Publications

We calculate the ground state of a two-dimensional electron gas in a shea-period lateral potential in magnetic field, with the Coulomb electron-electron interaction included in the Hartree-Fock approximation. For a sufficiently short period the dominant Coulomb effects are determined by the exchange interaction. We find numerical solutions of the self-consistent equations that have hysteresis properties when the magnetic field is tilted and increased, such that the perpendicular component is always constant. This behavior is a result of the interplay of the exchange interaction with the energy dispersion and the spin splitting. We suggest that hysteresis effects of this type could be observable in magnetotransport and magnetization experiments on quantum-wire and quantum-dot superlattices.

A far-from-equilibrium preparation route and using a particular combination of starting materials led us to obtain nonstoichiometric Y(Fe,V)(12) and its interstitial carbide. Structural analysis and EDAX results reveal the formation of the main 1:12 phase with the ThMn12-type structure, with 13-15% vacancies at the 3d metal sites. Insertion of interstitial carbon increases the T-C and magnetization of the 1:12 phase, whereas the unit cell volume remains unchanged. The effects of vacancies and interstitial carbon on the Mossbauer hyperfine fields and isomer shifts are discussed with reference to the calculated quantities. (C) 2000 Elsevier Science S.A. All rights reserved.

The morphology and structure of a La0.6Y0.07Ca0.33MnO3 thin film is studied by transmission electron microscopy. The film was grown by pulsed laser deposition (PLD) on an MgO-buffered silicon wafer. The as-deposited films are polycrystalline and show a columnar morphology. We present a detailed high-resolution transmission electron microscopy structural study concerning a possible monoclinic distortion of the orthorhombic LYCMO crystal cell. (Received February 22, 2000; accepted March 10, 2000).

Elastic recoil detection analysis (ERDA) has been applied to determine the composition of carbon nitride and hydrogenated carbon nitride films prepared by hollow cathode discharge process. The films were deposited on Si (111) substrates. The films were also characterized by Fourier transform infrared spectroscopy (FTIR). The influence of the main deposition parameters (substrate bias voltage, nitrogen injection mode) on the film properties was investigated. (C) 1000 Elsevier Science B.V. All rights reserved.

Carbon nitride thin films (CNx) were produced by UV laser (ArF, lambda=193 nm) induced chemical vapour deposition from C2H2/NH3 mixtures, on Si and TiN/Si substrates. The surface and bulk composition of the deposited layers were investigated by X-ray photoelectron spectrometry (XPS) and elastic recoil detection analysis (ERDA), respectively. Nitrogen was found to be singly and doubly bonded to carbon, as revealed by FTIR spectrometry. Specific morphology of the films was evidenced by transmission electron microscopy (TEM), while selected area electron diffraction (SAED) confirms the presence of nano-crystallites in the layers. The calculated d-lattice spacings fit rather well with theoretical and experimental data for alpha- and beta-C3N4. (C) 2000 Elsevier Science S.A. All rights reserved.

Photostimulated luminescence (PSL) and diffuse reflectance measurements on X-ray irradiated BaFCl:Eu2+ microcrystalline powders were performed. We have investigated the nature of radiation induced defects involved in the photostimulation effect and its recombination mechanism. The experimental data sustain that X-ray irradiation gives rise to (F(Cl-) - O-F(-) - EU2+) photostimulable complexes and we have proposed several geometrical configurations of these complexes. (Received January 27, 2000; accepted February 16, 2000).

The formation mechanisms of nanocrystalline composite structures are discussed, and results on co-deposited material system of C-60-Au, C-60-Cu as well as Al-SiOx are presented. Three type of structures are distinguished in the composite films, depending on the concentration of the additive. The decisive structure-forming phenomena are the kinetic segregation of the minority species and the chemical reaction of the deposited components. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Photodarkening relaxation under light exposure of a-As2Se3 amorphous films doped with 0.5 at.% tin, manganese, samarium and dysposium, separately introduced, was studied in dependence on the impurity type and thermal annealing. Both factors reduce photodarkening and the degree of reduction depends on the type of impurity. The relaxation process may be described by a stretched exponential with the dispersive parameter 0.5<alpha<l and time constant increasing by thermal annealing. The results are discussed in the frame of recently proposed "slip-motion" model of photodarkening in chalcogenide glasses. (Received February 11, 2000; accepted February 29, 2000).

This work is a comparative study of the structural and magneto-transport properties of La0.60Y0.07Ca0.33MnO3-delta (LYCMO) as bulk and thin film. The bulk samples were prepared by solid-state reaction between the corresponding metallic oxides mixed in stoichiometric ratios. The thin film was deposited by pulsed laser deposition on an MgO single crystal using an excimer laser. We show that the structure and stoichiometry of the bulk target are perfectly reproduced in the thin film. We measured the magnetoresistive effect on both the LYCMO pellet and the thin film by using the four-probe technique. The maximum of the MR effect is 680% on the polycrystalline thin film in a 2T magnetic field. (C) 2000 Published by Elsevier Science B.V. All rights reserved.