9,668 research outputs found
Molecular-Dynamics Simulation of a Glassy Polymer Melt: Incoherent Scattering Function
We present simulation results for a model polymer melt, consisting of short,
nonentangled chains, in the supercooled state. The analysis focuses on the
monomer dynamics, which is monitored by the incoherent intermediate scattering
function. The scattering function is recorded over six decades in time and for
many different wave-vectors. The lowest temperatures studied are slightly above
the critical temperature of mode-coupling theory (MCT), which was determined
from a quantitative analysis of the beta- and alpha-relaxations. We find
evidence for the space-time factorization theorem in the beta-relaxation
regime, and for the time-temperature superposition principle in the
alpha-regime, if the temperature is not too close to the critical temperature.
The wave-vector dependence of the nonergodicity parameter, of the critical
amplitude, and the alpha-relaxation time are in qualitative agreement with
calculations for hard spheres. For wave-vectors larger than the maximum of the
structure factor the alpha-relaxation time already agrees fairly well with the
asymptotic MCT-prediction. The behavior of the relaxation time at small
wave-vectors can be rationalized by the validity of the Gaussian approximation
and the value of the Kohlrausch stretching exponent.Comment: 23 pages of REVTeX, 13 PostScript figures, submitted to Phys. Rev.
Understanding the physics of oligonucleotide microarrays: the Affymetrix spike-in data reanalysed
The Affymetrix U95 and U133 Latin Square spike-in datasets are reanalysed,
together with a dataset from a version of the U95 spike-in experiment without a
complex non-specific background. The approach uses a physico-chemical model
which includes the effects the specific and non-specific hybridisation and
probe folding at the microarray surface, target folding and hybridisation in
the bulk RNA target solution, and duplex dissociation during the
post-hybridisatoin washing phase. The model predicts a three parameter
hyperbolic response function that fits well with fluorescence intensity data
from all three datasets. The importance of the various hybridisation and
washing effects in determining each of the three parameters is examined, and
some guidance is given as to how a practical algorithm for determining specific
target concentrations might be developed.Comment: 32 pages, 13 figures, minor amendment
Binary mixture of hard disks as a model glass former: Caging and uncaging
I have proposed a measure for the cage effect in glass forming systems. A
binary mixture of hard disks is numerically studied as a model glass former. A
network is constructed on the basis of the colliding pairs of disks. A rigidity
matrix is formed from the isostatic (rigid) sub--network, corresponding to a
cage. The determinant of the matrix changes its sign when an uncaging event
occurs. Time evolution of the number of the uncaging events is determined
numerically. I have found that there is a gap in the uncaging timescales
between the cages involving different numbers of disks. Caging of one disk by
two neighboring disks sustains for a longer time as compared with other cages
involving more than one disk. This gap causes two--step relaxation of this
system
Properties of iterative Monte Carlo single histogram reweighting
We present iterative Monte Carlo algorithm for which the temperature variable
is attracted by a critical point. The algorithm combines techniques of single
histogram reweighting and linear filtering. The 2d Ising model of ferromagnet
is studied numerically as an illustration. In that case, the iterations
uncovered stationary regime with invariant probability distribution function of
temperature which is peaked nearly the pseudocritical temperature of specific
heat. The sequence of generated temperatures is analyzed in terms of stochastic
autoregressive model. The error of histogram reweighting can be better
understood within the suggested model. The presented model yields a simple
relation, connecting variance of pseudocritical temperature and parameter of
linear filtering.Comment: 3 figure
Coupled Fluctuations near Critical Wetting
Recent work on the complete wetting transition has emphasized the role played
by the coupling of fluctuations of the order parameter at the wall and at the
depinning fluid interface. Extending this approach to the wetting transition
itself we predict a novel crossover effect associated with the decoupling of
fluctuations as the temperature is lowered towards the transition temperature
T_W. Using this we are able to reanalyse recent Monte-Carlo simulation studies
and extract a value \omega(T_W)=0.8 at T_W=0.9T_C in very good agreement with
long standing theoretical predictions.Comment: 4 pages, LaTex, 1 postscript figur
Optimal stochastic modelling with unitary quantum dynamics
Identifying and extracting the past information relevant to the future
behaviour of stochastic processes is a central task in the quantitative
sciences. Quantum models offer a promising approach to this, allowing for
accurate simulation of future trajectories whilst using less past information
than any classical counterpart. Here we introduce a class of phase-enhanced
quantum models, representing the most general means of causal simulation with a
unitary quantum circuit. We show that the resulting constructions can display
advantages over previous state-of-art methods - both in the amount of
information they need to store about the past, and in the minimal memory
dimension they require to store this information. Moreover, we find that these
two features are generally competing factors in optimisation - leading to an
ambiguity in what constitutes the optimal model - a phenomenon that does not
manifest classically. Our results thus simultaneously offer new quantum
advantages for stochastic simulation, and illustrate further qualitative
differences in behaviour between classical and quantum notions of complexity.Comment: 9 pages, 5 figure
Critical behavior of colloid-polymer mixtures in random porous media
We show that the critical behavior of a colloid-polymer mixture inside a
random porous matrix of quenched hard spheres belongs to the universality class
of the random-field Ising model. We also demonstrate that random-field effects
in colloid-polymer mixtures are surprisingly strong. This makes these systems
attractive candidates to study random-field behavior experimentally.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
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