68,891 research outputs found
Attosecond screening dynamics mediated by electron-localization
Transition metals with their densely confined and strongly coupled valence
electrons are key constituents of many materials with unconventional
properties, such as high-Tc superconductors, Mott insulators and
transition-metal dichalcogenides. Strong electron interaction offers a fast and
efficient lever to manipulate their properties with light, creating promising
potential for next-generation electronics. However, the underlying dynamics is
a fast and intricate interplay of polarization and screening effects, which is
poorly understood. It is hidden below the femtosecond timescale of electronic
thermalization, which follows the light-induced excitation. Here, we
investigate the many-body electron dynamics in transition metals before
thermalization sets in. We combine the sensitivity of intra-shell transitions
to screening effects with attosecond time resolution to uncover the interplay
of photo-absorption and screening. First-principles time-dependent calculations
allow us to assign our experimental observations to ultrafast electronic
localization on d-orbitals. The latter modifies the whole electronic structure
as well as the collective dynamic response of the system on a timescale much
faster than the light-field cycle. Our results demonstrate a possibility for
steering the electronic properties of solids prior to electron thermalization,
suggesting that the ultimate speed of electronic phase transitions is limited
only by the duration of the controlling laser pulse. Furthermore, external
control of the local electronic density serves as a fine tool for testing
state-of-the art models of electron-electron interactions. We anticipate our
study to facilitate further investigations of electronic phase transitions,
laser-metal interactions and photo-absorption in correlated electron systems on
its natural timescale
Simulation of networks of spiking neurons: A review of tools and strategies
We review different aspects of the simulation of spiking neural networks. We
start by reviewing the different types of simulation strategies and algorithms
that are currently implemented. We next review the precision of those
simulation strategies, in particular in cases where plasticity depends on the
exact timing of the spikes. We overview different simulators and simulation
environments presently available (restricted to those freely available, open
source and documented). For each simulation tool, its advantages and pitfalls
are reviewed, with an aim to allow the reader to identify which simulator is
appropriate for a given task. Finally, we provide a series of benchmark
simulations of different types of networks of spiking neurons, including
Hodgkin-Huxley type, integrate-and-fire models, interacting with current-based
or conductance-based synapses, using clock-driven or event-driven integration
strategies. The same set of models are implemented on the different simulators,
and the codes are made available. The ultimate goal of this review is to
provide a resource to facilitate identifying the appropriate integration
strategy and simulation tool to use for a given modeling problem related to
spiking neural networks.Comment: 49 pages, 24 figures, 1 table; review article, Journal of
Computational Neuroscience, in press (2007
Sub-nanosecond signal propagation in anisotropy engineered nanomagnetic logic chains
Energy efficient nanomagnetic logic (NML) computing architectures propagate
and process binary information by relying on dipolar field coupling to reorient
closely-spaced nanoscale magnets. Signal propagation in nanomagnet chains of
various sizes, shapes, and magnetic orientations has been previously
characterized by static magnetic imaging experiments with low-speed adiabatic
operation; however the mechanisms which determine the final state and their
reproducibility over millions of cycles in high-speed operation (sub-ns time
scale) have yet to be experimentally investigated. Monitoring NML operation at
its ultimate intrinsic speed reveals features undetectable by conventional
static imaging including individual nanomagnetic switching events and
systematic error nucleation during signal propagation. Here, we present a new
study of NML operation in a high speed regime at fast repetition rates. We
perform direct imaging of digital signal propagation in permalloy nanomagnet
chains with varying degrees of shape-engineered biaxial anisotropy using
full-field magnetic soft x-ray transmission microscopy after applying single
nanosecond magnetic field pulses. Further, we use time-resolved magnetic
photo-emission electron microscopy to evaluate the sub-nanosecond dipolar
coupling signal propagation dynamics in optimized chains with 100 ps time
resolution as they are cycled with nanosecond field pulses at a rate of 3 MHz.
An intrinsic switching time of 100 ps per magnet is observed. These
experiments, and accompanying macro-spin and micromagnetic simulations, reveal
the underlying physics of NML architectures repetitively operated on nanosecond
timescales and identify relevant engineering parameters to optimize performance
and reliability.Comment: Main article (22 pages, 4 figures), Supplementary info (11 pages, 5
sections
Magneto-acoustic wave energy from numerical simulations of an observed sunspot umbra
We aim at reproducing the height dependence of sunspot wave signatures
obtained from spectropolarimetric observations through 3D MHD numerical
simulations. A magneto-static sunspot model based on the properties of the
observed sunspot is constructed and perturbed at the photosphere introducing
the fluctuations measured with the \SiI\ 10827 \AA\ line. The results
of the simulations are compared with the oscillations observed simultaneously
at different heights from the \HeI\ 10830 \AA\ line, the \CaIIH\ core
and the \FeI\ blends in the wings of the \CaIIH\ line. The simulations show a
remarkable agreement with the observations. They reproduce the velocity maps
and power spectra at the formation heights of the observed lines, as well as
the phase and amplification spectra between several pair of lines. We find that
the stronger shocks at the chromosphere are accompanied with a delay between
the observed signal and the simulated one at the corresponding height,
indicating that shocks shift the formation height of the chromospheric lines to
higher layers. Since the simulated wave propagation matches very well the
properties of the observed one, we are able to use the numerical calculations
to quantify the energy contribution of the magneto-acoustic waves to the
chromospheric heating in sunspots. Our findings indicate that the energy
supplied by these waves is too low to balance the chromospheric radiative
losses. The energy contained at the formation height of the lowermost \SiI\
10827 \AA\ line in the form of slow magneto-acoustic waves is already
insufficient to heat the higher layers, and the acoustic energy which reaches
the chromosphere is around 3-9 times lower than the required amount of energy.
The contribution of the magnetic energy is even lower.Comment: Accepted for publication in The Astrophysical Journa
Fast network configuration in Software Defined Networking
Software Defined Networking (SDN) provides a framework to dynamically adjust and re-program the data plane with the use of flow rules. The realization of highly adaptive SDNs with the ability to respond to changing demands or recover after a network failure in a short period of time, hinges on efficient updates of flow rules. We model the time to deploy a set of flow rules by the update time at the bottleneck switch, and formulate the problem of selecting paths to minimize the deployment time under feasibility constraints as a mixed integer linear program (MILP). To reduce the computation time of determining flow rules, we propose efficient heuristics designed to approximate the minimum-deployment-time solution by relaxing the MILP or selecting the paths sequentially. Through extensive simulations we show that our algorithms outperform current, shortest path based solutions by reducing the total network configuration time up to 55% while having similar packet loss, in the considered scenarios. We also demonstrate that in a networked environment with a certain fraction of failed links, our algorithms are able to reduce the average time to reestablish disrupted flows by 40%
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