847 research outputs found
Reduced density matrix hybrid approach: Application to electronic energy transfer
Electronic energy transfer in the condensed phase, such as that occurring in
photosynthetic complexes, frequently occurs in regimes where the energy scales
of the system and environment are similar. This situation provides a challenge
to theoretical investigation since most approaches are accurate only when a
certain energetic parameter is small compared to others in the problem. Here we
show that in these difficult regimes, the Ehrenfest approach provides a good
starting point for a dynamical description of the energy transfer process due
to its ability to accurately treat coupling to slow environmental modes. To
further improve on the accuracy of the Ehrenfest approach, we use our reduced
density matrix hybrid framework to treat the faster environmental modes quantum
mechanically, at the level of a perturbative master equation. This combined
approach is shown to provide an efficient and quantitative description of
electronic energy transfer in a model dimer and the Fenna-Matthews-Olson
complex and is used to investigate the effect of environmental preparation on
the resulting dynamics.Comment: 11 pages, 8 figure
Nonlinear viscoelasticity of metastable complex fluids
Many metastable complex fluids such as colloidal glasses and gels show
distinct nonlinear viscoelasticity with increasing oscillatory-strain
amplitude; the storage modulus decreases monotonically as the strain amplitude
increases whereas the loss modulus has a distinct peak before it decreases at
larger strains. We present a qualitative argument to explain this ubiquitous
behavior and use mode coupling theory (MCT) to confirm it. We compare
theoretical predictions to the measured nonlinear viscoelasticity in a dense
hard sphere colloidal suspensions; reasonable agreement is obtained. The
argument given here can be used to obtain new information about linear
viscoelasticity of metastable complex fluids from nonlinear strain
measurements.Comment: 7 pages, 3 figures, accepted for publication in Europhys. Let
Expert-Augmented Machine Learning
Machine Learning is proving invaluable across disciplines. However, its
success is often limited by the quality and quantity of available data, while
its adoption by the level of trust that models afford users. Human vs. machine
performance is commonly compared empirically to decide whether a certain task
should be performed by a computer or an expert. In reality, the optimal
learning strategy may involve combining the complementary strengths of man and
machine. Here we present Expert-Augmented Machine Learning (EAML), an automated
method that guides the extraction of expert knowledge and its integration into
machine-learned models. We use a large dataset of intensive care patient data
to predict mortality and show that we can extract expert knowledge using an
online platform, help reveal hidden confounders, improve generalizability on a
different population and learn using less data. EAML presents a novel framework
for high performance and dependable machine learning in critical applications
Time reparametrization invariance in arbitrary range p-spin models: symmetric versus non-symmetric dynamics
We explore the existence of time reparametrization symmetry in p-spin models.
Using the Martin-Siggia-Rose generating functional, we analytically probe the
long-time dynamics. We perform a renormalization group analysis where we
systematically integrate over short timescale fluctuations. We find three
families of stable fixed points and study the symmetry of those fixed points
with respect to time reparametrizations. One of those families is composed
entirely of symmetric fixed points, which are associated with the low
temperature dynamics. The other two families are composed entirely of
non-symmetric fixed points. One of these two non-symmetric families corresponds
to the high temperature dynamics.
Time reparametrization symmetry is a continuous symmetry that is
spontaneously broken in the glass state and we argue that this gives rise to
the presence of Goldstone modes. We expect the Goldstone modes to determine the
properties of fluctuations in the glass state, in particular predicting the
presence of dynamical heterogeneity.Comment: v2: Extensively modified to discuss both high temperature
(non-symmetric) and low temperature (symmetric) renormalization group fixed
points. Now 16 pages with 1 figure. v1: 13 page
Cumulant Expansions and the Spin-Boson Problem
The dynamics of the dissipative two-level system at zero temperature is
studied using three different cumulant expansion techniques. The relative
merits and drawbacks of each technique are discussed. It is found that a new
technique, the non-crossing cumulant expansion, appears to embody the virtues
of the more standard cumulant methods.Comment: 26 pages, LaTe
Reduced density matrix hybrid approach: An efficient and accurate method for adiabatic and non-adiabatic quantum dynamics
We present a new approach to calculate real-time quantum dynamics in complex
systems. The formalism is based on the partitioning of a system's environment
into "core" and "reservoir" modes, with the former to be treated quantum
mechanically and the latter classically. The presented method only requires the
calculation of the system's reduced density matrix averaged over the quantum
core degrees of freedom which is then coupled to a classically evolved
reservoir to treat the remaining modes. We demonstrate our approach by applying
it to the spin-boson problem using the noninteracting blip approximation to
treat the system and core, and Ehrenfest dynamics to treat the reservoir. The
resulting hybrid methodology is accurate for both fast and slow baths, since it
naturally reduces to its composite methods in their respective regimes of
validity. In addition, our combined method is shown to yield good results in
intermediate regimes where neither approximation alone is accurate and to
perform equally well for both strong and weak system-bath coupling. Our
approach therefore provides an accurate and efficient methodology for
calculating quantum dynamics in complex systems.Comment: 10 pages, 7 figure
Glasslike Arrest in Spinodal Decomposition as a Route to Colloidal Gelation
Colloid-polymer mixtures can undergo spinodal decomposition into colloid-rich
and colloid-poor regions. Gelation results when interconnected colloid-rich
regions solidify. We show that this occurs when these regions undergo a glass
transition, leading to dynamic arrest of the spinodal decomposition. The
characteristic length scale of the gel decreases with increasing quench depth,
and the nonergodicity parameter exhibits a pronounced dependence on scattering
vector. Mode coupling theory gives a good description of the dynamics, provided
we use the full static structure as input.Comment: 14 pages, 4 figures; replaced with published versio
Pollen-Mediated Gene Flow from Genetically Modified Herbicide Resistant Creeping Bentgrass
Approximately 162 ha of multiple experimental fields of creeping bentgrass (Agrostis stolonifera L.) genetically modified for resistance to Roundup ®herbicide, were planted in central Oregon in 2002. When the fields flowered for the first time in the summer of 2003, a unique opportunity was presented to evaluate methods to monitor potential pollen-mediated gene flow from the experimental GM crop fields to compatible sentinel and resident plants that were located in surrounding, primarily non-agronomic areas
Growing dynamical length, scaling and heterogeneities in the 3d Edwards-Anderson model
We study numerically spatio-temporal fluctuations during the
out-of-equilibrium relaxation of the three-dimensional Edwards-Anderson model.
We focus on two issues. (1) The evolution of a growing dynamical length scale
in the glassy phase of the model, and the consequent collapse of the
distribution of local coarse-grained correlations measured at different pairs
of times on a single function using {\it two} scaling parameters, the value of
the global correlation at the measuring times and the ratio of the coarse
graining length to the dynamical length scale (in the thermodynamic limit). (2)
The `triangular' relation between coarse-grained local correlations at three
pairs of times taken from the ordered instants .
Property (1) is consistent with the conjecture that the development of
time-reparametrization invariance asymptotically is responsible for the main
dynamic fluctuations in aging glassy systems as well as with other mechanisms
proposed in the literature. Property (2), we stress, is a much stronger test of
the relevance of the time-reparametrization invariance scenario.Comment: 24 pages, 12 fig
Quantum fluctuations can promote or inhibit glass formation
The very nature of glass is somewhat mysterious: while relaxation times in
glasses are of sufficient magnitude that large-scale motion on the atomic level
is essentially as slow as it is in the crystalline state, the structure of
glass appears barely different than that of the liquid that produced it.
Quantum mechanical systems ranging from electron liquids to superfluid helium
appear to form glasses, but as yet no unifying framework exists connecting
classical and quantum regimes of vitrification. Here we develop new insights
from theory and simulation into the quantum glass transition that surprisingly
reveal distinct regions where quantum fluctuations can either promote or
inhibit glass formation.Comment: Accepted for publication in Nature Physics. 22 pages, 3 figures, 1
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