3,510 research outputs found
Gluon Field Strength Correlation Functions within a Constrained Instanton Model
We suggest a constrained instanton (CI) solution in the physical QCD vacuum
which is described by large-scale vacuum field fluctuations. This solution
decays exponentially at large distances. It is stable only if the interaction
of the instanton with the background vacuum field is small and additional
constraints are introduced. The CI solution is explicitly constructed in the
ansatz form, and the two-point vacuum correlator of gluon field strengths is
calculated in the framework of the effective instanton vacuum model. At small
distances the results are qualitatively similar to the single instanton (SI)
case, in particular, the form factor is small, which is in agreement
with the lattice calculations.Comment: 25 pages (RevTex), 7 ps figures; some references added, one figure
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MILITARY PROGRESS IN A GLOBAL WORLD: THE NEED FOR A PROFESSIONAL APPROACH TO THE ETERNAL PROBLEMS
The author stresses that military progress is a part of the social progress, therefore its analysis involves the understanding of common problems and criteria of the development of mankind at this or that stage of history. The article covers new trends characterizing the military progress of the modern world. In particular, peculiarities of military progress after the «cold war», after the abolition of bipolar world system are analyzed. However, the real aims are the same, although they assume more rational nuance.Автор подчеркивает, что военный прогресс представляет собой часть общественного прогресса, поэтому его анализ предполагает понимание общих проблем и критериев развития человечества на том или ином историческом этапе. В статье освещаются новые тенденции, характеризующие военный прогресс современности. В частности, анализируются особенности военного прогресса после «холодной войны», после упразднения мировой биполярной системы. Тем не менее, реальные цели при этом остаются прежними, хотя и приобрели более рациональную окраску
Multilevel Analysis of Oscillation Motions in Active Regions of the Sun
We present a new method that combines the results of an oscillation study
made in optical and radio observations. The optical spectral measurements in
photospheric and chromospheric lines of the line-of-sight velocity were carried
out at the Sayan Solar Observatory. The radio maps of the Sun were obtained
with the Nobeyama Radioheliograph at 1.76 cm. Radio sources associated with the
sunspots were analyzed to study the oscillation processes in the
chromosphere-corona transition region in the layer with magnetic field B=2000
G. A high level of instability of the oscillations in the optical and radio
data was found. We used a wavelet analysis for the spectra. The best
similarities of the spectra of oscillations obtained by the two methods were
detected in the three-minute oscillations inside the sunspot umbra for the
dates when the active regions were situated near the center of the solar disk.
A comparison of the wavelet spectra for optical and radio observations showed a
time delay of about 50 seconds of the radio results with respect to optical
ones. This implies a MHD wave traveling upward inside the umbral magnetic tube
of the sunspot. Besides three-minute and five-minute ones, oscillations with
longer periods (8 and 15 minutes) were detected in optical and radio records.Comment: 17 pages, 9 figures, accepted to Solar Physics (18 Jan 2011). The
final publication is available at http://www.springerlink.co
Low-field diffusion magneto-thermopower of a high mobility two-dimensional electron gas
The low magnetic field diffusion thermopower of a high mobility
GaAs-heterostructure has been measured directly on an electrostatically defined
micron-scale Hall-bar structure at low temperature (T = 1.6 K) in the low
magnetic field regime (B < 1.2 T) where delocalized quantum Hall states do not
influence the measurements. The sample design allowed the determination of the
field dependence of the thermopower both parallel and perpendicular to the
temperature gradient, denoted respectively by Sxx (longitudinal thermopower)
and Syx (Nernst-Ettinghausen coefficient). The experimental data show clear
oscillations in Sxx and Syx due to the formation of Landau levels for 0.3 T < B
< 1.2 T and reveal that Syx is approximately 120 times larger than Sxx at a
magnetic field of 1 T, which agrees well with the theoretical prediction.Comment: 4 pages, 4 figure
Longer growing seasons do not increase net carbon uptake in Northeastern Siberian tundra
With global warming, snowmelt is occurring earlier and growing seasons are becoming longer around the Arctic. It has been suggested that this would lead to more uptake of carbon due to a lengthening of the period in which plants photosynthesize. To investigate this suggestion, 8 consecutive years of eddy covariance measurements at a northeastern Siberian graminoid tundra site were investigated for patterns in net ecosystem exchange, gross primary production (GPP) and ecosystem respiration (Reco). While GPP showed no clear increase with longer growing seasons, it was significantly increased in warmer summers. Due to these warmer temperatures however, the increase in uptake was mostly offset by an increase in Reco. Therefore, overall variability in net carbon uptake was low, and no relationship with growing season length was found. Furthermore, the highest net uptake of carbon occurred with the shortest and the coldest growing season. Low uptake of carbon mostly occurred with longer or warmer growing seasons. We thus conclude that the net carbon uptake of this ecosystem is more likely to decrease rather than to increase under a warmer climate. These results contradict previous research that has showed more net carbon uptake with longer growing seasons. We hypothesize that this difference is due to site-specific differences, such as climate type and soil, and that changes in the carbon cycle with longer growing seasons will not be uniform around the Arcti
Forest decline caused by high soil water conditions in a permafrost region
In the permafrost region near Yakutsk, eastern Siberia, Russia, annual precipitation (June–May) in 2005–2006 and 2006–2007 exceeded the 26-year (1982–2008) mean of 222&plusmn;68 mm by 185 mm and 128 mm, respectively, whereas in 2007–2008 the excedent was only 48 mm, well within the range of variability. Yellowing and browning of larch (<I>Larix cajanderi</I> Mayr.) trees occurred in an undisturbed forest near Yakutsk in the 2007 summer growing season. Soil water content at a depth of 0.20 m was measured along a roughly 400 m long line transect running through areas of yellowing and browning larch trees (YBL) and of normal larch trees (NL). In the two years of supranormal precipitation, soil water content was very high compared to values recorded for the same area in previous studies. For both wet years, the mean degree of saturation (<I>s</I>) was significantly greater in YBL than NL areas, whereas the converse was the case for the gas diffusivity in soil. This implies that rather than mitigating water stress suffered during normal precipitation years, elevated soil water conditions adversely affected the growth of larch trees. Eastern Siberia's taiga forest extends widely into the permafrost region. Was such supranormal annual precipitation to extend for more than two years, as might be expected under impending global climate changes, forest recovery may not be expected and emission of greenhouse gas might continue in future
Tunable Bose-Einstein condensation and roton-like excitation spectra with dipolar exciton-polaritons in crossed fields
We develop the many-body theory of dipolar exciton-polaritons in an optical
microcavity in crossed transverse electric and in-plane magnetic fields. Even
for relatively weak fields, we reveal the existence of two minima in the bare
lower-polariton dispersion, which give rise to the tuneable transition between
the polariton Bose-Einstein condensate and that of excitons, produced by the
competition between these minima. We predict that such dipolar condensate
exhibits a roton-maxon character of the excitation spectrum, never before
observed for polaritons. We show that upon the transition between the two
condensation regimes, the weak correlations in the polariton gas give way to
the intermediate interparticle correlations characteristic for excitons, and
that the transition is accompanied by a sharp quenching of photoluminescence as
the lifetime is increased by several orders of magnitude. While in the
polariton regime, the luminescence peak corresponding to the condensate is
shifted to a non-zero angle. The angular dependence of the two-photon decay
time in the Hanbury Brown and Twiss experiment is calculated and used as a tool
to evidence the formation of the macroscopically-coherent state. Our proposal
opens opportunities towards manipulating the superfluid properties and
extended-range dipole-dipole correlations of exciton-polariton condensates.Comment: 16 pages, 6 figure
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