1,237 research outputs found
Boundary effect on CDW: Friedel oscillations, STM image
We study the effect of open boundary condition on charge density waves (CDW).
The electron density oscillates rapidly close to the boundary, and additional
non-oscillating terms (~ln(r)) appear. The Friedel oscillations survive beyond
the CDW coherence length (v_F/Delta), but their amplitude gets heavily
suppressed. The scanning tunneling microscopy image (STM) of CDW shows clear
features of the boundary. The local tunneling conductance becomes asymmetric
with respect to the Fermi energy, and considerable amount of spectral weight is
transferred to the lower gap edge. Also it exhibits additional zeros reflecting
the influence of the boundary.Comment: 7 pages, 6 figure
Surface acoustic waves for acousto-optic modulation in buried silicon nitride waveguides
We theoretically investigate the use of Rayleigh surface acoustic waves
(SAWs) for refractive index modulation in optical waveguides consisting of
amorphous dielectrics. Considering low-loss SiN waveguides with a
standard core cross section of 4.40.03 m size, buried 8 m
deep in a SiO cladding we compare surface acoustic wave generation in
various different geometries via a piezo-active, lead zirconate titanate film
placed on top of the surface and driven via an interdigitized transducer (IDT).
Using numerical solutions of the acoustic and optical wave equations, we
determine the strain distribution of the SAW under resonant excitation. From
the overlap of the acoustic strain field with the optical mode field we
calculate and maximize the attainable amplitude of index modulation in the
waveguide. For the example of a near-infrared wavelength of 840 nm, a maximum
shift in relative effective refractive index of 0.7x10 was obtained for
TE polarized light, using an IDT period of 30 - 35 m, a film thickness of
2.5 - 3.5 m, and an IDT voltage of 10 V. For these parameters, the
resonant frequency is in the range 70 - 85 MHz. The maximum shift increases to
1.2x10, with a corresponding resonant frequency of 87 MHz, when the
height of the cladding above the core is reduced to 3 m. The relative
index change is about 300-times higher than in previous work based on
non-resonant proximity piezo-actuation, and the modulation frequency is about
200-times higher. Exploiting the maximum relative index change of
1.210 in a low-loss balanced Mach-Zehnder modulator should allow
full-contrast modulation in devices as short as 120 m (half-wave voltage
length product = 0.24 Vcm).Comment: 19 pages, 8 figure
A Simple and Efficient Device and Method for Measuring the Kinetics of Gas-Producing Reactions
A Simple and Efficient Device and Method for Measuring the Kinetics of Gas-Producing Reactions
Controlled Nanoparticle Formation by Diffusion Limited Coalescence
Polymeric nanoparticles (NPs) have a great application potential in science
and technology. Their functionality strongly depends on their size. We present
a theory for the size of NPs formed by precipitation of polymers into a bad
solvent in the presence of a stabilizing surfactant. The analytical theory is
based upon diffusion-limited coalescence kinetics of the polymers.
Two relevant time scales, a mixing and a coalescence time, are identified and
their ratio is shown to determine the final NP diameter. The size is found to
scale in a universal manner and is predominantly sensitive to the mixing time
and the polymer concentration if the surfactant concentration is sufficiently
high. The model predictions are in good agreement with experimental data. Hence
the theory provides a solid framework for tailoring nanoparticles with a priori
determined size.Comment: 4 pages, 3 figure
Deep Saturated Free Electron Laser Oscillators and Frozen Spikes
We analyze the behavior of Free Electron Laser (FEL) oscillators operating in
the deep saturated regime and point out the formation of sub-peaks of the
optical pulse. They are very stable configurations, having a width
corresponding to a coherence length. We speculate on the physical mechanisms
underlying their growth and attempt an identification with FEL mode locked
structures associated with Super Modes. Their impact on the intra-cavity
nonlinear harmonic generation is also discussed along with the possibility of
exploiting them as cavity out-coupler.Comment: 28 page
Selective Aerobic Oxidation of Lactate to Pyruvate Catalyzed by Vanadium‐Nitrogen‐Doped Carbon Nanosheets
Cooperative Catalysis for Selective Alcohol Oxidation with Molecular Oxygen
The activation of dioxygen for selective oxidation of organic molecules is a major catalytic challenge. Inspired by the activity of nitrogen-doped carbons in electrocatalytic oxygen reduction, we combined such a carbon with metal-oxide catalysts to yield cooperative catalysts. These simple materials boost the catalytic oxidation of several alcohols, using molecular oxygen at atmospheric pressure and low temperature (80 degrees C). Cobalt and copper oxide demonstrate the highest activities. The high activity and selectivity of these catalysts arises from the cooperative action of their components, as proven by various control experiments and spectroscopic techniques. We propose that the reaction should not be viewed as occurring at an active site', but rather at an active doughnut'-the volume surrounding the base of a carbon-supported metal-oxide particle
Cluster size dependence of high-order harmonic generation
We investigate high-order harmonic generation (HHG) from noble gas clusters
in a supersonic gas jet. To identify the contribution of harmonic generation
from clusters versus that from gas monomers, we measure the high-order harmonic
output over a broad range of the total atomic number density in the jet (from
3*10^16 cm^{-3} to 3x10^18 cm{-3}) at two different reservoir temperatures (303
K and 363 K). For the firrst time in the evaluation of the harmonic yield in
such measurements, the variation of the liquid mass fraction, g, versus
pressure and temperature is taken into consideration, which we determine,
reliably and consistently, to be below 20% within our range of experimental
parameters. By comparing the measured harmonic yield from a thin jet with the
calculated corresponding yield from monomers alone, we find an increased
emission of the harmonics when the average cluster size is less than 3000.
Using g, under the assumption that the emission from monomers and clusters add
up coherently, we calculate the ratio of the average single-atom response of an
atom within a cluster to that of a monomer and find an enhancement of around 10
for very small average cluster size (~200). We do not find any dependence of
the cut-off frequency on the composition of the cluster jet. This implies that
HHG in clusters is based on electrons that return to their parent ions and not
to neighbouring ions in the cluster. To fully employ the enhanced average
single-atom response found for small average cluster sizes (~200), the nozzle
producing the cluster jet must provide a large liquid mass fraction at these
small cluster sizes for increasing the harmonic yield. Moreover, cluster jets
may allow for quasi-phase matching, as the higher mass of clusters allows for a
higher density contrast in spatially structuring the nonlinear medium.Comment: 16 pages, 6 figure
Поняття “леґітимність” та “леґітимація” як теоретичні інновації М.Вебера: зміст і роль у сучасній соціології
Forssman antigen, a neutral glycosphingolipid carrying five monosaccharides, was localized in epithelial MDCK cells by the immunogold technique. Labeling with a well defined mAb and protein A-gold after freeze-substitution and low temperature embedding in Lowicryl HM20 of aldehyde-fixed and cryoprotected cells, resulted in high levels of specific labeling and excellent retention of cellular ultrastructure compared to ultra-thin cryosections. No Forssman glycolipid was lost from the cells during freeze-substitution as measured by radio-immunostaining of lipid extracts. Redistribution of the glycolipid between membranes did not occur. Forssman glycolipid, abundantly expressed on the surface of MDCK II cells, did not move to neighboring cell surfaces in cocultures with Forssman negative MDCK I cells, even though they were connected by tight junctions. The labeling density on the apical plasma membrane was 1.4-1.6 times higher than basolateral. Roughly two-thirds of the gold particles were found intracellularly. The Golgi complex was labeled for Forssman as were endosomes, identified by endocytosed albumin-gold, and lysosomes, defined by double labeling for cathepsin D. In most cases, the nuclear envelope was Forssman positive, but the labeling density was 10-fold less than on the plasma membrane. Mitochondria and peroxisomes, the latter identified by catalase, remained free of label, consistent with the notion that they do not receive transport vesicles carrying glycosphingolipids. The present method of lipid immunolabeling holds great potential for the localization of other antigenic lipids
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