7,984 research outputs found
A simplified picture for Pi electrons in conjugated polymers : from PPP Hamiltonian to an effective molecular crystal approach
An excitonic method proper to study conjugated oligomers and polymers is
described and its applicability tested on the ground state and first excited
states of trans-polyacetylene, taken as a model. From the Pariser-Parr-Pople
Hamiltonian, we derive an effective Hamiltonian based on a local description of
the polymer in term of monomers; the relevant electronic configurations are
build on a small number of pertinent local excitations. The intuitive and
simple microscopic physical picture given by our model supplement recent
results, such as the Rice and Garstein ones. Depending of the parameters, the
linear absorption appears dominated by an intense excitonic peak.Comment: 41 Pages, 6 postscript figure
Persistent holes in a fluid
We observe stable holes in a vertically oscillated 0.5 cm deep aqueous
suspension of cornstarch for accelerations a above 10g. Holes appear only if a
finite perturbation is applied to the layer. Holes are circular and
approximately 0.5 cm wide, and can persist for more than 10^5 cycles. Above a =
17g the rim of the hole becomes unstable producing finger-like protrusions or
hole division. At higher acceleration, the hole delocalizes, growing to cover
the entire surface with erratic undulations. We find similar behavior in an
aqueous suspension of glass microspheres.Comment: 4 pages, 6 figure
On the potential vegetation feedbacks that enhance phosphorus availability – insights from a process-based model linking geological and ecological timescales
In old and heavily weathered soils, the availability of P might be
so small that the primary production of plants is limited. However,
plants have evolved several mechanisms to actively take up P from
the soil or mine it to overcome this limitation. These mechanisms
involve the active uptake of P mediated by mycorrhiza, biotic
de-occlusion through root clusters, and the biotic enhancement of
weathering through root exudation. The objective of this paper is to
investigate how and where these processes contribute to alleviate P
limitation on primary productivity. To do so, we propose
a process-based model accounting for the major processes of the
carbon, water, and P cycles including chemical weathering at the
global scale. Implementing P limitation on biomass synthesis allows
the assessment of the efficiencies of biomass production across
different ecosystems. We use simulation experiments to assess the relative
importance of the different uptake mechanisms to alleviate P
limitation on biomass production.
We find that active P uptake is an essential mechanism for
sustaining P availability on long timescales, whereas biotic
de-occlusion might serve as a buffer on timescales shorter than
10 000 yr. Although active P uptake is essential for
reducing P losses by leaching, humid lowland soils reach P
limitation after around 100 000 yr of soil evolution. Given
the generalized modelling framework, our model results compare
reasonably with observed or independently estimated patterns and
ranges of P concentrations in soils and vegetation. Furthermore, our
simulations suggest that P limitation might be an important driver
of biomass production efficiency (the fraction of the gross primary
productivity used for biomass growth), and that vegetation on old
soils has a smaller biomass production rate when P becomes limiting.
With this study, we provide a theoretical basis for investigating
the responses of terrestrial ecosystems to P availability linking
geological and ecological timescales under different environmental
settings
Research on the Geography of Agricultural Change: Redundant or Revitalized?
Future research directions for agricultural geography were the subject of debate in Area in the late 1980s. The subsequent application of political economy ideas undoubtedly revived interest in agricultural research. This paper argues that agricultural geography contains greater diversity than the dominant political economy discourse would suggest. It reviews ‘other’ areas of agricultural research on policy, post-productivism, people, culture and animals, presenting future suggestions for research. They should ensure that agricultural research continues revitalized rather than redundant into the next millennium
An examination of the effects of self-regulatory focus on the perception of the media richness: the case of email
Communication is a key element in organizations’ business success. The media richness theory and the channel expansion theory are two of the most influential theories regarding the selection and use of communication media in organizations; however, literature has focused little on the effects of self-regulation by managers and employees in these theories. To analyze these topics, this study develops an empirical investigation by gathering data from 600 managers and employees using a questionnaire. The results suggest that the perception of media richness is positively affected when the individual shows a promotion focus or strategy.Peer ReviewedPostprint (author’s final draft
Generalized "Quasi-classical" Ground State for an Interacting Two Level System
We treat a system (a molecule or a solid) in which electrons are coupled
linearly to any number and type of harmonic oscillators and which is further
subject to external forces of arbitrary symmetry. With the treatment restricted
to the lowest pair of electronic states, approximate "vibronic"
(vibration-electronic) ground state wave functions are constructed having the
form of simple, closed expressions. The basis of the method is to regard
electronic density operators as classical variables. It extends an earlier
"guessed solution", devised for the dynamical Jahn-Teller effect in cubic
symmetry, to situations having lower (e.g., dihedral) symmetry or without any
symmetry at all. While the proposed solution is expected to be quite close to
the exact one, its formal simplicity allows straightforward calculations of
several interesting quantities, like energies and vibronic reduction (or Ham)
factors. We calculate for dihedral symmetry two different -factors (""
and "") and a -factor. In simplified situations we obtain . The formalism enables quantitative estimates to be made for the dynamical
narrowing of hyperfine lines in the observed ESR spectrum of the dihedral
cyclobutane radical cation.Comment: 28 pages, 4 figure
Phospholipid asymmetry in rough- and smooth-endoplasmic-reticulum membranes of untreated and phenobarbital-treated rat liver
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