3,221 research outputs found
Numerical Latent Heat Observation of the q=5 Potts Model
Site energy of the five-state ferromagnetic Potts model is numerically
calculated at the first-order transition temperature using corner transfer
matrix renormalization group (CTMRG) method. The calculated energy of the
disordered phase is clearly different from that of the ordered phase
. The obtained latent heat is 0.027, which
quantitatively agrees with the exact solution.Comment: 2 pages, Latex(JPSJ style files are included), 2 ps figures,
submitted to J. Phys. Soc. Jpn.(short note
Incommensurate structures studied by a modified Density Matrix Renormalization Group Method
A modified density matrix renormalization group (DMRG) method is introduced
and applied to classical two-dimensional models: the anisotropic triangular
nearest- neighbor Ising (ATNNI) model and the anisotropic triangular
next-nearest-neighbor Ising (ANNNI) model. Phase diagrams of both models have
complex structures and exhibit incommensurate phases. It was found that the
incommensurate phase completely separates the disordered phase from one of the
commensurate phases, i. e. the non-existence of the Lifshitz point in phase
diagrams of both models was confirmed.Comment: 14 pages, 14 figures included in text, LaTeX2e, submitted to PRB,
presented at MECO'24 1999 (Wittenberg, Germany
Two-scale momentum theory for time-dependent modelling of large wind farms
This paper presents a theory based on the law of momentum conservation to
define and help analyse the problem of large wind farm aerodynamics. The theory
splits the problem into two sub-problems; namely an 'external' (or farm-scale)
problem, which is a time-dependent problem considering large-scale motions of
the atmospheric boundary layer (ABL) to assess the amount of momentum available
to the ABL's bottom resistance (due to wind turbines and land/sea surface) at a
certain time; and an 'internal' (or turbine-scale) problem, which is a
quasi-steady (in terms of large-scale motions of the ABL) problem describing
the breakdown of the ABL's bottom resistance into wind turbine drag and
land/sea surface friction. The two sub-problems are coupled to each other
through a non-dimensional parameter called 'farm wind-speed reduction factor,'
for which a simple analytic equation is derived that can be solved iteratively
using information obtained from both sub-problems. This general form of
coupling allows us to use the present theory with various types of flow models
at each scale, such as a numerical weather prediction (NWP) model for the
external problem and a computational fluid dynamics (CFD) model for the
internal problem. The theory is presented for a simplified wind farm situation
first, followed by a discussion on how the theory can be applied (in an
approximate manner) to real-world problems; for example, how to estimate the
power loss due to the so-called 'wind farm blockage effect' for a given large
wind farm under given environmental conditions.Comment: Under consideration for publication in J. Fluid Mech. (16 pages, 5
figures
Electronic Structure, Local Moments and Transport in Fe_2VAl
Local spin density approximation calculations are used to elucidate
electronic and magnetic properties of Heusler structure Fe_2VAl. The compound
is found to be a low carrier density semimetal. The Fermi surface has small
hole pockets derived from a triply degenerate Fe derived state at Gamma
compensated by an V derived electron pocket at the X point. The ideal compound
is found to be stable against ferromagnetism. Fe impurities on V sites,
however, behave as local moments. Because of the separation of the hole and
electron pockets the RKKY interaction between such local moments should be
rapidly oscillating on the scale of its decay, leading to the likelihood of
spin-glass behavior for moderate concentrations of Fe on V sites. These
features are discussed in relation to experimental observations of an unusual
insulating state in this compound.Comment: 16 pages, RevTeX, 5 figure
An analytical model of momentum availability for predicting large wind farm power
Turbine-wake and farm-atmosphere interactions influence wind farm power
production. For large offshore farms, the farm-atmosphere interaction is
usually the more significant effect. This study proposes an analytical model of
the `momentum availability factor' to predict the impact of farm-atmosphere
interactions. It models the effects of net advection, pressure gradient forcing
and turbulent entrainment, using steady quasi-1D flow assumptions. Turbulent
entrainment is modelled by assuming self-similar vertical shear stress
profiles. We used the model with the `two-scale momentum theory' to predict the
power of large finite-sized farms. The model compared well with existing
results of large-eddy simulations (LES) of finite wind farms in conventionally
neutral boundary layers. The model captured most of the effects of atmospheric
boundary layer (ABL) height on farm performance by considering the undisturbed
vertical shear stress profile of the ABL as an input. In particular, the model
predicted the power of staggered wind farms with a typical error of 5% or less.
The developed model provides a novel way of instantly predicting the power of
large wind farms, including the farm blockage effects. A further simplification
of the model to analytically predict the 'wind extractability factor' is also
presented. This study provides a novel framework for modelling farm-atmosphere
interactions. Future studies can use the framework to better model large wind
farms.Comment: 22 pages, 12 figures, 4 table
Two-Time Physics with gravitational and gauge field backgrounds
It is shown that all possible gravitational, gauge and other interactions
experienced by particles in ordinary d-dimensions (one-time) can be described
in the language of two-time physics in a spacetime with d+2 dimensions. This is
obtained by generalizing the worldline formulation of two-time physics by
including background fields. A given two-time model, with a fixed set of
background fields, can be gauged fixed from d+2 dimensions to (d-1) +1
dimensions to produce diverse one-time dynamical models, all of which are
dually related to each other under the underlying gauge symmetry of the unified
two-time theory. To satisfy the gauge symmetry of the two-time theory the
background fields must obey certain coupled differential equations that are
generally covariant and gauge invariant in the target d+2 dimensional
spacetime. The gravitational background obeys a null homothety condition while
the gauge field obeys a differential equation that generalizes a similar
equation derived by Dirac in 1936. Explicit solutions to these coupled
equations show that the usual gravitational, gauge, and other interactions in d
dimensions may be viewed as embedded in the higher d+2 dimensional space, thus
displaying higher spacetime symmetries that otherwise remain hidden.Comment: Latex, 19 pages, references adde
Censoring Distances Based on Labeled Cortical Distance Maps in Cortical Morphometry
Shape differences are manifested in cortical structures due to
neuropsychiatric disorders. Such differences can be measured by labeled
cortical distance mapping (LCDM) which characterizes the morphometry of the
laminar cortical mantle of cortical structures. LCDM data consist of signed
distances of gray matter (GM) voxels with respect to GM/white matter (WM)
surface. Volumes and descriptive measures (such as means and variances) for
each subject and the pooled distances provide the morphometric differences
between diagnostic groups, but they do not reveal all the morphometric
information contained in LCDM distances. To extract more information from LCDM
data, censoring of the distances is introduced. For censoring of LCDM
distances, the range of LCDM distances is partitioned at a fixed increment
size; and at each censoring step, and distances not exceeding the censoring
distance are kept. Censored LCDM distances inherit the advantages of the pooled
distances. Furthermore, the analysis of censored distances provides information
about the location of morphometric differences which cannot be obtained from
the pooled distances. However, at each step, the censored distances aggregate,
which might confound the results. The influence of data aggregation is
investigated with an extensive Monte Carlo simulation analysis and it is
demonstrated that this influence is negligible. As an illustrative example, GM
of ventral medial prefrontal cortices (VMPFCs) of subjects with major
depressive disorder (MDD), subjects at high risk (HR) of MDD, and healthy
control (Ctrl) subjects are used. A significant reduction in laminar thickness
of the VMPFC and perhaps shrinkage in MDD and HR subjects is observed when
compared to Ctrl subjects. The methodology is also applicable to LCDM-based
morphometric measures of other cortical structures affected by disease.Comment: 25 pages, 10 figure
Density Matrices for a Chain of Oscillators
We consider chains with an optical phonon spectrum and study the reduced
density matrices which occur in density-matrix renormalization group (DMRG)
calculations. Both for one site and for half of the chain, these are found to
be exponentials of bosonic operators. Their spectra, which are correspondingly
exponential, are determined and discussed. The results for large systems are
obtained from the relation to a two-dimensional Gaussian model.Comment: 15 pages,8 figure
Symmetry adapted finite-cluster solver for quantum Heisenberg model in two-dimensions: a real-space renormalization approach
We present a quantum cluster solver for spin- Heisenberg model on a
two-dimensional lattice. The formalism is based on the real-space
renormalization procedure and uses the lattice point group-theoretical analysis
and nonabelian SU(2) spin symmetry technique. The exact diagonalization
procedure is used twice at each renormalization group step. The method is
applied to the spin-half antiferromagnet on a square lattice and a calculation
of local observables is demonstrated. A symmetry based truncation procedure is
suggested and verified numerically.Comment: willm appear in J. Phys.
Gravitational fields as generalized string models
We show that Einstein's main equations for stationary axisymmetric fields in
vacuum are equivalent to the motion equations for bosonic strings moving on a
special nonflat background. This new representation is based on the analysis of
generalized harmonic maps in which the metric of the target space explicitly
depends on the parametrization of the base space. It is shown that this
representation is valid for any gravitational field which possesses two
commuting Killing vector fields. We introduce the concept of dimensional
extension which allows us to consider this type of gravitational fields as
strings embedded in D-dimensional nonflat backgrounds, even in the limiting
case where the Killing vector fields are hypersurface orthogonal.Comment: latex, 25 page
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