4,293 research outputs found
Optimal Energy Estimation in Path-Integral Monte Carlo Simulations
We investigate the properties of two standard energy estimators used in
path-integral Monte Carlo simulations. By disentangling the variance of the
estimators and their autocorrelation times we analyse the dependence of the
performance on the update algorithm and present a detailed comparison of
refined update schemes such as multigrid and staging techniques. We show that a
proper combination of the two estimators leads to a further reduction of the
statistical error of the estimated energy with respect to the better of the two
without extra cost.Comment: 45 pp. LaTeX, 22 Postscript Figure
Witnessing the birth of a supermassive protostar
The detection of quasars reveals the existence of supermassive
black holes of a few . One of the potential pathways to
explain their formation in the infant universe is the so-called direct collapse
model which provides massive seeds of . An isothermal
direct collapse mandates that halos should be of a primordial composition and
the formation of molecular hydrogen remains suppressed in the presence of a
strong Lyman Werner flux. In this study, we perform high resolution
cosmological simulations for two massive primordial halos employing a detailed
chemical model which includes cooling as well as realistic opacities
for both the bound-free emission and the Rayleigh scattering of
hydrogen atoms. We are able to resolve the collapse up to unprecedentedly high
densities of and to scales of about AU.
Our results show that the gas cools down to 5000 K in the presence
of cooling, and induces fragmentation at scales of about 8000 AU in
one of the two simulated halos, which may lead to the formation of a binary. In
addition, fragmentation also occurs on the AU scale in one of the halos but the
clumps are expected to merge on short time scales. Our results confirm that
cooling does not prevent the formation of a supermassive star and the
trapping of cooling radiation stabilises the collapse on small scales.Comment: Accpeted version, to appear in MNRAS, comments are still welcome and
high resolution version is available at
http://www2.iap.fr/users/latif/DCBH.pd
Binary spinning black hole Hamiltonian in canonical center-of-mass and rest-frame coordinates through higher post-Newtonian order
The recently constructed Hamiltonians for spinless binary black holes through
third post-Newtonian order and for spinning ones through formal second
post-Newtonian order, where the spins are counted of zero post-Newtonian order,
are transformed into fully canonical center-of-mass and rest-frame variables.
The mixture terms in the Hamiltonians between center-of-mass and rest-frame
variables are in accordance with the relation between the total linear momentum
and the center-of-mass velocity as demanded by global Lorentz invariance. The
various generating functions for the center-of-mass and rest-frame canonical
variables are explicitly given in terms of the single-particle canonical
variables. The no-interaction theorem does not apply because the world-line
condition of Lorentz covariant position variables is not imposed.Comment: 18 pages, no figure
Temperature induced phase averaging in one-dimensional mesoscopic systems
We analyse phase averaging in one-dimensional interacting mesoscopic systems
with several barriers and show that for incommensurate positions an independent
average over several phases can be induced by finite temperature. For three
strong barriers with conductances G_i and mutual distances larger than the
thermal length, we obtain G ~ sqrt{G_1 G_2 G_3} for the total conductance G.
For an interacting wire, this implies power laws in G(T) with novel exponents,
which we propose as an experimental fingerprint to distinguish temperature
induced phase averaging from dephasing.Comment: 6 pages, 5 figures; added one figure; slightly extende
Stable periodic density waves in dipolar Bose-Einstein condensates trapped in optical lattices
Density-wave patterns in (quasi-) discrete media with local interactions are
known to be unstable. We demonstrate that \emph{stable} double- and triple-
period patterns (DPPs and TPPs), with respect to the period of the underlying
lattice, exist in media with nonlocal nonlinearity. This is shown in detail for
dipolar Bose-Einstein condensates (BECs), loaded into a deep one-dimensional
(1D) optical lattice (OL), by means of analytical and numerical methods in the
tight-binding limit. The patterns featuring multiple periodicities are
generated by the modulational instability of the continuous-wave (CW) state,
whose period is identical to that of the OL. The DPP and TPP emerge via phase
transitions of the second and first kind, respectively. The emerging patterns
may be stable provided that the dipole-dipole (DD) interactions are repulsive
and sufficiently strong, in comparison with the local repulsive nonlinearity.
Within the set of the considered states, the TPPs realize a minimum of the free
energy. Accordingly, a vast stability region for the TPPs is found in the
parameter space, while the DPP\ stability region is relatively narrow. The same
mechanism may create stable density-wave patterns in other physical media
featuring nonlocal interactions, such as arrayed optical waveguides with
thermal nonlinearity.Comment: 7 pages, 4 figures, Phys. Rev. Lett., in pres
The species composition of Antarctic phytoplankton interpreted in terms of Tilman's competition theory
An attempt was made, to test for the impact of resource competition on Antarctic marine phytoplankton. According to theory, species composition near competitive equilibrium should be determined by the ratios of limiting resources. Enrichment bioassays identified silicon and nitrogen as limiting nutrients for some of the most important phytoplankton species during early austral summer in the region near the Antarctic Peninsula. Together with the generally acknowledged limiting resource light, this gave three meaningful ratios of essential resources (Si:N, Si:light, N:light) and one ratio of substitutable resources (NO sub(3):NH sub(4)). Phytoplankton species assemblages were found to be well separated by the ratios of the essential resources and by mixing depth
Parathyroid hormone, vitamin D, and cardiovascular disease in chronic renal failure
Parathyroid hormone, vitamin D, and cardiovascular disease in chronic renal failure.BackgroundParathyroid hormone and vitamin D have been shown to influence cardiac and vascular growth and function experimentally in human subjects with normal renal function. Because of the increased prevalence of hyperparathyroidism and altered vitamin D status in chronic renal failure, these alterations have been considered to contribute to the increased prevalence of cardiovascular disease and hypertension seen in this patient population.Methods and ResultsIn this article, we review experimental and clinical literature on the cardiovascular effects of parathyroid hormone and vitamin D and relate them to the development of cardiac and vascular dysfunction in uremia, such as: cardiomyopathy, myocardial hypertrophy, and fibrosis, as well as to myocardial ischemia; uremic glucose intolerance, dyslipidemia, and atherosclerosis; hypertension; and vascular and cardiac calcifications.ConclusionsThe hyperparathyroid state and altered vitamin D status found in uremia contribute to the cardiovascular pathology seen clinically in uremia and also to the excess mortality from cardiovascular causes found in this patient group. The therapeutic implications of these observations are also discussed
Self-organized patterns of coexistence out of a predator-prey cellular automaton
We present a stochastic approach to modeling the dynamics of coexistence of
prey and predator populations. It is assumed that the space of coexistence is
explicitly subdivided in a grid of cells. Each cell can be occupied by only one
individual of each species or can be empty. The system evolves in time
according to a probabilistic cellular automaton composed by a set of local
rules which describe interactions between species individuals and mimic the
process of birth, death and predation. By performing computational simulations,
we found that, depending on the values of the parameters of the model, the
following states can be reached: a prey absorbing state and active states of
two types. In one of them both species coexist in a stationary regime with
population densities constant in time. The other kind of active state is
characterized by local coupled time oscillations of prey and predator
populations. We focus on the self-organized structures arising from
spatio-temporal dynamics of the coexistence. We identify distinct spatial
patterns of prey and predators and verify that they are intimally connected to
the time coexistence behavior of the species. The occurrence of a prey
percolating cluster on the spatial patterns of the active states is also
examined.Comment: 19 pages, 11 figure
Blurred Lines Between Competition and Parasitism
Accurately describing the ecological relationships between species is more than mere semantics-doing so has profound practical and applied implications, not the least of which is that inaccurate descriptions can lead to fundamentally incorrect predicted outcomes of community composition and functioning. Accurate ecological classifications are particularly important in the context of global change, where species interactions can change rapidly following shifts in species composition. Here, we argue that many common ecological interactions-particularly competition and parasitism-can be easily confused and that we often lack empirical evidence for the full reciprocal interaction among species. To make our case and to propose a theoretical framework for addressing this problem, we use the interactions between lianas and trees, whose outcomes have myriad implications for the ecology and conservation of tropical forests (e.g., Schnitzer et al. 2015)
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