156 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,
200
Longitudinal associations between body mass index, physical activity, and healthy dietary behaviors in adults: A parallel latent growth curve modeling approach
Background Physical activity (PA) and healthy dietary behaviors (HDB) are two well-documented lifestyle factors influencing body mass index (BMI). This study examined 7-year longitudinal associations between changes in PA, HDB, and BMI among adults using a parallel latent growth curve modeling (LGCM).
Methods We used prospective cohort data collected by a private company (SimplyWell LLC, Omaha, NE, USA) implementing a workplace health screening program. Data from a total of 2,579 adults who provided valid BMI, PA, and HDB information for at least 5 out of 7 follow-up years from the time they entered the program were analyzed. PA and HDB were subjectively measured during an annual online health survey. Height and weight measured during an annual onsite health screening were used to calculate BMI (kgm2). The parallel LGCMs stratified by gender and baseline weight status (normal: BMI30) were fitted to examine the longitudinal associations of changes in PA and HDB with change in BMI over years.
Results On average, BMI gradually increased over years, at rates ranging from 0.06 to 0.20 kgm 2year, with larger increases observed among those of normal baseline weight status across genders. The increases in PA and HDB were independently associated with a smaller increase in BMI for obese males (b = -1.70 and -1.98, respectively), and overweight females (b = -1.85 and -2.46, respectively) and obese females (b = -2.78 and -3.08,respectively). However, no significant associations of baseline PA and HDB with changes in BMI were observed.
Conclusions Our study suggests that gradual increases in PA and HDB are independently associated with smaller increases in BMI in overweight and obese adults, but not in normal weight individuals. Further study is warranted to address factors that check increases in BMI in normal weight adults
Interface magnetic anisotropy in cobalt clusters embedded in a platinum or niobium matrix
A low concentration of cobalt clusters with a fcc structure and containing
almost one thousand atoms are embedded in two different metallic matrices:
platinum and niobium. Samples have been prepared using a co-deposition
technique. Cobalt clusters preformed in the gas phase and matrix atoms are
simultaneously deposited on a silicon substrate under Ultra High Vacuum
conditions. This original technique allows to prepare nanostructured systems
from miscible elements such as Co/Pt and Co/Nb in which clusters keep a pure
cobalt core surrounded with an alloyed interface. Magnetic measurements
performed using a Vibrating Sample Magnetometer (VSM) reveal large differences
in the magnetic properties of cobalt clusters in Pt and Nb pointing out the key
role of cluster/matrix interfaces.Comment: 7 pages (LaTeX), 12 PostScript figures, 1 PostScript tabl
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
Cavity-Enhanced Vernier Spectroscopy with a Chip-Scale Mid-Infrared Frequency Comb
Chip-scale optical frequency combs can provide broadband spectroscopy for
diagnosing complex organic molecules. They are also promising as miniaturized
laser spectrometers in applications ranging from atmospheric chemistry to
geological science and the search for extraterrestrial life. While optical
cavities are commonly used to boost sensitivity, it is challenging to realize a
compact cavity-enhanced comb-based spectrometer. Here, we apply the Vernier
technique to free-running operation of an interband cascade laser frequency
comb in a simple linear geometry that performs cavity-enhanced chemical
sensing. A centimeter-scale high-finesse cavity simultaneously provides
selective mode filtering and enhancement of the path length to 30 meters. As a
proof-of-concept, we sense transient open-path releases of ppm-level
difluoroethane with 2 ms temporal resolution over a 1 THz optical bandwidth
centered at 3.64 m.Comment: 10 pages, 5 figure
X-Ray Analysis of Oxygen-induced Perpendicular Magnetic Anisotropy in Pt/Co/AlOx trilayer
X-ray spectroscopy measurements have been performed on a series of Pt/Co/AlOx
trilayers to investigate the role of Co oxidation in the perpendicular magnetic
anisotropy of the Co/AlOx interface. It is observed that high temperature
annealing modifies the magnetic properties of the Co layer, inducing an
enhancement of the perpendicular magnetic anisotropy. The microscopic
structural properties are analyzed via X-ray Absorption Spectroscopy, X-ray
Magnetic Circular Dichroism and X-ray Photoelectron Spectroscopy measurements.
It is shown that annealing enhances the amount of interfacial oxide, which may
be at the origin of a strong perpendicular magnetic anisotropy
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
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