177 research outputs found
Quantum Monte Carlo treatment of elastic exciton-exciton scattering
We calculate cross sections for low energy elastic exciton-exciton scattering
within the effective mass approximation. Unlike previous theoretical
approaches, we give a complete, non-perturbative treatment of the four-particle
scattering problem. Diffusion Monte Carlo is used to calculate the essentially
exact energies of scattering states, from which phase shifts are determined.
For the case of equal-mass electrons and holes, which is equivalent to
positronium-positronium scattering, we find a_s = 2.1 a_x for scattering of
singlet-excitons and a_s= 1.5 a_x for triplet-excitons, where a_x is the
excitonic radius. The spin dependence of the cross sections arises from the
spatial exchange symmetry of the scattering wavefunctions. A significant
triplet-triplet to singlet-singlet scattering process is found, which is
similar to reported effects in recent experiments and theory for excitons in
quantum wells. We also show that the scattering length can change sign and
diverge for some values of the mass ratio m_h/m_e, an effect not seen in
previous perturbative treatments.Comment: 6 pages, 6 figures. Revision has updated figures, improved paper
structure, some minor correction
Method to compute the stress-energy tensor for the massless spin 1/2 field in a general static spherically symmetric spacetime
A method for computing the stress-energy tensor for the quantized, massless,
spin 1/2 field in a general static spherically symmetric spacetime is
presented. The field can be in a zero temperature state or a non-zero
temperature thermal state. An expression for the full renormalized
stress-energy tensor is derived. It consists of a sum of two tensors both of
which are conserved. One tensor is written in terms of the modes of the
quantized field and has zero trace. In most cases it must be computed
numerically. The other tensor does not explicitly depend on the modes and has a
trace equal to the trace anomaly. It can be used as an analytic approximation
for the stress-energy tensor and is equivalent to other approximations that
have been made for the stress-energy tensor of the massless spin 1/2 field in
static spherically symmetric spacetimes.Comment: 34 pages, no figure
Self-organized criticality in deterministic systems with disorder
Using the Bak-Sneppen model of biological evolution as our paradigm, we
investigate in which cases noise can be substituted with a deterministic signal
without destroying Self-Organized Criticality (SOC). If the deterministic
signal is chaotic the universality class is preserved; some non-universal
features, such as the threshold, depend on the time correlation of the signal.
We also show that, if the signal introduced is periodic, SOC is preserved but
in a different universality class, as long as the spectrum of frequencies is
broad enough.Comment: RevTex, 8 pages, 8 figure
A system of ODEs for a Perturbation of a Minimal Mass Soliton
We study soliton solutions to a nonlinear Schrodinger equation with a
saturated nonlinearity. Such nonlinearities are known to possess minimal mass
soliton solutions. We consider a small perturbation of a minimal mass soliton,
and identify a system of ODEs similar to those from Comech and Pelinovsky
(2003), which model the behavior of the perturbation for short times. We then
provide numerical evidence that under this system of ODEs there are two
possible dynamical outcomes, which is in accord with the conclusions of
Pelinovsky, Afanasjev, and Kivshar (1996). For initial data which supports a
soliton structure, a generic initial perturbation oscillates around the stable
family of solitons. For initial data which is expected to disperse, the finite
dimensional dynamics follow the unstable portion of the soliton curve.Comment: Minor edit
Signals for Lorentz Violation in Electrodynamics
An investigation is performed of the Lorentz-violating electrodynamics
extracted from the renormalizable sector of the general Lorentz- and
CPT-violating standard-model extension. Among the unconventional properties of
radiation arising from Lorentz violation is birefringence of the vacuum. Limits
on the dispersion of light produced by galactic and extragalactic objects
provide bounds of 3 x 10^{-16} on certain coefficients for Lorentz violation in
the photon sector. The comparative spectral polarimetry of light from
cosmologically distant sources yields stringent constraints of 2 x 10^{-32}.
All remaining coefficients in the photon sector are measurable in
high-sensitivity tests involving cavity-stabilized oscillators. Experimental
configurations in Earth- and space-based laboratories are considered that
involve optical or microwave cavities and that could be implemented using
existing technology.Comment: 23 pages REVTe
Avalanche Dynamics in Evolution, Growth, and Depinning Models
The dynamics of complex systems in nature often occurs in terms of
punctuations, or avalanches, rather than following a smooth, gradual path. A
comprehensive theory of avalanche dynamics in models of growth, interface
depinning, and evolution is presented. Specifically, we include the Bak-Sneppen
evolution model, the Sneppen interface depinning model, the Zaitsev flux creep
model, invasion percolation, and several other depinning models into a unified
treatment encompassing a large class of far from equilibrium processes. The
formation of fractal structures, the appearance of noise, diffusion with
anomalous Hurst exponents, Levy flights, and punctuated equilibria can all be
related to the same underlying avalanche dynamics. This dynamics can be
represented as a fractal in spatial plus one temporal dimension. We develop
a scaling theory that relates many of the critical exponents in this broad
category of extremal models, representing different universality classes, to
two basic exponents characterizing the fractal attractor. The exact equations
and the derived set of scaling relations are consistent with numerical
simulations of the above mentioned models.Comment: 27 pages in revtex, no figures included. Figures or hard copy of the
manuscript supplied on reques
An interlaboratory study of TEX86 and BIT analysis of sediments, extracts and standard mixtures.
Two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) are the TEX86 (TetraEther indeX of 86 carbon atoms) paleothermometer for sea surface temperature reconstructions and the BIT (Branched Isoprenoid Tetraether) index for reconstructing soil organic matter input to the ocean. An initial round-robin study of two sediment extracts, in which 15 laboratories participated, showed relatively consistent TEX86 values (reproducibility ±3-4°C when translated to temperature) but a large spread in BIT measurements (reproducibility ±0.41 on a scale of 0-1). Here we report results of a second round-robin study with 35 laboratories in which three sediments, one sediment extract, and two mixtures of pure, isolated GDGTs were analyzed. The results for TEX86 and BIT index showed improvement compared to the previous round-robin study. The reproducibility, indicating interlaboratory variation, of TEX86 values ranged from 1.3 to 3.0°C when translated to temperature. These results are similar to those of other temperature proxies used in paleoceanography. Comparison of the results obtained from one of the three sediments showed that TEX86 and BIT indices are not significantly affected by interlaboratory differences in sediment extraction techniques. BIT values of the sediments and extracts were at the extremes of the index with values close to 0 or 1, and showed good reproducibility (ranging from 0.013 to 0.042). However, the measured BIT values for the two GDGT mixtures, with known molar ratios of crenarchaeol and branched GDGTs, had intermediate BIT values and showed poor reproducibility and a large overestimation of the "true" (i.e., molar-based) BIT index. The latter is likely due to, among other factors, the higher mass spectrometric response of branched GDGTs compared to crenarchaeol, which also varies among mass spectrometers. Correction for this different mass spectrometric response showed a considerable improvement in the reproducibility of BIT index measurements among laboratories, as well as a substantially improved estimation of molar-based BIT values. This suggests that standard mixtures should be used in order to obtain consistent, and molar-based, BIT values
An updated radiocarbon-based ice margin chronology for the last deglaciation of the North American Ice Sheet Complex
The North American Ice Sheet Complex (NAISC; consisting of the Laurentide, Cordilleran and Innuitian ice sheets) was the largest ice mass to repeatedly grow and decay in the Northern Hemisphere during the Quaternary. Understanding its pattern of retreat following the Last Glacial Maximum is critical for studying many facets of the Late Quaternary, including ice sheet behaviour, the evolution of Holocene landscapes, sea level, atmospheric circulation, and the peopling of the Americas. Currently, the most up-to-date and authoritative margin chronology for the entire ice sheet complex is featured in two publications (Geological Survey of Canada Open File 1574 [Dyke et al., 2003]; ‘Quaternary Glaciations – Extent and Chronology, Part II’ [Dyke, 2004]). These often-cited datasets track ice margin recession in 36 time slices spanning 18 ka to 1 ka (all ages in uncalibrated radiocarbon years) using a combination of geomorphology, stratigraphy and radiocarbon dating. However, by virtue of being over 15 years old, the ice margin chronology requires updating to reflect new work and important revisions. This paper updates the aforementioned 36 ice margin maps to reflect new data from regional studies. We also update the original radiocarbon dataset from the 2003/2004 papers with 1541 new ages to reflect work up to and including 2018. A major revision is made to the 18 ka ice margin, where Banks and Eglinton islands (once considered to be glacial refugia) are now shown to be fully glaciated. Our updated 18 ka ice sheet increased in areal extent from 17.81 to 18.37 million km2, which is an increase of 3.1% in spatial coverage of the NAISC at that time. Elsewhere, we also summarize, region-by-region, significant changes to the deglaciation sequence. This paper integrates new information provided by regional experts and radiocarbon data into the deglaciation sequence while maintaining consistency with the original ice margin positions of Dyke et al. (2003) and Dyke (2004) where new information is lacking; this is a pragmatic solution to satisfy the needs of a Quaternary research community that requires up-to-date knowledge of the pattern of ice margin recession of what was once the world’s largest ice mass. The 36 updated isochrones are available in PDF and shapefile format, together with a spreadsheet of the expanded radiocarbon dataset (n = 5195 ages) and estimates of uncertainty for each interval
Flavor Production in Pb(160AGeV) on Pb Collisions: Effect of Color Ropes and Hadronic Rescattering
Collective interactions in the preequilibrium quark matter and hadronic
resonance gas stage of ultrarelativistic nucleus-nucleus collisions are studied
in the framework of the the transport theoretical approach RQMD. The paper
reviews string fusion into color ropes and hadronic rescattering which serve as
models for these interactions. Hadron production in central Pb(160AGeV) on Pb
collisions has been calculated. The changes of the final flavor composition are
more pronounced than in previous RQMD studies of light ion induced reactions at
200AGeV. The ratio of created quark pairs /(+) is
enhanced by a factor of 2.4 in comparison to results. Color rope formation
increases the initially produced antibaryons to 3 times the value in the `NN
mode', but only one quarter of the produced antibaryons survives because of
subsequent strong absorption. The differences in the final particle composition
for Pb on Pb collisions compared to S induced reactions are attributed to the
hadronic resonance gas stage which is baryon-richer and lasts longer.Comment: 60 pages + 11 postscript figures (uuencoded and included
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