104,665 research outputs found
Coherent Graphene Devices: Movable Mirrors, Buffers and Memories
We theoretically report that, at a sharp electrostatic step potential in
graphene, massless Dirac fermions can obtain Goos-H\"{a}nchen-like shifts under
total internal reflection. Based on these results, we study the coherent
propagation of the quasiparticles along a sharp graphene \emph{p-n-p} waveguide
and derive novel dispersion relations for the guided modes. Consequently,
coherent graphene devices (e.g. movable mirrors, buffers and memories) induced
only by the electric field effect can be proposed.Comment: 12 pages, 5 figure
Coexistence of full which-path information and interference in Wheelers delayed choice experiment with photons
We present a computer simulation model that is a one-to-one copy of an
experimental realization of Wheeler's delayed choice experiment that employs a
single photon source and a Mach-Zehnder interferometer composed of a 50/50
input beam splitter and a variable output beam splitter with adjustable
reflection coefficient (V. Jacques {\sl et al.}, Phys. Rev. Lett. 100,
220402 (2008)). For , experimentally measured values of the
interference visibility and the path distinguishability , a parameter
quantifying the which-path information WPI, are found to fulfill the
complementary relation , thereby allowing to obtain partial WPI
while keeping interference with limited visibility. The simulation model that
is solely based on experimental facts, that satisfies Einstein's criterion of
local causality and that does not rely on any concept of quantum theory or of
probability theory, reproduces quantitatively the averages calculated from
quantum theory. Our results prove that it is possible to give a particle-only
description of the experiment, that one can have full WPI even if D=0, V=1 and
therefore that the relation cannot be regarded as quantifying
the notion of complementarity.Comment: Physica E, in press; see also http://www.compphys.ne
Distributed Clustering in Cognitive Radio Ad Hoc Networks Using Soft-Constraint Affinity Propagation
Absence of network infrastructure and heterogeneous spectrum availability in cognitive radio ad hoc networks (CRAHNs) necessitate the self-organization of cognitive radio users (CRs) for efficient spectrum coordination. The cluster-based structure is known to be effective in both guaranteeing system performance and reducing communication overhead in variable network environment. In this paper, we propose a distributed clustering algorithm based on soft-constraint affinity propagation message passing model (DCSCAP). Without dependence on predefined common control channel (CCC), DCSCAP relies on the distributed message passing among CRs through their available channels, making the algorithm applicable for large scale networks. Different from original soft-constraint affinity propagation algorithm, the maximal iterations of message passing is controlled to a relatively small number to accommodate to the dynamic environment of CRAHNs. Based on the accumulated evidence for clustering from the message passing process, clusters are formed with the objective of grouping the CRs with similar spectrum availability into smaller number of clusters while guaranteeing at least one CCC in each cluster. Extensive simulation results demonstrate the preference of DCSCAP compared with existing algorithms in both efficiency and robustness of the clusters
Diffusion induced decoherence of stored optical vortices
We study the coherence properties of optical vortices stored in atomic
ensembles. In the presence of thermal diffusion, the topological nature of
stored optical vortices is found not to guarantee slow decoherence. Instead the
stored vortex state has decoherence surprisingly larger than the stored
Gaussian mode. Generally, the less phase gradient, the more robust for stored
coherence against diffusion. Furthermore, calculation of coherence factor shows
that the center of stored vortex becomes completely incoherent once diffusion
begins and, when reading laser is applied, the optical intensity at the center
of the vortex becomes nonzero. Its implication for quantum information is
discussed. Comparison of classical diffusion and quantum diffusion is also
presented.Comment: 5 pages, 2 figure
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Modelling human behaviours and reactions under dangerous environment
This paper describes the framework of a real-time simulation system to model human behavior and reactions in dangerous environments. The system utilizes the latest 3D computer animation techniques, combined with artificial intelligence, robotics and psychology, to model human behavior, reactions and decision making under expected/unexpected dangers in real-time in virtual environments. The development of the system includes: classification on the conscious/subconscious behaviors and reactions of different people; capturing different motion postures by the Eagle Digital System; establishing 3D character animation models; establishing 3D models for the scene; planning the scenario and the contents; and programming within Virtools (TM) Dev. Programming within Virtools (TM) Dev is subdivided into modeling dangerous events, modeling character's perceptions, modeling character's decision making, modeling character's movements, modeling character's interaction with environment and setting up the virtual cameras. The real-time simulation of human reactions in hazardous environments is invaluable in military defense, fire escape, rescue operation planning, traffic safety studies, and safety planning in chemical factories, the design of buildings, airplanes, ships and trains. Currently, human motion modeling can be realized through established technology, whereas to integrate perception and intelligence into virtual human's motion is still a huge undertaking. The challenges here are the synchronization of motion and intelligence, the accurate modeling of human's vision, smell, touch and hearing, the diversity and effects of emotion and personality in decision making. There are three types of software platforms which could be employed to realize the motion and intelligence within one system, and their advantages and disadvantages are discussed
Calibrating the {\alpha} parameter of convective efficiency using observed stellar properties
Context. Synthetic model atmosphere calculations are still the most commonly
used tool when determining precise stellar parameters and stellar chemical
compositions. Besides three-dimensional models that consistently solve for
hydrodynamic processes, one-dimensional models that use an approximation for
convective energy transport play the major role.
Aims. We use modern Balmer-line formation theory as well as spectral energy
distribution (SED) measurements for the Sun and Procyon to calibrate the model
parameter {\alpha} that describes the efficiency of convection in our 1D
models. Convection was calibrated over a significant range in parameter space,
reaching from F-K along the main sequence and sampling the turnoff and giant
branch over a wide range of metallicities. This calibration was compared to
theoretical evaluations and allowed an accurate modeling of stellar
atmospheres.
Methods. We used Balmer-line fitting and SED fits to determine the convective
efficiency parameter {\alpha}. Both methods are sensitive to the structure and
temperature stratification of the deeper photosphere.
Results. While SED fits do not allow a precise determination of the
convective parameter for the Sun and Procyon, they both favor values
significantly higher than 1.0. Balmer-line fitting, which we find to be more
sensitive, suggests that the convective efficiency parameter {\alpha} is
2.0 for the main sequence and quickly decreases to 1.0 for
evolved stars. These results are highly consistent with predictions from 3D
models. While the values on the main sequence fit predictions very well,
measurements suggest that the decrease of convective efficiency as stars evolve
to the giant branch is more dramatic than predicted by models.Comment: 14 pages, 16 figures, accepted for publication in A&
Pavlov's dog associative learning demonstrated on synaptic-like organic transistors
In this letter, we present an original demonstration of an associative
learning neural network inspired by the famous Pavlov's dogs experiment. A
single nanoparticle organic memory field effect transistor (NOMFET) is used to
implement each synapse. We show how the physical properties of this dynamic
memristive device can be used to perform low power write operations for the
learning and implement short-term association using temporal coding and spike
timing dependent plasticity based learning. An electronic circuit was built to
validate the proposed learning scheme with packaged devices, with good
reproducibility despite the complex synaptic-like dynamic of the NOMFET in
pulse regime
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