3,136 research outputs found
Instability of frozen-in states in synchronous Hebbian neural networks
The full dynamics of a synchronous recurrent neural network model with Ising
binary units and a Hebbian learning rule with a finite self-interaction is
studied in order to determine the stability to synaptic and stochastic noise of
frozen-in states that appear in the absence of both kinds of noise. Both, the
numerical simulation procedure of Eissfeller and Opper and a new alternative
procedure that allows to follow the dynamics over larger time scales have been
used in this work. It is shown that synaptic noise destabilizes the frozen-in
states and yields either retrieval or paramagnetic states for not too large
stochastic noise. The indications are that the same results may follow in the
absence of synaptic noise, for low stochastic noise.Comment: 14 pages and 4 figures; accepted for publication in J. Phys. A: Math.
Ge
Symmetric sequence processing in a recurrent neural network model with a synchronous dynamics
The synchronous dynamics and the stationary states of a recurrent attractor
neural network model with competing synapses between symmetric sequence
processing and Hebbian pattern reconstruction is studied in this work allowing
for the presence of a self-interaction for each unit. Phase diagrams of
stationary states are obtained exhibiting phases of retrieval, symmetric and
period-two cyclic states as well as correlated and frozen-in states, in the
absence of noise. The frozen-in states are destabilised by synaptic noise and
well separated regions of correlated and cyclic states are obtained. Excitatory
or inhibitory self-interactions yield enlarged phases of fixed-point or cyclic
behaviour.Comment: Accepted for publication in Journal of Physics A: Mathematical and
Theoretica
Aliquoting structure for centrifugal microfluidics based on a new pneumatic valve
We present a new microvalve that can be monolithically integrated in centrifugally driven lab-on-a-chip systems. In contrast to existing operation principles that use hydrophobic patches, geometrically defined capillary stops or siphons, here we present a pneumatic principle. It needs neither additional local coatings nor expensive micro sized geometries. The valve is controlled by the spinning frequency and can be switched to be open when the centrifugal pressure overcomes the pneumatic pressure inside an unvented reaction cavity. We designed and characterized valves ranging in centrifugal burst pressure from 6700 Pa to 2100 Pa. Based on this valving principle we present a new structure for aliquoting of liquids. We experimentally demonstrated this by splitting 105 muL volumes into 16 aliquots with a volume CV of 3 %
Structure analysis of the virtual Compton scattering amplitude at low energies
We analyze virtual Compton scattering off the nucleon at low energies in a
covariant, model-independent formalism.
We define a set of invariant functions which, once the irregular nucleon pole
terms have been subtracted in a gauge-invariant fashion, is free of poles and
kinematical zeros.
The covariant treatment naturally allows one to implement the constraints due
to Lorentz and gauge invariance, crossing symmetry, and the discrete
symmetries.
In particular, when applied to the reaction,
charge-conjugation symmetry in combination with nucleon crossing generates four
relations among the ten originally proposed generalized polarizabilities of the
nucleon.Comment: 19 pages, LaTeX2e/RevTeX, no figures, original sections IV.-VI.
removed, to be discussed in a separate publication, none of the conclusions
change
Nucleon generalized polarizabilities within a relativistic Constituent Quark Model
Nucleon generalized polarizabilities are investigated within a relativistic
framework, defining such quantities through a Lorentz covariant multipole
expansion of the amplitude for virtual Compton scattering. The key physical
ingredients in the calculation of the nucleon polarizabilities are the Lorentz
invariant reduced matrix elements of the electromagnetic transition current,
which can be evaluated from off-energy-shell helicity amplitudes. The evolution
of the proton paramagnetic polarizability, , as a function of
the virtual-photon three-momentum transfer is explicitly evaluated within
a relativistic constituent quark model by adopting transition form factors
obtained in the light-front formalism. The discussion is focussed on the role
played by the effects due to the relativistic approach and to the transition
form factors, derived within different models.Comment: 14 pages and three figures (included), to appear in Phys. Rev. C (May
1998
Field-dependent heat transport in the Kondo insulator SmB6 : phonons scattered by magnetic impurities
The thermal conductivity of the Kondo insulator SmB was measured
at low temperature, down to 70 mK, in magnetic fields up to 15 T, on single
crystals grown using both the floating-zone and the flux methods. The residual
linear term at is found to be zero in all samples, for
all magnetic fields, in agreement with previous studies. There is therefore no
clear evidence of fermionic heat carriers. In contrast to some prior data, we
observe a large enhancement of with increasing field. The effect of
field is anisotropic, depending on the relative orientation of field and heat
current (parallel or perpendicular), and with respect to the cubic crystal
structure. We interpret our data in terms of heat transport predominantly by
phonons, which are scattered by magnetic impurities.Comment: publish versio
Rabi Oscillations at Exceptional Points in Microwave Billiards
We experimentally investigated the decay behavior with time t of resonances
near and at exceptional points, where two complex eigenvalues and also the
associated eigenfunctions coalesce. The measurements were performed with a
dissipative microwave billiard, whose shape depends on two parameters. The
t^2-dependence predicted at the exceptional point on the basis of a two-state
matrix model could be verified. Outside the exceptional point the predicted
Rabi oscillations, also called quantum echoes in this context, were detected.
To our knowledge this is the first time that quantum echoes related to
exceptional points were observed experimentally.Comment: 10 pages, 3 figure
Progressive multifocal leukoencephalopathy in a multiple Sclerosis patient diagnosed after switching from natalizumab to fingolimod
Background: Natalizumab- (NTZ-) associated progressive multifocal leukoencephalopathy (PML) is a severe and often disabling infectious central nervous system disease that can become evident in multiple sclerosis (MS) patients after NTZ discontinuation. Recently, novel diagnostic biomarkers for the assessment of PML risk in NTZ treated MS patients such as the anti-JC virus antibody index have been reported, and the clinical relevance of milky-way lesions detectable by MRI has been discussed. Case Presentation and Conclusion: We report a MS patient in whom PML was highly suspected solely based on MRI findings after switching from NTZ to fingolimod despite repeatedly negative (ultrasensitive) polymerase chain reaction (PCR) testing for JC virus DNA in cerebrospinal fluid. The PML diagnosis was histopathologically confirmed by brain biopsy. The occurrence of an immune reconstitution inflammatory syndrome (IRIS) during fingolimod therapy, elevated measures of JCV antibody indices, and the relevance of milky-way-like lesions detectable by (7âT) MRI are discussed
Birth, life and survival of Tidal Dwarf Galaxies
Advances on the formation and survival of the so-called Tidal Dwarf Galaxies
(TDGs) are reviewed. The understanding on how objects of the mass of dwarf
galaxies may form in debris of galactic collisions has recently benefited from
the coupling of multi-wavelength observations with numerical simulations of
galaxy mergers. Nonetheless, no consensual scenario has yet emerged and as a
matter of fact the very definition of TDGs remains elusive. Their real
cosmological importance is also a matter of debate, their presence in our Local
Group of galaxies as well. Identifying old, evolved, TDGs among the population
of regular dwarf galaxies and satellites may not be straightforward. However a
number of specific properties (location, dark matter and metal content) that
objects of tidal origin should have are reminded here. Examples of newly
discovered genuine old TDGs around a nearby elliptical galaxy are finally
presented.Comment: 9 pages, 5 figures, invited talk at JENAM 2010 symposium on "Dwarf
Galaxies", v2:reference and acknowledgements update
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