50,652 research outputs found
The phase transition in the anisotropic Heisenberg model with long range dipolar interactions
In this work we have used extensive Monte Carlo calculations to study the
planar to paramagnetic phase transition in the two-dimensional anisotropic
Heisenberg model with dipolar interactions (AHd) considering the true
long-range character of the dipolar interactions by means of the Ewald
summation. Our results are consistent with an order-disorder phase transition
with unusual critical exponents in agreement with our previous results for the
Planar Rotator model with dipolar interactions. Nevertheless, our results
disagrees with the Renormalization Group results of Maier and Schwabl [PRB, 70,
134430 (2004)] and the results of Rapini et. al. [PRB, 75, 014425 (2007)],
where the AHd was studied using a cut-off in the evaluation of the dipolar
interactions. We argue that besides the long-range character of dipolar
interactions their anisotropic character may have a deeper effect in the system
than previously believed. Besides, our results shows that the use of a cut-off
radius in the evaluation of dipolar interactions must be avoided when analyzing
the critical behavior of magnetic systems, since it may lead to erroneous
results.Comment: Accepted for publication in the Journal of Magnetism and Magnetic
Materials. arXiv admin note: substantial text overlap with arXiv:1109.184
Nucleosynthesis in Fast Expansions of High-Entropy, Proton Rich Matter
We demonstrate that nucleosynthesis in rapid, high-entropy expansions of
proton-rich matter from high temperature and density can result in a wider
variety of abundance patterns than heretofore appreciated. In particular, such
expansions can produce iron-group nuclides, p-process nuclei, or even heavy,
neutron-rich isotopes. Such diversity arises because the nucleosynthesis enters
a little explored regime in which the free nucleons are not in equilibrium with
the abundant alpha particles. This allows nuclei significantly heavier than
iron to form in t he presence of abundant free nucleons early in the expansion.
As the temperature drops, nucleons increasingly assemble into alpha particles
and heavier nuclei. If the assembly is efficient, the resulting depletion of
free neutrons allows disintegrat ion flows to drive nuclei back down to iron
and nickel. If this assembly is inefficient, then the large abundance of free
nucleons prevents the disintegration flows and leaves a distribution of heavy
nuclei after reaction freezeout. For cases in between, an intermediate
abundance distribution, enriched in p-process isotopes, is frozen out. These
last expansions may contribute to the solar system's supply of the p-process
nuclides if mildly proton-rich, high-entropy matter is ejected from
proto-neutron stars winds or other astrophysical sites. Also sign ificant is
the fact that, because the nucleosynthesis is primary, the signature of this
nucleosyn thesis may be evident in metal poor stars.Comment: 11 pages, 2 tables, 1 figure. Submitted to ApJ Letter
Using zeros of the canonical partition function map to detect signatures of a Berezinskii-Kosterlitz-Thouless transition
Using the two dimensional model as a test case, we show that
analysis of the Fisher zeros of the canonical partition function can provide
signatures of a transition in the Berezinskii-Kosterlitz-Thouless ()
universality class. Studying the internal border of zeros in the complex
temperature plane, we found a scenario in complete agreement with theoretical
expectations which allow one to uniquely classify a phase transition as in the
class of universality. We obtain in excellent accordance with
previous results. A careful analysis of the behavior of the zeros for both
regions and in the
thermodynamic limit show that goes to zero in the former
case and is finite in the last one
Scalar fields in the Lense-Thirring background with a cosmic string and Hawking radiation
We analyze the influence of the gravitational field produced by a slowly
rotating black hole with a cosmic string along the axis of symmetry on a
massive scalar field. Exact solutions of both angular and radial parts of the
Klein-Gordon equation in this spacetime are obtained, and are given in terms of
the confluent Heun functions. We emphasize the role of the presence of the
cosmic string in these solutions. We also investigate the solutions in regions
near and far from the event horizon. From the radial solution, we obtain the
exact wave solutions near the exterior horizon of the black hole, and discuss
the Hawking radiation of massive scalar particles.Comment: 6 page
Predicting Intermediate Storage Performance for Workflow Applications
Configuring a storage system to better serve an application is a challenging
task complicated by a multidimensional, discrete configuration space and the
high cost of space exploration (e.g., by running the application with different
storage configurations). To enable selecting the best configuration in a
reasonable time, we design an end-to-end performance prediction mechanism that
estimates the turn-around time of an application using storage system under a
given configuration. This approach focuses on a generic object-based storage
system design, supports exploring the impact of optimizations targeting
workflow applications (e.g., various data placement schemes) in addition to
other, more traditional, configuration knobs (e.g., stripe size or replication
level), and models the system operation at data-chunk and control message
level.
This paper presents our experience to date with designing and using this
prediction mechanism. We evaluate this mechanism using micro- as well as
synthetic benchmarks mimicking real workflow applications, and a real
application.. A preliminary evaluation shows that we are on a good track to
meet our objectives: it can scale to model a workflow application run on an
entire cluster while offering an over 200x speedup factor (normalized by
resource) compared to running the actual application, and can achieve, in the
limited number of scenarios we study, a prediction accuracy that enables
identifying the best storage system configuration
Graphene-based spin-pumping transistor
We demonstrate with a fully quantum-mechanical approach that graphene can
function as gate-controllable transistors for pumped spin currents, i.e., a
stream of angular momentum induced by the precession of adjacent
magnetizations, which exists in the absence of net charge currents.
Furthermore, we propose as a proof of concept how these spin currents can be
modulated by an electrostatic gate. Because our proposal involves nano-sized
systems that function with very high speeds and in the absence of any applied
bias, it is potentially useful for the development of transistors capable of
combining large processing speeds, enhanced integration and extremely low power
consumption
Graphene as a non-magnetic spin-current lens
In spintronics, the ability to transport magnetic information often depends
on the existence of a spin current traveling between two different magnetic
objects acting as source and probe. A large fraction of this information never
reaches the probe and is lost because the spin current tends to travel
omni-directionally. We propose that a curved boundary between a gated and a
non-gated region within graphene acts as an ideal lens for spin currents
despite being entirely of non-magnetic nature. We show as a proof of concept
that such lenses can be utilized to redirect the spin current that travels away
from a source onto a focus region where a magnetic probe is located, saving a
considerable fraction of the magnetic information that would be otherwise lost.Comment: 9 pages, 3 figure
Theory of Local Dynamical Magnetic Susceptibilities from the Korringa-Kohn-Rostoker Green Function Method
Within the framework of time-dependent density functional theory combined
with the Korringa-Kohn-Rostoker Green function formalism, we present a real
space methodology to investigate dynamical magnetic excitations from
first-principles. We set forth a scheme which enables one to deduce the correct
effective Coulomb potential needed to preserve the spin-invariance signature in
the dynamical susceptibilities, i.e. the Goldstone mode. We use our approach to
explore the spin dynamics of 3d adatoms and different dimers deposited on a
Cu(001) with emphasis on their decay to particle-hole pairs.Comment: 32 pages (preprint), 6 figures, one tabl
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