9,092 research outputs found
Phase diagram of an Ising model for ultrathin magnetic films
We study the critical properties of a two--dimensional Ising model with
competing ferromagnetic exchange and dipolar interactions, which models an
ultra-thin magnetic film with high out--of--plane anisotropy in the monolayer
limit. In this work we present a detailed calculation of the phase
diagram, being the ratio between exchange and dipolar interactions
intensities. We compare the results of both mean field approximation and Monte
Carlo numerical simulations in the region of low values of ,
identifying the presence of a recently detected phase with nematic order in
different parts of the phase diagram, besides the well known striped and
tetragonal liquid phases. A remarkable qualitative difference between both
calculations is the absence, in this region of the Monte Carlo phase diagram,
of the temperature dependency of the equilibrium stripe width predicted by the
mean field approximation. We also detected the presence of an increasing number
of metastable striped states as the value of increases.Comment: 9 pages, 9 figure
Unconventional cosmology on the (thick) brane
We consider the cosmology of a thick codimension 1 brane. We obtain the
matching conditions leading to the cosmological evolution equations and show
that when one includes matter with a pressure component along the extra
dimension in the brane energy-momentum tensor, the cosmology is of non-standard
type. In particular one can get acceleration when a dust of non-relativistic
matter particles is the only source for the (modified) Friedman equation. Our
equations would seem to violate the conservation of energy-momentum from a 4D
perspective, but in 5D the energy-momentum is conserved. One could write down
an effective conserved 4D energy-momentum tensor attaching a ``dark energy''
component to the energy-momentum tensor of matter that has pressure along the
extra dimension. This extra component could, on a cosmological scale, be
interpreted as matter-coupled quintessence. We comment on the effective 4D
description of this effect in terms of the time evolution of a scalar field
(the 5D radion) coupled to this kind of matter.Comment: 9 pages, v2. eq.(17) corrected, comments on effective theory change
Chiral phase transitions: focus driven critical behavior in systems with planar and vector ordering
The fixed point that governs the critical behavior of magnets described by
the -vector chiral model under the physical values of () is
shown to be a stable focus both in two and three dimensions. Robust evidence in
favor of this conclusion is obtained within the five-loop and six-loop
renormalization-group analysis in fixed dimension. The spiral-like approach of
the chiral fixed point results in unusual crossover and near-critical regimes
that may imitate varying critical exponents seen in physical and computer
experiments.Comment: 4 pages, 5 figures. Discussion enlarge
On The Existence of Roton Excitations in Bose Einstein Condensates: Signature of Proximity to a Mott Insulating Phase
Within the last decade, artificially engineered Bose Einstein Condensation
has been achieved in atomic systems. Bose Einstein Condensates are superfluids
just like bosonic Helium is and all interacting bosonic fluids are expected to
be at low enough temperatures. One difference between the two systems is that
superfluid Helium exhibits roton excitations while Bose Einstein Condensates
have never been observed to have such excitations. The reason for the roton
minimum in Helium is its proximity to a solid phase. The roton minimum is a
consequence of enhanced density fluctuations at the reciprocal lattice vector
of the stillborn solid. Bose Einstein Condensates in atomic traps are not near
a solid phase and therefore do not exhibit roton minimum. We conclude that if
Bose Einstein Condensates in an optical lattice are tuned near a transition to
a Mott insulating phase, a roton minimum will develop at a reciprocal lattice
vector of the lattice. Equivalently, a peak in the structure factor will appear
at such a wavevector. The smallness of the roton gap or the largeness of the
structure factor peak are experimental signatures of the proximity to the Mott
transition.Comment: 4 pages, 5 figure
Noncommutative spaces, the quantum of time and the Lorentz symmetry
We introduce three space-times that are discrete in time and compatible with
the Lorentz symmetry. We show that these spaces are no commutative, with
commutation relations similar to the relations of the Snyder and Yang spaces.
Furthermore, using a reparametrized relativistic particle we obtain a
realization of the Snyder type spaces and we construct an action for them.Comment: 8 pages, to appear in PR
Interação entre efeito materno e sexo do bezerro para pesos e taxas de crescimento do nascimento à desmama em animais cruzados 5/8 Zebu + 3/8 Charolês.
Analisaram-se os dados de 130 animais nascidos de agosto a novembro dos anos de 1950 a 1952, com o objetivo de avaliar o efeito do tipo de cruzamento no desempenho ate a desmama de bezerros 5/8 Zebu + 3/8 Charoles, criados a pasto na Fazenda Canchim, SĂŁo Carlos, SP
Performance Metrics for the Objective Assessment of Capacitive Deionization Systems
In the growing field of capacitive deionization (CDI), a number of
performance metrics have emerged to describe the desalination process.
Unfortunately, the separation conditions under which these metrics are measured
are often not specified, resulting in optimal performance at minimal removal.
Here we outline a system of performance metrics and reporting conditions that
resolves this issue. Our proposed system is based on volumetric energy
consumption (Wh/m) and throughput productivity (L/h/m) reported for a
specific average concentration reduction, water recovery, and feed salinity. To
facilitate and rationalize comparisons between devices, materials, and
operation modes, we propose a nominal standard testing condition of removing 5
mM from a 20 mM NaCl feed solution at 50% water recovery for CDI research.
Using this separation, we compare the desalination performance of a
flow-through electrode (fte-CDI) cell and a flow between membrane (fb-MCDI)
device, showing how significantly different systems can be compared in terms of
generally desirable desalination characteristics. In general, we find that
performance analysis must be considered carefully so to not allow for ambiguous
separation conditions or the maximization of one metric at the expense of
another. Additionally, for context we discuss a number of important underlying
performance indicators and cell characteristics that are not performance
measures in and of themselves but can be examined to better understand
differences in performance
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