5,195 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
Spin chain simulations with a meron cluster algorithm
We apply a meron cluster algorithm to the XY spin chain, which describes a
quantum rotor. This is a multi-cluster simulation supplemented by an improved
estimator, which deals with objects of half-integer topological charge. This
method is powerful enough to provide precise results for the model with a
theta-term - it is therefore one of the rare examples, where a system with a
complex action can be solved numerically. In particular we measure the
correlation length, as well as the topological and magnetic susceptibility. We
discuss the algorithmic efficiency in view of the critical slowing down. Due to
the excellent performance that we observe, it is strongly motivated to work on
new applications of meron cluster algorithms in higher dimensions.Comment: 18 pages, 9 figures, published versio
Green's Functions from Quantum Cluster Algorithms
We show that cluster algorithms for quantum models have a meaning independent
of the basis chosen to construct them. Using this idea, we propose a new method
for measuring with little effort a whole class of Green's functions, once a
cluster algorithm for the partition function has been constructed. To explain
the idea, we consider the quantum XY model and compute its two point Green's
function in various ways, showing that all of them are equivalent. We also
provide numerical evidence confirming the analytic arguments. Similar
techniques are applicable to other models. In particular, in the recently
constructed quantum link models, the new technique allows us to construct
improved estimators for Wilson loops and may lead to a very precise
determination of the glueball spectrum.Comment: 15 pages, LaTeX, with four figures. Added preprint numbe
Universality in the Gross-Neveu model
We consider universal finite size effects in the large-N limit of the
continuum Gross-Neveu model as well as in its discretized versions with Wilson
and with staggered fermions. After extrapolation to zero lattice spacing the
lattice results are compared to the continuum values.Comment: Lattice2004(theory
Ventilation times scales for a subtropical bay from 3-D modelling
[Abstract]: We applied a multi-purpose three-dimensional ocean general circulation model to
compute water renewal time scales for a large coastal embayment situated off the
central eastern coast of Australia (Hervey Bay) that shows features of an inverse estuary.
Water renewal or ventilation time scales are not directly observable but can easily
be diagnosed from numerical simulations. Improved knowledge of these time scales
can assists in evaluating the water quality of coastal environments and can be utilised
in sustainable marine resource management.
The numerical studies are performed with the COupled Hydrodynamical Ecological
model for REgioNal Shelf seas (COHERENS). The model, adopted for Hervey Bay,
provided insight into ventilation pathways, and renewal time scales were found to exhibit
a strong spatial variability. More than 80 % of the coastal embayment was fully
ventilated after about 70-100 days, with the eastern and western shallow coastal regions
ventilated more rapidly than the central, deeper part of the bay.
The concept of a single ’typical’ ventilation timescale characterising this particular
coastal embayment is inadequate and the consideration of spatial variability is clearly
important, hence in a second set of simulations local monitoring boxes and Lagrangian
tracers have been used to focus on this spatial variability. Simple parameters are derived
to estimate local sedimentation, transport processes or places of high/low biological
production
A Cluster Method for the Ashkin--Teller Model
A cluster Monte Carlo algorithm for the Ashkin-Teller (AT) model is
constructed according to the guidelines of a general scheme for such
algorithms. Its dynamical behaviour is tested for the square lattice AT model.
We perform simulations on the line of critical points along which the exponents
vary continuously, and find that critical slowing down is significantly
reduced. We find continuous variation of the dynamical exponent along the
line, following the variation of the ratio , in a manner which
satisfies the Li-Sokal bound , that was so far
proved only for Potts models.Comment: 18 pages, Revtex, figures include
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