National Institute Of Materials Physics - Romania

An EPR study, of the paramagnetic centers formed by irradiation of polycrystalline K2Cr2O7 and BaCr2O7 was made, In the EPR spectra of the gamma irradiated powder samples, at room temperature, the presence of three types of paramagnetic centers iv as observed. According to their EPR parameters and thermal behaviour. these centers were attributed to the Cr(O)over dot(4)(3-), Cr-2(O)over dot(7)(3-) and Cr(O)over dot(3)(-) radicals respectively,. An adequate mechanism radiolysis has been established.

Trapping levels which govern the vertical transport in (nc-Si/CaF2)(n) multi-quantum wells were investigated using two different techniques: (a) the temperature dependence of the dark current and (b) the thermally stimulated depolarization current technique (TSDC). Measurements by the first technique showed that the conduction mechanism was thermally activated above 200 K with activation energies of 0.35-0.7 eV, These activation energies were found to increase with increasing electric field. TSDC measurements showed also the existence of at least one broad peak above 200 K with estimated activation energies in the range of 0.4-0.45 eV. Analysis of the peak by the fractional heating method showed a continuous distribution of defect states from 0.3 to 0.83 eV. (C) 2001 Elsevier Science B.V. All rights reserved.

Recent studies of mechanically alloyed Fe-Cu powder mixtures have suggested differences in the local magnetic environment of iron atoms. For a more accurate definition of this point, ball-milled Cu70Fe30 and Cu50Fe50 alloys were investigated by Mossbauer spectroscopy in the, temperature range 4.2-300 K. The low temperature Mossbauer spectra exhibit a broad magnetic pattern. typical of a defect structural configuration. The magnetic splitting strongly decreases with increasing temperature, especially in the case of Cu70Fe30 alloy. But even for this composition there is, at room temperature, an unresolved magnetic pattern. Applying a magnetic field of 3 T, parallel to gamma rays, at 4.2 K a rotation of all magnetic moments along the external field is observed. The samples behave as an alloy with continuously distributed local fields. (C) 2001 Elsevier Science BY. All rights reserved.

The high resolution spectroscopic and emission decay properties of Nd:YAG ceramics with up to 9 at.% Nd show that no major changes from the single crystals other than those induced by the increased Nd concentration occur and indicate that these materials can be used for construction of efficient and high power lasers. The laser emission of a 3.8 at.% Nd ceramic laser component under hot band resonant pump at 885 nm with a Ti:sapphire laser is demonstrated. (C) 2001 Elsevier Science B.V. All rights reserved.

The magnetic interactions between the nanograins in heterogeneous granular Cu-(SmCo5)-Fe ribbons were evaluated with a classical mean field model to obtain the exchange coupling as well as the magnetic moment per nanoparticle. The samples were subjected to various thermal treatments and a significant magnetoresistive effect was obtained when the optimal annealing conditions had been achieved. (C) 2001 Elsevier Science B.V. All rights reserved.

We consider semiconductor devices composed of a small quantum structure as the active device region and two classical environments constituting the source- and the drain contact. The contacts are taken as free electron gases with infinite conductivity defining the chemical potentials in the contacts. The transport through the quantum structure is described in the Landauer-Buttiker formalism using electronic scattering wave functions which determine the electron density in the quantum system. In our Hartree approximation these charges and the induced charges in the contacts are the sources of the self-consistent Coulomb field. As a particular quantum structure we study a GaAs heterostructure device consisting of a two-dimensional electron gas sandwiched between a gate contact and an AlGaAs blocking barrier [see V.T. Dolgopolov ct al., Phys. Low-Dim. Struct. 6 (1996) 1]. We demonstrate the quantitative agreement of our theory with the experimental results. (C) 2001 Elsevier Science B.V. All rights reserved.

The Letter presents an attempt to include the concentration dependence in the Landau-de Gennes theory for the nematic to isotropic phase transition for nematic lyotropic liquid crystals by introducing a critical concentration for the lyotropic solutions. This allows to describe phase transitions of micellar solutions as depending on temperature and concentration. An estimation of phenomenological coefficients of the CsPFO/water system is made, and we obtained the trends of the order parameter curves in function of temperature and concentration in good agreement with experimental results from literature. (C) 2001 Elsevier Science B.V. All rights reserved.

Polycrystalline diamond thick films were subjected to annealing in vacuum at temperatures of 1350-1450 degreesC. The films were examined by optical absorption, Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. The formation of amorphous carbon and/or of well-crystallized graphite layers up to 20 nm thick was evidenced along grain boundaries. Intra-granular nanometer-sized graphite islands were also observed, sometimes as transformed micro-twin bands. The diamond-to-graphite transition occurs in such a way that three (111) diamond planes transform into two (0002) graphitic sheets. The internal graphitization causes a severe degradation of the optical quality of the diamond films.

The CO consumption of tin dioxide, thick film sensors and their sensor signals have been investigated in order to study the GO-water interaction on the SnO2 surface. The mechanism of CO combustion, and the influence of the sensor's operation temperature (200-400 degreesC) and ambient humidity (0-50% r.h.) on the combustion has been characterised in detail. Here, the overall CO consumption of the sensors has been divided according to the contributions of the various individual sensor parts: alumina substrate; electrodes: heater; and the SnO2 based sensitive layer. It can be shown that CO combustion is a linear function of CO concentration. The influence of the substrate on total CO consumption has been proven to be significant. (C) 2001 Elsevier Science B.V. All rights reserved.

An Fe+-type center with axial (001) symmetry has been studied by electron nuclear double resonance (ENDOR) in the microwave X and Q bands. The Fe+(I) center is produced only after irradiation with x or gamma rays at 80 K of SrCl2:Fe2+ crystals grown in a Cl-2 atmosphere. It has a 3d(7) F-4 Gamma (8) ground state with spin S =3/2. As shown by the correlated analysis of the ENDOR data and electron paramagnetic resonance superhyperfine structure, the Fe+ ion is situated in the center of a tetragonally compressed cube of eight nearest Cl- ions. The simplest model of the Fe+(I) center, compatible with the magnetic resonance results, consists of an interstitial Fe+ ion with two substitutional monovalent cations, very likely K+ ions, in two opposite nearest-neighbor Sr2+ sites along the tetragonal axis of the center.