850 research outputs found
Seismoelectric measurements on artifical porous media and Berea sandstone
Seismic to electromagnetic wave conversion has been proposed
as a possible exploration method. We present laboratory measurements
of the conversion of an acoustic wave into electric potential,
which occurs at a fluid/porous-medium interface. The
conversion is measured for artificial porous media and Berea
sandstone. Recorded electric potentials are smaller for large grain
samples. We also show variability of the conversion over
a time span of 120 hours.Shell Refining Compan
Guiding center picture of magnetoresistance oscillations in rectangular superlattices
We calculate the magneto-resistivities of a two-dimensional electron gas
subjected to a lateral superlattice (LSL) of rectangular symmetry within the
guiding-center picture, which approximates the classical electron motion as a
rapid cyclotron motion around a slowly drifting guiding center. We explicitly
evaluate the velocity auto-correlation function along the trajectories of the
guiding centers, which are equipotentials of a magnetic-field dependent
effective LSL potential. The existence of closed equipotentials may lead to a
suppression of the commensurability oscillations, if the mean free path and the
LSL modulation potential are large enough. We present numerical and analytical
results for this suppression, which allow, in contrast to previous quantum
arguments, a classical explanation of similar suppression effects observed
experimentally on square-symmetric LSL. Furthermore, for rectangular LSLs of
lower symmetry they lead us to predict a strongly anisotropic resistance
tensor, with high- and low-resistance directions which can be interchanged by
tuning the externally applied magnetic field.Comment: 12 pages, 9 figure
Magnetoresistance of a two-dimensional electron gas with spatially periodic lateral modulations: Exact consequences of Boltzmann's equation
On the basis of Boltzmann's equation, and including anisotropic scattering in
the collision operator, we investigate the effect of one-dimensional
superlattices on two-dimensional electron systems. In addition to superlattices
defined by static electric and magnetic fields, we consider mobility
superlattices describing a spatially modulated density of scattering centers.
We prove that magnetic and electric superlattices in -direction affect only
the resistivity component if the mobility is homogeneous, whereas a
mobility lattice in -direction in the absence of electric and magnetic
modulations affects only . Solving Boltzmann's equation numerically,
we calculate the positive magnetoresistance in weak magnetic fields and the
Weiss oscillations in stronger fields within a unified approach.Comment: submitted to PR
Weiss Oscillations in Surface Acoustic Wave Propagation
The interaction of a surface acoustic wave (SAW) with a a two-dimensional
electron gas in a periodic electric potential and a classical magnetic field is
considered. We calculate the attenuation of the SAW and its velocity change and
show that these quantities exhibit Weiss oscillations.Comment: 4 pages REVTEX, 2 figures included as eps file
Planar cyclotron motion in unidirectional superlattices defined by strong magnetic and electric fields: Traces of classical orbits in the energy spectrum
We compare the quantum and the classical description of the two-dimensional
motion of electrons subjected to a perpendicular magnetic field and a
one-dimensional lateral superlattice defined by spatially periodic magnetic and
electric fields of large amplitudes. We explain in detail the complicated
energy spectra, consisting of superimposed branches of strong and of weak
dispersion, by the correspondence between the respective eigenstates and the
``channeled'' and ``drifting'' orbits of the classical description.Comment: 11 pages, 11 figures, to appear in Physical Review
Effect on gastric function and symptoms of drinking wine, black tea, or schnapps with a Swiss cheese fondue: randomised controlled crossover trial
OBJECTIVE: To compare the effects of drinking white wine or black tea with Swiss cheese fondue followed by a shot of cherry schnapps on gastric emptying, appetite, and abdominal symptoms.
DESIGN: Randomised controlled crossover study.
PARTICIPANTS: 20 healthy adults (14 men) aged 23-58.
INTERVENTIONS: Cheese fondue (3260 kJ, 32% fat) labelled with 150 mg sodium (13)Carbon-octanoate was consumed with 300 ml of white wine (13%, 40 g alcohol) or black tea in randomised order, followed by 20 ml schnapps (40%, 8 g alcohol) or water in randomised order.
MAIN OUTCOME MEASURES: Cumulative percentage dose of (13)C substrate recovered over four hours (higher values indicate faster gastric emptying) and appetite and dyspeptic symptoms (visual analogue scales).
RESULTS: Gastric emptying was significantly faster when fondue was consumed with tea or water than with wine or schnapps (cumulative percentage dose of (13)C recovered 18.1%, 95% confidence interval 15.2% to 20.9% v 7.4%, 4.6% to 10.3%; P<0.001). An inverse dose-response relation between alcohol intake and gastric emptying was evident. Appetite was similar with consumption of wine or tea (difference 0.11, -0.12 to 0.34; P=0.35), but reduced if both wine and schnapps were consumed (difference -0.40, -0.01 to -0.79; P<0.046). No difference in dyspeptic symptoms was present.
CONCLUSIONS: Gastric emptying after a Swiss cheese fondue is noticeably slower and appetite suppressed if consumed with higher doses of alcohol. This effect was not associated with dyspeptic symptoms.
TRIAL REGISTRATION: ClinicalTrials.gov NCT00943696
Anisotropic scattering and quantum magnetoresistivities of a periodically modulated 2D electron gas
We calculate the longitudinal conductivities of a two-dimensional
noninteracting electron gas in a uniform magnetic field and a lateral electric
or magnetic periodic modulation in one spatial direction, in the quantum
regime. We consider the effects of the electron-impurity scattering anisotropy
through the vertex corrections on the Kubo formula, which are calculated with
the Bethe-Salpeter equation, in the self-consistent Born approximation. We find
that due to the scattering anisotropy the band conductivity increases, and the
scattering conductivities decrease and become anisotropic. Our results are in
qualitative agreement with recent experiments.Comment: 19 pages, 8 figures, Revtex, to appear in Phys. Rev.
The Reliability of Global and Hemispheric Surface Temperature Records
The purpose of this review article is to discuss the development and associated estimation of uncertainties in the global and hemispheric surface temperature records. The review begins by detailing the groups that produce surface temperature datasets. After discussing the reasons for similarities and differences between the various products, the main issues that must be addressed when deriving accurate estimates, particularly for hemispheric and global averages, are then considered. These issues are discussed in the order of their importance for temperature records at these spatial scales: biases in SST data, particularly before the 1940s; the exposure of land-based thermometers before the development of louvred screens in the late 19th century; and urbanization effects in some regions in recent decades. The homogeneity of land-based records is also discussed; however, at these large scales it is relatively unimportant. The article concludes by illustrating hemispheric and global temperature records from the four groups that produce series in near-real time
Weak Localization and Integer Quantum Hall Effect in a Periodic Potential
We consider magnetotransport in a disordered two-dimensional electron gas in
the presence of a periodic modulation in one direction. Existing quasiclassical
and quantum approaches to this problem account for Weiss oscillations in the
resistivity tensor at moderate magnetic fields, as well as a strong
modulation-induced modification of the Shubnikov-de Haas oscillations at higher
magnetic fields. They do not account, however, for the operation at even higher
magnetic fields of the integer quantum Hall effect, for which quantum
interference processes are responsible. We then introduce a field-theory
approach, based on a nonlinear sigma model, which encompasses naturally both
the quasiclassical and quantum-mechanical approaches, as well as providing a
consistent means of extending them to include quantum interference corrections.
A perturbative renormalization-group analysis of the field theory shows how
weak localization corrections to the conductivity tensor may be described by a
modification of the usual one-parameter scaling, such as to accommodate the
anisotropy of the bare conductivity tensor. We also show how the two-parameter
scaling, conjectured as a model for the quantum Hall effect in unmodulated
systems, may be generalized similarly for the modulated system. Within this
model we illustrate the operation of the quantum Hall effect in modulated
systems for parameters that are realistic for current experiments.Comment: 15 pages, 4 figures, ReVTeX; revised version with condensed
introduction; two figures taken out; reference adde
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