GHz) capability. The sensitivity of
the microscope to changes in relative permittivity is 2 at permittivity = 500,
while the nonlinear dielectric tunability sensitivity is 10^-3 cm/kV.Comment: 12 pages, 10 figures, to be published in Rev. Sci. Instrum., July,
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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
Large Anomalous Hall effect in a silicon-based magnetic semiconductor
Magnetic semiconductors are attracting high interest because of their
potential use for spintronics, a new technology which merges electronics and
manipulation of conduction electron spins. (GaMn)As and (GaMn)N have recently
emerged as the most popular materials for this new technology. While Curie
temperatures are rising towards room temperature, these materials can only be
fabricated in thin film form, are heavily defective, and are not obviously
compatible with Si. We show here that it is productive to consider transition
metal monosilicides as potential alternatives. In particular, we report the
discovery that the bulk metallic magnets derived from doping the narrow gap
insulator FeSi with Co share the very high anomalous Hall conductance of
(GaMn)As, while displaying Curie temperatures as high as 53 K. Our work opens
up a new arena for spintronics, involving a bulk material based only on
transition metals and Si, and which we have proven to display a variety of
large magnetic field effects on easily measured electrical properties.Comment: 19 pages with 5 figure
Determining small refractive index contrast in chalcogenide-glass pairs at mid-infrared wavelengths
A two-composition thin film (Ge20Sb10Se70/Ge20Sb10Se67S3 atomic%core/cladding glasses) was fabricated using a hot-fibre-pressing technique in which both glasses follow the same post-fibre processing. A simple approach is proposed that uses normal incidence transmission spectra to determine their refractive index contrast over the wavelength range from 2 to 25 μm with an error of less than _ 0.002. Using an improved Swanepoel method, the calculated numerical aperture of these two compositions was within _ 0.011 of that obtained from prism minimum deviation measurements. Results show that introducing 3 atomic % S into the Ge-Sb-Se glass system lowered the refractive index and blue-shifted the visible optical bandgap, the far-infrared fundamental vibrational absorption bands and the zero-dispersion wavelength
Accounting fraud, business failure and creative auditing: A microanalysis of the strange case of the Sunbeam Corporation
This article closely examines the Sunbeam Corporation’s path to failure and explores the reasons for its singularity. From the analysis of US fraud cases included in the UCLA-LoPucki Bankruptcy Research Database, this corporate case appears as an outlier. For Sunbeam, the time-lapse between fraud disclosure and its final bankruptcy is the longest of the entire sample; it is unique because of its length. This article uses a historical microanalysis to evaluate different hypotheses about the Sunbeam Corporation’s path to failure. The relationships between acquisitions and fraud, ‘scapegoat dynamics’ and ‘creative auditing’ are identified as the most relevant issues to be examined against a changing institutional context. The resulting reconstruction of the events provides unexpected insights and recommendations for future research on auditing and accounting fraud
Stimulatory effect of Echinacea purpurea extract on the trafficking activity of mouse dendritic cells: revealed by genomic and proteomic analyses
<p>Abstract</p> <p>Background</p> <p>Several <it>Echinacea </it>species have been used as nutraceuticals or botanical drugs for "immunostimulation", but scientific evidence supporting their therapeutic use is still controversial. In this study, a phytocompound mixture extracted from the butanol fraction (BF) of a stem and leaf (S+L) extract of <it>E. purpurea </it>([BF/S+L/Ep]) containing stringently defined bioactive phytocompounds was obtained using standardized and published procedures. The transcriptomic and proteomic effects of this phytoextract on mouse bone marrow-derived dendritic cells (BMDCs) were analyzed using primary cultures.</p> <p>Results</p> <p>Treatment of BMDCs with [BF/S+L/Ep] did not significantly influence the phenotypic maturation activity of dendritic cells (DCs). Affymetrix DNA microarray and bioinformatics analyses of genes differentially expressed in DCs treated with [BF/S+L/Ep] for 4 or 12 h revealed that the majority of responsive genes were related to cell adhesion or motility (<it>Cdh10</it>, <it>Itga6</it>, <it>Cdh1</it>, <it>Gja1 </it>and <it>Mmp8</it>), or were chemokines (<it>Cxcl2, Cxcl7) </it>or signaling molecules (<it>Nrxn1, Pkce </it>and <it>Acss1</it>). TRANSPATH database analyses of gene expression and related signaling pathways in treated-DCs predicted the JNK, PP2C-α, AKT, ERK1/2 or MAPKAPK pathways as the putative targets of [BF/S+L/Ep]. In parallel, proteomic analysis showed that the expressions of metabolic-, cytoskeleton- or NF-κB signaling-related proteins were regulated by treatment with [BF/S+L/Ep]. <it>In vitro </it>flow cytometry analysis of chemotaxis-related receptors and <it>in vivo </it>cell trafficking assay further showed that DCs treated with [BF/S+L/Ep] were able to migrate more effectively to peripheral lymph node and spleen tissues than DCs treated as control groups.</p> <p>Conclusion</p> <p>Results from this study suggest that [BF/S+L/Ep] modulates DC mobility and related cellular physiology in the mouse immune system. Moreover, the signaling networks and molecules highlighted here are potential targets for nutritional or clinical application of <it>Echinacea </it>or other candidate medicinal plants.</p
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