Leone Fronzoni

Molecular orientation in nematic liquid crystal films with two free surfaces

Abstract The director orientation in nematic layers with two free surfaces has been studied by optical methods for different thicknesses of the film. A sharp structure transition, obtained by reducing the thickness of the film under a critical value, is reported for the first time. The same structural transition for fixed thickness is also shown to occur below a critical temperature.

Critical behaviour of the anchoring energy of the director at the free surface of a nematic liquid crystal

Abstract A new structural transition occurs at the free surface of some nematic liquid crystals when the temperature reaches a critical value T 0 . In this work we study the temperature dependence of the anchoring energy of the director at the free surface close to the critical point. We find that the anchoring energy tends to zero with the critical exponent δ = 1 when the temperature approaches the critical value T 0 . The experimental results are interpreted in terms of the Parsons and Mada theories.

The influence of the astrocyte field on neuronal dynamics and synchronization.

Astrocytes can sense local synaptic release of glutamate by metabotropic glutamate receptors. Receptor activation in turn can mediate transient increases of astrocytic intracellular calcium concentration through inositol 1,4,5-trisphosphate production. Notably, the perturbation of calcium concentration can propagate to other adjacent astrocytes. Astrocytic calcium signaling can therefore be linked to synaptic information transfer between neurons. On the other hand, astrocytes can also modulate neuronal activity by feeding back onto synaptic terminals in a fashion that depends on their intracellular calcium concentration. Thus, astrocytes can also be active partners in neuronal network activity. The aim of our study is to provide a computationally simple network model of mutual neuron–astrocyte interactions, in order to investigate the possible roles of astrocytes in neuronal network dynamics. In particular, we focus on the information entropy of neuronal firing of the whole network, considering how it could be affected by neuron–glial interactions.

Electrohydrodynamic domain patterns in freely suspended layers of nematic liquid crystals with negative dielectric anisotropy

The electrohydrodynamic domain patterns observed in freely suspended layers of nematic liquid crystals with negative dielectric anisotropy are investigated. Two different domain patterns, in low and high frequency electric fields, occur. Both the director lines and the hydrodynamic flow lines associated with these patterns are investigated in detail. The threshold voltage of the instability is measured as a function of thickness and distance between the electrodes. The experimental results agree with a two‐dimensional theoretical approach based on the Carr–Helfrich’s model.

Static and dynamic behavior of the vortex–electrohydrodynamic instability in freely suspended layers of nematic liquid crystals

The vortex–electrohydrodynamic instability occurring in thin freely suspended layers of some nematic liquid crystals is investigated. The vortex pattern is observed both in the anisotropic and in the isotropic phase of the nematic liquid crystal. In this paper, we study the behavior of the vortex pattern when dc and ac electric fields are applied parallel to the free surfaces. The threshold electric field of the instability is measured as a function of the temperature, the thickness and the electric conductivity of the sample. In order to explain the main features of the vortex regime, we develop a new isotropic electrohydrodynamic model based on surface electric forces, which shows a satisfactory agreement with the experimental results.

A lower bound for the velocity of quantum communications in the preferred frame

Abstract An EPR experiment with polarized entangled photons is performed to test the Eberhard model. According to the Eberhard model, quantum correlations between space-like separated events are due to a superluminal communication signal propagating in a preferred frame. The coincidences between entangled photons passing through two polarizers aligned along a East–West axis are measured as a function of time during 21 sidereal days. No deviation from the predictions of the Quantum Theory is observed. Tacking into account for the experimental uncertainties, we infer that, if a preferred frame for superluminal signals exists which moves at velocity v → with respect to the Earth, the modulus of the velocity of quantum communications in this frame has to be greater than v t ≃ 0.6 × 10 4 c for v 0.1 c and for any arbitrary direction of v → .

The projection approach to the Fokker-Planck equation. I: Colored gaussian noise

It is shown that the Fokker-Planck operator can be derived via a projection-perturbation approach, using the repartition of a more detailed operatorℒ into a perturbationℒ1 and an unperturbed partℒ0. The standard Fokker-Planck structure is recovered at the second order inℒ1, whereas the perturbation terms of higher order are shown to provoke the breakdown of this structure. To get rid of these higher order terms, a key approximation, local linearization (LL), is made. In general, to evaluate at the second order inℒ1 the exact expression of the diffusion coefficient which simulates the influence of a Gaussian noise with a finite correlation timeτ, a resummation up to infinite order inτ must be carried out, leading to what other authors call the best Fokker-Planck approximation (BFPA). It is shown that, due to the role of terms of higher order inℒ1, the BFPA leads to predictions on the equilibrium distributions that are reliable only up to the first order in t. The LL, on the contrary, in addition to making the influence of terms of higher order inℒ1 vanish, results in a simple analytical expression for the term of second order that is formally coincident with the complete resummation over all the orders in t provided by the Fox theory. The corresponding diffusion coefficient in turn is shown to lead in the limiting case τ→∞ to exact results for the steady-state distributions. Therefore, over the whole range 0⩽τ⩽∞ the LL turns out to be an approximation much more accurate than the global linearization proposed by other authors for the same purpose of making the terms of higher order inℒ1 vanish. In the short-τ region the LL leads to results virtually coincident with those of the BFPA. In the large-τ region the LL is a more accurate approximation than the BFPA itself. These theoretical arguments are supported by the results of both analog and digital simulation.

Transient Bimodality in a Bistable Laser with Saturable Absorber

In a laser bistable operation the transient response is investigated when a control parameter is step-switched to a value outside the bistability region. A two-peaked probability distribution for the laser intensity at a given delay time is measured under the influence of internal noise or an externally applied noise. The phenomenon is briefly discussed on the basis of the theory for optically bistable passive systems.

Noisy cooperative intermittent processes: From blinking quantum dots to human consciousness

We study the superposition of a non-Poisson renewal process with the presence of a superimposed Poisson noise. The non-Poisson renewals mark the passage between meta-stable states in system with self-organization. We propose methods to measure the amount of information due to the two independent processes independently, and we see that a superficial study based on the survival probabilities yield stretched-exponential relaxations. Our method is in fact able to unravel the inverse-power law relaxation of the isolated non-Poisson processes, even when noise is present. We provide examples of this behavior in system of diverse nature, from blinking nano-crystals to weak turbulence. Finally we focus our discussion on events extracted from human electroencephalograms, and we discuss their connection with emerging properties of integrated neural dynamics, i.e. consciousness.

The complexity of the logistic map at the chaos threshold

We apply a generalized version of the Kolmogorov–Sinai entropy, based on a non-extensive form, to analyzing the dynamics of the logistic map at the chaotic threshold, the paradigm of power-law sensitivity to initial conditions. We make the statistical averages on the distribution of the power indexes β, and we show that the resulting entropy time evolution becomes a linear function of time if we assign to the non-extensive index q the value Q< 1 prescribed by the heuristic arguments of earlier work. We also show that the emerging entropy index Q is determined by the asymptotic mean value of the index β ,a nd that this same mean value determines the strength of the logarithmic time increase of entropy, stemming from the adoption of the ordinary Shannon form.  2001 Elsevier Science B.V. All rights reserved.