3,320 research outputs found
Topological frustration of artificial spin ice
Frustrated systems, typically characterized by competing interactions that
cannot all be simultaneously satisfied, display rich behaviours not found
elsewhere in nature. Artificial spin ice takes a materials-by-design approach
to studying frustration, where lithographically patterned bar magnets mimic the
frustrated interactions in real materials but are also amenable to direct
characterization. Here, we introduce controlled topological defects into square
artificial spin ice lattices in the form of lattice edge dislocations and
directly observe the resulting spin configurations. We find the presence of a
topological defect produces extended frustration within the system caused by a
domain wall with indeterminate configuration. Away from the dislocation, the
magnets are locally unfrustrated, but frustration of the lattice persists due
to its topology. Our results demonstrate the non-trivial nature of topological
defects in a new context, with implications for many real systems in which a
typical density of dislocations could fully frustrate a canonically
unfrustrated system.Comment: 12 pages, 6 figures, 3 supplemental figures. For supplemental movies,
see http://dx.doi.org/10.13016/M25H7
SPATIAL HEDONIC ANALYSIS OF VETERINARIAN INCOME
We investigate a hedonic model for veterinarian income using survey data from the American Veterinarian Medical Association. Diagnostic testing indicates the presence of spatial autoregression in the hedonic income model, which is accounted for by incorporating a spatial component into the regression model. The results provide unique empirical findings about determinants of veterinarian income and spatial patterns, as well as insight useful for governments and academic institutions planning programs and the veterinarian industry.Teaching/Communication/Extension/Profession,
Efficient computation of updated lower expectations for imprecise continuous-time hidden Markov chains
We consider the problem of performing inference with imprecise
continuous-time hidden Markov chains, that is, imprecise continuous-time Markov
chains that are augmented with random output variables whose distribution
depends on the hidden state of the chain. The prefix `imprecise' refers to the
fact that we do not consider a classical continuous-time Markov chain, but
replace it with a robust extension that allows us to represent various types of
model uncertainty, using the theory of imprecise probabilities. The inference
problem amounts to computing lower expectations of functions on the state-space
of the chain, given observations of the output variables. We develop and
investigate this problem with very few assumptions on the output variables; in
particular, they can be chosen to be either discrete or continuous random
variables. Our main result is a polynomial runtime algorithm to compute the
lower expectation of functions on the state-space at any given time-point,
given a collection of observations of the output variables
Proton transport in biological systems can be probed by two-dimensional infrared spectroscopy
We propose a new method to determine the proton transfer (PT) rate in channel proteins by two-dimensional infrared (2DIR) spectroscopy. Proton transport processes in biological systems, such as proton channels, trigger numerous fundamental biochemical reactions. Due to the limitation in both spatial and time resolution of the traditional experimental approaches, describing the whole proton transport process and identifying the rate limiting steps at the molecular level is challenging. In the present paper, we focus on proton transport through the Gramicidin A channel. Using a kinetic PT model derived from all-atom molecular dynamics simulations, we model the amide I region of the 2DIR spectrum of the channel protein to examine its sensitivity to the proton transport process. We demonstrate that the 2DIR spectrum of the isotope-labeled channel contain information on the PT rate, which may be extracted by analyzing the antidiagonal linewidth of the spectral feature related to the labeled site. Such experiments in combination with detailed numerical simulations should allow the extraction of site dependent PT rates, providing a method for identifying possible rate limiting steps for proton channel transfer.
Hitting Times and Probabilities for Imprecise Markov Chains
We consider the problem of characterising expected hitting times and hitting
probabilities for imprecise Markov chains. To this end, we consider three
distinct ways in which imprecise Markov chains have been defined in the
literature: as sets of homogeneous Markov chains, as sets of more general
stochastic processes, and as game-theoretic probability models. Our first
contribution is that all these different types of imprecise Markov chains have
the same lower and upper expected hitting times, and similarly the hitting
probabilities are the same for these three types. Moreover, we provide a
characterisation of these quantities that directly generalises a similar
characterisation for precise, homogeneous Markov chains
Hitting times and probabilities for imprecise Markov chains
We consider the problem of characterising expected hitting times and hitting probabilities for imprecise Markov chains. To this end, we consider three distinct ways in which imprecise Markov chains have been defined in the literature: as sets of homogeneous Markov chains, as sets of more general stochastic processes, and as game-theoretic probability models. Our first contribution is that all these different types of imprecise Markov chains have the same lower and upper expected hitting times, and similarly the hitting probabilities are the same for these three types. Moreover, we provide a characterisation of these quantities that directly generalises a similar characterisation for precise, homogeneous Markov chains
Imprecise Continuous-Time Markov Chains
Continuous-time Markov chains are mathematical models that are used to
describe the state-evolution of dynamical systems under stochastic uncertainty,
and have found widespread applications in various fields. In order to make
these models computationally tractable, they rely on a number of assumptions
that may not be realistic for the domain of application; in particular, the
ability to provide exact numerical parameter assessments, and the applicability
of time-homogeneity and the eponymous Markov property. In this work, we extend
these models to imprecise continuous-time Markov chains (ICTMC's), which are a
robust generalisation that relaxes these assumptions while remaining
computationally tractable.
More technically, an ICTMC is a set of "precise" continuous-time finite-state
stochastic processes, and rather than computing expected values of functions,
we seek to compute lower expectations, which are tight lower bounds on the
expectations that correspond to such a set of "precise" models. Note that, in
contrast to e.g. Bayesian methods, all the elements of such a set are treated
on equal grounds; we do not consider a distribution over this set.
The first part of this paper develops a formalism for describing
continuous-time finite-state stochastic processes that does not require the
aforementioned simplifying assumptions. Next, this formalism is used to
characterise ICTMC's and to investigate their properties. The concept of lower
expectation is then given an alternative operator-theoretic characterisation,
by means of a lower transition operator, and the properties of this operator
are investigated as well. Finally, we use this lower transition operator to
derive tractable algorithms (with polynomial runtime complexity w.r.t. the
maximum numerical error) for computing the lower expectation of functions that
depend on the state at any finite number of time points
Exciton-Exciton Annihilation Is Coherently Suppressed in H-Aggregates, but Not in J-Aggregates
We theoretically demonstrate a strong dependence of the annihilation rate
between (singlet) excitons on the sign of dipole-dipole couplings between
molecules. For molecular H-aggregates, where this sign is positive, the phase
relation of the delocalized two-exciton wavefunctions causes a destructive
interference in the annihilation probability. For J-aggregates, where this sign
is negative, the interference is constructive instead, as a result of which no
such coherent suppression of the annihilation rate occurs. As a consequence,
room temperature annihilation rates of typical H- and J-aggregates differ by a
factor of ~3, while an order of magnitude difference is found for
low-temperature aggregates with a low degree of disorder. These findings, which
explain experimental observations, reveal a fundamental principle underlying
exciton-exciton annihilation, with major implications for technological devices
and experimental studies involving high excitation densities
DETERMINANTS OF FAST FOOD CONSUMPTION
Replaced with revised version of paper 03/24/03.Food Consumption/Nutrition/Food Safety,
COMMERCIAL BEEF HERD REPLACEMENT STRATEGIES
Annualized cow ownership costs represent a large component of the total costs in a cow-calf enterprise, and therefore impact profitability. Annualized cow costs are determined in large part by the price or cost of that cow when it entered the herd. We find that ownership costs, and in turn cow-herd profitability, can be significantly impacted by heifer replacement strategies. Timing (within the cattle cycle) and method (raise vs. purchase) are both important considerations.Livestock Production/Industries,
- …