Publications

Multilayer Cr/CrN/Cr/Cr(N,C) and Cr/CrN with 8 and 32 layer coatings were deposited on austenite substrates using pulsed laser deposition (PLD) technique. The microstructure observations were performed using Philips CM20 (TM), TECNAI G(2) F20 - TWIN (TM) and JEOL EX4000 (TM) transmission microscopes. The performed experiments indicated that lowering the argon flow from 60 to 30 cm(3)/s during chromium ablation changes buffer layers microstructure from nearly amorphous to nano-crystalline. The nitride or carbo-nitride layers turned out to be less sensitive to changes in nitrogen flow during deposition. The columnar microstructure of Cr layers is coarser than those in CrN ones under the same deposition condition. This observation proved also that relying on PLD technique as thin as 30 nm layers might be formed within multilayer Cr/CrN coatings.

The dependence of the driven-current through a tunnel junction on the relative orientation of the magnetization in the electrodes deviates from the Julliere formula when the current is resonant through impurity levels. For asymmetric coupling of the impurities to the electrodes, the magnetoresistance can even be negative on the resonance. Such inversion in sign is due to the fact that not only the density of states in the electrodes is important (for both orientations of spins), but also the density of states at the impurity position, which has a specific behavior. We find the exact conditions under which the negative magnetoresistance occurs, as function of both polarization and coupling asymmetry. The resonant magnetoresistance may have single or double-dip aspect, depending on the mentioned parameters.

Shallow defect levels in floating zone (FZ) and diffusion oxygenated FZ (DOFZ) silicon, before and after irradiation with a Co-60 gamma-source up to 300 Mrad, have been studied by thermally stimulated currents (TSC) and deep level transient spectroscopy (DLTS) in the temperature range 4.2-110 K. Besides vacancy oxygen (VO) and interstitial-substitutional carbon (CiCs) emissions, several TSC peaks have been observed. A trap with an activation energy of 11 meV has been observed at 6 K only in irradiated DOFZ. Two hole traps at 80 meV and 95 meV have been observed both in irradiated FZ and DOFZ, while a trap at 100 meV, related to an interstitial-oxygen (IO2) complex, has been revealed only in irradiated DOFZ. A TSC peak close to 24 K has been resolved into two components, whose concentrations are independent of irradiation fluence: a trap at 55 meV and a level which remains charged after emission at 80 meV. Our measurements confirm the formation, only in DOFZ, of a radiation induced donor at 230 meV. It appears to be responsible for the improved radiation hardness of oxygenated Si together with the suppression of deep acceptors, since no shallower radiation-induced donors have been detected in DOFZ samples.

The Mn2+ EPR spectra of 12 marbles and limestones (calcitic, dolomitic and calci-dolomitic) collected from different Greek quarries have been analysed by means of cluster analyses numerical taxonomy. By means of Mn2+ EPR line shape this technique allowed classifying all samples in concordance with their mineralogical structure. Accordingly, we have found that while Euclidean and power distances showed to be more adequate in differentiating samples belonging to the same petrologic group, the Perason's correlation coefficient is most suitable to discriminate samples having different mineralogical composition.

We report two ion beam analysis techniques, elastic recoil detection analysis (ERDA) and Rutherford backscattering (RBS), to characterize YBa2CU3O7-x (YBCO) films, obtained in situ by pulsed laser deposition (PLD). Initially, ERDA measurements were performed on a thin film to evaluate the PLD rate. RBS measurements correlated with complementary scanning electron microscopy and X-ray diffraction measurements were performed afterwards on a good quality thick YBCO film to determine its stoichiometry, thickness, crystalline structure and surface morphology. (c) 2006 Elsevier B.V. All rights reserved.

Nonlinear optical borate crystals of Gd1-xScxCa4O(BO3)(3) and Gd1-xLuxCa4O(BO3)(3) have been grown and characterized. By changing the composition of these crystals, their optical birefringence can be controlled to obtain second-harmonic generations (SHG) in the ranges of 790-824 mn along the Y axis and 920-963 nm along the Z axis through noncritical phase matching (NUM). The refractive indices of grown crystals were measured with a minimum-deviation technique. The experimental results of NCPM conditions for SHG are in good agreement with the theoretical predictions. (c) 2006 Optical Society of America.

In the paper, the physico-chemical orientation mechanism on solid surfaces for a liquid crystal (NP5 from Merck) doped for homeotropic alignment, is tested. For that, the dispersion and polar parts of surface tension of the liquid crystal are determined. Using substrates with and without adsorbed surfactant, the dispersion and polar parts of the surface tension of the substrates were also measured. Comparison of liquid and solid surface tensions, involved in the physico-chemical mechanism, failed to explain the difference of the molecular alignment for the two substrates. In the presence of the surfactant adsorbed on the surface, the steric model probably gives the explanation for the homeotropic alignment.

The principal obstacle to long-time operation of silicon detectors at the highest energies in the next generation of experiments arises from bulk displacement damage which causes significant degradation of their macroscopic properties. The analysis of the behaviour of silicon detectors after irradiation conduces to a good or reasonable agreement between theoretical calculations and experimental data for the time evolution of the leakage current and effective carrier concentration after lepton and gamma irradiation and large discrepancies after hadron irradiation and this in conditions where a reasonable agreement is obtained between experimental and calculated concentrations of complex defects. In this paper, we argue that the main discrepancies could be solved naturally considering as primary defects the self-interstitials, classical vacancies and the new predicted fourfold coordinated silicon pseudo-vacancy defects. This new defect is supposed to be introduced uniformly in the bulk during irradiation, has deep energy level(s) in the gap and it is stable in time. Considering the mechanisms of production of defects and their kinetics, it was possible to determine indirectly the characteristics of the Si-FFCD defect: energy level in the band gap and cross-section for minority carrier capture. In the frame of the model, the effects of primary defects on the degradation of silicon detectors are important in conditions of continuous long-time irradiation and/or high fluences.

Amorphous SnSe2 films were prepared by pulsed laser deposition (PLD) and pulsed electron deposition (PED) from solid polycrystalline targets. The atomic scale structure has been revealed by X-ray diffraction. Hardness properties and electrical properties were measured. The peculiarities of the structures of the films prepared by PLD and PED methods were investigated discussed.