1,356 research outputs found
Stability of mode-locked kinks in the ac driven and damped sine-Gordon lattice
Kink dynamics in the underdamped and strongly discrete sine-Gordon lattice
that is driven by the oscillating force is studied. The investigation is
focused mostly on the properties of the mode-locked states in the {\it
overband} case, when the driving frequency lies above the linear band. With the
help of Floquet theory it is demonstrated that the destabilizing of the
mode-locked state happens either through the Hopf bifurcation or through the
tangential bifurcation. It is also observed that in the overband case the
standing mode-locked kink state maintains its stability for the bias amplitudes
that are by the order of magnitude larger than the amplitudes in the
low-frequency case.Comment: To appear in Springer Series on Wave Phenomena, special volume
devoted to the LENCOS'12 conference; 6 figure
Two coupled Josephson junctions: dc voltage controlled by biharmonic current
We study transport properties of two Josephson junctions coupled by an
external shunt resistance. One of the junction (say, the first) is driven by an
unbiased ac current consisting of two harmonics. The device can rectify the ac
current yielding a dc voltage across the first junction. For some values of
coupling strength, controlled by an external shunt resistance, a dc voltage
across the second junction can be generated. By variation of system parameters
like the relative phase or frequency of two harmonics, one can conveniently
manipulate both voltages with high efficiency, e.g., changing the dc voltages
across the first and second junctions from positive to negative values and vice
versa.Comment: 15 pages, 7 figures, to appear in J. Phys. Condens. Matter (2012
NASA space station automation: AI-based technology review
Research and Development projects in automation for the Space Station are discussed. Artificial Intelligence (AI) based automation technologies are planned to enhance crew safety through reduced need for EVA, increase crew productivity through the reduction of routine operations, increase space station autonomy, and augment space station capability through the use of teleoperation and robotics. AI technology will also be developed for the servicing of satellites at the Space Station, system monitoring and diagnosis, space manufacturing, and the assembly of large space structures
NASA space station automation: AI-based technology review. Executive summary
Research and Development projects in automation technology for the Space Station are described. Artificial Intelligence (AI) based technologies are planned to enhance crew safety through reduced need for EVA, increase crew productivity through the reduction of routine operations, increase space station autonomy, and augment space station capability through the use of teleoperation and robotics
Mineral Licks as a Potential Nidus for Parasite Transmission
Discrete landscape features can concentrate animals in time and space, leading to non-random interspecific encounters. These encounters have implications for predator-prey interactions, habitat selection, intraspecific competition, and transmission of parasites and other pathogens. The lifecycle of the parasitic nematode Parelaphostrongylus tenuis requires an intermediate host of a terrestrial gastropod. Natural hosts of P. tenuis are whitetailed deer, and an aberrant host of conservation concern is moose, which are susceptible to high levels of mortality as a naive host to the parasite. Intermediate hosts become infected when P. tenuis larvae are shed in deer feces, then consumed or enter the gastropod through the foot. Incidental (or perhaps intentional) ingestion of infected gastropod intermediate hosts by aberrant or dead-end hosts often results in mortality of that animal. We present photographic evidence depicting a potential mechanism for transmission from infected white-tailed deer to moose, heretofore not examined in the literature. We deployed remote cameras at mineral licks around Grand Portage Indian Reservation in northeastern Minnesota, USA. We observed white-tailed deer defecating at mineral lick sites and geophagous moose at the same sites. We hypothesize that mineral licks may act as a nidus for P. tenuis transmission between deer and moose in this system and call for further research into the potential role of mineral licks in parasite transmission. The Grand Portage Band of Lake Superior Chippewa is a federally recognized Indian tribe in extreme northeastern Minnesota, USA, and proudly exercises its rights to food sovereignty through subsistence hunting and fishing. Mooz (Moose) are a primary subsistence food used by the Anishinaabeg (people) of Grand Portage Band historically and presently. Management for and research on maintaining this moose population as a vital subsistence species thus sets the context for this paper examining potential for disease transmission between whitetailed deer and moose through shared use of mineral licks
Status of Acropora palmata Populations off the Coast of South Caicos, Turks and Caicos Islands
This study is the first detailed assessment of A. palmata populations of the Turks and Caicos Islands. A total of 203 individual colonies and 62 thickets were tagged on five shallow reefs. Depth, percentages of living tissue, recent mortality and old skeleton were estimated. Presence of disease and predatory snails was noted, and disease spread and grazing rates of the snails estimated. Colonies were found in depths of 0.2 - 4 m. Living tissue for individual colonies (75.9% ± 2.2 SE) was significantly greater than for thickets (58.6% ± 3.6) and in both cases exceeded old skeleton (individuals: 22.7% ± 2.1 SE, thickets: 38.0% ± 3.4 SE). Percentage of recent mortality was very low (individuals: 1.3% ± 0.3 SE, thickets: 3.4% ± 0.7%). We found WBD (n = 2), white pox disease a (WPDa) (n = 7) and white pox disease b (WPDb) (n = 14) with greatly varying spreading rates. The WBD infected colonies showed an atypical spread from the top of the branch towards the base. Coralliophila abbreviata and C. caribaea affected 3 .7 54.7% of the populations (grazing rate: 4.29 cm 2 /day/snail ± 1.16 SE). South Caicos’ A. palmata populations are still in good condition, though increasing human disturbances combined with disease and predatory snails may threaten these populations
Escape from a zero current state in a one dimensional array of Josephson junctions
A long one dimensional array of small Josephson junctions exhibits Coulomb
blockade of Cooper pair tunneling. This zero current state exists up to a
switching voltage, Vsw, where there is a sudden onset of current. In this paper
we present histograms showing how Vsw changes with temperature for a long array
and calculations of the corresponding escape rates. Our analysis of the problem
is based on the existence of a voltage dependent energy barrier and we do not
make any assumptions about its shape. The data divides up into two temperature
regimes, the higher of which can be explained with Kramers thermal escape
model. At low temperatures the escape becomes independent of temperature.Comment: 4 pages 5 figure
Palette-colouring: a belief-propagation approach
We consider a variation of the prototype combinatorial-optimisation problem
known as graph-colouring. Our optimisation goal is to colour the vertices of a
graph with a fixed number of colours, in a way to maximise the number of
different colours present in the set of nearest neighbours of each given
vertex. This problem, which we pictorially call "palette-colouring", has been
recently addressed as a basic example of problem arising in the context of
distributed data storage. Even though it has not been proved to be NP complete,
random search algorithms find the problem hard to solve. Heuristics based on a
naive belief propagation algorithm are observed to work quite well in certain
conditions. In this paper, we build upon the mentioned result, working out the
correct belief propagation algorithm, which needs to take into account the
many-body nature of the constraints present in this problem. This method
improves the naive belief propagation approach, at the cost of increased
computational effort. We also investigate the emergence of a satisfiable to
unsatisfiable "phase transition" as a function of the vertex mean degree, for
different ensembles of sparse random graphs in the large size ("thermodynamic")
limit.Comment: 22 pages, 7 figure
Appearance and Stability of Anomalously Fluctuating States in Shor's Factoring Algorithm
We analyze quantum computers which perform Shor's factoring algorithm, paying
attention to asymptotic properties as the number L of qubits is increased.
Using numerical simulations and a general theory of the stabilities of
many-body quantum states, we show the following: Anomalously fluctuating states
(AFSs), which have anomalously large fluctuations of additive operators, appear
in various stages of the computation. For large L, they decohere at anomalously
great rates by weak noises that simulate noises in real systems. Decoherence of
some of the AFSs is fatal to the results of the computation, whereas
decoherence of some of the other AFSs does not have strong influence on the
results of the computation. When such a crucial AFS decoheres, the probability
of getting the correct computational result is reduced approximately
proportional to L^2. The reduction thus becomes anomalously large with
increasing L, even when the coupling constant to the noise is rather small.
Therefore, quantum computations should be improved in such a way that all AFSs
appearing in the algorithms do not decohere at such great rates in the existing
noises.Comment: 11 figures. A few discussions were added in verion 2. Version 3 is
the SAME as version 2; only errors during the Web-upload were fixed. Version
4 is the publised version, in which several typos are fixed and the reference
list is update
Tree defence and bark beetles in a drying world: carbon partitioning, functioning and modelling
Drought has promoted large‐scale, insect‐induced tree mortality in recent years, with severe consequences for ecosystem function, atmospheric processes, sustainable resources and global biogeochemical cycles. However, the physiological linkages among drought, tree defences, and insect outbreaks are still uncertain, hindering our ability to accurately predict tree mortality under on‐going climate change. Here we propose an interdisciplinary research agenda for addressing these crucial knowledge gaps. Our framework includes field manipulations, laboratory experiments, and modelling of insect and vegetation dynamics, and focuses on how drought affects interactions between conifer trees and bark beetles. We build upon existing theory and examine several key assumptions: 1) there is a trade‐off in tree carbon investment between primary and secondary metabolites (e.g. growth vs. defence); 2) secondary metabolites are one of the main component of tree defence against bark beetles and associated microbes; and 3) implementing conifer‐bark beetle interactions in current models improves predictions of forest disturbance in a changing climate. Our framework provides guidance for addressing a major shortcoming in current implementations of large‐scale vegetation models, the under‐representation of insect‐induced tree mortality
- …