1,619 research outputs found
Zener transitions between dissipative Bloch bands. II: Current Response at Finite Temperature
We extend, to include the effects of finite temperature, our earlier study of
the interband dynamics of electrons with Markoffian dephasing under the
influence of uniform static electric fields. We use a simple two-band
tight-binding model and study the electric current response as a function of
field strength and the model parameters. In addition to the Esaki-Tsu peak,
near where the Bloch frequency equals the damping rate, we find current peaks
near the Zener resonances, at equally spaced values of the inverse electric
field. These become more prominenent and numerous with increasing bandwidth (in
units of the temperature, with other parameters fixed). As expected, they
broaden with increasing damping (dephasing).Comment: 5 pages, LateX, plus 5 postscript figure
Can surface flux transport account for the weak polar field in cycle 23?
To reproduce the weak magnetic field on the polar caps of the Sun observed
during the declining phase of cycle 23 poses a challenge to surface flux
transport models since this cycle has not been particularly weak. We use a
well-calibrated model to evaluate the parameter changes required to obtain
simulated polar fields and open flux that are consistent with the observations.
We find that the low polar field of cycle 23 could be reproduced by an increase
of the meridional flow by 55% in the last cycle. Alternatively, a decrease of
the mean tilt angle of sunspot groups by 28% would also lead to a similarly low
polar field, but cause a delay of the polar field reversals by 1.5 years in
comparison to the observations.Comment: 9 pages, 8 figures, Space Science Reviews, accepte
Holstein polarons in a strong electric field: delocalized and stretched states
The coherent dynamics of a Holstein polaron in strong electric fields is
considered under different regimes. Using analytical and numerical analysis, we
show that even for small hopping constant and weak electron-phonon interaction,
the original discrete Wannier-Stark (WS) ladder electronic states are each
replaced by a semi-continuous band if a resonance condition is satisfied
between the phonon frequency and the ladder spacing. In this regime, the
original localized WS states can become {\em delocalized}, yielding both
`tunneling' and `stretched' polarons. The transport properties of such a system
would exhibit a modulation of the phonon replicas in typical tunneling
experiments. The modulation will reflect the complex spectra with
nearly-fractal structure of the semi-continuous band. In the off-resonance
regime, the WS ladder is strongly deformed, although the states are still
localized to a degree which depends on the detuning: Both the spacing between
the levels in the deformed ladder and the localization length of the resulting
eigenfunctions can be adjusted by the applied electric field. We also discuss
the regime beyond small hopping constant and weak coupling, and find an
interesting mapping to that limit via the Lang-Firsov transformation, which
allows one to extend the region of validity of the analysis.Comment: 10 pages, 13 figures, submitted to PR
Neutrino Halos in Clusters of Galaxies and their Weak Lensing Signature
We study whether non-linear gravitational effects of relic neutrinos on the
development of clustering and large-scale structure may be observable by weak
gravitational lensing. We compute the density profile of relic massive
neutrinos in a spherical model of a cluster of galaxies, for several neutrino
mass schemes and cluster masses. Relic neutrinos add a small perturbation to
the mass profile, making it more extended in the outer parts. In principle,
this non-linear neutrino perturbation is detectable in an all-sky weak lensing
survey such as EUCLID by averaging the shear profile of a large fraction of the
visible massive clusters in the universe, or from its signature in the general
weak lensing power spectrum or its cross-spectrum with galaxies. However,
correctly modeling the distribution of mass in baryons and cold dark matter and
suppressing any systematic errors to the accuracy required for detecting this
neutrino perturbation is severely challenging.Comment: 13 pages, 11 figures. Submitted to JCA
Comparison of large-scale flows on the Sun measured by time-distance helioseismology and local correlation tracking technique
We present a direct comparison between two different techniques time-distance
helioseismology and a local correlation tracking method for measuring mass
flows in the solar photosphere and in a near-surface layer: We applied both
methods to the same dataset (MDI high-cadence Dopplergrams covering almost the
entire Carrington rotation 1974) and compared the results. We found that after
necessary corrections, the vector flow fields obtained by these techniques are
very similar. The median difference between directions of corresponding vectors
is 24 degrees, and the correlation coefficients of the results for mean zonal
and meridional flows are 0.98 and 0.88 respectively. The largest discrepancies
are found in areas of small velocities where the inaccuracies of the computed
vectors play a significant role. The good agreement of these two methods
increases confidence in the reliability of large-scale synoptic maps obtained
by them.Comment: 14 pages, 6 figures, just before acceptance in Solar Physic
Integrating fluctuations into distribution of resources in transportation networks
We propose a resource distribution strategy to reduce the average travel time
in a transportation network given a fixed generation rate. Suppose that there
are essential resources to avoid congestion in the network as well as some
extra resources. The strategy distributes the essential resources by the
average loads on the vertices and integrates the fluctuations of the
instantaneous loads into the distribution of the extra resources. The
fluctuations are calculated with the assumption of unlimited resources, where
the calculation is incorporated into the calculation of the average loads
without adding to the time complexity. Simulation results show that the
fluctuation-integrated strategy provides shorter average travel time than a
previous distribution strategy while keeping similar robustness. The strategy
is especially beneficial when the extra resources are scarce and the network is
heterogeneous and lowly loaded.Comment: 14 pages, 4 figure
Photosynthetic characteristics of summer maize under different planting patterns and the responses to nitrogen application of previous crop
Maize (Zea mays L.) is one of the most important grain crops in the North China Plain. Management practices affect the photosynthetic characteristics and the production of summer maize. This two-year (2014-2015) study examined the effects of different planting patterns and the application of nitrogen to previous winter wheat (Triticum aestivum L.) on the photosynthetic characteristics, yield and radiation use efficiency (RUE) of summer maize. Field experiments used a two-factor split-plot design with three replicates at Taian, Shandong Province, China (36°09′ N, 117°09′ E). The experiments involved two planting patterns (ridge planting, RP; and uniform row planting, UR) and two nitrogen application levels of previous winter wheat (N1, 112.50 kg ha-1; N2, 225.00 kg ha-1). The results indicated that the application of nitrogen on previous crop and ridge planting of the following crop had significant effects on the photosynthetic characteristics and yields of summer maize. Compared with UR, this study found that RP increased the chlorophyll content index (CCI), leaf area index (LAI), net photosynthetic rate (Pn), dry matter (DM), yield and grain RUE by 4.1%, 6.3%, 5.2%, 6.4%, 8.9% and 9.4%, respectively. The CCI, LAI, Pn, yield, and grain RUE of N2 were 9.7%, 3.3%, 3.7%, 10.0% and 10.1% higher than those of N1, respectively. RP combined with the application of nitrogen on previous crop of winter wheat could increase the CCI, LAI, Pn, DM, ultimately increasing the grain yield and RUE of the following summer’s maize. It was concluded that previous crop nitrogen application and RP pattern treatment resulted in optimal cropping conditions for the North China plain
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