6,727 research outputs found
Boundary effects on the scaling of the superfluid density
We study numerically the influence of the substrate (boundary conditions) on
the finite--size scaling properties of the superfluid density in
superfluid films of thickness within the XY model employing the Monte Carlo
method. Our results suggest that the jump at the
Kosterlitz--Thouless transition temperature depends on the boundary
conditions.Comment: 2 pages, 1 Latex file, 1 postscript figure, 2 style file
Numerical relativity with characteristic evolution, using six angular patches
The characteristic approach to numerical relativity is a useful tool in
evolving gravitational systems. In the past this has been implemented using two
patches of stereographic angular coordinates. In other applications, a
six-patch angular coordinate system has proved effective. Here we investigate
the use of a six-patch system in characteristic numerical relativity, by
comparing an existing two-patch implementation (using second-order finite
differencing throughout) with a new six-patch implementation (using either
second- or fourth-order finite differencing for the angular derivatives). We
compare these different codes by monitoring the Einstein constraint equations,
numerically evaluated independently from the evolution. We find that, compared
to the (second-order) two-patch code at equivalent resolutions, the errors of
the second-order six-patch code are smaller by a factor of about 2, and the
errors of the fourth-order six-patch code are smaller by a factor of nearly 50.Comment: 12 pages, 5 figures, submitted to CQG (special NFNR issue
Optical Properties of Crystals with Spatial Dispersion: Josephson Plasma Resonance in Layered Superconductors
We derive the transmission coefficient, , for grazing incidence of
crystals with spatial dispersion accounting for the excitation of multiple
modes with different wave vectors for a given frequency . The
generalization of the Fresnel formulas contains the refraction indices of these
modes as determined by the dielectric function . Near
frequencies , where the group velocity vanishes, depends
also on an additional parameter determined by the crystal microstructure. The
transmission is significantly suppressed, if one of the excited modes is
decaying into the crystal. We derive these features microscopically for the
Josephson plasma resonance in layered superconductors.Comment: 4 pages, 2 figures, epl.cls style file, minor change
Screen-Printed Soft-Nitrided Carbon Electrodes for Detection of Hydrogen Peroxide
Nitrogen-doped carbon materials have garnered much interest due to their electrocatalytic activity towards important reactions such as the reduction of hydrogen peroxide. N-doped carbon materials are typically prepared and deposited on solid conductive supports, which can sometimes involve time-consuming, complex, and/or costly procedures. Here, nitrogen-doped screen-printed carbon electrodes (N-SPCEs) were fabricated directly from a lab-formulated ink composed of graphite that was modified with surface nitrogen groups by a simple soft nitriding technique. N-SPCEs prepared from inexpensive starting materials (graphite powder and urea) demonstrated good electrocatalytic activity towards hydrogen peroxide reduction. Amperometric detection of H2O2 using N-SPCEs with an applied potential of −0.4 V (vs. Ag/AgCl) exhibited good reproducibility and stability as well as a reasonable limit of detection (2.5 µM) and wide linear range (0.020 to 5.3 mM)
Reduction of laser intensity scintillations in turbulent atmospheres using time averaging of a partially coherent beam
We demonstrate experimentally and numerically that the application of a
partially coherent beam (PCB) in combination with time averaging leads to a
significant reduction in the scintillation index. We use a simplified
experimental approach in which the atmospheric turbulence is simulated by a
phase diffuser. The role of the speckle size, the amplitude of the phase
modulation, and the strength of the atmospheric turbulence are examined. We
obtain good agreement between our numerical simulations and our experimental
results. This study provides a useful foundation for future applications of
PCB-based methods of scintillation reduction in physical atmospheres.Comment: 18 pages, 14 figure
Collapse of an Instanton
We construct a two parameter family of collapsing solutions to the 4+1
Yang-Mills equations and derive the dynamical law of the collapse. Our
arguments indicate that this family of solutions is stable. The latter fact is
also supported by numerical simulations.Comment: 17 pages, 1 figur
Dynamical Ordering of Driven Stripe Phases in Quenched Disorder
We examine the dynamics and stripe formation in a system with competing short
and long range interactions in the presence of both an applied dc drive and
quenched disorder. Without disorder, the system forms stripes organized in a
labyrinth state. We find that, when the disorder strength exceeds a critical
value, an applied dc drive can induce a dynamical stripe ordering transition to
a state that is more ordered than the originating undriven, unpinned pattern.
We show that signatures in the structure factor and transport properties
correspond to this dynamical reordering transition, and we present the dynamic
phase diagram as a function of strengths of disorder and dc drive.Comment: 4 pages, 4 postscript figure
- …