1,244 research outputs found
Tunneling between edge states in a quantum spin Hall system
We analyze a quantum spin Hall (QSH) device with a point contact connecting
two of its edges. The contact supports a net spin tunneling current that can be
probed experimentally via a two-terminal resistance measurement. We find that
the low-bias tunneling current and the differential conductance exhibit scaling
with voltage and temperature that depend nonlinearly on the strength of the
electron-electron interaction.Comment: 4 pages, 3 figures; published versio
Edge Dynamics in a Quantum Spin Hall State: Effects from Rashba Spin-Orbit Interaction
We analyze the dynamics of the helical edge modes of a quantum spin Hall
state in the presence of a spatially non-uniform Rashba spin-orbit (SO)
interaction. A randomly fluctuating Rashba SO coupling is found to open a
scattering channel which causes localization of the edge modes for a weakly
screened electron-electron (e-e) interaction. A periodic modulation of the SO
coupling, with a wave number commensurate with the Fermi momentum, makes the
edge insulating already at intermediate strengths of the e-e interaction. We
discuss implications for experiments on edge state transport in a HgTe quantum
well.Comment: 4 pages, 2 figures; published versio
Efficient evaluation of the error probability for pilot-assisted URLLC with Massive MIMO
We propose a numerically efficient method for evaluating the random-coding
union bound with parameter on the error probability achievable in the
finite-blocklength regime by a pilot-assisted transmission scheme employing
Gaussian codebooks and operating over a memoryless block-fading channel. Our
method relies on the saddlepoint approximation, which, differently from
previous results reported for similar scenarios, is performed with respect to
the number of fading blocks (a.k.a. diversity branches) spanned by each
codeword, instead of the number of channel uses per block. This different
approach avoids a costly numerical averaging of the error probability over the
realizations of the fading process and of its pilot-based estimate at the
receiver and results in a significant reduction of the number of channel
realizations required to estimate the error probability accurately. Our
numerical experiments for both single-antenna communication links and massive
multiple-input multiple-output (MIMO) networks show that, when two or more
diversity branches are available, the error probability can be estimated
accurately with the saddlepoint approximation with respect to the number of
fading blocks using a numerical method that requires about two orders of
magnitude fewer Monte-Carlo samples than with the saddlepoint approximation
with respect to the number of channel uses per block
A review of sea-spray aerosol source functions using a large global set of sea salt aerosol concentration measurements
Sea-spray aerosols (SSA) are an important part of the climate system because
of their effects on the global radiative budget – both directly as scatterers and
absorbers of solar and terrestrial radiation, and indirectly as cloud
condensation nuclei (CCN) influencing cloud formation, lifetime, and
precipitation. In terms of their global mass, SSA have the largest
uncertainty of all aerosols. In this study we review 21 SSA source functions
from the literature, several of which are used in current climate models. In
addition, we propose a~new function. Even excluding outliers, the global annual
SSA mass produced spans roughly 3–70 Pg yr<sup>−1</sup> for the different
source functions, for particles with dry diameter <i>D</i><sub>p</sub> < 10 μm,
with relatively little interannual variability for a given
function. The FLEXPART Lagrangian particle dispersion model was run in
backward mode for a large global set of observed SSA concentrations,
comprised of several station networks and ship cruise measurement campaigns.
FLEXPART backward calculations produce gridded emission sensitivity fields,
which can subsequently be multiplied with gridded SSA production fluxes in order to
obtain modeled SSA concentrations. This allowed us to efficiently and simultaneously evaluate all
21 source functions against the measurements. Another
advantage of this method is that source-region information on wind speed and
sea surface temperatures (SSTs) could be stored and used for improving the
SSA source function parameterizations. The best source functions reproduced
as much as 70% of the observed SSA concentration variability at
several stations, which is comparable with "state of the art" aerosol
models. The main driver of SSA production is wind, and we found that the best
fit to the observation data could be obtained when the SSA production is
proportional to <i>U</i><sub>10</sub><sup>3.5</sup>, where
<i>U</i><sub>10</sub> is the source region averaged
10 m wind speed. A strong influence of SST on SSA production, with
higher temperatures leading to higher production, could be detected as well,
although the underlying physical mechanisms of the SST influence remains
unclear. Our new source function with wind speed and temperature dependence
gives a global SSA production for particles smaller than
<i>D</i><sub>p</sub> < 10 μm of 9 Pg yr<sup>−1</sup>, and is the best fit to the observed
concentrations
Explicit simulation of aerosol physics in a cloud-resolving model
International audienceThe role of convection in introducing aerosols and promoting the formation of new particles to the upper troposphere has been examined using a cloud-resolving model coupled with an interactive explicit aerosol module. A baseline simulation suggests good agreement in the upper troposphere between modeled and observed results including concentrations of aerosols in different size ranges, mole fractions of key chemical species, and concentrations of ice particles. In addition, a set of 34 sensitivity simulations has been carried out to investigate the sensitivity of modeled results to the treatment of various aerosol physical and chemical processes in the model. The size distribution of aerosols is proved to be an important factor in determining the aerosols' fate within the convective cloud. Nucleation mode aerosols (02SO4. Accumulation mode aerosols (d>?31.0 nm) are almost completely removed by nucleation (activation of cloud droplets) and impact scavenging. However, a substantial part (up to 10% of the boundary layer concentration) of the Aitken mode aerosol population (5.84 nm<?d<?31.0 nm) reaches the top of the cloud and the free troposphere. These particles may continually survive in the upper troposphere, or over time form ice crystals, both that could impact the atmospheric radiative budget. The sensitivity simulations performed indicate that critical processes in the model causing a substantial change in the upper tropospheric Aitken mode number concentration are coagulation, condensation, nucleation scavenging, nucleation of aerosols and the transfer of aerosol mass and number between different aerosol bins. In particular, for aerosols in the Aitken mode to grow to CCN size, coagulation appears to be more important than condensation. Less important processes are dry deposition, impact scavenging and the initial vertical distribution and concentration of aerosols. It is interesting to note that in order to sustain a vigorous storm cloud, the supply of CCN must be continuous over a considerably long time period of the simulation. Hence, the treatment of the growth of particles is in general much more important than the initial aerosol concentration itself
Seasonal variation of aerosol water uptake and its impact on the direct radiative effect at Ny-Ã…lesund, Svalbard
© Author(s) 2014. This work is distributed under the Creative Commons Attribution 3.0 LicenseIn this study we investigated the impact of water uptake by aerosol particles in ambient atmosphere on their optical properties and their direct radiative effect (ADRE, W m-2) in the Arctic at Ny-Ålesund, Svalbard, during 2008. To achieve this, we combined three models, a hygroscopic growth model, a Mie model and a radiative transfer model, with an extensive set of observational data. We found that the seasonal variation of dry aerosol scattering coefficients showed minimum values during the summer season and the beginning of fall (July-August-September), when small particles (< 100 nm in diameter) dominate the aerosol number size distribution. The maximum scattering by dry particles was observed during the Arctic haze period (March-April-May) when the average size of the particles was larger. Considering the hygroscopic growth of aerosol particles in the ambient atmosphere had a significant impact on the aerosol scattering coefficients: the aerosol scattering coefficients were enhanced by on average a factor of 4.30 ± 2.26 (mean ± standard deviation), with lower values during the haze period (March-April-May) as compared to summer and fall. Hygroscopic growth of aerosol particles was found to cause 1.6 to 3.7 times more negative ADRE at the surface, with the smallest effect during the haze period (March-April-May) and the highest during late summer and beginning of fall (July-August-September).Peer reviewe
Hairy and enhancer of split homolog-1 (HES-1) mediates the proliferative effect of Beta-estradiol on breast cancer cell lines
Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity
International audienceFactors controlling the distribution of relative humidity above ice saturation in the upper troposphere and lower stratosphere in the presence of cirrus clouds are examined with the help of microphysical trajectory simulations using a box model. Our findings are related to results from recent field campaigns and global model studies. We suggest that the relative humidities at which ice crystals form in the atmosphere can be inferred from in situ measurements of water vapor and temperature close to, but outside of, cirrus clouds. The comparison with similar measurements performed inside cirrus clouds provides a clue to freezing mechanisms active in cirrus. The comparison with field data reveals distinct interhemispheric differences in cirrus cloud freezing thresholds. Combining the present findings with recent results addressing the frequency distributions of updraft speeds and cirrus ice crystal number densities (Kärcher and Ström, 2993} provides evidence for the existence of complex heterogeneous freezing mechanisms in cirrus, at least in the polluted northern hemisphere, and further emphasizes the key role of gravity wave-induced dynamical variability in vertical air motion at the mesoscale. The key features of distributions of upper tropospheric relative humidity simulated by a global climate model are shown to be in general agreement with both, microphysical simulations and field observations, delineating a feasible method to include and validate ice supersaturation in other large-scale models of the atmosphere, in particular chemistry-transport and weather forecast models
Is Synchronization a Bottleneck for Pilot-Assisted URLLC Links?
We propose a framework to evaluate the random-coding union bound with
parameter on the achievable error probability in the finite-blocklength
regime for a pilot-assisted transmission scheme operating over an imperfectly
synchronized and memoryless block-fading waveform channel. Unlike previous
results, which disregard the effects of imperfect synchronization, our
framework utilizes pilots for both synchronization and channel estimation.
Specifically, we provide an algorithm to perform joint synchronization and
channel estimation and verify its accuracy by observing its tightness in
comparison with the Cramer-Rao bound. Then, we develop an RCUs bound on the
error probability, which applies for a receiver that treats the estimates
provided by the algorithm as accurate. Additionally, we utilize the saddlepoint
approximation to provide a numerically efficient method for evaluating the RCUs
bound in this scenario. Our numerical experiments verify the accuracy of the
proposed approximation. Moreover, when transmission blocks are received
synchronously, numerical results indicate that the number of pilot symbols
needed to estimate the fading channel gains to the level of accuracy required
in ultra-reliable low-latency communication is also sufficient to acquire
sufficiently good synchronization. However, when the blocks are received
asynchronously, synchronization becomes the bottleneck for the system
performance
Determination of tungsten sources in the JET-ILW divertor by spectroscopic imaging in the presence of a strong plasma continuum
The identification of the sources of atomic tungsten and the measurement of their radiation distribution in front of all plasma-facing components has been performed in JET with the help of two digital cameras with the same two-dimensional view, equipped with interference filters of different bandwidths centred on the W I (400.88 nm) emission line. A new algorithm for the subtraction of the continuum radiation was successfully developed and is now used to evaluate the W erosion even in the inner divertor region where the strong recombination emission is dominating over the tungsten emission. Analysis of W sputtering and W redistribution in the divertor by video imaging spectroscopy with high spatial resolution for three different magnetic configurations was performed. A strong variation of the emission of the neutral tungsten in toroidal direction and corresponding W erosion has been observed. It correlates strongly with the wetted area with a maximal W erosion at the edge of the divertor tile.EURATOM 63305
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