585 research outputs found
Monte Carlo Simulations for the Magnetic Phase Diagram of the Double Exchange Hamiltonian
We have used Monte Carlo simulation techniques to obtain the magnetic phase
diagram of the double exchange Hamiltonian. We have found that the Berry's
phase of the hopping amplitude has a negligible effect in the value of the
magnetic critical temperature. To avoid finite size problems in our simulations
we have also developed an approximated expression for the double exchange
energy. This allows us to obtain the critical temperature for the ferromagnetic
to paramagnetic transition more accurately. In our calculations we do not
observe any strange behavior in the kinetic energy, chemical potential or
electron density of states near the magnetic critical temperature. Therefore,
we conclude that other effects, not included in the double exchange
Hamiltonian, are needed to understand the metal-insulator transition which
occurs in the manganites.Comment: 6 pages Revtex, 8 PS figure
Accelerated avian invasion into the Mediterranean region endangers biodiversity and mandates international collaboration
Policy DirectionDespite posing a serious threat to global biodiversity, national and international
management efforts have not been able to limit the spread of most invasive
species. In highly dispersive species, local invasions may be followed by regional
range expansion that crosses international borders. In such cases, independent
management efforts of the invading population may be futile unless international
collaboration is practiced.
2. We focus on the ongoing human-mediated
invasion of the common myna
Acridotheres tristis into the Mediterranean basin, a region rich in overall numbers
of species and endemic species, where common mynas have been introduced
into a handful of countries. Some introductions were followed by subsequent
range expansions into neighbouring countries. This species poses major threats
to the biodiversity of the Mediterranean which is already susceptible to biodiversity
loss as the result of ongoing land use and climate changes. Without
action, this species and possibly others similar to it, could have severe consequences
for native ecosystems.
3. Policy implications. Given the regional scope of its invasion in the Mediterranean
basin, common myna management requires an international collaboration to
successfully prevent additional introductions and range expansions and to avoid
accelerating threats to Mediterranean biodiversity, already at risk as a result of ongoing changes in land use and climate. We argue that international reciprocal
transfer of information and the development of regional mitigation are essential
for the successful management of the invasion of the common myna and other
species into the Mediterraneaninfo:eu-repo/semantics/publishedVersio
Spectral functions, Fermi surface and pseudogap in the t-J model
Spectral functions within the generalized t-J model as relevant to cuprates
are analyzed using the method of equations of motion for projected fermion
operators. In the evaluation of the self energy the decoupling of spin and
single-particle fluctuations is performed. It is shown that in an undoped
antiferromagnet (AFM) the method reproduces the selfconsistent Born
approximation. For finite doping with short range AFM order the approximation
evolves into a paramagnon contribution which retains large incoherent
contribution in the hole part of the spectral function as well as the
hole-pocket-like Fermi surface at low doping. On the other hand, the
contribution of (longitudinal) spin fluctuations, with the coupling mostly
determined predominantly by J and next-neighbor hopping t', is essential for
the emergence of the pseudogap. The latter shows at low doping in the effective
truncation of the large Fermi surface, reduced electron density of states and
at the same time quasiparticle density of states at the Fermi level.Comment: RevTex, 13 pages, 11 figures (5 color
Dispersion of Ordered Stripe Phases in the Cuprates
A phase separation model is presented for the stripe phase of the cuprates,
which allows the doping dependence of the photoemission spectra to be
calculated. The idealized limit of a well-ordered array of magnetic and charged
stripes is analyzed, including effects of long-range Coulomb repulsion.
Remarkably, down to the limit of two-cell wide stripes, the dispersion can be
interpreted as essentially a superposition of the two end-phase dispersions,
with superposed minigaps associated with the lattice periodicity. The largest
minigap falls near the Fermi level; it can be enhanced by proximity to a (bulk)
Van Hove singularity. The calculated spectra are dominated by two features --
this charge stripe minigap plus the magnetic stripe Hubbard gap. There is a
strong correlation between these two features and the experimental
photoemission results of a two-peak dispersion in LaSrCuO, and
the peak-dip-hump spectra in BiSrCaCuO. The
differences are suggestive of the role of increasing stripe fluctuations. The
1/8 anomaly is associated with a quantum critical point, here expressed as a
percolation-like crossover. A model is proposed for the limiting minority
magnetic phase as an isolated two-leg ladder.Comment: 24 pages, 26 PS figure
Pulsar-wind nebulae and magnetar outflows: observations at radio, X-ray, and gamma-ray wavelengths
We review observations of several classes of neutron-star-powered outflows:
pulsar-wind nebulae (PWNe) inside shell supernova remnants (SNRs), PWNe
interacting directly with interstellar medium (ISM), and magnetar-powered
outflows. We describe radio, X-ray, and gamma-ray observations of PWNe,
focusing first on integrated spectral-energy distributions (SEDs) and global
spectral properties. High-resolution X-ray imaging of PWNe shows a bewildering
array of morphologies, with jets, trails, and other structures. Several of the
23 so far identified magnetars show evidence for continuous or sporadic
emission of material, sometimes associated with giant flares, and a few
possible "magnetar-wind nebulae" have been recently identified.Comment: 61 pages, 44 figures (reduced in quality for size reasons). Published
in Space Science Reviews, "Jets and Winds in Pulsar Wind Nebulae, Gamma-ray
Bursts and Blazars: Physics of Extreme Energy Release
Active Brownian Particles. From Individual to Collective Stochastic Dynamics
We review theoretical models of individual motility as well as collective
dynamics and pattern formation of active particles. We focus on simple models
of active dynamics with a particular emphasis on nonlinear and stochastic
dynamics of such self-propelled entities in the framework of statistical
mechanics. Examples of such active units in complex physico-chemical and
biological systems are chemically powered nano-rods, localized patterns in
reaction-diffusion system, motile cells or macroscopic animals. Based on the
description of individual motion of point-like active particles by stochastic
differential equations, we discuss different velocity-dependent friction
functions, the impact of various types of fluctuations and calculate
characteristic observables such as stationary velocity distributions or
diffusion coefficients. Finally, we consider not only the free and confined
individual active dynamics but also different types of interaction between
active particles. The resulting collective dynamical behavior of large
assemblies and aggregates of active units is discussed and an overview over
some recent results on spatiotemporal pattern formation in such systems is
given.Comment: 161 pages, Review, Eur Phys J Special-Topics, accepte
Biodiversity increases the resistance of ecosystem productivity to climate extremes
It remains unclear whether biodiversity buffers ecosystems against climate extremes, which are becoming increasingly frequent worldwide1. Early results suggested that the ecosystem productivity of diverse grassland plant communities was more resistant, changing less during drought, and more resilient, recovering more quickly after drought, than that of depauperate communities2. However, subsequent experimental tests produced mixed results3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13. Here we use data from 46 experiments that manipulated grassland plant diversity to test whether biodiversity provides resistance during and resilience after climate events. We show that biodiversity increased ecosystem resistance for a broad range of climate events, including wet or dry, moderate or extreme, and brief or prolonged events. Across all studies and climate events, the productivity of low-diversity communities with one or two species changed by approximately 50% during climate events, whereas that of high-diversity communities with 16–32 species was more resistant, changing by only approximately 25%. By a year after each climate event, ecosystem productivity had often fully recovered, or overshot, normal levels of productivity in both high- and low-diversity communities, leading to no detectable dependence of ecosystem resilience on biodiversity. Our results suggest that biodiversity mainly stabilizes ecosystem productivity, and productivity-dependent ecosystem services, by increasing resistance to climate events. Anthropogenic environmental changes that drive biodiversity loss thus seem likely to decrease ecosystem stability14, and restoration of biodiversity to increase it, mainly by changing the resistance of ecosystem productivity to climate events
The Baikal Deep Underwater Neutrino Experiment: Results, Status, Future
We review the present status of the Baikal Underwater Neutrino Experiment and
present results obtained with the various stages of the stepwise increasing
detector: NT-36 (1993-95), NT-72 (1995-96) and NT-96 (1996-97). Results cover
atmospheric muons, first clear neutrino events, search for neutrinos from WIMP
annihilation in the center of the Earth, search for magnetic monopoles, and --
far from astroparticle physics -- limnology.Comment: Talk given at the Int. School on Nuclear Physics, Erice, Sept.199
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