319 research outputs found
Spectroscopic investigations of a luminescent conjugated polymer
This work presents experimental results and comprehensive analysis of the first electroabsorption and photocurrent measurements on the conjugated polymer poly(2,5-pyridinediyl) (PPY). In the electroabsorption experiment the change of the UV/vis absorption spectrum of disordered (spin-cast) and oriented (stretched), 100 nm thin PPY films due to the presence of a strong electric field was measured. The spectra were successfully fitted to a linear combination of the linear absorption spectrum and its first and second derivative. From the analysis of the obtained linear coefficients it is concluded that the electroabsorption spectrum is dominated by a Stark-red-shift of the first allowed optical transition at 3.2 eV and by the emergence of a normally one-photon forbidden 2A(_g) state at 3.7 eV. In addition polarised electroabsorption spectroscopy on oriented films and on fihns in sandwich configuration have been used to investigate the alignment of the polymer chains and the directions of the relevant transition dipole moments in PPY. The photoconductivity measurements have been carried out on thin films of PPY m the sandwich configuration of the structure ITO/PPY/semitransparent metal. Photocurrent spectra have been recorded using the four possible directions of the applied electric field and illumination directions and the dependence on temperature, film thickness and applied electric field has been investigated. The combined analysis of the experimental results shows that the photocurrent originates from the dissociation of excitons, which are photogenerated inside the polymer near the negative electrode. Finally evidence is given for the electron fransporting, semiconducting properties of PPY and the photocurrent-voltage-characteristic have been successfully modelled with an Onsager-theory yielding an apparent exciton binding energy of 0.14 eV. All results of the work are compared and contrasted with those reported for other conjugated polymers
Ethernet - a survey on its fields of application
During the last decades, Ethernet progressively became the most widely used local area networking (LAN) technology. Apart from LAN installations, Ethernet became also attractive for many other fields of application, ranging from industry to avionics, telecommunication, and multimedia. The expanded application of this technology is mainly due to its significant assets like reduced cost, backward-compatibility, flexibility, and expandability. However, this new trend raises some problems concerning the services of the protocol and the requirements for each application. Therefore, specific adaptations prove essential to integrate this communication technology in each field of application. Our primary objective is to show how Ethernet has been enhanced to comply with the specific requirements of several application fields, particularly in transport, embedded and multimedia contexts. The paper first describes the common Ethernet LAN technology and highlights its main features. It reviews the most important specific Ethernet versions with respect to each application fieldâs requirements. Finally, we compare these different fields of application and we particularly focus on the fundamental concepts and the quality of service capabilities of each proposal
Markov vs. nonMarkovian processes A comment on the paper Stochastic feedback, nonlinear families of Markov processes, and nonlinear Fokker-Planck equations by T.D. Frank
The purpose of this comment is to correct mistaken assumptions and claims
made in the paper Stochastic feedback, nonlinear families of Markov processes,
and nonlinear Fokker-Planck equations by T. D. Frank. Our comment centers on
the claims of a nonlinear Markov process and a nonlinear Fokker-Planck
equation. First, memory in transition densities is misidentified as a Markov
process. Second, Frank assumes that one can derive a Fokker-Planck equation
from a Chapman-Kolmogorov equation, but no proof was given that a
Chapman-Kolmogorov equation exists for memory-dependent processes. A nonlinear
Markov process is claimed on the basis of a nonlinear diffusion pde for a
1-point probability density. We show that, regardless of which initial value
problem one may solve for the 1-point density, the resulting stochastic
process, defined necessarily by the transition probabilities, is either an
ordinary linearly generated Markovian one, or else is a linearly generated
nonMarkovian process with memory. We provide explicit examples of diffusion
coefficients that reflect both the Markovian and the memory-dependent cases. So
there is neither a nonlinear Markov process nor nonlinear Fokker-Planck
equation for a transition density. The confusion rampant in the literature
arises in part from labeling a nonlinear diffusion equation for a 1-point
probability density as nonlinear Fokker-Planck, whereas neither a 1-point
density nor an equation of motion for a 1-point density defines a stochastic
process, and Borland misidentified a translation invariant 1-point density
derived from a nonlinear diffusion equation as a conditional probability
density. In the Appendix we derive Fokker-Planck pdes and Chapman-Kolmogorov
eqns. for stochastic processes with finite memory
First passage time for subdiffusion: The nonextensive entropy approach versus the fractional model
We study the similarities and differences between different models concerning
subdiffusion. More particularly, we calculate first passage time (FPT)
distributions for subdiffusion, derived from Greens' functions of nonlinear
equations obtained from Sharma-Mittal's, Tsallis's and Gauss's nonadditive
entropies. Then we compare these with FPT distributions calculated from a
fractional model using a subdiffusion equation with a fractional time
derivative. All of Greens' functions give us exactly the same standard relation
which characterizes subdiffusion
(), but generally FPT's are not equivalent to one another. We will
show here that the FPT distribution for the fractional model is asymptotically
equal to the Sharma--Mittal model over the long time limit only if in the
latter case one of the three parameters describing Sharma--Mittal entropy
depends on , and satisfies the specific equation derived in this paper,
whereas the other two models mentioned above give different FTPs with the
fractional model. Greens' functions obtained from the Sharma-Mittal and
fractional models - for obtained from this particular equation - are very
similar to each other. We will also discuss the interpretation of subdiffusion
models based on nonadditive entropies and the possibilities of experimental
measurement of subdiffusion models parameters.Comment: 12 pages, 8 figure
Biological Flora of the Tropical and Subtropical Intertidal Zone: Literature Review for Rhizophora mangle L.
Rhizophora mangle L. is a tropical and subtropical mangrove species that occurs as a dominant tree species in the intertidal zone of low-energy shorelines. Rhizophora mangle plays an important role in coastal zones as habitat for a wide range of organisms of intertidal food webs, as a natural barrier to coastal erosion, and as carbon sequestration. A review of mangrove literature has been performed, but a review specifically on red mangroves has not. The approach was to cover a broad range of topics with a focus on topics that have seen significant work since the 1970s. This review includes a brief introduction to red mangroves and then focuses on the following topics: biogeography, habitats and zonation, geomorphological interactions, taxonomy, histology, anatomy, physiological ecology, productivity, biomass, litter, reproduction, population biology, plant communities, interactions with other species, impacts of storms, reforestation, remote sensing, modelling, and economic importance
When translocation dynamics becomes anomalous
Recent single molecule experiments probing the passage process of a short
single-stranded DNA (ssDNA) through a membrane channel (translocation) allow to
measure the passage time distribution. Building on a recent modelling approach
(D. K. Lubensky and D. R. Nelson, Biophys. J. 77, 1824 (1999)), which has been
demonstrated to be valid for chains of up to nucleotides and
therefore well applies to the system we have in mind, we discuss the
consequences if the associated dynamics is not of Markov origin, but if strong
memory effects prevail during the translocation. Motivation is drawn from
recent results indicating that the distribution of translocation times is
broader than predicted by simple Markovian models based on Brownian motion.Comment: 5 pages, 2 figures, RevTeX4, submitted to Biophys.
Minimal model of associative learning for cross-situational lexicon acquisition
An explanation for the acquisition of word-object mappings is the associative
learning in a cross-situational scenario. Here we present analytical results of
the performance of a simple associative learning algorithm for acquiring a
one-to-one mapping between objects and words based solely on the
co-occurrence between objects and words. In particular, a learning trial in our
learning scenario consists of the presentation of objects together
with a target word, which refers to one of the objects in the context. We find
that the learning times are distributed exponentially and the learning rates
are given by in the case the target
words are sampled randomly and by in the
case they follow a deterministic presentation sequence. This learning
performance is much superior to those exhibited by humans and more realistic
learning algorithms in cross-situational experiments. We show that introduction
of discrimination limitations using Weber's law and forgetting reduce the
performance of the associative algorithm to the human level
A probabilistic approach to Zhang's sandpile model
The current literature on sandpile models mainly deals with the abelian
sandpile model (ASM) and its variants. We treat a less known - but equally
interesting - model, namely Zhang's sandpile. This model differs in two aspects
from the ASM. First, additions are not discrete, but random amounts with a
uniform distribution on an interval . Second, if a site topples - which
happens if the amount at that site is larger than a threshold value
(which is a model parameter), then it divides its entire content in equal
amounts among its neighbors. Zhang conjectured that in the infinite volume
limit, this model tends to behave like the ASM in the sense that the stationary
measure for the system in large volumes tends to be peaked narrowly around a
finite set. This belief is supported by simulations, but so far not by
analytical investigations.
We study the stationary distribution of this model in one dimension, for
several values of and . When there is only one site, exact computations
are possible. Our main result concerns the limit as the number of sites tends
to infinity, in the one-dimensional case. We find that the stationary
distribution, in the case , indeed tends to that of the ASM (up
to a scaling factor), in agreement with Zhang's conjecture. For the case ,
we provide strong evidence that the stationary expectation tends to
.Comment: 47 pages, 3 figure
Fermionic quantum criticality and the fractal nodal surface
The complete lack of theoretical understanding of the quantum critical states
found in the heavy fermion metals and the normal states of the high-T
superconductors is routed in deep fundamental problem of condensed matter
physics: the infamous minus signs associated with Fermi-Dirac statistics render
the path integral non-probabilistic and do not allow to establish a connection
with critical phenomena in classical systems. Using Ceperley's constrained
path-integral formalism we demonstrate that the workings of scale invariance
and Fermi-Dirac statistics can be reconciled. The latter is self-consistently
translated into a geometrical constraint structure. We prove that this "nodal
hypersurface" encodes the scales of the Fermi liquid and turns fractal when the
system becomes quantum critical. To illustrate this we calculate nodal surfaces
and electron momentum distributions of Feynman backflow wave functions and
indeed find that with increasing backflow strength the quasiparticle mass
gradually increases, to diverge when the nodal structure becomes fractal. Such
a collapse of a Fermi liquid at a critical point has been observed in the
heavy-fermion intermetallics in a spectacular fashion.Comment: 14 pages, 6 figure
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