Publications

The small-signal intrinsic impedance of GaAs/AlAs based resonant tunnelling diodes with thin barriers has been measured at room temperature over the full 0-2V bias-voltage and 0.05-40.05GHz frequency ranges, on stable, non-oscillating devices. The classical Esaki and the quantum-inductance equivalent circuits were used to model the impedance for CAD purposes. Information about the quasibound-state lifetime against bias-voltage was extracted.

Au-substituted Al-Cu-Fe icosahedral phases, Al62Cu25.5-xAuxFe12.5 (x less than or equal to 7) were prepared by melt spinning. EDS data showed that Cu as well as Al atoms are effectively substituted for with Au. The hypercubic lattice parameter of as-quenched samples slowly increases with x, while phonon-and phason-type disorders are not affected by Au substitution. Experimental structure factors for 'sum' lines were used to derive an average radius of the atomic hypersurfaces. Comparison of experimental versus calculated ratios of structure factors for different models yielded information about the Au distribution over the icosahedral sites. It was inferred that a fraction of the gold shapes its Al surrounding in a AuAl2-type configuration. Evidence was found for d-sp hybridization of Au-Al bonds, on the basis of the 'white-line' area at the Au L-3 absorption edge.

A fast numerical algorithm is presented for calculating the TDS correction to Bragg peaks recorded in time-of-flight neutron diffraction studies on single crystals. The algorithm allows two average sound velocities, together with the structural parameters, to be treated as free parameters in a refinement program. The correction does not require, therefore, a prior knowledge of the elastic constants of the sample. The model is tested on a simulated set of reflection data.

In usual chemoresistive SnO2 gas sensors, space charge accumulation at the electrodes and at intergranular contacts can contribute in the hystereses, irreproducibilities and time variations in electrical conductance which hinder the operation of these devices as transducers. For a thorough investigation of the effects of electrical polarisation, samples with highly asymmetric electrode contacts, in which polarisation is strong and which behave like diodes were used. On the other hand, we tentatively found the operation principle of a gas sensitive SnO2 device which avoids polarisation phenomena. (C) 1997 Elsevier Science S.A.

We present an accurate calculation of the intensity of the second harmonic radiation generated by the reflection of the incident radiation from a metal surface, This radiation is due to two currents, The first current results from the rapid variation of the normal component of the oscillating electric field in the vicinity of the surface and depends on both conduction and bound electrons, The second one results from the action of the Lorentz force on conduction electrons, We show that if the adiabatic and the Thomas-Fermi assumptions are considered, then an accurate calculation of these currents is possible and leads to a simple relation of the intensity of the second harmonic radiation.

The embedding of a enantioselective homogeneous catalyst - the chiral manganese(III) cationic complex of salen type (Jacobsen complex I) - into the pores of mesoporous substituted silicates is investigated. The host/guest interaction is studied by different methods: TG-DTA, UV-VIS, and FTIR. The results are consistent with a location of the large Mn complex inside the mesopores. Catalytic tests in olefin epoxidation prove the fully maintenance of the catalytic activity and stereoselectivity of the complex after embedding. This promising result indicates that MCM-like materials might have perspectives as supports for the heterogenization of homogeneous catalysts. (C) 1997 Published by Elsevier Science B.V.

A lead sulphide/lead titanate/silicon (PbS/PbTiO3/Si) heterostructure was manufactured by successive deposition of PbTiO3 and PbS thin film on a single crystalline, p-type Si substrate. Chemical methods were used for deposition. A three electrode configuration was used to control the photoconductivity of PbS thin film by field effect, through the ferroelectric PbTiO3 thin film. The spectral distribution of the photoconductive signal shows two maxima, situated at 1.1 and 2.45 mu m. It was found that the photoconductive signal at 1 mu m varies of about eight times when the voltage applied on the Si substrate is varied between -1 and +1 V. At 2 mu m the photoconductive signal is almost independent of the applied voltage on the Si substrate. The observed results are explained considering a ferroelectric field effect by which the photogenerated carriers in Si influence the photoconductive signal in the PbS thin film. (C) 1997 American Institute of Physics.

The optimal resonant tunneling, or the complete tunneling transparence of a biased double-barrier resonant-tunneling (DBRT) structure, is discussed. It is shown that its physics does not rest on the departure from the constant potential within the barriers and well, due to the applied electric field, but on the effective symmetry of the rectangular-barrier profile, which approximates the real potential profile for the corresponding applied bias.

Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) have been used to evidence the occurrence, morphology and microcomposition of the superconducting phases (Bi,Pb)(2)Sr2Ca2Cu3O10+delta (2223) and (Bi,Pb)(2)Sr2CaCu2O8+delta (2212), and of other non-superconducting phases, in the sintered pellets obtained from nitrate solution by spray frozen, freeze drying technique. For decomposition of the nitrate powder four different heat treatments were used. Superconducting and structure properties of the pellets have been tested by AC susceptibility measurements (610 Hz, 0.5 Oe) and X-ray diffraction analysis, respectively. A correlation between the SEM and EDS observations and the superconducting properties has been established. (C) 1997 Elsevier Science B.V.

Electron spin resonance investigations on ion beam irradiated polymers are reviewed from a critical point of view. The nature of free radicals produced by heavy ion bombardment is discussed in detail, in connection with the main hypotheses (thermal spikes and secondary electrons). ESR investigations on cellulose nitrate irradiated with accelerated oxygen ions are reported. From the temperature dependence of resonance line parameters, the presence of exchange interaction is noticed and the generation of free radicals at 200 degrees C is studied. (C) 1997 Elsevier Science B.V.