2,279 research outputs found
Spins coupled to a -Regge lattice in 4d
We study an Ising spin system coupled to a fluctuating four-dimensional
-Regge lattice and compare with the results of the four-dimensional Ising
model on a regular lattice. Particular emphasis is placed on the phase
transition of the spin system and the associated critical exponents. We present
results from finite-size scaling analyses of extensive Monte Carlo simulations
which are consistent with mean-field predictions.Comment: Lattice2001(surfaces), 3 pages, 2 figure
Parallel-tempering cluster algorithm for computer simulations of critical phenomena
In finite-size scaling analyses of Monte Carlo simulations of second-order
phase transitions one often needs an extended temperature range around the
critical point. By combining the parallel tempering algorithm with cluster
updates and an adaptive routine to find the temperature window of interest, we
introduce a flexible and powerful method for systematic investigations of
critical phenomena. As a result, we gain one to two orders of magnitude in the
performance for 2D and 3D Ising models in comparison with the recently proposed
Wang-Landau recursion for cluster algorithms based on the multibondic
algorithm, which is already a great improvement over the standard
multicanonical variant.Comment: pages, 5 figures, and 2 table
Ising spins coupled to a four-dimensional discrete Regge skeleton
Regge calculus is a powerful method to approximate a continuous manifold by a
simplicial lattice, keeping the connectivities of the underlying lattice fixed
and taking the edge lengths as degrees of freedom. The discrete Regge model
employed in this work limits the choice of the link lengths to a finite number.
To get more precise insight into the behavior of the four-dimensional discrete
Regge model, we coupled spins to the fluctuating manifolds. We examined the
phase transition of the spin system and the associated critical exponents. The
results are obtained from finite-size scaling analyses of Monte Carlo
simulations. We find consistency with the mean-field theory of the Ising model
on a static four-dimensional lattice.Comment: 19 pages, 7 figure
Verbal Episodic Memory and Endogenous Estradiol: An Association in Patients with Mild Cognitive Impairment and Alzheimer's Disease
In the continuum of patients with Alzheimer's disease (AD), mild cognitive impairment (MCI), and normal controls, a possible association of verbal memory and endogenous estradiol (E2) levels was investigated. Verbal episodic memory was measured with a german version of the California verbal memory test (CVLT). Results were controlled for apolipoprotein E (ApoE) phenotype. We studied 37 controls, 32 MCIs and 117 ADs. Groups differed in all trials of the CVLT (P < 0.001) and in E2 levels (P < 0.001). E2 levels differed significantly between groups only among females (P < 0.001). In females correcting for age and ApoE, there was an overall correlation between CVLT delayed recall and level of E2 (P = 0.025). Stepwise regression analyses found E2 level to be a significant predictor for CVLT delayed recall (P < 0.001). It may be concluded that lower E2 levels occur more in the course of the disease than may be considered as a risk factor per se
Exciton Dissociation Dynamics in Model Donor-Acceptor Polymer Heterojunctions: I. Energetics and Spectra
In this paper we consider the essential electronic excited states in parallel
chains of semiconducting polymers that are currently being explored for
photovoltaic and light-emitting diode applications. In particular, we focus
upon various type II donor-acceptor heterojunctions and explore the relation
between the exciton binding energy to the band off-set in determining the
device characteristic of a particular type II heterojunction material. As a
general rule, when the exciton binding energy is greater than the band off-set
at the heterojunction, the exciton will remain the lowest energy excited state
and the junction will make an efficient light-emitting diode. On the other
hand, if the off-set is greater than the exciton binding energy, either the
electron or hole can be transferred from one chain to the other. Here we use a
two-band exciton to predict the vibronic absorption and emission spectra of
model polymer heterojunctions. Our results underscore the role of vibrational
relaxation and suggest that intersystem crossings may play some part in the
formation of charge-transfer states following photoexcitation in certain cases
Lattice Models of Quantum Gravity
Standard Regge Calculus provides an interesting method to explore quantum
gravity in a non-perturbative fashion but turns out to be a CPU-time demanding
enterprise. One therefore seeks for suitable approximations which retain most
of its universal features. The -Regge model could be such a desired
simplification. Here the quadratic edge lengths of the simplicial complexes
are restricted to only two possible values , with
, in close analogy to the ancestor of all lattice theories, the
Ising model. To test whether this simpler model still contains the essential
qualities of the standard Regge Calculus, we study both models in two
dimensions and determine several observables on the same lattice size. In order
to compare expectation values, e.g. of the average curvature or the Liouville
field susceptibility, we employ in both models the same functional integration
measure. The phase structure is under current investigation using mean field
theory and numerical simulation.Comment: 4 pages, 1 figure
Boundary field induced first-order transition in the 2D Ising model: numerical study
In a recent paper, Clusel and Fortin [J. Phys. A.: Math. Gen. 39 (2006) 995]
presented an analytical study of a first-order transition induced by an
inhomogeneous boundary magnetic field in the two-dimensional Ising model. They
identified the transition that separates the regime where the interface is
localized near the boundary from the one where it is propagating inside the
bulk. Inspired by these results, we measured the interface tension by using
multimagnetic simulations combined with parallel tempering to determine the
phase transition and the location of the interface. Our results are in very
good agreement with the theoretical predictions. Furthermore, we studied the
spin-spin correlation function for which no analytical results are available.Comment: 12 pages, 7 figures, 2 table
Make life simple: unleash the full power of the parallel tempering algorithm
We introduce a new update scheme to systematically improve the efficiency of
parallel tempering simulations. We show that by adapting the number of sweeps
between replica exchanges to the canonical autocorrelation time, the average
round-trip time of a replica in temperature space can be significantly
decreased. The temperatures are not dynamically adjusted as in previous
attempts but chosen to yield a 50% exchange rate of adjacent replicas. We
illustrate the new algorithm with results for the Ising model in two and the
Edwards-Anderson Ising spin glass in three dimensionsComment: 4 pages, 5 figure
Phorbol Esters Enhance Exocytosis from Chromaffin Cells by Two Mechanisms
Treatment with phorbol esters such as 12- O -tet-radecanoylphorbol acetate (TPA) rapidly enhances [ 3 H]norepinephrine secretion from digitonin-permeabilized adrenal chromaffin cells. When TPA treatment was prolonged for several hours, a second distinct enhancing effect was observed. This later enhancement was most prominent at intracellular Ca 2+ concentrations of 3–30 ΜM , and did not require the continued presence of membrane-bound protein kinase C for its expression. The effect could be elicited by as little as 30-min exposure to TPA, followed by several hours in TPA- free medium. This effect of TPA was blocked by actinomycin D and cycloheximide, indicating a requirement for RNA and protein synthesis. Similar effects were seen when intact cells that had been pretreated with TPA were stimulated to secrete by depolarizing concentrations of K + . Thus, protein kinase C enhances secretion by two mechanisms. One is rapid and probably reflects the effects of immediate protein phosphorylation. The other occurs over several hours and requires gene transcription and protein synthesis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66341/1/j.1471-4159.1990.tb13302.x.pd
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