National Institute Of Materials Physics - Romania

The effect of low amounts of dysprosium, samarium and manganese (0.1 - 0.5 at.%) on the properties of the chalcogenide glassy semiconductors As2S3 and As2Se3 is investigated The metals have significant influence on the structural, optical, electrical and photoelectric characteristics. The fundamental absorption edge shifts towards longer wavelengths with doping. The largest shift was observed for As2S3 glasses doped with Dy and Mn. The conductivity is significantly increased by addition of Mn and Dy. All the dopants hamper the photoconductivity with the exception of 0.5 at. % Dy in As2Se3 which generates a broad secondary band in the photoconductivity spectrum near 1.05 eV, ascribed to small amounts of crystalline phase. Close correlation was revealed between the medium range order in the network, the metal electronegativity and the activation energy of conductivity.

Structural and magnetic properties of melt-spun Cu-Co-(Fe-Si) ribbons, as-quenched and after appropriate annealing, are investigated and correlated with their magnetoresistive properties. The effect of structural refinement of the magnetic phases and of the coexistence of multiple magnetic phases on the magnetoresistance is discussed. The existence of an optimum in the annealing conditions for obtaining a significant magnetoresistive effect, is inferred. (C) 1999 Elsevier Science B.V. All rights reserved.

A reactive DC magnetron discharge produced in a TIN deposition system was investigated using optical emission spectroscopy. The influence of He, Ne and Kr addition into an Ar + N-2 working atmosphere was investigated. Addition of small percentages of He or Ne in the working atmosphere of Ar + N-2, caused a decrease of the N-2(+) (B)/N-2(C) ratio, whle with Kr addition the ratio increased. The known decrease of the relative intensity of Ti lines for N-2 addition was also observed for He addition, while it increases when Ne and Kr are added. XRD analysis of TiN thin films revealed that He addition determines the same effect as N-2. This behavior is explained by N and N+ density increase for He addition. (C) 1999 Elsevier Science Ltd. All rights reserved.

We consider the class of Hamiltonians composed by a quadratic form in the canonical operators and a linear interaction term with time periodic external fields of an arbitrary strength. The Hamiltonian can be made time independent by a unitary transformation in the extended Hilbert space, which has the meaning of a time-dependent translation in the classical phase space. We solve the eigenvalue problem for the Floquet Hamiltonian and we find general expressions for the time-dependent wave functions, of interest for the optical and transport properties of quantum nanostructures.

Cyclic voltammetry and impedance spectroscopy were used for study of polyaniline in acid solutions. Impedance spectra were presented both as Bode diagrams and Nyquist representations. The capacitive charging behaviour and anion (ClO4-, HSO4-) insertion/elimination reaction of the polymer film were discussed on the basis of a simplified Randles circuit model. The examined scanning electron micrographs have also shown that the nature of dopant anion changes the film morphology.

Metal structures of diffusion limited aggregation type were obtained by controlled electrodifussion in quartz crystal without macroscopic damages, Conditions of temperature, applied voltage and the form, shape and dimension of electrodes were found, in order to obtain fractal branches. Similarity with fractal DLA structures are described and discussed.

The frustration of the trinuclear clusters with antiferromagnetic intracluster interaction was investigated at low temperature and in applied magnetic field, by Mossbauer, susceptibility and magnetisation measurements. The spin configuration and the electronic state are described as a function of the temperature and the intensity of the applied field. The values of the exchange integrals are provided. (C) 1999 Elsevier Science B.V. All rights reserved.

After being dispersed in liquid solvents (Ethanol, CHCl3), conducting polymers and singlewalled nanotubes have been deposited on ruguous substrats (Au, Ag, Cu). With FT Raman technique (laser excitation : 1064 nm), we have obtained SERS signal of both conductiong polymers and singlewalled nanotubes. The Raman signals change with substrat and/or solvent, which could be correlated with the size of clusters formed at the surface and then to the deposition mechanism. The main advantage of the >SERS technique will be emphasized in terms of enhancement as a powerful technique to study nanoparticles.

Melt-spun Cu80Co10Fe10 ribbons are shown to exhibit a nanogranular structure with magnetic alpha-Fe(Co) grains segregated from the Cu-rich matrix after appropriate annealing. The Cu matrix purification as well as the magnetic properties of the alpha-Fe(Co) grains existing in a range of sizes are studied using X-ray diffraction, Mossbauer spectrometry and magnetic measurements. There is an optimum annealing for obtaining a high magnetoresistive effect. (C) 1999 Elsevier Science B.V. All rights reserved.

Two sets of p-type silicon thigh resistivity bulk and low resistivity epitaxial) samples and one set of n(+)-p junctions have been irradiated with fast neutrons up to 8 x 10(13) cm(-2). I-V and C-V characteristics as well as Thermally Stimulated Currents (TSC) and Hall Effect (HE) analyses have been performed on the irradiated samples and diodes in view to determine the radiation-induced damage and the change in the electrical properties. A change in the effective carrier concentration and in the leakage current after irradiation similar to the one found for p(+)-n detectors has been observed in p-type diodes. An increase with the fluence of the resistivity and Hall coefficient was measured at room temperature both for the low and high resistivity sets. This evidence has been explained in terms of a two-level model taking into account a linear increase in concentration with the fluence of the main radiation-induced defects observed with TSC, probably related to divacancy and carbon-oxygen complex. (C) 1999 Elsevier Science B.V. All rights reserved.