665 research outputs found
Supersymmetry and Localization in the Quantum Hall Effect
We study the localization transition in the integer quantum Hall effect as
described by the network model of quantum percolation. Starting from a path
integral representation of transport Green's functions for the network model,
which employs both complex (bosonic) and Grassman (fermionic) fields, we map
the problem of localization to the problem of diagonalizing a one-dimensional
non-Hermitian Hamiltonian of interacting bosons and fermions. An exact solution
is obtained in a restricted subspace of the Hilbert space which preserves
boson-fermion supersymmetry. The physically relevant regime is investigated
using the density matrix renormalization group (DMRG) method, and critical
behavior is found at the plateau transition.Comment: 14 RevTex pages, 3 eps figures; This revised version contains an
extended disussion of supersymmetry and improved numerical result
Stealth Branes
We discuss the brane world model of Dvali, Gabadadze and Porrati in which
branes evolve in an infinite bulk and the brane curvature term is added to the
action. If Z_2 symmetry between the two sides of the brane is not imposed, we
show that the model admits the existence of "stealth branes" which follow the
standard 4D internal evolution and have no gravitational effect on the bulk
space. Stealth branes can nucleate spontaneosly in a Minkowski bulk. This
process is described by the standard 4D quantum cosmology formalism with
tunneling boundary conditions for the brane world wave function. The notorious
ambiguity in the choice of boundary conditions is fixed in this case due to the
presence of the embedding spacetime. We also point to some problematic aspects
of models admitting stealth brane solutions.Comment: 24 pages; Final version, to appear in Phys. Rev. D. The discussion of
"embeddability obstruction" is removed (thanks to Takahiro Tanaka who
convinced us that there is no such obstruction
H110alpha recombination-line emission and 4.8-GHz continuum emission in the Carina Nebula
We present results from observations of H110alpha recombination-line emission
at 4.874 GHz and the related 4.8-GHz continuum emission towards the Carina
Nebula using the Australia Telescope Compact Array. These data provide
information on the velocity, morphology and excitation parameters of the
ionized gas associated with the two bright HII regions within the nebula, Car I
and Car II. They are consistent with both Car I and Car II being expanding
ionization fronts arising from the massive star clusters Trumpler 14 and
Trumpler 16, respectively. The overall continuum emission distribution at 4.8
GHz is similar to that at lower frequencies. For Car I, two compact sources are
revealed that are likely to be young HII regions associated with triggered star
formation. These results provide the first evidence of ongoing star formation
in the northern region of the nebula. A close association between Car I and the
molecular gas is consistent with a scenario in which Car I is currently carving
out a cavity within the northern molecular cloud. The complicated kinematics
associated with Car II point to expansion from at least two different centres.
All that is left of the molecular cloud in this region are clumps of dense gas
and dust which are likely to be responsible for shaping the striking morphology
of the Car II components.Comment: 10 pages, 7 figures, accepted for publication in MNRA
Putting theory oriented evaluation into practice
Evaluations of gaming simulations and business games as teaching devices are typically end-state driven. This emphasis fails to detect how the simulation being evaluated does or does not bring about its desired consequences. This paper advances the use of a logic model approach which possesses a holistic perspective that aims at including all elements associated with the situation created by a game. The use of the logic model approach is illustrated as applied to Simgame, a board game created for secondary school level business education in six European Union countries
Exploring nu signals in dark matter detectors
We investigate standard and non-standard solar neutrino signals in direct
dark matter detection experiments. It is well known that even without new
physics, scattering of solar neutrinos on nuclei or electrons is an irreducible
background for direct dark matter searches, once these experiments each the ton
scale. Here, we entertain the possibility that neutrino interactions are
enhanced by new physics, such as new light force carriers (for instance a "dark
photon") or neutrino magnetic moments. We consider models with only the three
standard neutrino flavors, as well as scenarios with extra sterile neutrinos.
We find that low-energy neutrino--electron and neutrino--nucleus scattering
rates can be enhanced by several orders of magnitude, potentially enough to
explain the event excesses observed in CoGeNT and CRESST. We also investigate
temporal modulation in these neutrino signals, which can arise from geometric
effects, oscillation physics, non-standard neutrino energy loss, and
direction-dependent detection efficiencies. We emphasize that, in addition to
providing potential explanations for existing signals, models featuring new
physics in the neutrino sector can also be very relevant to future dark matter
searches, where, on the one hand, they can be probed and constrained, but on
the other hand, their signatures could also be confused with dark matter
signals.Comment: 38 pages, 8 figures, 1 table; v3: eq 3 and nuclear recoil plots
corrected, footnote added, conclusions unchange
Summer CO2 evasion from streams and rivers in the Kolyma River basin, north-east Siberia
Inland water systems are generally supersaturated in carbon dioxide (CO2) and are increasingly recognized as playing an important role in the global carbon cycle. The Arctic may be particularly important in this respect, given the abundance of inland waters and carbon contained in Arctic soils; however, a lack of trace gas measurements from small streams in the Arctic currently limits this understanding.We investigated the spatial variability of CO2 evasion during the summer low-flow period from streams and rivers in the northern portion of the Kolyma River basin in north-eastern Siberia. To this end, partial pressure of carbon dioxide (pCO2) and gas exchange velocities (k) were measured at a diverse set of streams and rivers to calculate CO2 evasion fluxes.
We combined these CO2 evasion estimates with satellite remote sensing and geographic information system techniques to calculate total areal CO2 emissions. Our results show that small streams are substantial sources of atmospheric CO2 owing to high pCO2 and k, despite being a small portion of total inland water surface area. In contrast, large rivers were generally near equilibrium with atmospheric CO2. Extrapolating our findings across the Panteleikha-Ambolikha sub-watersheds demonstrated that small streams play a major role in CO2 evasion, accounting for 86% of the total summer CO2 emissions from inland waters within these two sub-watersheds. Further expansion of these regional CO2 emission estimates across time and space will be critical to accurately quantify and understand the role of Arctic streams and rivers in the global carbon budget
Electrode Polarization Effects in Broadband Dielectric Spectroscopy
In the present work, we provide broadband dielectric spectra showing strong
electrode polarization effects for various materials, belonging to very
different material classes. This includes both ionic and electronic conductors
as, e.g., salt solutions, ionic liquids, human blood, and
colossal-dielectric-constant materials. These data are intended to provide a
broad data base enabling a critical test of the validity of phenomenological
and microscopic models for electrode polarization. In the present work, the
results are analyzed using a simple phenomenological equivalent-circuit
description, involving a distributed parallel RC circuit element for the
modeling of the weakly conducting regions close to the electrodes. Excellent
fits of the experimental data are achieved in this way, demonstrating the
universal applicability of this approach. In the investigated ionically
conducting materials, we find the universal appearance of a second dispersion
region due to electrode polarization, which is only revealed if measuring down
to sufficiently low frequencies. This indicates the presence of a second
charge-transport process in ionic conductors with blocking electrodes.Comment: 9 pages, 6 figures, experimental data are provided in electronic form
(see "Data Conservancy"
Carbon cycle uncertainty in the Alaskan Arctic
Climate change is leading to a disproportionately
large warming in the high northern latitudes, but the magnitude and sign of the future carbon balance of the Arctic are highly uncertain. Using 40 terrestrial biosphere models for the Alaskan Arctic from four recent model intercomparison projects – NACP (North American Carbon Program) site and regional syntheses, TRENDY (Trends in net land atmosphere carbon exchanges), and WETCHIMP (Wetland and Wetland CH4 Inter-comparison of Models Project) – we provide a baseline of terrestrial carbon cycle uncertainty, defined as the multi-model standard deviation (o) for each quantity that follows. Mean annual absolute uncertainty was largest for soil carbon (14.0±9.2 kgCm−2), then gross primary production (GPP) (0.22±0.50 kgCm−2 yr−1), ecosystem respiration (Re) (0.23±0.38 kgCm−2 yr−1), net primary production (NPP) (0.14±0.33 kgCm−2 yr−1), autotrophic respiration (Ra) (0.09±0.20 kgCm−2 yr−1), heterotrophic respiration (Rh) (0.14±0.20 kgCm−2 yr−1), net ecosystem exchange (NEE) (−0.01±0.19 kgCm−2 yr−1), and CH4 flux
(2.52±4.02 g CH4 m−2 yr−1). There were no consistent spatial
patterns in the larger Alaskan Arctic and boreal regional
carbon stocks and fluxes, with some models showing NEE
for Alaska as a strong carbon sink, others as a strong carbon source, while still others as carbon neutral. Finally, AmeriFlux data are used at two sites in the Alaskan Arctic to evaluate the regional patterns; observed seasonal NEE was captured within multi-model uncertainty. This assessment of carbon cycle uncertainties may be used as a baseline for the improvement of experimental and modeling activities, as well as a reference for future trajectories in carbon cycling with climate change in the Alaskan Arctic and larger boreal region
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