8,783 research outputs found
Quantum dynamics in macrosystems with several coupled electronic states: hierarchy of effective Hamiltonians
We address the nonadiabatic quantum dynamics of macrosystems with several
coupled electronic states, taking into account the possibility of multi-state
conical intersections. The general situation of an arbitrary number of states
and arbitrary number of nuclear degrees of freedom (modes) is considered. The
macrosystem is decomposed into a system part carrying a few, strongly coupled
modes, and an environment, comprising the vast number of remaining modes. By
successively transforming the modes of the environment, a hierarchy of
effective Hamiltonians for the environment is constructed. Each effective
Hamiltonian depends on a reduced number of effective modes, which carry
cumulative effects. By considering the system's Hamiltonian along with a few
members of the hierarchy, it is shown mathematically by a moment analysis that
the quantum dynamics of the entire macrosystem can be numerically exactly
computed on a given time-scale. The time scale wanted defines the number of
effective Hamiltonians to be included. The contribution of the environment to
the quantum dynamics of the macrosystem translates into a sequential coupling
of effective modes. The wavefunction of the macrosystem is known in the full
space of modes, allowing for the evaluation of observables such as the
time-dependent individual excitation along modes of interest, as well a spectra
and electronic-population dynamics
Predicting coexistence of plants subject to a tolerance-competition trade-off
Ecological trade-offs between species are often invoked to explain species
coexistence in ecological communities. However, few mathematical models have
been proposed for which coexistence conditions can be characterized explicitly
in terms of a trade-off. Here we present a model of a plant community which
allows such a characterization. In the model plant species compete for sites
where each site has a fixed stress condition. Species differ both in stress
tolerance and competitive ability. Stress tolerance is quantified as the
fraction of sites with stress conditions low enough to allow establishment.
Competitive ability is quantified as the propensity to win the competition for
empty sites. We derive the deterministic, discrete-time dynamical system for
the species abundances. We prove the conditions under which plant species can
coexist in a stable equilibrium. We show that the coexistence conditions can be
characterized graphically, clearly illustrating the trade-off between stress
tolerance and competitive ability. We compare our model with a recently
proposed, continuous-time dynamical system for a tolerance-fecundity trade-off
in plant communities, and we show that this model is a special case of the
continuous-time version of our model.Comment: To be published in Journal of Mathematical Biology. 30 pages, 5
figures, 5 appendice
Application of Laguerre based adaptive predictive control to Shape Memory Alloy (SMA) actuators
This paper discusses the use of an existing adaptive predictive controller to control some Shape Memory Alloy (SMA) linear actuators. The model consists in a truncated linear combination of Laguerre filters identified online. The controller stability is studied in details. It is proven that the tracking error is asymptotically stable under some conditions on the modelling error. Moreover, the tracking error converge toward zero for step references, even if the identified model is inaccurate. Experimentalcresults obtained on two different kind of actuator validate the proposed control. They also show that it is robust with regard to input constraints.ANR MAFESM
Event-Triggered Observers and Observer-Based Controllers for a Class of Nonlinear Systems
In this paper, we investigate the stabilization of a nonlinear plant subject
to network constraints, under the assumption of partial knowledge of the plant
state. The event triggered paradigm is used for the observation and the control
of the system. Necessary conditions, making use of the ISS property, are given
to guarantee the existence of a triggering mechanism, leading to asymptotic
convergence of the observer and system states. The proposed triggering
mechanism is illustrated in the stabilization of a robot with a flexible link
robot.Comment: Proceedings of the 2015 American Control Conference - ACC 201
The Hydraulic Jump in Liquid Helium
We present the results of some experiments on the circular hydraulic jump in
normal and superfluid liquid helium. The radius of the jump and the depth of
the liquid outside the jump are measured through optical means. Although the
scale of the apparatus is rather small, the location of the jump is found to be
consistent with the assumption that the jump can be treated as a shock, if the
surface tension is taken into account. The radius of the jump does not change
when going down in temperature through the lambda point; we think that the flow
is supercritical. A remarkable feature of the experiment is the observation of
stationary ripples within the jump when the liquid is superfluid.Comment: Submitted to the proceedings of the 24th International Conference on
Low Temperature Physics. 2 figure
Phylogenetic congruence between subtropical trees and their associated fungi.
Recent studies have detected phylogenetic signals in pathogen-host networks for both soil-borne and leaf-infecting fungi, suggesting that pathogenic fungi may track or coevolve with their preferred hosts. However, a phylogenetically concordant relationship between multiple hosts and multiple fungi in has rarely been investigated. Using next-generation high-throughput DNA sequencing techniques, we analyzed fungal taxa associated with diseased leaves, rotten seeds, and infected seedlings of subtropical trees. We compared the topologies of the phylogenetic trees of the soil and foliar fungi based on the internal transcribed spacer (ITS) region with the phylogeny of host tree species based on matK, rbcL, atpB, and 5.8S genes. We identified 37 foliar and 103 soil pathogenic fungi belonging to the Ascomycota and Basidiomycota phyla and detected significantly nonrandom host-fungus combinations, which clustered on both the fungus phylogeny and the host phylogeny. The explicit evidence of congruent phylogenies between tree hosts and their potential fungal pathogens suggests either diffuse coevolution among the plant-fungal interaction networks or that the distribution of fungal species tracked spatially associated hosts with phylogenetically conserved traits and habitat preferences. Phylogenetic conservatism in plant-fungal interactions within a local community promotes host and parasite specificity, which is integral to the important role of fungi in promoting species coexistence and maintaining biodiversity of forest communities
X-ray and UV investigation into the magnetic connectivity of a solar flare
We investigate the X-ray and UV emission detected by RHESSI and TRACE in the
context of a solar flare on the 16th November 2002 with the goal of better
understanding the evolution of the flare. We analysed the characteristics of
the X-ray emission in the 12-25 and 25-50 keV energy range while we looked at
the UV emission at 1600 {\AA}. The flare appears to have two distinct phases of
emission separated by a 25-second time delay, with the first phase being
energetically more important. We found good temporal and spatial agreement
between the 25-50 keV X-rays and the most intense areas of the 1600 {\AA} UV
emission. We also observed an extended 100-arcsecond < 25 keV source that
appears coronal in nature and connects two separated UV ribbons later in the
flare. Using the observational properties in X-ray and UV wavelengths, we
propose two explanations for the flare evolution in relation to the spine/fan
magnetic field topology and the accelerated electrons. We find that a
combination of quasi separatrix layer reconnection and null-point reconnection
is required to account for the observed properties of the X-ray and UV
emission.Comment: 8 pages, 8 figures, published in Astronomy and Astrophysic
- âŠ