Synchronisation in networks of delay-coupled type-I excitable systems

We use a generic model for type-I excitability (known as the SNIPER or SNIC model) to describe the local dynamics of nodes within a network in the presence of non-zero coupling delays. Utilising the method of the Master Stability Function, we investigate the stability of the zero-lag synchronised dynamics of the network nodes and its dependence on the two coupling parameters, namely the coupling strength and delay time. Unlike in the FitzHugh-Nagumo model (a model for type-II excitability), there are parameter ranges where the stability of synchronisation depends on the coupling strength and delay time. One important implication of these results is that there exist complex networks for which the adding of inhibitory links in a small-world fashion may not only lead to a loss of stable synchronisation, but may also restabilise synchronisation or introduce multiple transitions between synchronisation and desynchronisation. To underline the scope of our results, we show using the Stuart-Landau model that such multiple transitions do not only occur in excitable systems, but also in oscillatory ones.     

An approximate solution scheme for the algebraic random eigenvalue problem

A reduced basis formulation is presented for the efficient solution of large-scale algebraic random eigenvalue problems. This formulation aims to improve the accuracy of the first order perturbation method, and also allow the efficient computation of higher order statistical moments of the eigenparameters. In the present method, the two terms of the first order perturbation approximation for the eigenvector are used as basis vectors for Ritz analysis of the governing random eigenvalue problem. This leads to a sequence of reduced order random eigenvalue problems to be solved for each eigenmode of interest. Since, only two basis vectors are used to represent each eigenvector, explicit expressions for the random eigenvalues and eigenvectors can readily be derived. This enables the statistics of the random eigenparameters and the forced response to be efficiently computed. Numerical studies are presented for free and forced vibration analysis of a linear stochastic structural system. It is demonstrated that the reduced basis method gives better results as compared to the first order perturbation method.     

PRESS-MAG-O: A new facility to probe materials and phenomena under extreme conditions

In the recent years, several experiments performed under high magnetic fields (HMFs), at high pressure (HP) and/or at low temperature (LT) have led to spectacular discoveries in condensed matter. In many new systems, although challenging, it is strategic to perform a magneto-optical analysis, to investigate the phonon behavior in the far infrared (IR) domain. By combining HMF and HP in a wide temperature (T) range to perform concurrently IR magneto-optics and ac-magneto-dynamic experiments, it will be possible to achieve unique information on systems and/or new phenomena, almost impossible to obtain with standard spectroscopic methods. Here we present PRESS-MAG-O, a new facility under construction that will perform HP experiments under HMF in a wide T range. The system is expected to be operational by the end of 2008 and will be tested at SINBAD, the IR synchrotron radiation beamline operational since 2001 at DAΦNE (Double Annular Φ-factory for Nice Experiments), the storage ring of the INFN Frascati National Laboratory (LNF). While for IR experiments an interferometer will be used, for the magneto-dynamic experiments a SQUID magnetometer in the 10 Hz-2 KHz frequency range will be utilized. HP will be applied to samples by a Cu-Be diamond anvil cell (DAC), so that the device will be able to collect FTIR spectra and high harmonic ac susceptibility data in a dc magnetic field up to 8 T and to about 20 GPa in a wide temperature range (4.2-200 K).     

Strongly mixingg-measures

LetT be ann-to-1 covering transformation of the compact metric spaceX (e.g. (X, T) then-shift). For suitable functionsg onX an inverse _g ofT is defined: _g is a Markov kernel. Ifg is strictly positive and satisfies a Lipschitz condition, then there exists a unique _g measure, strongly mixing underT. Conversely, we associate to anyT-invariant probability measure a suitableg, and ifg is nice, then strong mixing is present. Examples include all Bernoulli and Markov measures on then-shift. The strong mixing criterion is useful, and applications to harmonic analysis, ergodic theory, and symbolic dynamics are given. For example: if is any infinite subgroup of the group of roots of unity, there exist uncountably many (explicitly constructible) continuous Morse sequences whose corresponding dynamical systems are pairwise non-isomorphic and all have as eigenvalue group exactly the given group .C.N.R.S. Équipe Associée 250.Research supported in part by NSF grant GP-16392 while the author was visiting at Yale University.     

A scanning electron and atomic force microscopy study of the surface morphology and composition of CsI films as affected by evaporation rate and humid-air exposure.

Evaporation rate and subsequent exposure to humid air affect the surface morphology and composition of cesium iodide (CsI) films and, in turn, their photoemissive efficiency when used as photocathodes. The surface morphology and elemental composition of 300-nm-thick CsI films grown at two different rates (1 nm/s and 0.04 nm/s), both freshly evaporated and after 24-h exposure to humid air were investigated by means of atomic force microscopy and scanning electron microscopy/electron diffraction spectroscopy. The CsI film freshly evaporated at a slow rate exhibited a granular surface presenting circular holes or craters where the CsI material was moved from the center to the boundaries. After 24-h exposure to humid air, this film coalesced in large grain showing a marked increase of surface roughness. Conversely, the CsI film grown at a fast rate mostly retained its original surface uniformity and homogeneity with no presence of holes and craters after 24-h exposure to humid air. Further, surface roughness and average peak height decreased, but the surface coalesced in large grains spaced by small fractures where the CsI coverage was almost lost. In conclusion, the films grown at a fast evaporation rate were affected by 24-h exposure to humid air less than those grown at a slow rate, and are thus expected to possess a greater long-term stability.     

Simulations of Differential Gain and Linewidth Enhancement Factor of Quantum Dot Semiconductor Lasers

We present a multi-population rate equation model for the analysis of the static and dynamic characteristics of a quantum dot (QD) semiconductor laser. The model is applied to the extraction of the differential gain and linewidth enhancement factor of the QD laser simulating the same kind of experiments usually done in the laboratory. Coherently with the experimental results, we show the difference between the values of the differential parameters extracted in below and above threshold characterizations. We demonstrate that such discrepancy is due the complex dynamics of the carriers in those energy states, whose carrier concentration is not clamped by the stimulated emission process above threshold.     

The centrifugal pendulum as a non linear torsional vibration absorber

Summary In this paper we study the centrifugal pendulum as a torsional vibrations absorber of an engine. A non-autonomous two degrees of freedom mechanical system with viscous-Coulomb damping is considered. The motion equations are written and we analyse for small oscillations of the pendulum, the effect of the non linear damping on the response curves in amplitude. Several numerical examples are ratio and the damping coefficients on the resonance peaks.

---

Sommario In questo lavoro si studia il pendolo centrifugo come ammortizzatore di vibrazioni torsionali di un motore. Si considera un sistema meccanico non autonomo a due gradi di libertà con dissipazione coulombiana-viscosa. Si scrivono le equazioni del moto e si analizza, per piccole oscillazioni del pendolo, l'effetto dell'attrito non lineare sulle curve di risposta in ampiezza. Vengono presentati numerosi esempi numerici per illustrare l'influenza del rapporto di massa e dei coefficienti di attrito sui picchi di risonanza.

---

---

This work has been realized within the activities of the Italian Council for the Research, C.N.R., G.N.F.M.     

Functional histology of glioma vasculature by FTIR imaging

Fourier-transform infrared (FTIR) imaging has been used to investigate brain tumor angiogenesis using a mice solid tumor model and bare-gold (∅ 25 nm) or BaSO₄ (∅ 500 nm) nanoparticles (NP) injected into blood vasculature. FTIR images of 20-μm-thick tissue sections were used for chemical histology of healthy and tumor areas. Distribution of BaSO₄-NP (using the 1,218–1,159 cm⁻¹ spectral interval) revealed clearly all details of blood vasculature with morphological abnormalities of tumor capillaries, while Au-NP (using the 1,046–1,002 cm⁻¹ spectral interval) revealed also diffusion properties of leaky blood vessels. Diffusion of Au-NP out of vascular space reached 64 ± 29 μm, showing the fenestration of “leaky” tumor blood vessels, which should allow small NP (<100 nm, as for Au-NP) to diffuse almost freely, while large NP should not (as for BaSO₄-NP in this study). Therefore, we propose to develop FTIR imaging as a convenient tool for functional molecular histology imaging of brain tumor vasculature, both for identifying blood capillaries and for determining the extravascular diffusion space offered by vessel fenestration.