AC conductivity of porous silicon: A fractal and surface transport mechanism?

Summary In this paper we report on the frequency dependence of the AC conductivity of porous silicon in the range 10 Hz-100 kHz. Two types of testing devices have been fabricated on three different series of samples formed electrochemically using as a starting materialptype,n ⁻-type andn ⁺-type silicon substrates. For frequencies less than 20-40 kHz the conductivity is found to follow a sublinear frequency dependence. This behaviour is typical of a carrier transport mechanism determined by an anomalous diffusion process. At higher frequencies we find that surface states influence the transport mechanism. This suggests a double-channel transport mechanism: one related to porous-silicon “volume” properties and the other more connected to the “surface” itself. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettiore di Luce) Torino, 12–13 October 1995.     

Investigations of azimuthal asymmetry in semi-inclusive leptoproduction

We consider the azimuthal asymmetries in semi-inclusive deep inelastic leptoproduction arising due to both perturbative and nonperturbative effects at HERMES energies and show that the order corrections to and are significant. We also reconsider the results of perturbative effects for at large momentum transfers [1] using the more recent sets of scale-dependent distribution and fragmentation functions, which bring up to 18% difference in . In the same approach we calculate the as well. Received: 24 October 1997 / Revised version: 10 February 1998 / Published online: 30 March 1998     

Differentiability of solutions of some evolution equations in nonreflexive Banach spaces

Summary The purpose of this paper is to present a regularity result that provides a unified treatment of the Cauchy problem for certain nonlinear partial differential equations that appear in kinetic theory. Part. 1 contain the main theorem, based on the theory of evolution equations. In part 2 it is indicated how these abstract results are applicable to the spatially homogeneous Boltzmann equazion and to the kinetic equation of vehicular traffic. Entrata in Redazione il 28 giugno 1978. Work performed under the auspices of the G.N.A.F.A. of the National Research Couucil.     

Geometrical theory of energy launching and pulse distortion in dielectric optical waveguides

In this paper the geometrical theory of pulse distortion and energy launching into multimode optical fibres is generalized when skew rays are taken into account. To this purpose, within the geometrical theory of skew rays, we firstly obtained a new expression for the numerical aperture. From this expression we derived an extensive analysis of the energy launched and the pulse response of a multimode optical fibre. A better launching efficiency and a different pulse form at the output of the fibre together with a larger broadening are obtained in comparison with the results derived from theories only for meridional rays. The theory is also applied to sources having planar geometry (such as LED) and linear geometry (such as semiconductor lasers), and the response to some rectangular-shaped pulses is investigated. Finally some considerations concerning scattering effects are described.     

Characterization of macromolecules in liquid solutions by thermal diffusion. I. Dependence of the soret coefficient on the nature of the dispersing medium

Experiments have been carried out on thermal diffusion of macromolecular particles dispersed in various liquids, with the object of checking some predictions of the radiation-pressure theory of Soret effect in liquids and of establishing a method of physical characterization of macromolecules in liquid solutions. The experimental results confirm the importance of the ratio G between thermal conductivity K and (phase) velocity v of high-frequency elastic waves of the materials composing the mixture in determining the thermodiffusive behavior of a liquid solution. We have shown that the migration of the macromolecular component takes place in the same direction in which thermal energy is flowing or opposite to it, depending on whether G of the dispersed particles is smaller or larger relative to the G of the liquid. Another aspect of the same phenomenon may be observed when macroscopic pieces of nonmetallic materials are suspended in a liquid, and heat is made to flow through this solid plunger and the surrounding liquid. The experiments performed with molecular solutions and with macroscopic plungers mutually complement and confirm each other. Anomalous results obtained in the case of solutions of polyvinylpyrrolidone in methanol are also discussed, and the possibility that this might be the consequence of the existence of a marked velocity dispersion in the high-frequency region of the spectrum of thermal waves in both water and methyl alcohol is indicated. Finally the possibility is hinted that thermal diffusion might have been responsible for the phenomena of molecular selection and evolution which ultimately led to the origin of life on our planet.     

Anisotropic magnetic susceptibility and phase transitions in intercalated graphite: KC24

We measured the magnetic susceptibility of KC₂₄ from 4.2 to 300 K and found no anomalies near the phase transitions at 95 and 123 K as observed in the resistivity. We conclude that the transitions must be due to order- disorder transitions of the K atoms and not charge density wave formation. The susceptibility is anisotropic; at room temperature $\text{χ}{}_{\mathrm{g}}\text{(}\text{H}\text{6}\text{c}\text{)= + 1.50 × 10}{}^{-6}\text{emu g}{}^{-1}\text{± 2\%}$ and $\text{χ}{}_{\mathrm{g}}\text{(}\text{H}\text{⊥}\text{c}\text{)= + 0.045 × 10}{}^{-6}\text{emu g}{}^{-1}\text{± 50\%}$. This anisotropy is not understood in terms of simple rigid band extensions of the band structure of graphite.     

Analysis of thiols by HPLC after fluorescent prelabelling with 4-(6-methylnaphthalen-2-yl)-4-oxobuten-2-oic acid

Summary The use of 4-(6-methylnaphthalen-2-yl)-4-oxobuten-2-oic acid as a fluorogenic reagent in pre-column derivatization for the high-performance liquid chromatography (HPLC) of biologically important thiols (L-cysteine, glutathione, N-acetylcysteine, homocysteine and mercaptopropionylglycine) was investigated. The aroylacrylic acid reacts selectively and rapidly (15 min. at room temperature) with the thiol compounds to give stable fluorescent adducts which can be separated by reversedphase HPLC and detected fluorometrically (_ex 300nm); _em 445nm). The experimental conditions for the thiol derivatization and chromatographic separation are discussed. Applications to the determination of N-acetylcysteine, mercaptopropionylglycine and cysteine are described.Presented in part at Bononiachem, XVI Congresso Nazionale di Chimica, Bologna, October 9–14, 1988.     

Observation of pairs of charmed particles produced by high-energy photons in nuclear emulsions coupled with a magnetic spectrometer

Preliminary results of an experiment using nuclear emulsions exposed to a tagged photon beam at the CERN SPS (experiment WA58) are presented. Six events showing pairs of charmed particles have been found. One of these has been interpreted as an associated production of Λ_c⁺ and $\text{D}$⁰. The mean lifetime of the neutral D meson based on three events is τ⁰ = (0.58_?0.2^+0.8 × 10^?13 s,, whereas a statistical analysis of eight charged charmed particles, assumed to be D mesons, leads to an estimate of the mean lifetime of τ^± ≈ 4.4 × 10^?13 s.