220 research outputs found
Stabilizing continuous-wave output in semiconductor lasers by time-delayed feedback
The stabilization of steady states is studied in a modified Lang-Kobayashi
model of a semiconductor laser. We show that multiple time-delayed feedback,
realized by a Fabry-Perot resonator coupled to the laser, provides a valuable
tool for the suppression of unwanted intensity pulsations, and leads to stable
continuous-wave operation. The domains of control are calulated in dependence
on the feedback strength, delay time (cavity round trip time), memory parameter
(mirror reflectivity), latency time, feedback phase, and bandpass filtering,
Due to the optical feedback, multistable behavior can also occur in the form of
delay-induced intensity pulsations or other modes for certain choices of the
control parameters. Control may then still be achieved by slowly ramping the
injection current during turn-on.Comment: 12 pages, 17 figure
Temperature and frequency dependent optical properties of ultra-thin Au films
While the optical properties of thin metal films are well understood in the
visible and near-infrared range, little has been done in the mid- and
far-infrared region. Here we investigate ultra-thin gold films prepared on
Si(111)(7 x 7) in UHV by measuring in the frequency range between 500 cm-1 and
7000 cm-1 and for temperatures between 300 K and 5 K. The nominal thickness of
the gold layers ranges between one monolayer and 9 nm. The frequency and
temperature dependences of the thicker films can be well described by the Drude
model of a metal, when taking into account classical size effects due to
surface scattering. The films below the percolation threshold exhibit a
non-metallic behavior: the reflection increases with frequency and decreases
with temperature. The frequency dependence can partly be described by a
generalized Drude model. The temperature dependence does not follow a simple
activation process. For monolayers we observe a transition between surface
states around 1100 cm-1.Comment: 7 pages, 10 figure
Disease spread through animal movements: a static and temporal network analysis of pig trade in Germany
Background: Animal trade plays an important role for the spread of infectious
diseases in livestock populations. As a case study, we consider pig trade in
Germany, where trade actors (agricultural premises) form a complex network. The
central question is how infectious diseases can potentially spread within the
system of trade contacts. We address this question by analyzing the underlying
network of animal movements.
Methodology/Findings: The considered pig trade dataset spans several years
and is analyzed with respect to its potential to spread infectious diseases.
Focusing on measurements of network-topological properties, we avoid the usage
of external parameters, since these properties are independent of specific
pathogens. They are on the contrary of great importance for understanding any
general spreading process on this particular network. We analyze the system
using different network models, which include varying amounts of information:
(i) static network, (ii) network as a time series of uncorrelated snapshots,
(iii) temporal network, where causality is explicitly taken into account.
Findings: Our approach provides a general framework for a
topological-temporal characterization of livestock trade networks. We find that
a static network view captures many relevant aspects of the trade system, and
premises can be classified into two clearly defined risk classes. Moreover, our
results allow for an efficient allocation strategy for intervention measures
using centrality measures. Data on trade volume does barely alter the results
and is therefore of secondary importance. Although a static network description
yields useful results, the temporal resolution of data plays an outstanding
role for an in-depth understanding of spreading processes. This applies in
particular for an accurate calculation of the maximum outbreak size.Comment: main text 33 pages, 17 figures, supporting information 7 pages, 7
figure
Control of unstable steady states by time-delayed feedback methods
We show that time-delayed feedback methods, which have successfully been used
to control unstable periodic ortbits, provide a tool to stabilize unstable
steady states. We present an analytical investigation of the feedback scheme
using the Lambert function and discuss effects of both a low-pass filter
included in the control loop and non-zero latency times associated with the
generation and injection of the feedback signal.Comment: 8 pages, 11 figure
Tip-induced distortions in STM imaging of carbon nanotubes
By means of STM measurements and fully self-consistent
transport calculations we analyze how STM trajectories for the
mapping of nanostructures on surfaces are affected by the
atomic structure of the tip.
For the particular case of carbon nanotubes we show that
considerable distortions of the STM trajectory with respect to
the actual structure, position and diameter of the nanotube
can occur for certain tip geometries. Comparison between
theory and experiment can allow to characterize and correct
these distortions
Early warning of infectious disease outbreaks on cattle-transport networks.
Surveillance of infectious diseases in livestock is traditionally carried out at the farms, which are the typical units of epidemiological investigations and interventions. In Central and Western Europe, high-quality, long-term time series of animal transports have become available and this opens the possibility to new approaches like sentinel surveillance. By comparing a sentinel surveillance scheme based on markets to one based on farms, the primary aim of this paper is to identify the smallest set of sentinel holdings that would reliably and timely detect emergent disease outbreaks in Swiss cattle. Using a data-driven approach, we simulate the spread of infectious diseases according to the reported or available daily cattle transport data in Switzerland over a four year period. Investigating the efficiency of surveillance at either market or farm level, we find that the most efficient early warning surveillance system [the smallest set of sentinels that timely and reliably detect outbreaks (small outbreaks at detection, short detection delays)] would be based on the former, rather than the latter. We show that a detection probability of 86% can be achieved by monitoring all 137 markets in the network. Additional 250 farm sentinels-selected according to their risk-need to be placed under surveillance so that the probability of first hitting one of these farm sentinels is at least as high as the probability of first hitting a market. Combining all markets and 1000 farms with highest risk of infection, these two levels together will lead to a detection probability of 99%. We conclude that the design of animal surveillance systems greatly benefits from the use of the existing abundant and detailed animal transport data especially in the case of highly dynamic cattle transport networks. Sentinel surveillance approaches can be tailored to complement existing farm risk-based and syndromic surveillance approaches
Oscillatory Size-Dependence of the Surface Plasmon Linewidth in Metallic Nanoparticles
We study the linewidth of the surface plasmon resonance in the optical
absorption spectrum of metallic nanoparticles, when the decay into
electron-hole pairs is the dominant channel. Within a semiclassical approach,
we find that the electron-hole density-density correlation oscillates as a
function of the size of the particles, leading to oscillations of the
linewidth. This result is confirmed numerically for alkali and noble metal
particles. While the linewidth can increase strongly, the oscillations persist
when the particles are embedded in a matrix.Comment: RevTeX4, 5 pages, 2 figures, final versio
Diffusion of gold nanoclusters on graphite
We present a detailed molecular-dynamics study of the diffusion and
coalescence of large (249-atom) gold clusters on graphite surfaces. The
diffusivity of monoclusters is found to be comparable to that for single
adatoms. Likewise, and even more important, cluster dimers are also found to
diffuse at a rate which is comparable to that for adatoms and monoclusters. As
a consequence, large islands formed by cluster aggregation are also expected to
be mobile. Using kinetic Monte Carlo simulations, and assuming a proper scaling
law for the dependence on size of the diffusivity of large clusters, we find
that islands consisting of as many as 100 monoclusters should exhibit
significant mobility. This result has profound implications for the morphology
of cluster-assembled materials
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