5,343 research outputs found
Critical exponents of a three dimensional O(4) spin model
By Monte Carlo simulation we study the critical exponents governing the
transition of the three-dimensional classical O(4) Heisenberg model, which is
considered to be in the same universality class as the finite-temperature QCD
with massless two flavors. We use the single cluster algorithm and the
histogram reweighting technique to obtain observables at the critical
temperature. After estimating an accurate value of the inverse critical
temperature \Kc=0.9360(1), we make non-perturbative estimates for various
critical exponents by finite-size scaling analysis. They are in excellent
agreement with those obtained with the expansion method with
errors reduced to about halves of them.Comment: 25 pages with 8 PS figures, LaTeX, UTHEP-28
Diffusion-limited reactions on a two-dimensional lattice with binary disorder
Reaction-diffusion systems where transition rates exhibit quenched disorder
are common in physical and chemical systems. We study pair reactions on a
periodic two-dimensional lattice, including continuous deposition and
spontaneous desorption of particles. Hopping and desorption are taken to be
thermally activated processes. The activation energies are drawn from a binary
distribution of well depths, corresponding to `shallow' and `deep' sites. This
is the simplest non-trivial distribution, which we use to examine and explain
fundamental features of the system. We simulate the system using kinetic Monte
Carlo methods and provide a thorough understanding of our findings. We show
that the combination of shallow and deep sites broadens the temperature window
in which the reaction is efficient, compared to either homogeneous system. We
also examine the role of spatial correlations, including systems where one type
of site is arranged in a cluster or a sublattice. Finally, we show that a
simple rate equation model reproduces simulation results with very good
accuracy.Comment: 9 pages, 5 figure
Population structure, biomass and production of the West African lucinid Keletistes rhizoecus (Bivalvia, Mollusca) in Sivibilagbara swamp at Bodo Creek, Niger Delta, Nigeria
The West African lucinid bivalve Keletistes rhizoecus (Oliver, Basteria 50:47-64, 1986) is only known from the Niger Delta in Nigeria. Due to inaccessibility of its habitat population biology, growth parameters, biomass, and annual secondary production are unknown. The danger of oil pollution threatens the localities where this species occurs. Hence, ecological characteristics of the species were investigated quantitatively from May 2007 to April 2008 at Sivibilagbara, a protected mangrove swamp at Bodo Creek in the lower Niger Delta. Density of this chemosymbiotic lucinid was significantly higher than data previously reported. Temporal size distribution of the population showed minor changes due chiefly to recruitment and growth increments. Recruits peaked in February and September. The species lifespan is estimated to be 1.2 years. The biomass and production values are relatively high, but comparable to those of other bivalve species, especially those from nearby Andoni intertidal flats
The chiral Anomalous Hall effect in re-entrant AuFe alloys
The Hall effect has been studied in a series of AuFe samples in the
re-entrant concentration range, as well as in part of the spin glass range. An
anomalous Hall contribution linked to the tilting of the local spins can be
identified, confirming theoretical predictions of a novel topological Hall term
induced when chirality is present. This effect can be understood in terms of
Aharonov-Bohm-like intrinsic current loops arising from successive scatterings
by canted local spins. The experimental measurements indicate that the chiral
signal persists, meaning scattering within the nanoscopic loops remains
coherent, up to temperatures of the order of 150 K.Comment: 7 pages, 11 eps figures Published version. Minor change
Cluster algorithms
Cluster algorithms for classical and quantum spin systems are discussed. In
particular, the cluster algorithm is applied to classical O(N) lattice actions
containing interactions of more than two spins. The performance of the
multi-cluster and single--cluster methods, and of the standard and improved
estimators are compared. (Lecture given at the summer school on `Advances in
Computer Simulations', Budapest, July 1996.)Comment: 17 pages, Late
Robust Magnetic Polarons in Type-II (Zn,Mn)Te Quantum Dots
We present evidence of magnetic ordering in type-II (Zn, Mn) Te quantum dots.
This ordering is attributed to the formation of bound magnetic polarons caused
by the exchange interaction between the strongly localized holes and Mn within
the dots. In our photoluminescence studies, the magnetic polarons are detected
at temperatures up to ~ 200 K, with a binding energy of ~ 40 meV. In addition,
these dots display an unusually small Zeeman shift with applied field (2 meV at
10 T). This behavior is explained by a small and weakly temperature-dependent
magnetic susceptibility due to anti-ferromagnetic coupling of the Mn spins
Magnetic Breakdown in the electron-doped cuprate superconductor NdCeCuO: the reconstructed Fermi surface survives in the strongly overdoped regime
We report on semiclassical angle-dependent magnetoresistance oscillations
(AMRO) and the Shubnikov-de Haas effect in the electron-overdoped cuprate
superconductor NdCeCuO. Our data provide convincing evidence
for magnetic breakdown in the system. This shows that a reconstructed
multiply-connected Fermi surface persists, at least at strong magnetic fields,
up to the highest doping level of the superconducting regime. Our results
suggest an intimate relation between translational symmetry breaking and the
superconducting pairing in the electron-doped cuprate superconductors.Comment: 5 pages, 4 figures, submitted to PR
Origin of the reduced exchange bias in epitaxial FeNi(111)/CoO(111) bilayer
We have employed Soft and Hard X-ray Resonant Magnetic Scattering and
Polarised Neutron Diffraction to study the magnetic interface and the bulk
antiferromagnetic domain state of the archetypal epitaxial
NiFe(111)/CoO(111) exchange biased bilayer. The combination of
these scattering tools provides unprecedented detailed insights into the still
incomplete understanding of some key manifestations of the exchange bias
effect. We show that the several orders of magnitude difference between the
expected and measured value of exchange bias field is caused by an almost
anisotropic in-plane orientation of antiferromagnetic domains. Irreversible
changes of their configuration lead to a training effect. This is directly seen
as a change in the magnetic half order Bragg peaks after magnetization
reversal. A 30 nm size of antiferromagnetic domains is extracted from the width
the (1/2 1/2 1/2) antiferromagnetic magnetic peak measured both by neutron and
x-ray scattering. A reduced blocking temperature as compared to the measured
antiferromagnetic ordering temperature clearly corresponds to the blocking of
antiferromagnetic domains. Moreover, an excellent correlation between the size
of the antiferromagnetic domains, exchange bias field and frozen-in spin ratio
is found, providing a comprehensive understanding of the origin of exchange
bias in epitaxial systems.Comment: 8 pages, 5 figures, submitte
2-D Radiative Transfer in Protostellar Envelopes: I. Effects of Geometry on Class I Sources
We present 2-D radiation transfer models of Class I Protostars and show the
effect of including more realistic geometries on the resulting spectral energy
distributions and images. We begin with a rotationally flattened infalling
envelope as our comparison model, and add a flared disk and bipolar cavity. The
disk affects the spectral energy distribution most strongly at edge-on
inclinations, causing a broad dip at about 10 um (independent of the silicate
feature) due to high extinction and low scattering albedo in this wavelength
region. The bipolar cavities allow more direct stellar+disk radiation to emerge
into polar directions, and more scattering radiation to emerge into all
directions. The wavelength-integrated flux, often interpreted as luminosity,
varies with viewing angle, with pole-on viewing angles seeing 2-4 times as much
flux as edge-on, depending on geometry. Thus, observational estimates of
luminosity should take into account the inclination of a source. The envelopes
with cavities are significantly bluer in near-IR and mid-IR color-color plots
than those without cavities. Using 1-D models to interpret Class I sources with
bipolar cavities would lead to an underestimate of envelope mass and an
overestimate of the implied evolutionary state. We compute images at near-,
mid-, and far-IR wavelengths. We find that the mid-IR colors and images are
sensitive to scattering albedo, and that the flared disk shadows the midplane
on large size scales at all wavelengths plotted. Finally, our models produce
polarization spectra which can be used to diagnose dust properties, such as
albedo variations due to grain growth. Our results of polarization across the
3.1 um ice feature agree well with observations for ice mantles covering 5% of
the radius of the grains.Comment: Accepted for publication in ApJ, 37 pages, 13 figures (several
figures reduced in quality; find original version at
http://gemelli.colorado.edu/~bwhitney/preprints.html
Large-q asymptotics of the random bond Potts model
We numerically examine the large-q asymptotics of the q-state random bond
Potts model. Special attention is paid to the parametrisation of the critical
line, which is determined by combining the loop representation of the transfer
matrix with Zamolodchikov's c-theorem. Asymptotically the central charge seems
to behave like c(q) = 1/2 log_2(q) + O(1). Very accurate values of the bulk
magnetic exponent x_1 are then extracted by performing Monte Carlo simulations
directly at the critical point. As q -> infinity, these seem to tend to a
non-trivial limit, x_1 -> 0.192 +- 0.002.Comment: 9 pages, no figure
- âŠ