355 research outputs found
Thermo-elasticity for anisotropic media in higher dimensions
In this note we develop tools to study the Cauchy problem for the system of
thermo-elasticity in higher dimensions. The theory is developed for general
homogeneous anisotropic media under non-degeneracy conditions.
For degenerate cases a method of treatment is sketched and for the cases of
cubic media and hexagonal media detailed studies are provided.Comment: 33 pages, 5 figure
Relaxation energies and excited state structures of poly(para-phenylene)
We investigate the relaxation energies and excited state geometries of the
light emitting polymer, poly(para-phenylene). We solve the
Pariser-Parr-Pople-Peierls model using the density matrix renormalization group
method. We find that the lattice relaxation of the dipole-active
state is quite different from that of the state and the
dipole-inactive state. In particular, the state is
rather weakly coupled to the lattice and has a rather small relaxation energy
ca. 0.1 eV. In contrast, the and states are strongly
coupled with relaxation energies of ca. 0.5 and ca. 1.0 eV, respectively. By
analogy to linear polyenes, we argue that this difference can be understood by
the different kind of solitons present in the , and
states. The difference in relaxation energies of the
and states accounts for approximately one-third of the exchange
gap in light-emitting polymers.Comment: Submitted to Physical Review
Rebuttal related to “Traits and Stress: Keys to identify community effects of low levels of toxicants in test systems” by Liess and Beketov (2011)
Trend analysis using non-stationary time series clustering based on the finite element method
In order to analyze low-frequency variability of climate, it is useful to
model the climatic time series with multiple linear trends and locate the
times of significant changes. In this paper, we have used non-stationary time
series clustering to find change points in the trends. Clustering in a
multi-dimensional non-stationary time series is challenging, since the
problem is mathematically ill-posed. Clustering based on the finite element
method (FEM) is one of the methods that can analyze multidimensional time
series. One important attribute of this method is that it is not dependent on
any statistical assumption and does not need local stationarity in the time
series. In this paper, it is shown how the FEM-clustering method can be used
to locate change points in the trend of temperature time series from in
situ observations. This method is applied to the temperature time
series of North Carolina (NC) and the results represent region-specific
climate variability despite higher frequency harmonics in climatic time
series. Next, we investigated the relationship between the climatic indices
with the clusters/trends detected based on this clustering method. It appears
that the natural variability of climate change in NC during 1950–2009 can be
explained mostly by AMO and solar activity
Theory of nonlinear optical properties of phenyl-substituted polyacetylenes
In this paper we present a theoretical study of the third-order nonlinear
optical properties of poly(diphenyl)polyacetylene (PDPA) pertaining to the
third-harmonic-generation (THG) process. We study the aforesaid process in
PDPA's using both the independent electron Hueckel model, as well as
correlated-electron Pariser-Parr-Pople (P-P-P) model. The P-P-P model based
calculations were performed using various configuration interaction (CI)
methods such as the the multi-reference-singles-doubles CI (MRSDCI), and the
quadruples-CI (QCI) methods, and the both longitudinal and the transverse
components of third-order susceptibilities were computed. The Hueckel model
calculations were performed on oligo-PDPA's containing up to fifty repeat
units, while correlated calculations were performed for oligomers containing up
to ten unit cells. At all levels of theory, the material exhibits highly
anisotropic nonlinear optical response, in keeping with its structural
anisotropy. We argue that the aforesaid anisotropy can be divided over two
natural energy scales: (a) the low-energy response is predominantly
longitudinal and is qualitatively similar to that of polyenes, while (b) the
high-energy response is mainly transverse, and is qualitatively similar to that
of trans-stilbene.Comment: 13 pages, 7 figures (included), to appear in Physical Review B (April
15, 2004
Exciton dynamics in soluble poly(p-phenylene-vinylene) : Towards an ultrafast excitonic switch
Electromodulated photoinduced absorption : A new spectroscopy in π- conjugated polymer/C60 blends
Optical Science, Engineering and Instrumentation '97, 1997, San Diego, CA, United StatesMartin Liess, Paul A. Lane, Zakya H. Kafafi, M. Hamaguchi, Masanori Ozaki, Katsumi Yoshino, and Z. Valy Vardeny "Electromodulated photoinduced absorption: a new spectroscopy in π-conjugated polymer/C60 blends", Proc. SPIE 3142, Fullerenes and Photonics IV, (1 November 1997). DOI: https://doi.org/10.1117/12.29335
Traits and stress: keys to identify community effects of low levels of toxicants in test systems
Community effects of low toxicant concentrations are obscured by a multitude of confounding factors. To resolve this issue for community test systems, we propose a trait-based approach to detect toxic effects. An experiment with outdoor stream mesocosms was established 2-years before contamination to allow the development of biotic interactions within the community. Following pulse contamination with the insecticide thiacloprid, communities were monitored for additional 2 years to observe long-term effects. Applying a priori ecotoxicological knowledge species were aggregated into trait-based groups that reflected stressor-specific vulnerability of populations to toxicant exposure. This reduces inter-replicate variation that is not related to toxicant effects and enables to better link exposure and effect. Species with low intrinsic sensitivity showed only transient effects at the highest thiacloprid concentration of 100 μg/l. Sensitive multivoltine species showed transient effects at 3.3 μg/l. Sensitive univoltine species were affected at 0.1 μg/l and did not recover during the year after contamination. Based on these results the new indicator SPEARmesocosm was calculated as the relative abundance of sensitive univoltine taxa. Long-term community effects of thiacloprid were detected at concentrations 1,000 times below those detected by the PRC (Principal Response Curve) approach. We also found that those species, characterised by the most vulnerable trait combination, that were stressed were affected more strongly by thiacloprid than non-stressed species. We conclude that the grouping of species according to toxicant-related traits enables identification and prediction of community response to low levels of toxicants and that additionally the environmental context determines species sensitivity to toxicants
Interspecific competition delays recovery of Daphnia spp. populations from pesticide stress
Xenobiotics alter the balance of competition between species and induce shifts in community composition. However, little is known about how these alterations affect the recovery of sensitive taxa. We exposed zooplankton communities to esfenvalerate (0.03, 0.3, and 3 μg/L) in outdoor microcosms and investigated the long-term effects on populations of Daphnia spp. To cover a broad and realistic range of environmental conditions, we established 96 microcosms with different treatments of shading and periodic harvesting. Populations of Daphnia spp. decreased in abundance for more than 8 weeks after contamination at 0.3 and 3 μg/L esfenvalerate. The period required for recovery at 0.3 and 3 μg/L was more than eight and three times longer, respectively, than the recovery period that was predicted on the basis of the life cycle of Daphnia spp. without considering the environmental context. We found that the recovery of sensitive Daphnia spp. populations depended on the initial pesticide survival and the related increase of less sensitive, competing taxa. We assert that this increase in the abundance of competing species, as well as sub-lethal effects of esfenvalerate, caused the unexpectedly prolonged effects of esfenvalerate on populations of Daphnia spp. We conclude that assessing biotic interactions is essential to understand and hence predict the effects and recovery from toxicant stress in communities
Toward a stoichiometric framework for evolutionary biology. Oikos
2005. Toward a stoichiometric framework for evolutionary biology. Á/ Oikos 109: 6 Á/17. Ecological stoichiometry, the study of the balance of energy and materials in living systems, may serve as a useful synthetic framework for evolutionary biology. Here, we review recent work that illustrates the power of a stoichiometric approach to evolution across multiple scales, and then point to important open questions that may chart the way forward in this new field. At the molecular level, stoichiometry links hereditary changes in the molecular composition of organisms to key phenotypic functions. At the level of evolutionary ecology, a simultaneous focus on the energetic and material underpinnings of evolutionary tradeoffs and transactions highlights the relationship between the cost of resource acquisition and the functional consequences of biochemical composition. At the macroevolutionary level, a stoichiometric perspective can better operationalize models of adaptive radiation and escalation, and elucidate links between evolutionary innovation and the development of global biogeochemical cycles. Because ecological stoichiometry focuses on the interaction of energetic and multiple material currencies, it should provide new opportunities for coupling evolutionary dynamics across scales from genomes to the biosphere
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