4,323 research outputs found
Magnetic Raman scattering of the ordered tetrahedral spin-1/2 clusters in Cu_2Te_2O_5(Br_(1-x)Cl_x)_2 compounds
Raman light-scattering experiments in the antiferromagnetic phase of the
Cu_2Te_2O_5(Br_(1-x)Cl_x)_2 compounds are analyzed in terms of a dimerized spin
model for the tetrahedral Cu-clusters. It is shown that the longitudinal
magnetic excitation in the pure Br system hybridizes with a localized singlet
excitation due to the presence of a Dzyaloshinskii-Moriya anisotropy term. The
drastic change of the magnetic scattering intensities observed when a
proportion of Br is replaced by Cl ions, is proposed to be caused by a change
of the magnetic order parameter. Instead of being parallel/antiparallel with
each other, the spins in the two pairs of spin-1/2 order perpendicular to each
other, when the composition x is larger than about 0.25.Comment: EPL, in pres
Intrinsic adaptation in autonomous recurrent neural networks
A massively recurrent neural network responds on one side to input stimuli
and is autonomously active, on the other side, in the absence of sensory
inputs. Stimuli and information processing depends crucially on the qualia of
the autonomous-state dynamics of the ongoing neural activity. This default
neural activity may be dynamically structured in time and space, showing
regular, synchronized, bursting or chaotic activity patterns.
We study the influence of non-synaptic plasticity on the default dynamical
state of recurrent neural networks. The non-synaptic adaption considered acts
on intrinsic neural parameters, such as the threshold and the gain, and is
driven by the optimization of the information entropy. We observe, in the
presence of the intrinsic adaptation processes, three distinct and globally
attracting dynamical regimes, a regular synchronized, an overall chaotic and an
intermittent bursting regime. The intermittent bursting regime is characterized
by intervals of regular flows, which are quite insensitive to external stimuli,
interseeded by chaotic bursts which respond sensitively to input signals. We
discuss these finding in the context of self-organized information processing
and critical brain dynamics.Comment: 24 pages, 8 figure
‘Big Think’, Disjointed Incrementalism: Chinese Economic Success and Policy Lessons for Africa, or the Case for Pan-Africanism
Chinese economic success is not the product of free market accidental coincidence. Rather, it is orchestrated by the State through a mixture of nationalism (‘big think’) and pragmatic decisions (disjointed incrementalism) in agriculture, finance and industry. Furthermore, these decisions build upon existing institutions (e.g. the Household Responsibility System, Township Village Enterprises, etc), some dating back to pre-revolutionary China (e.g. Special Economic Zones), rather than imported ones from outside China. The article explores the utility (and lack thereof) of the Chinese model in the African context, as well as the possibilities of an Africa-centred ‘big think’ (Pan-Africanism) capable of mobilizing the continent for development
Real time localization of Gamma Ray Bursts with INTEGRAL
The INTEGRAL satellite has been successfully launched in October 2002 and has
recently started its operational phase. The INTEGRAL Burst Alert System (IBAS)
will distribute in real time the coordinates of the GRBs detected with
INTEGRAL. After a brief introduction on the INTEGRAL instruments, we describe
the main IBAS characteristics and report on the initial results. During the
initial performance and verification phase of the INTEGRAL mission, which
lasted about two months, two GRBs have been localized with accuracy of about
2-4 arcmin. These observations have allowed us to validate the IBAS software,
which is now expected to provide quick (few seconds delay) and precise (few
arcmin) localization for about 10-15 GRBs per year.Comment: 6 pages, latex, 3 figures, submitted to Adv. Sp. Res., Proceedings of
the 34th COSPAR Scientific Assembly, Houston, 10-19 October 200
Statistics of the electromagnetic response of a chaotic reverberation chamber
This article presents a study of the electromagnetic response of a chaotic
reverberation chamber (RC) in the presence of losses. By means of simulations
and of experiments, the fluctuations in the maxima of the field obtained in a
conventional mode-stirred RC are compared with those in a chaotic RC in the
neighborhood of the Lowest Useable Frequency (LUF). The present work
illustrates that the universal spectral and spatial statistical properties of
chaotic RCs allow to meet more adequately the criteria required by the Standard
IEC 61000-4-21 to perform tests of electromagnetic compatibility.Comment: 6 pages, 9 figure
Attractor Metadynamics in Adapting Neural Networks
Slow adaption processes, like synaptic and intrinsic plasticity, abound in
the brain and shape the landscape for the neural dynamics occurring on
substantially faster timescales. At any given time the network is characterized
by a set of internal parameters, which are adapting continuously, albeit
slowly. This set of parameters defines the number and the location of the
respective adiabatic attractors. The slow evolution of network parameters hence
induces an evolving attractor landscape, a process which we term attractor
metadynamics. We study the nature of the metadynamics of the attractor
landscape for several continuous-time autonomous model networks. We find both
first- and second-order changes in the location of adiabatic attractors and
argue that the study of the continuously evolving attractor landscape
constitutes a powerful tool for understanding the overall development of the
neural dynamics
Properties of the energetic particle distributions during the October 28, 2003 solar flare from INTEGRAL/SPI observations
Analysis of spectra obtained with the gamma-ray spectrometer SPI onboard
INTEGRAL of the GOES X17-class flare on October 28, 2003 is presented. In the
energy range 600 keV - 8 MeV three prominent narrow lines at 2.223, 4.4 and 6.1
MeV, resulting from nuclear interactions of accelerated ions within the solar
atmosphere could be observed. Time profiles of the three lines and the
underlying continuum indicate distinct phases with several emission peaks and
varying continuum-to-line ratio for several minutes before a smoother decay
phase sets in. Due to the high-resolution Ge detectors of SPI and the
exceptional intensity of the flare, detailed studies of the 4.4 and 6.1 MeV
line shapes was possible for the first time. Comparison with calculated line
shapes using a thick target interaction model and several energetic particle
angular distributions indicates that the nuclear interactions were induced by
downward-directed particle beams with alpha-to-proton ratios of the order of
0.1. There are also indications that the 4.4 MeV to 6.1 MeV line fluence ratio
changed between the beginning and the decay phase of the flare, possibly due to
a temporal evolution of the energetic particle alpha-to-proton ratio.Comment: 24 pages, 10 figures, accepted for publication by A&
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