A new reverse mode light shutter from silica-dispersed liquid crystals

Nanoparticle dispersions in liquid crystalline materials at low concentrations allow both investigating the formation of defects in liquid crystal (LC) and enhancing the light-scattering properties of LC optical devices. Reverse mode LC dispersions are LC devices, which look like transparent in their OFF state, when no electric field is applied, and opaque in their ON state. In this paper, a new reverse mode device, formed by a dispersion of a LC mixture in a silica nanoparticle crosslinked network, is presented. The morphology and the electro-optical properties of these silica nanoparticle/LC composites were investigated for two different LC mixtures with a negative dielectric anisotropy. The observed transmittances and relaxation times were found to depend strongly on the silica amount and chemical–physical properties of LC used in the sample preparation.     

Element distributions for the reactions32S+100Mo,32S+93Nb and28Si+93Nb near the coulomb barrier

The element distributions for the reactions³²S+¹⁰⁰Mo,³²S+⁹³Nb, and²⁸Si+⁹³Nb have been measured atE/B～1.4. The correlations between the first moment of the measured distributions and TKEL are consistent with the predictions of the liquid drop potential energy surfaces showing the charge equilibration of the dinuclear systems. The measured second moments are compared with a recent systematics.     

High-energy spectral changes in three Seyfert galaxies observed with GINGA

Summary We have analysed observations of the three Seyfert Galaxies IC4329A, MCG-6-30-15 and NGC 4051 extracted from the GINGA archive. The variations in the (4⋎18) keV spectrum of the three sources can be interpreted in the framework of a model with two components: a ?direct? power law with constant spectral index (α=1.9⋎2.1), and the ?reflection bump? predicted by Guilbert and Rees, and Lightman and White which is produced by reprocessing of the direct power law in optically thick matter in the surroundings of the central source. The differences found in the behaviour of the three sources would arise, in this scenario, from the different size of the ?reprocessing region?. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Activity inAplysia buccal ganglia characterized by signal processing techniques

Summary Two procedures aimed at a synthetic, quantitative characterization of the integrative activity in a small system of nerve cells are proposed, and applications to the buccal ganglion of the molluskAplysia are described. Both methods are based on cross-correlation evaluation. The first method processes intracellular signals simultaneously recorded from pairs of neurones, and reveals and measures the occurrence of shared synaptic input. The second method processes extracellular multiunit signals recorded by two cuff electrodes placed around one nerve, and evaluates the impulse traffic, subdivided according to direction and speed of propagation. The outcome of both methods may be used to represent the state of the system, and may provide a basis to further studies on its dynamics. Paper presented at the International Workshop ?Fluctuations in Physics and Biology: Stochastic Resonance, Signal Processing and Related Phenomena?, Elba, 5–10 June 1994.     

Comparing different protocols of temperature selection in the parallel tempering method

Parallel tempering Monte Carlo simulations have been applied to a variety of systems presenting rugged free-energy landscapes. Despite this, its efficiency depends strongly on the temperature set. With this query in mind, we present a comparative study among different temperature selection schemes in three lattice-gas models. We focus our attention in the constant entropy method (CEM), proposed by Sabo et al. In the CEM, the temperature is chosen by the fixed difference of entropy between adjacent replicas. We consider a method to determine the entropy which avoids numerical integrations of the specific heat and other thermodynamic quantities. Different analyses for first- and second-order phase transitions have been undertaken, revealing that the CEM may be an useful criterion for selecting the temperatures in the parallel tempering.     

Preparation and characterization of Fe-based bulk metallic glasses in plate form

Amorphous alloys with composition (at%) Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Gd₂ (alloy A) and Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Y₂ (alloy B) were prepared either using pure elements (A and B1) and a commercial AISI430 steel as a base material (B2). When prepared from pure elements both alloys (A and B1) could be cast in plate form with a fixed thickness of 2 mm and variable lengths between 10 and 20 mm by means of copper-mold injection in air atmosphere. In the case of alloy B2, prepared using commercial grade raw materials, rods of 2 mm diameter were obtained.     

Flow-induced grating from cholesteric mixtures

In this work we present a new technique for obtaining large diffraction gratings (some cm) by means of a simple filling of cells having a planar treatment of their inner surfaces. A homogeneous mixture, composed of a cholesteric liquid crystal and a nematic liquid crystal monomer, was used. During the filling process, the flow induces a phase separation between the cholesteric liquid crystal and the liquid crystal monomer and, at the same time, the latter is oriented planar to the surfaces of the cell. Phase separation produces alternate arrays constituted by the cholesteric liquid crystal and the nematic liquid crystal monomer. Successive UV polymerization of these films yields a permanent grating. We have investigated the transmitted and first order diffracted beam efficiency for films obtained at different temperatures. The morphology of the films was studied by using an optical microscope equipped with crossed polarizers and by electron microscopy in order to control the shape of the arrays and the alignment of the oriented polymer.