1,392 research outputs found
Impact of human population density on fire frequency at the global scale
Human impact on wildfires, a major earth system component, remains
poorly understood. While local studies have found more fires close
to settlements and roads, assimilated charcoal records and analyses
of regional fire patterns from remote-sensing observations point to
a decline in fire frequency with increasing human population. Here,
we present a global analysis using three multi-year satellite-based
burned-area products combined with a parameter estimation and
uncertainty analysis with a non-linear model. We show that at the
global scale, the impact of increasing population density is mainly
to reduce fire frequency. Only for areas with up to 0.1 people
per km2, we find that fire frequency increases by 10 to
20% relative to its value at no population. The results are
robust against choice of burned-area data set, and indicate that at
only very few places on earth, fire frequency is limited by human
ignitions. Applying the results to historical population estimates
results in a moderate but accelerating decline of global burned area
by around 14% since 1800, with most of the decline since 1950
Updated NNLO QCD predictions for the weak radiative B-meson decays
Weak radiative decays of the B mesons belong to the most important flavor
changing processes that provide constraints on physics at the TeV scale. In the
derivation of such constraints, accurate standard model predictions for the
inclusive branching ratios play a crucial role. In the current Letter we
present an update of these predictions, incorporating all our results for the
O(alpha_s^2) and lower-order perturbative corrections that have been calculated
after 2006. New estimates of nonperturbative effects are taken into account,
too. For the CP- and isospin-averaged branching ratios, we find B_{s gamma} =
(3.36 +_ 0.23) * 10^-4 and B_{d gamma} = 1.73^{+0.12}_{-0.22} * 10^-5, for
E_gamma > 1.6GeV. Both results remain in agreement with the current
experimental averages. Normalizing their sum to the inclusive semileptonic
branching ratio, we obtain R_gamma = ( B_{s gamma} + B_{d gamma})/B_{c l nu} =
(3.31 +_ 0.22) * 10^-3. A new bound from B_{s gamma} on the charged Higgs boson
mass in the two-Higgs-doublet-model II reads M_{H^+} > 480 GeV at 95%C.L.Comment: journal version, 5 pages, no figure
Non-dispersive Fermi arcs and absence of charge ordering in the pseudogap phase of Bi2Sr2CaCu2O8+d
The autocorrelation of angle resolved photoemission data from the high
temperature superconductor Bi2Sr2CaCu2O8+d shows distinct peaks in momentum
space which disperse with binding energy in the superconducting state, but not
in the pseudogap phase. Although it is tempting to attribute a non-dispersive
behavior in momentum space to some ordering phenomenon, a de-construction of
the autocorrelation reveals that the non-dispersive peaks arise not from
ordering, but rather from the tips of the Fermi arcs, which themselves do not
change with binding energy.Comment: 4 pages, 3 figure
Momentum anisotropy of the scattering rate in cuprate superconductors
We examine the momentum and energy dependence of the scattering rate of the
high temperature cuprate superconductors using angle resolved photoemission
spectroscopy. The scattering rate is of the form a + b*w. The inelastic
coefficient b is found to be isotropic. The elastic term, a, however, is found
to be highly anisotropic in the pseudogap phase of optimal doped samples, with
an anisotropy which correlates with that of the pseudogap. This can be
contrasted with heavily overdoped samples, which show an isotropic scattering
rate in the normal state
Interference in interacting quantum dots with spin
We study spectral and transport properties of interacting quantum dots with
spin. Two particular model systems are investigated: Lateral multilevel and two
parallel quantum dots. In both cases different paths through the system can
give rise to interference. We demonstrate that this strengthens the multilevel
Kondo effect for which a simple two-stage mechanism is proposed. In parallel
dots we show under which conditions the peak of an interference-induced orbital
Kondo effect can be split.Comment: 8 pages, 8 figure
Statistics of Coulomb blockade peak spacings for a partially open dot
We show that randomness of the electron wave functions in a quantum dot
contributes to the fluctuations of the positions of the conductance peaks. This
contribution grows with the conductance of the junctions connecting the dot to
the leads. It becomes comparable with the fluctuations coming from the
randomness of the single particle spectrum in the dot while the Coulomb
blockade peaks are still well-defined. In addition, the fluctuations of the
peak spacings are correlated with the fluctuations of the conductance peak
heights.Comment: 13 pages, 1 figur
Suppression of interleukin-2 by the putative endogenous cannabinoid 2-arachidonyl-glycerol is mediated through down-regulation of the nuclear factor of activated T cells.
ABSTRACT 2-Arachidonyl-glycerol (2-Ara-Gl) recently was identified as a putative endogenous ligand for cannabinoid receptor types CB1 and CB2 by competitive binding. More recent immune function assays demonstrated that 2-Ara-Gl possessed immunomodulatory activity. Because several plant-derived cannabinoids inhibit interleukin-2 (IL-2) expression, 2-Ara-Gl was investigated for its ability to modulate this cytokine. The direct addition of 2-Ara-Gl to mouse splenocyte cultures suppressed phorbol-12-myristate-13-acetate plus ionomycin-induced IL-2 secretion and steady state mRNA expression in a dose-dependent manner. 2-Ara-Gl also produced a marked inhibition of IL-2 promotor activity as determined by transient transfection of EL4.IL-2 cells with a pIL-2-CAT construct. 2-Ara-Gl at 5, 10, 20, and 50 M suppressed phorbol-12-myristate-13-acetate plus ionomycin-induced IL-2 promotor activity by 18%, 28%, 39%, and 54%, respectively. To further characterize the mechanism for the transcriptional regulation of IL-2 by 2-Ara-Gl, the DNA-binding activity of transcription factors, nuclear factor of activated T cells (NF-AT), nuclear factor for immunoglobulin chain in B cells (NF-B/Rel), activator protein-1(AP-1), octamer, and cAMP-response element binding protein was evaluated by electrophoretic mobility shift assay in mouse splenocytes. In addition, a reporter gene expression system for p(NF-B) 3 -CAT, p(NF-AT) 3 -CAT, and p(AP-1) 3 -CAT was used in transiently transfected EL4.IL-2 cells to determine the effect of 2-Ara-Gl on promoter activity for each of the specific transcription factors. 2-Ara-Gl reduced both the NF-AT-binding and promoter activity in a dose-dependent manner and, to a lesser degree, NF-B/Rel-binding and promoter activity. No significant effect was observed on octamer-and cAMP-response element-binding activity. AP-1 DNA-binding activity was not inhibited by 2-AraGl, but a modest inhibition of promoter activity was observed
Linear conductance in Coulomb-blockade quantum dots in the presence of interactions and spin
We discuss the calculation of the linear conductance through a
Coulomb-blockade quantum dot in the presence of interactions beyond the
charging energy. In the limit where the temperature is large compared with a
typical tunneling width, we use a rate-equations approach to describe the
transitions between the corresponding many-body states. We discuss both the
elastic and rapid-thermalization limits, where the rate of inelastic scattering
in the dot is either small or large compared with the elastic transition rate,
respectively. In the elastic limit, we find several cases where a closed
solution for the conductance is possible, including the case of a constant
exchange interaction. In the rapid-thermalization limit, a closed solution is
possible in the general case. We show that the corresponding expressions for
the linear conductance simplify for a Hamiltonian that is invariant under spin
rotations.Comment: 11 pages, no figures, revtex
Electron transport and energy relaxation in dilute magnetic alloys
We consider the effect of the RKKY interaction between magnetic impurities on
the electron relaxation rates in a normal metal. The interplay between the RKKY
interaction and the Kondo effect may result in a non-monotonic temperature
dependence of the electron momentum relaxation rate, which determines the Drude
conductivity. The electron phase relaxation rate, which determines the
magnitude of the weak localization correction to the resistivity, is also a
non-monotonic function of temperature. For this function, we find the
dependence of the position of its maximum on the concentration of magnetic
impurities. We also relate the electron energy relaxation rate to the
excitation spectrum of the system of magnetic impurities. The energy relaxation
determines the distribution function for the out-of-equilibrium electrons.
Measurement of the electron distribution function thus may provide information
about the excitations in the spin glass phase.Comment: 15 pages, 5 figure
Spin Effects and Transport in Quantum Dots with overlapping Resonances
The role of spin is investigated in the transport through a quantum dot with
two overlapping resonances (one having a width larger than the level separation
and the other very narrow, cf. Silvestrov and Imry, Phys. Rev. Lett. {\bf 85},
2565 (2000)). For a series of consecutive charging resonances, one electron
from the leads populates one and the same broad level in the dot. Moreover,
there is the tendency to occupy the same level also by the second electron
within the same resonance. This second electron is taken from the narrow levels
in the dot. The narrow levels are populated (and broad level is depopulated)
via sharp rearrangements of the electronic configuration in the Coulomb
blockade valleys. Possible experimental manifestations of this scenario are
considered. Among these there are sharp features in the valleys and in the
Mixed Valence regime and an unusual Kondo effect.Comment: 7 pages, 3 figures, just a published versio
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