8,784 research outputs found
Influence of Complex Exciton-Phonon Coupling on Optical Absorption and Energy Transfer of Quantum Aggregates
We present a theory that efficiently describes the quantum dynamics of an
electronic excitation that is coupled to a continuous, highly structured phonon
environment. Based on a stochastic approach to non-Markovian open quantum
systems, we develop a dynamical framework that allows us to handle realistic
systems where a fully quantum treatment is desired yet the usual approximation
schemes fail. The capability of the method is demonstrated by calculating
spectra and energy transfer dynamics of mesoscopic molecular aggregates,
elucidating the transition from fully coherent to incoherent transfer
Fluctuations of Spatial Patterns as a Measure of Classical Chaos
In problems where the temporal evolution of a nonlinear system cannot be
followed, a method for studying the fluctuations of spatial patterns has been
developed. That method is applied to well-known problems in deterministic chaos
(the logistic map and the Lorenz model) to check its effectiveness in
characterizing the dynamical behaviors. It is found that the indices
are as useful as the Lyapunov exponents in providing a quantitative measure of
chaos.Comment: 10 pages + 7 figures (in ps file), LaTex, Submitted to Phys. Rev.
Critical Exponents of the Classical 3D Heisenberg Model: A Single-Cluster Monte Carlo Study
We have simulated the three-dimensional Heisenberg model on simple cubic
lattices, using the single-cluster Monte Carlo update algorithm. The expected
pronounced reduction of critical slowing down at the phase transition is
verified. This allows simulations on significantly larger lattices than in
previous studies and consequently a better control over systematic errors. In
one set of simulations we employ the usual finite-size scaling methods to
compute the critical exponents from a few
measurements in the vicinity of the critical point, making extensive use of
histogram reweighting and optimization techniques. In another set of
simulations we report measurements of improved estimators for the spatial
correlation length and the susceptibility in the high-temperature phase,
obtained on lattices with up to spins. This enables us to compute
independent estimates of and from power-law fits of their
critical divergencies.Comment: 33 pages, 12 figures (not included, available on request). Preprint
FUB-HEP 19/92, HLRZ 77/92, September 199
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
Chandra and Swift observations of the quasi-persistent neutron star transient EXO 0748-676 back to quiescence
The quasi-persistent neutron star X-ray transient and eclipsing binary EXO
0748-676 recently started the transition to quiescence following an accretion
outburst that lasted more than 24 years. We report on two Chandra and twelve
Swift observations performed within five months after the end of the outburst.
The Chandra spectrum is composed of a soft, thermal component that fits to a
neutron star atmosphere model with kT^inf~0.12 keV, joined by a hard powerlaw
tail that contributes ~20% of the total 0.5-10 keV unabsorbed flux. The
combined Chandra/Swift data set reveals a relatively hot and luminous quiescent
system with a temperature of kT^inf~0.11-0.13 keV and a bolometric thermal
luminosity of ~8.1E33-1.6E34 (d/7.4 kpc)^2 erg/s. We discuss our results in the
context of cooling neutron star models.Comment: Accepted for publication in MNRAS Letters, moderate revision
according to referee report, added one plot to figure 2 and included new
Swift observations, 5 pages, 2 figure
Monte Carlo simulation of ice models
We propose a number of Monte Carlo algorithms for the simulation of ice
models and compare their efficiency. One of them, a cluster algorithm for the
equivalent three colour model, appears to have a dynamic exponent close to
zero, making it particularly useful for simulations of critical ice models. We
have performed extensive simulations using our algorithms to determine a number
of critical exponents for the square ice and F models.Comment: 32 pages including 15 postscript figures, typeset in LaTeX2e using
the Elsevier macro package elsart.cl
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
O(N) and RP^{N-1} Models in Two Dimensions
I provide evidence that the 2D model for is equivalent
to the -invariant non-linear -model in the continuum limit. To
this end, I mainly study particular versions of the models, to be called
constraint models. I prove that the constraint and models are
equivalent for sufficiently weak coupling. Numerical results for their
step-scaling function of the running coupling are
presented. The data confirm that the constraint model is in the samei
universality class as the model with standard action. I show that the
differences in the finite size scaling curves of i and models
observed by Caracciolo et al. can be explained as a boundary effect. It is
concluded, in contrast to Caracciolo et al., that and models
share a unique universality class.Comment: 14 pages (latex) + 1 figure (Postscript) ,uuencode
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