8,371 research outputs found
Random walks with thermalizing collisions in bounded regions; physical applications valid from the ballistic to diffusive regimes
The behavior of a spin undergoing Larmor precession in the presence of
fluctuating fields is of interest to workers in many fields. The fluctuating
fields cause frequency shifts and relaxation which are related to their power
spectrum, which can be determined by taking the Fourier transform of the
auto-correlation functions of the field fluctuations. Recently we have shown
how to calculate these correlation functions for all values of mean free path
(ballistic to diffusive motion) in finite bounded regions, using the model of
persistent continuous time random walks (CTRW) for particles subject to
scattering by fixed (frozen) scattering centers so that the speed of the moving
particles is not changed by the collisions. In this work we show how scattering
with energy exchange from an ensemble of scatterers in thermal equilibrium can
be incorporated into the CTRW. We present results for 1,2 and 3 dimensions. The
results agree for all these cases contrary to the previously studied 'frozen'
models. Our results for the velocity autocorrelation function show a long time
tail , which we also obtain from conventional
diffusion theory, with the same power, independent of dimensionality. Our
results are valid for any Markovian scattering kernel as well as any kernel
based on a scattering cross section Comment: 43 pages, 12 figure
Size-independence of statistics for boundary collisions of random walks and its implications for spin-polarized gases
A bounded random walk exhibits strong correlations between collisions with a
boundary. For an one-dimensional walk, we obtain the full statistical
distribution of the number of such collisions in a time t. In the large t
limit, the fluctuations in the number of collisions are found to be
size-independent (independent of the distance between boundaries). This occurs
for any inter-boundary distance, including less and greater than the
mean-free-path, and means that this boundary effect does not decay with
increasing system-size. As an application, we consider spin-polarized gases,
such as 3-Helium, in the three-dimensional diffusive regime. The above results
mean that the depolarizing effect of rare magnetic-impurities in the container
walls is orders of magnitude larger than a Smoluchowski assumption (to neglect
correlations) would imply. This could explain why depolarization is so
sensitive to the container's treatment with magnetic fields prior to its use.Comment: 5 page manuscript with extra details in appendices (additional 3
pages
Specific Rab GTPase-activating proteins define the Shiga toxin and epidermal growth factor uptake pathways
Rab family guanosine triphosphatases (GTPases) together with their regulators define specific pathways of membrane traffic within eukaryotic cells. In this study, we have investigated which Rab GTPase-activating proteins (GAPs) can interfere with the trafficking of Shiga toxin from the cell surface to the Golgi apparatus and studied transport of the epidermal growth factor (EGF) from the cell surface to endosomes. This screen identifies 6 (EVI5, RN-tre/USP6NL, TBC1D10A–C, and TBC1D17) of 39 predicted human Rab GAPs as specific regulators of Shiga toxin but not EGF uptake. We show that Rab43 is the target of RN-tre and is required for Shiga toxin uptake. In contrast, RabGAP-5, a Rab5 GAP, was unique among the GAPs tested and reduced the uptake of EGF but not Shiga toxin. These results suggest that Shiga toxin trafficking to the Golgi is a multistep process controlled by several Rab GAPs and their target Rabs and that this process is discrete from ligand-induced EGF receptor trafficking
Recommended from our members
A Sec14 domain protein is required for photoautotrophic growth and chloroplast vesicle formation in Arabidopsis thaliana.
In eukaryotic photosynthetic organisms, the conversion of solar into chemical energy occurs in thylakoid membranes in the chloroplast. How thylakoid membranes are formed and maintained is poorly understood. However, previous observations of vesicles adjacent to the stromal side of the inner envelope membrane of the chloroplast suggest a possible role of membrane transport via vesicle trafficking from the inner envelope to the thylakoids. Here we show that the model plant Arabidopsis thaliana has a chloroplast-localized Sec14-like protein (CPSFL1) that is necessary for photoautotrophic growth and vesicle formation at the inner envelope membrane of the chloroplast. The cpsfl1 mutants are seedling lethal, show a defect in thylakoid structure, and lack chloroplast vesicles. Sec14 domain proteins are found only in eukaryotes and have been well characterized in yeast, where they regulate vesicle budding at the trans-Golgi network. Like the yeast Sec14p, CPSFL1 binds phosphatidylinositol phosphates (PIPs) and phosphatidic acid (PA) and acts as a phosphatidylinositol transfer protein in vitro, and expression of Arabidopsis CPSFL1 can complement the yeast sec14 mutation. CPSFL1 can transfer PIP into PA-rich membrane bilayers in vitro, suggesting that CPSFL1 potentially facilitates vesicle formation by trafficking PA and/or PIP, known regulators of membrane trafficking between organellar subcompartments. These results underscore the role of vesicles in thylakoid biogenesis and/or maintenance. CPSFL1 appears to be an example of a eukaryotic cytosolic protein that has been coopted for a function in the chloroplast, an organelle derived from endosymbiosis of a cyanobacterium
The Sun is less active than other solar-like stars
Magnetic activity of the Sun and other stars causes their brightness to vary.
We investigate how typical the Sun's variability is compared to other
solar-like stars, i.e. those with near-solar effective temperatures and
rotation periods. By combining four years of photometric observations from the
Kepler space telescope with astrometric data from the Gaia spacecraft, we
measure photometric variabilities of 369 solar-like stars. Most of the
solar-like stars with well-determined rotation periods show higher variability
than the Sun and are therefore considerably more active. These stars appear
nearly identical to the Sun, except for their higher variability. Their
existence raises the question of whether the Sun can also experience epochs of
such high variability.Comment: Accepted for publication in Science. 3 (main) + 10 (supplementary)
figure
Geometric potential and transport in photonic topological crystals
We report on the experimental realization of an optical analogue of a quantum
geometric potential for light wave packets constrained on thin dielectric
guiding layers fabricated in silica by the femtosecond laser writing
technology. We further demonstrate the optical version of a topological
crystal, with the observation of Bloch oscillations and Zener tunneling of
purely geometric nature
- …