10,345 research outputs found
The Anomalous Hall effect in re-entrant AuFe alloys and the real space Berry phase
The Hall effect has been studied in a series of AuFe samples in the
re-entrant concentration range, as well as in the spin glass range. The data
demonstrate that the degree of canting of the local spins strongly modifies the
anomalous Hall effect, in agreement with theoretical predictions associating
canting, chirality and a real space Berry phase. The canonical parametrization
of the Hall signal for magnetic conductors becomes inappropriate when local
spins are canted.Comment: 4 pages, 1 eps figur
Stochastic delocalization of finite populations
Heterogeneities in environmental conditions often induce corresponding
heterogeneities in the distribution of species. In the extreme case of a
localized patch of increased growth rates, reproducing populations can become
strongly concentrated at the patch despite the entropic tendency for population
to distribute evenly. Several deterministic mathematical models have been used
to characterize the conditions under which localized states can form, and how
they break down due to convective driving forces. Here, we study the
delocalization of a finite population in the presence of number fluctuations.
We find that any finite population delocalizes on sufficiently long time
scales. Depending on parameters, however, populations may remain localized for
a very long time. The typical waiting time to delocalization increases
exponentially with both population size and distance to the critical wind speed
of the deterministic approximation. We augment these simulation results by a
mathematical analysis that treats the reproduction and migration of individuals
as branching random walks subject to global constraints. For a particular
constraint, different from a fixed population size constraint, this model
yields a solvable first moment equation. We find that this solvable model
approximates very well the fixed population size model for large populations,
but starts to deviate as population sizes are small. The analytical approach
allows us to map out a phase diagram of the order parameter as a function of
the two driving parameters, inverse population size and wind speed. Our results
may be used to extend the analysis of delocalization transitions to different
settings, such as the viral quasi-species scenario
The 2-dimensional non-linear sigma-model on a random latice
The O(n) non-linear -model is simulated on 2-dimensional regular and
random lattices. We use two different levels of randomness in the construction
of the random lattices and give a detailed explanation of the geometry of such
lattices. In the simulations, we calculate the mass gap for and 8,
analysing the asymptotic scaling of the data and computing the ratio of Lambda
parameters . These ratios are in
agreement with previous semi-analytical calculations. We also numerically
calculate the topological susceptibility by using the cooling method.Comment: REVTeX file, 23 pages. 13 postscript figures in a separate compressed
tar fil
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
Exchange Monte Carlo Method and Application to Spin Glass Simulations
We propose an efficient Monte Carlo algorithm for simulating a
``hardly-relaxing" system, in which many replicas with different temperatures
are simultaneously simulated and a virtual process exchanging configurations of
these replica is introduced. This exchange process is expected to let the
system at low temperatures escape from a local minimum. By using this algorithm
the three-dimensional Ising spin glass model is studied. The ergodicity
time in this method is found much smaller than that of the multi-canonical
method. In particular the time correlation function almost follows an
exponential decay whose relaxation time is comparable to the ergodicity time at
low temperatures. It suggests that the system relaxes very rapidly through the
exchange process even in the low temperature phase.Comment: 10 pages + uuencoded 5 Postscript figures, REVTe
Finite-size scaling of the helicity modulus of the two-dimensional O(3) model
Using Monte Carlo methods, we compute the finite-size scaling function of the
helicity modulus of the two-dimensional O(3) model and compare it to
the low temperature expansion prediction. From this, we estimate the range of
validity for the leading terms of the low temperature expansion of the
finite-size scaling function and for the low temperature expansion of the
correlation length. Our results strongly suggest that a Kosterlitz-Thouless
transition at a temperature is extremely unlikely in this model.Comment: 4 pages, 3 Postscript figures, to appear in Phys. Rev. B Jan. 1997 as
a Brief Repor
The adsorption and desorption of ethanol ices from a model grain surface
Reflection absorption infrared spectroscopy (RAIRS) and temperature programed desorption (TPD) have been used to probe the adsorption and desorption of ethanol on highly ordered pyrolytic graphite (HOPG) at 98 K. RAIR spectra for ethanol show that it forms physisorbed multilayers on the surface at 98 K. Annealing multilayer ethanol ices (exposures > 50 L) beyond 120 K gives rise to a change in morphology before crystallization within the ice occurs. TPD shows that ethanol adsorbs and desorbs molecularly on the HOPG surface and shows four different species in desorption. At low coverage, desorption of monolayer ethanol is observed and is described by first-order kinetics. With increasing coverage, a second TPD peak is observed at a lower temperature, which is assigned to an ethanol bilayer. When the coverage is further increased, a second multilayer, less strongly bound to the underlying ethanol ice film, is observed. This peak dominates the TPD spectra with increasing coverage and is characterized by fractional-order kinetics and a desorption energy of 56.3 +/- 1.7 kJ mol(-1). At exposures exceeding 50 L, formation of crystalline ethanol is also observed as a high temperature shoulder on the TPD spectrum at 160 K. (c) 2008 American Institute of Physics
Bound Magnetic Polaron Interactions in Insulating Doped Diluted Magnetic Semiconductors
The magnetic behavior of insulating doped diluted magnetic semiconductors
(DMS) is characterized by the interaction of large collective spins known as
bound magnetic polarons. Experimental measurements of the susceptibility of
these materials have suggested that the polaron-polaron interaction is
ferromagnetic, in contrast to the antiferromagnetic carrier-carrier
interactions that are characteristic of nonmagnetic semiconductors. To explain
this behavior, a model has been developed in which polarons interact via both
the standard direct carrier-carrier exchange interaction (due to virtual
carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due
to the interactions of polarons with magnetic ions in an interstitial region).
Using a variational procedure, the optimal values of the model parameters were
determined as a function of temperature. At temperatures of interest, the
parameters describing polaron-polaron interactions were found to be nearly
temperature-independent. For reasonable values of these constant parameters, we
find that indirect ferromagnetic interactions can dominate the direct
antiferromagnetic interactions and cause the polarons to align. This result
supports the experimental evidence for ferromagnetism in insulating doped DMS.Comment: 11 pages, 7 figure
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
Probing the Crust of the Neutron Star in EXO 0748-676
X-ray observations of quiescent X-ray binaries have the potential to provide
insight into the structure and the composition of neutron stars. EXO 0748-676
had been actively accreting for over 24 yr before its outburst ceased in late
2008. Subsequent X-ray monitoring revealed a gradual decay of the quiescent
thermal emission that can be attributed to cooling of the accretion-heated
neutron star crust. In this work, we report on new Chandra and Swift
observations that extend the quiescent monitoring to ~5 yr post-outburst. We
find that the neutron star temperature remained at ~117 eV between 2009 and
2011, but had decreased to ~110 eV in 2013. This suggests that the crust has
not fully cooled yet, which is supported by the lower temperature of ~95 eV
that was measured ~4 yr prior to the accretion phase in 1980. Comparing the
data to thermal evolution simulations reveals that the apparent lack of cooling
between 2009 and 2011 could possibly be a signature of convection driven by
phase separation of light and heavy nuclei in the outer layers of the neutron
star.Comment: 9 pages, 4 tables, 3 figures. Minor revisions according to referee
report. Accepted to Ap
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