7,994 research outputs found
The onset of synchronization in large networks of coupled oscillators
We study the transition from incoherence to coherence in large networks of
coupled phase oscillators. We present various approximations that describe the
behavior of an appropriately defined order parameter past the transition, and
generalize recent results for the critical coupling strength. We find that,
under appropriate conditions, the coupling strength at which the transition
occurs is determined by the largest eigenvalue of the adjacency matrix. We show
how, with an additional assumption, a mean field approximation recently
proposed is recovered from our results. We test our theory with numerical
simulations, and find that it describes the transition when our assumptions are
satisfied. We find that our theory describes the transition well in situations
in which the mean field approximation fails. We study the finite size effects
caused by nodes with small degree and find that they cause the critical
coupling strength to increase.Comment: To appear in PRE; Added an Appendix, a reference, modified two
figures and improved the discussion of the range of validity of perturbative
approache
Electron Correlations in the Low Carrier Density LaFeAsO0.89F0.11 Superconductor (Tc = 28 K)
The crystal structure and numerous normal and superconducting state
properties of layered tetragonal (P4/nmm) LaFeAsO, with F-doping of 11 %, are
reported. Resistivity measurements give an onset transition temperature Tc =
28.2 K, and low field magnetic susceptibility data indicate bulk
superconductivity. In applied magnetic field, analysis of the resistive
transition results in a critical field Hc2 = 30 T and a coherence length 35 A.
An upper limit for the electron carrier concentration of 1 x 10^21 cm-3 is
inferred from Hall data just above Tc. Strong electron-electron correlations
are suggested from temperature-dependent resistivity, Seebeck coefficient, and
thermal conductivity data. Anomalies near Tc are observed in both Seebeck
coefficient and thermal conductivity data.Comment: 13 pages, 8 figure
Conformal symmetry and light flavor baryon spectra
The degeneracy among parity pairs systematically observed in the N and Delta
spectra is interpreted to hint on a possible conformal symmetry realization in
the light flavor baryon sector in line with AdS_5/CFT_4. The case is made by
showing that all the observed N and Delta resonances with masses below 2500 MeV
distribute fairly well each over the first levels of a unitary representation
of the conformal group, a representation that covers the spectrum of a
quark-diquark system, placed directly on the AdS_5 cone, conformally
compactified to R^1*S^3. The free geodesic motion on the S^3 manifold is
described by means of the scalar conformal equation there, which is of the
Klein-Gordon type. The equation is then gauged by the "curved" Coulomb
potential that has the form of a cotangent function. Conformal symmetry is not
exact, this because the gauge potential slightly modifies the conformal
centrifugal barrier of the free geodesic motion. Thanks to this, the degeneracy
between P11-S11 pairs from same level is relaxed, while the remaining states
belonging to same level remain practically degenerate. The model describes the
correct mass ordering in the P11-S11 pairs through the nucleon spectrum as a
combined effect of the above conformal symmetry breaking, on the one side, and
a parity change of the diquark from a scalar at low masses, to a pseudoscalar
at higher masses, on the other. The quality of the wave functions is
illustrated by calculations of realistic mean-square charge radii and electric
charge form-factors on the examples of the proton, and the protonic P11(1440),
and S11(1535) resonances. The scheme also allows for a prediction of the
dressing function of an effective instantaneous gluon propagator from the
Fourier transform of the gauge potential. We find a dressing function that is
finite in the infrared and tends to zero at infinity.Comment: Latex, 5 figures, 2 tables; Paper upgraded in accord with the
published version. Discussion on the meson sector include
Recovery From Monocular Deprivation Using Binocular Deprivation: Experimental Observations and Theoretical Analysis
Ocular dominance (OD) plasticity is a robust paradigm for examining the functional consequences of synaptic plasticity. Previous experimental and theoretical results have shown that OD plasticity can be accounted for by known synaptic plasticity mechanisms, using the assumption that deprivation by lid suture eliminates spatial structure in the deprived channel. Here we show that in the mouse, recovery from monocular lid suture can be obtained by subsequent binocular lid suture but not by dark rearing. This poses a significant challenge to previous theoretical results. We therefore performed simulations with a natural input environment appropriate for mouse visual cortex. In contrast to previous work we assume that lid suture causes degradation but not elimination of spatial structure, whereas dark rearing produces elimination of spatial structure. We present experimental evidence that supports this assumption, measuring responses through sutured lids in the mouse. The change in assumptions about the input environment is sufficient to account for new experimental observations, while still accounting for previous experimental results
Hybrid fuzzy and sliding-mode control for motorised tether spin-up when coupled with axial vibration
A hybrid fuzzy sliding mode controller is applied to the control of motorised tether spin-up coupled with an axial oscillation phenomenon. A six degree of freedom dynamic model of a motorised momentum exchange tether is used as a basis for interplanetary payload exchange. The tether comprises a symmetrical double payload configuration, with an outrigger counter inertia and massive central facility. It is shown that including axial elasticity permits an enhanced level of performance prediction accuracy and a useful departure from the usual rigid body representations, particularly for accurate payload positioning at strategic points. A special simulation program has been devised in MATLAB and MATHEMATICA for a given initial condition data case
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