1,962 research outputs found
Frustration Effects in Antiferromagnetic FCC Heisenberg Films
We study the effects of frustration in an antiferromagnetic film of FCC
lattice with Heisenberg spin model including an Ising-like anisotropy. Monte
Carlo (MC) simulations have been used to study thermodynamic properties of the
film. We show that the presence of the surface reduces the ground state (GS)
degeneracy found in the bulk. The GS is shown to depend on the surface in-plane
interaction with a critical value at which ordering of type I coexists
with ordering of type II. Near this value a reentrant phase is found. Various
physical quantities such as layer magnetizations and layer susceptibilities are
shown and discussed. The nature of the phase transition is also studied by
histogram technique. We have also used the Green's function (GF) method for the
quantum counterpart model. The results at low- show interesting effects of
quantum fluctuations. Results obtained by the GF method at high are
compared to those of MC simulations. A good agreement is observed.Comment: 11 pages, 19 figures, submitted to J. Phys.: Condensed Matte
Effects of Frustrated Surface in Heisenberg Thin Films
We study by extensive Monte Carlo (MC) simulations and analytical Green
function (GF) method effects of frustrated surfaces on the properties of thin
films made of stacked triangular layers of atoms bearing Heisenberg spins with
an Ising-like interaction anisotropy. We suppose that the in-plane surface
interaction can be antiferromagnetic or ferromagnetic while all other
interactions are ferromagnetic. We show that the ground-state spin
configuration is non linear when is lower than a critical value .
The film surfaces are then frustrated. In the frustrated case, there are two
phase transitions related to disorderings of surface and interior layers. There
is a good agreement between MC and GF results. In addition, we show from MC
histogram calculation that the value of the ratio of critical exponents
of the observed transitions is deviated from the values of two and
three Ising universality classes. The origin of this deviation is discussed
with general physical arguments.Comment: 9 pages, 16 figure
Effect of Dipolar Interaction in Molecular Crystals
We investigate in this paper the ground state and the nature of the
transition from an orientational ordered phase at low temperature to the
disordered state at high temperature in a molecular crystal. Our model is a
Potts model which takes into account the exchange interaction between
nearest-neighbor molecules and a dipolar interaction between molecular axes in
three dimensions. The dipolar interaction is characterized by two parameters:
its amplitude and the cutoff distance . If the molecular axis at a
lattice site has three orientations, say the , or axes, then when
D=0, the system is equivalent to the 3-state Potts model: the transition to the
disordered phase is known to be of first order. When , the
ground-state configuration is shown to be composed of two independent
interpenetrating layered subsystems which form a sandwich whose periodicity
depends on and . We show by extensive Monte Carlo simulation with a
histogram method that the phase transition remains of first order at relatively
large values of .Comment: 6 pages, 7 figure
Re-orientation Transition in Molecular Thin Films: Potts Model with Dipolar Interaction
We study the low-temperature behavior and the phase transition of a thin film
by Monte Carlo simulation. The thin film has a simple cubic lattice structure
where each site is occupied by a Potts parameter which indicates the molecular
orientation of the site. We take only three molecular orientations in this
paper which correspond to the 3-state Potts model. The Hamiltonian of the
system includes: (i) the exchange interaction between nearest-neighbor
sites and (ii) the long-range dipolar interaction of amplitude
truncated at a cutoff distance (iii) a single-ion perpendicular
anisotropy of amplitude . We allow between surface spins, and
otherwise. We show that the ground state depends on the the ratio
and . For a single layer, for a given , there is a critical value
below (above) which the ground-state (GS) configuration of molecular axes
is perpendicular (parallel) to the film surface. When the temperature is
increased, a re-orientation transition occurs near : the low- in-plane
ordering undergoes a transition to the perpendicular ordering at a finite ,
below the transition to the paramagnetic phase. The same phenomenon is observed
in the case of a film with a thickness. We show that the surface phase
transition can occur below or above the bulk transition depending on the ratio
. Surface and bulk order parameters as well as other physical quantities
are shown and discussed.Comment: 7 pages, 11 figures, submitted for publicatio
Monte Carlo studies of the Ising square lattice with competing interactions
We use improved Monte-Carlo algorithms to study the antiferromagnetic
2D-Ising model with competing interactions on nearest neighbour and
on next-nearest neighbour bonds. The finite-temperature phase diagram is
divided by a critical point at where the groundstate is highly
degenerate. To analyse the phase boundaries we look at the specific heat and
the energy distribution for various ratios of . We find a first order
transition for small and the transition temperature suppressed to
at the critical point.Comment: 4 pages, 4 figures, accepted for publication in the proceedings of
the conference on Highly Frustrated Magnets 2008 in Braunschwei
Flat Energy-Histogram Simulation of the Phase Transition in an Ising Fully Frustrated Lattice
We show in this paper the results on the phase transition of the so-called
fully frustrated simple cubic lattice with the Ising spin model. We use here
the Monte Carlo method with the flat energy-histogram Wang-Landau technique
which is very powerful to detect weak first-order phase transition. We show
that the phase transition is clearly of first order, providing a definite
answer to a question raised 25 years ago.Comment: Submitted for publicatio
EPIC 220204960: A Quadruple Star System Containing Two Strongly Interacting Eclipsing Binaries
We present a strongly interacting quadruple system associated with the K2
target EPIC 220204960. The K2 target itself is a Kp = 12.7 magnitude star at
Teff ~ 6100 K which we designate as "B-N" (blue northerly image). The host of
the quadruple system, however, is a Kp = 17 magnitude star with a composite
M-star spectrum, which we designate as "R-S" (red southerly image). With a 3.2"
separation and similar radial velocities and photometric distances, 'B-N' is
likely physically associated with 'R-S', making this a quintuple system, but
that is incidental to our main claim of a strongly interacting quadruple system
in 'R-S'. The two binaries in 'R-S' have orbital periods of 13.27 d and 14.41
d, respectively, and each has an inclination angle of >89 degrees. From our
analysis of radial velocity measurements, and of the photometric lightcurve, we
conclude that all four stars are very similar with masses close to 0.4 Msun.
Both of the binaries exhibit significant ETVs where those of the primary and
secondary eclipses 'diverge' by 0.05 days over the course of the 80-day
observations. Via a systematic set of numerical simulations of quadruple
systems consisting of two interacting binaries, we conclude that the outer
orbital period is very likely to be between 300 and 500 days. If sufficient
time is devoted to RV studies of this faint target, the outer orbit should be
measurable within a year.Comment: 20 pages, 18 figures, 7 tables; accepted for publication in MNRA
Innovative modeling and visualization platform for sustainable cities - Mutopia
Now more than half the world’s population lives in towns and cities and this proportion will rise to nearly two thirds by 2030. Many cities worldwide are facing acute challenges, and therefore it is essential that all future developments are carried out on a sustainable footing. Through a web-based platform, MUtopia visualises and demonstrates in a quantifiable manner what impact a planned site development would have by representing best practice in all aspects of sustainable urban living on a relatively large scale. Sites may be new suburbs or rebuilt sections of the city large enough to require systematic planning. The project focuses on the development of an integrated modelling, analysis and visualization tool that helps the government and developers to make informed decisions to achieve such sustainable urban development and implementation. MUtopia integrates the streams of energy, waste, water and transport, based on land use, as well as social and environmental factors so that various planning scenarios or dependencies between factors can be tested. It is an integrated BIM and GIS tool. MUtopia would be an international first in an area of growing interest and need
Noninvasive Markers of Hepatic Fibrosis in Chronic Hepatitis B
A serum biomarker (FibroTest; Biopredictive, Paris, France; FibroSure; LabCorp, Burlington, USA) and liver stiffness measurement (LSM) by Fibroscan (Echosens, Paris, France) have been extensively validated in chronic hepatitis C. This review updates the clinical validation of serum biomarkers and LSM in patients with chronic hepatitis B (CHB). One meta-analysis combined all published studies and another used a database combining FibroTest individual data. Sensitivity analysis assessed the impact of several factors, including authors’ independence, length of biopsy, ethnicity, hepatitis B early antigen status, viral load, and alanine aminotransferase value. Only two biomarkers had several validations: FibroTest (8 studies, 1,842 patients), and Fibroscan (5 studies, 618 patients). For the diagnosis of advanced fibrosis, the standardized area under the receiver operating curve was 0.84 (0.79–0.86) for FibroTest and 0.89 (0.83–0.96) for LSM, without significant difference. No significant factors of variability were identified for FibroTest’s performance. In conclusion, FibroTest and LSM were the most validated biomarkers of fibrosis in CHB. However, the reliability of Fibroscan must be better assessed
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