714 research outputs found
Resonant and non-resonant Tunneling through a double barrier
An explicit expression is obtained for the phase-time corresponding to
tunneling of a (non-relativistic) particle through two rectangular barriers,
both in the case of resonant and in the case of non-resonant tunneling. It is
shown that the behavior of the transmission coefficient and of the tunneling
phase-time near a resonance is given by expressions with "Breit-Wigner type"
denominators. By contrast, it is shown that, when the tunneling probability is
low (but not negligible), the non-resonant tunneling time depends on the
barrier width and on the distance between the barriers only in a very weak
(exponentially decreasing) way: This can imply in various cases, as well-known,
the highly Superluminal tunneling associated with the so-called "generalized
Hartman Effect"; but we are now able to improve and modify the mathematical
description of such an effect, and to compare more in detail our results with
the experimental data for non-resonant tunneling of photons. Finally, as a
second example, the tunneling phase-time is calculated, and compared with the
available experimental results, in the case of the quantum-mechanical tunneling
of neutrons through two barrier-filters at the resonance energy of the set-up.Comment: replaced with some improvements in the text and in the references:
pdf (11 pages) produced from a source-file in Word; including one Figur
Four-path interference and uncertainty principle in photodetachment microscopy
We study the quantal motion of electrons emitted by a pointlike monochromatic
isotropic source into parallel uniform electric and magnetic fields. The
two-path interference pattern in the emerging electron wave due to the electric
force is modified by the magnetic lens effect which periodically focuses the
beam into narrow filaments along the symmetry axis. There, four classical paths
interfere. With increasing electron energy, the current distribution changes
from a quantum regime governed by the uncertainty principle, to an intricate
spatial pattern that yields to a semiclassical analysis.Comment: submitted to Europhysics Letter
The lysosomotrope, GPN, mobilises Ca2+ from acidic organelles
Lysosomes are acidic Ca2+ stores often mobilised in conjunction with endoplasmic reticulum (ER) Ca2+ stores. GPN is a widely used lysosomotropic agent that evokes cytosolic Ca2+ signals in many cells. But whether these signals are due to a primary action on lysosomes is unclear in light of recent evidence showing GPN mediates direct ER Ca2+ release through changes in cytosolic pH. Here, we show that GPN evoked rapid increases in cytosolic pH but slower Ca2+ signals. NH4Cl evoked comparable changes in pH but failed to affect Ca2+ The V-type ATPase inhibitor, bafilomycin A1, increased lysosomal pH over a period of hours. Acute treatment modestly affected lysosomal pH and potentiated Ca2+ signals evoked by GPN. In contrast, chronic treatment led to more profound changes in luminal pH and selectively inhibited GPN-action. GPN blocked Ca2+ responses evoked by the novel NAADP-like agonist, TPC2-A1-N. GPN-evoked Ca2+ signals were thus better correlated with associated pH changes in the lysosome compared to the cytosol and coupled to lysosomal Ca2+ release. We conclude that Ca2+ signals evoked by GPN most likely derive from acidic organelles
Biogenic halocarbons from the Peruvian upwelling region as tropospheric halogen source
Halocarbons are produced naturally in the oceans by biological and chemical processes. They are emitted from surface seawater into the atmosphere, where they take part in numerous chemical processes such as ozone destruction and the oxidation of mercury and dimethyl sulfide. Here we present oceanic and atmospheric halocarbon data for the Peruvian upwelling zone obtained during the M91 cruise onboard the research vessel METEOR in December 2012. Surface waters during the cruise were characterized by moderate concentrations of bromoform (CHBr3) and dibromomethane (CH2Br2) correlating with diatom biomass derived from marker pigment concentrations, which suggests this phytoplankton group is a likely source. Concentrations measured for the iodinated compounds methyl iodide (CH3I) of up to 35.4 pmol L−1, chloroiodomethane (CH2ClI) of up to 58.1 pmol L−1 and diiodomethane (CH2I2) of up to 32.4 pmol L−1 in water samples were much higher than previously reported for the tropical Atlantic upwelling systems. Iodocarbons also correlated with the diatom biomass and even more significantly with dissolved organic matter (DOM) components measured in the surface water. Our results suggest a biological source of these compounds as a significant driving factor for the observed large iodocarbon concentrations. Elevated atmospheric mixing ratios of CH3I (up to 3.2 ppt), CH2ClI (up to 2.5 ppt) and CH2I2 (3.3 ppt) above the upwelling were correlated with seawater concentrations and high sea-to-air fluxes. During the first part of the cruise, the enhanced iodocarbon production in the Peruvian upwelling contributed significantly to tropospheric iodine levels, while this contribution was considerably smaller during the second part
Photophysiological state of natural phytoplankton communities in the South China Sea and Sulu Sea
In recent years, an increasing number of studies on phytoplankton in the tropical South China Sea (SCS) and Sulu Sea (SS) have been conducted. However, still little is known about the photophysiological state of natural phytoplankton communities under varying environmental conditions. This study investigates the photophysiological state of natural phytoplankton communities in the southern SCS and SS based on high horizontal and vertical resolution field observations collected during the SHIVA (Stratosphere ozone: Halogens in a Varying Atmosphere) cruise (SO 218) in November 2011 on board RV Sonne. At the surface, pigment results revealed that total chlorophyll a (TChl a ) concentrations at all offshore stations were low at the surface and were generally dominated by cyanobacteria. Enhanced concentrations of TChl a were only observed below the upper mixed layer and above the euphotic depth with haptophytes, prochlorophytes and prasinophytes contributing most of the biomass. At stations close to the coast and river outflows, surface phytoplankton blooms (between 1 to 2.2 mg m−3) dominated by diatoms were observed. Overall, the study region exhibited strong nitrate + nitrite (NOx, 2 μmol L−1) were observed in conjunction with increased TChl a and diatoms concentrations. Surface NOx concentrations were observed to correlate positively with temperature (τ = 0.22, p 0.4
Direct measurement of plasmon propagation lengths on lithographically defined metallic waveguides on GaAs
We present optical investigations of rectangular surface plasmon polariton
waveguides lithographically defined on GaAs substrates. The plasmon propagation
length is directly determined using a confocal microscope, with independent
polarization control in both excitation and detection channels. Surface plasmon
polaritons are launched along the waveguide using a lithographically defined
defect at one end. At the remote end of the waveguide they scatter into the
far-field, where they are imaged using a CCD camera. By monitoring the length
dependence of the intensity of scattered light from the waveguide end, we
directly extract the propagation length, obtaining values ranging from LSPP =
10-40 {\mu}m depending on the waveguide width (w=2-5 {\mu}m) and excitation
wavelength (760-920 nm). Results are in good accord with theoretical
expectations demonstrating the high quality of the lithographically defined
structures. The results obtained are of strong relevance for the development of
future semiconductor based integrated plasmonic technologies
Aminopropylindenes derived from Grundmann’s ketone as a novel chemotype of oxidosqualene cyclase inhibitors.
Multibarrier tunneling
We study the tunneling through an arbitrary number of finite rectangular
opaque barriers and generalize earlier results by showing that the total
tunneling phase time depends neither on the barrier thickness nor on the
inter-barrier separation. We also predict two novel peculiar features of the
system considered, namely the independence of the transit time (for non
resonant tunneling) and the resonant frequency on the number of barriers
crossed, which can be directly tested in photonic experiments. A thorough
analysis of the role played by inter-barrier multiple reflections and a
physical interpretation of the results obtained is reported, showing that
multibarrier tunneling is a highly non-local phenomenon.Comment: RevTex, 7 pages, 1 eps figur
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