5,714 research outputs found
Localized systems coupled to small baths: from A to Z
We investigate what happens if an Anderson localized system is coupled to a
small bath, with a discrete spectrum, when the coupling between system and bath
is specially chosen so as to never localize the bath. We find that the effect
of the bath on localization in the system is a non-monotonic function of the
coupling between system and bath. At weak couplings, the bath facilitates
transport by allowing the system to 'borrow' energy from the bath. But above a
certain coupling the bath produces localization, because of an orthogonality
catastrophe, whereby the bath 'dresses' the system and hence suppresses the
hopping matrix element. We call this last regime the regime of
"Zeno-localization", since the physics of this regime is akin to the quantum
Zeno effect, where frequent measurements of the position of a particle impede
its motion. We confirm our results by numerical exact diagonalization
Improving photon-hadron discrimination based on cosmic ray surface detector data
The search for photons at EeV energies and beyond has considerable
astrophysical interest and will remain one of the key challenges for ultra-high
energy cosmic ray (UHECR) observatories in the near future. Several upper
limits to the photon flux have been established since no photon has been
unambiguously observed up to now. An improvement in the reconstruction
efficiency of the photon showers and/or better discrimination tools are needed
to improve these limits apart from an increase in statistics. Following this
direction, we analyze in this work the ability of the surface parameter Sb,
originally proposed for hadron discrimination, for photon search.
Semi-analytical and numerical studies are performed in order to optimize Sb
for the discrimination of photons from a proton background in the energy range
from 10^18.5 to 10^19.6 eV. Although not shown explicitly, the same analysis
has been performed for Fe nuclei and the corresponding results are discussed
when appropriate. The effects of different array geometries and the
underestimation of the muon component in the shower simulations are analyzed,
as well as the Sb dependence on primary energy and zenith angle.Comment: 9 pages, 19 Figures. Accepted in Astroparticle Physics on May 31th,
201
Mind over chatter: plastic up-regulation of the fMRI alertness network by EEG neurofeedback
EEG neurofeedback (NFB) is a brain-computer interface (BCI) approach used to shape brain oscillations by means of real-time feedback from the electroencephalogram (EEG), which is known to reflect neural activity across cortical networks. Although NFB is being evaluated as a novel tool for treating brain disorders, evidence is scarce on the mechanism of its impact on brain function. In this study with 34 healthy participants, we examined whether, during the performance of an attentional auditory oddball task, the functional connectivity strength of distinct fMRI networks would be plastically altered after a 30-min NFB session of alpha-band reduction (n=17) versus a sham-feedback condition (n=17). Our results reveal that compared to sham, NFB induced a specific increase of functional connectivity within the alertness/salience network (dorsal anterior and mid cingulate), which was detectable 30 minutes after termination of training. Crucially, these effects were significantly correlated with reduced mind-wandering 'on-task' and were coupled to NFB-mediated resting state reductions in the alpha-band (8-12 Hz). No such relationships were evident for the sham condition. Although group default-mode network (DMN) connectivity was not significantly altered following NFB, we observed a positive association between modulations of resting alpha amplitude and precuneal connectivity, both correlating positively with frequency of mind-wandering. Our findings demonstrate a temporally direct, plastic impact of NFB on large-scale brain functional networks, and provide promising neurobehavioral evidence supporting its use as a noninvasive tool to modulate brain function in health and disease
Searching for low mass objects around nearby dMe radio stars
Nearby M-dwarfs are best suited for searches of low mass companions. VLBI
phase-referencing observations with sensitive telescopes are able to detect
radio star flux-densities of tenths of mJy as well as to position the star on
the sky with submilliarcsecond precision. We have initiated a long-term
observational program, using EVN telescopes in combination with NASA DSN
dishes, to revisit the kinematics of nearby, single M dwarfs. The precision of
the astrometry allows us to search for possible companions with masses down to
1 Jupiter mass. In this contribution we report preliminary results of the first
observation epochs, in which we could detect some of the radio stars included
in our program.Comment: Proceedings of the 6th European VLBI Network Symposium, Ros E.,
Porcas R.W., Lobanov A.P., & Zensus J.A. (eds.), MPIfR, Bonn, Germany, p.
255-258 (2002). 4 pages, 3 figures, needs evn2002.cl
Variability and Velocity of Superluminal Sources
We investigate the relation between the Doppler factor determined from
variations in total flux at 22 and 37 GHz, and the apparent transverse velocity
determined from VLBA observations at 2 cm. The data are consistent with the
relativistic beaming theory for compact radio sources, in that the distribution
of beta_{app}/delta_{var}, for 30 quasars, is roughly consistent with a Monte
Carlo simulation. The intrinsic temperature appears to be ~2x10^{10} K, close
to the "equipartition value" calculated by Readhead (1994). We deduce the
distribution of Lorentz factors for a group of 48 sources; the values range up
to about gamma=40.Comment: To be published in "Radio Astronomy at the Fringe", ASP Conf. Ser.
Vol. 300, J. A. Zensus, M. H. Cohen, & E. Ros (eds.), 8 pages, 3 figures,
needs rafringe.st
Fractional-order operators: Boundary problems, heat equations
The first half of this work gives a survey of the fractional Laplacian (and
related operators), its restricted Dirichlet realization on a bounded domain,
and its nonhomogeneous local boundary conditions, as treated by
pseudodifferential methods. The second half takes up the associated heat
equation with homogeneous Dirichlet condition. Here we recall recently shown
sharp results on interior regularity and on -estimates up to the boundary,
as well as recent H\"older estimates. This is supplied with new higher
regularity estimates in -spaces using a technique of Lions and Magenes,
and higher -regularity estimates (with arbitrarily high H\"older estimates
in the time-parameter) based on a general result of Amann. Moreover, it is
shown that an improvement to spatial -regularity at the boundary is
not in general possible.Comment: 29 pages, updated version, to appear in a Springer Proceedings in
Mathematics and Statistics: "New Perspectives in Mathematical Analysis -
Plenary Lectures, ISAAC 2017, Vaxjo Sweden
Distance and spectral power profile shaping using machine learning enabled Raman amplifiers
We propose a Convolutional Neural Network (CNN) to learn the mapping between the 2D power profiles, (distance and frequency), and the Raman pumps. Using the CNN, the pump powers and wavelengths for arbitrary 2D profiles can be determined with high accuracy
Inverse design of a Raman amplifier in frequency and distance domains using convolutional neural networks
We present a convolutional neural network architecture for inverse Raman amplifier design. This model aims at finding the pump powers and wavelengths required for a target signal power evolution in both distance along the fiber and in frequency. Using the proposed framework, the prediction of the pump configuration required to achieve a target power profile is demonstrated numerically with high accuracy in C-band considering both counter-propagating and bidirectional pumping schemes. For a distributed Raman amplifier based on a 100 km single-mode fiber, a low mean set (0.51, 0.54, and 0.64 dB) and standard deviation set (0.62, 0.43, and 0.38 dB) of the maximum test error are obtained numerically employing two and three counter-, and four bidirectional propagating pumps, respectively
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