457 research outputs found
Evaluating Poverty Impacts of Bottom-of-the-Pyramid Irrigation Technology Supply: IDE’s Rolling Baseline Approach to Household Income Impact Assessment
Localization properties of one-dimensional Frenkel excitons: Gaussian versus Lorentzian diagonal disorder
We compare localization properties of one-dimensional Frenkel excitons with
Gaussian and Lorentzian uncorrelated diagonal disorder. We focus on the states
of the Lifshits tail, which dominate the optical response and low-temperature
energy transport in molecular J-aggregates. The absence of exchange narrowing
in chains with Lorentzian disorder is shown to manifest itself in the disorder
scaling of the localization length distribution. Also, we show that the local
exciton level structure of the Lifshits tail differs substantially for these
two types of disorder: In addition to the singlets and doublets of localized
states near the bare band edge, strongly resembling those found for Gaussian
disorder, for Lorentzian disorder two other types of states are found in this
energy region as well, namely multiplets of three or four states localized on
the same chain segment and isolated states localized on short segments.
Finally, below the Lifshits tail, Lorentzian disorder induces strongly
localized exciton states, centered around low energy sites, with localization
properties that strongly depend on energy. For Gaussian disorder with a
magnitude that does not exceed the exciton bandwidth, the likelihood to find
such very deep states is exponentially small.Comment: 9 two-column pages, 4 figures, to appear in Phys. Rev.
Excitons in Molecular Aggregates with L\'evy Disorder: Anomalous Localization and Exchange Broadening of Optical Spectra
We predict the existence of exchange broadening of optical lineshapes in
disordered molecular aggregates and a nonuniversal disorder scaling of the
localization characteristics of the collective electronic excitations
(excitons). These phenomena occur for heavy-tailed L\'evy disorder
distributions with divergent second moments - distributions that play a role in
many branches of physics. Our results sharply contrast with aggregate models
commonly analyzed, where the second moment is finite. They bear a relevance for
other types of collective excitations as well
Response to the Comment on "Excitons in Molecular Aggregates with L\'evy Disorder: Anomalous Localization and Exchange Broadening of Optical Spectra"
In previous work, we have predicted novel effects, such as exchange
broadening, anomalous scaling of the localization length and a blue shift of
the absorption spectrum with increasing disorder strength, for static disorder
models described by stable distributions with stability index {\alpha}<1. The
main points of the Comment are that the outliers introduced by heavy tails in
the disorder distribution (i) do not lead to deviations from the conventional
scaling law for the half width at half maximum (HWHM) of the absorption
spectrum and (ii) do not lead to non-universality of the distribution of
localization lengths. We show below that the findings reported by us in the
Letter are correct and that the wrong conclusions of the Comment arise from
focusing on small {\sigma} values.Comment: Based on our response submitted to Physical Review Letters on January
20, 2012. We now also take into account the modifications made to the Comment
upon resubmission of the Comment. The Reply has been accepted in Physical
Review Letter
Correlated interaction fluctuations in photosynthetic complexes
The functioning and efficiency of natural photosynthetic complexes is
strongly influenced by their embedding in a noisy protein environment, which
can even serve to enhance the transport efficiency. Interactions with the
environment induce fluctuations of the transition energies of and interactions
between the chlorophyll molecules, and due to the fact that different
fluctuations will partially be caused by the same environmental factors,
correlations between the various fluctuations will occur. We argue that
fluctuations of the interactions should in general not be neglected, as these
have a considerable impact on population transfer rates, decoherence rates and
the efficiency of photosynthetic complexes. Furthermore, while correlations
between transition energy fluctuations have been studied, we provide the first
quantitative study of the effect of correlations between interaction
fluctuations and transition energy fluctuations, and of correlations between
the various interaction fluctuations. It is shown that these additional
correlations typically lead to changes in interchromophore transfer rates,
population oscillations and can lead to a limited enhancement of the light
harvesting efficiency
Nonmonotonic energy harvesting efficiency in biased exciton chains
We theoretically study the efficiency of energy harvesting in linear exciton
chains with an energy bias, where the initial excitation is taking place at the
high-energy end of the chain and the energy is harvested (trapped) at the other
end. The efficiency is characterized by means of the average time for the
exciton to be trapped after the initial excitation. The exciton transport is
treated as the intraband energy relaxation over the states obtained by
numerically diagonalizing the Frenkel Hamiltonian that corresponds to the
biased chain. The relevant intraband scattering rates are obtained from a
linear exciton-phonon interaction. Numerical solution of the Pauli master
equation that describes the relaxation and trapping processes, reveals a
complicated interplay of factors that determine the overall harvesting
efficiency. Specifically, if the trapping step is slower than or comparable to
the intraband relaxation, this efficiency shows a nonmonotonic dependence on
the bias: it first increases when introducing a bias, reaches a maximum at an
optimal bias value, and then decreases again because of dynamic (Bloch)
localization of the exciton states. Effects of on-site (diagonal) disorder,
leading to Anderson localization, are addressed as well.Comment: 9 pages, 6 figures, to appear in Journal of Chemical Physic
Thermal effects in exciton harvesting in biased one-dimensional systems
The study of energy harvesting in chain-like structures is important due to
its relevance to a variety of interesting physical systems. Harvesting is
understood as the combination of exciton transport through intra-band exciton
relaxation (via scattering on phonon modes) and subsequent quenching by a trap.
Previously, we have shown that in the low temperature limit different
harvesting scenarios as a function of the applied bias strength (slope of the
energy gradient towards the trap) are possible \cite{Vlaming07}. This paper
generalizes the results for both homogeneous and disordered chains to nonzero
temperatures. We show that thermal effects are appreciable only for low bias
strengths, particularly so in disordered systems, and lead to faster
harvesting.Comment: 8 pages, 2 fugures, to appear in Journal of Luminescenc
Signature of Anomalous Exciton Localization in the Optical Response of Self-Assembled Organic Nanotubes
We show that the disorder scaling of the low-temperature optical absorption linewidth of tubular molecular assemblies sharply contrasts with that known for one-dimensional aggregates. The difference can be explained by an anomalous localization of excitons, which arises from the combination of long-range intermolecular interactions and the tube's higher-dimensional geometry. As a result, the exciton density of states near the band bottom drops to zero, leading to a strong suppression of exciton localization. Our results explain the strong linear dichroism and weak exciton-exciton scattering in tubular J aggregates observed in experiments and suggest that for nanoscale wirelike applications a tubular shape is to be preferred over a truly one-dimensional chain
第9章 大学コンソーシアムひょうご神戸 社会連携助成事業 : 「平常時・災害時における歴史資料の保全・修復ができる人材の育成事業
In recent years, there has been a considerable amount of research on the use of regularization methods for inference and prediction in quantitative genetics. Such research mostly focuses on selection of markers and shrinkage of their effects. In this review paper, the use of ridge regression for prediction in quantitative genetics using single-nucleotide polymorphism data is discussed. In particular, we consider (i) the theoretical foundations of ridge regression, (ii) its link to commonly used methods in animal breeding, (iii) the computational feasibility, and (iv) the scope for constructing prediction models with nonlinear effects (e.g., dominance and epistasis). Based on a simulation study we gauge the current and future potential of ridge regression for prediction of human traits using genome-wide SNP data. We conclude that, for outcomes with a relatively simple genetic architecture, given current sample sizes in most cohorts (i.e., N<10,000) the predictive accuracy of ridge regression is slightly higher than the classical genome-wide association study approach of repeated simple regression (i.e., one regression per SNP). However, both capture only a small proportion of the heritability. Nevertheless, we find evidence that for large-scale initiatives, such as biobanks, sample sizes can be achieved where ridge regression compared to the classical approach improves predictive accuracy substantially
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