National Institute Of Materials Physics - Romania

We investigate the transport properties of quantum dots placed in a strong magnetic field using a quantum-mechanical approach based on the two-dimensional tight-binding Hamiltonian with direct Coulomb interaction and the Landauer-Buttiker formalism. The electronic transmittance and the Hall resistance show Coulomb oscillations and also prove multiple addition processes. We identify this feature as the "bunching" of electrons observed in recent experiments and give an elementary explanation in terms of spectral characteristics of the dot. The spatial distribution of the added electrons may distinguish between the edge and bulk states and it has specific features for bunched electrons. The dependence of the charging energy on the number of electrons is discussed for a strong magnetic field. The crossover from the tunneling to quantum Hall regime is analyzed in terms of dot-lead coupling.

We have studied the nonlinear current transport in Ga0.47In0.53As/Al0.48In0.52As quantum-well structures near filling factor 1. A strictly bistable switching between the nearly nondissipative quantum Hall conduction and dissipative conduction was observed at filling factors between 1 and 1.5 at the critical current. The steepness of the transitions was found to be sharper than 4 ppm (limited by the resolution of the setup) with respect to the filling factor. In contrast to thermal or density-distribution instabilities, which can lead to similar bistabilities at any integer filling factor, the effect observed in this study occurred exclusively around filling factor 1. Consequently, we attribute the switching to a feedback effect between the tunneling rate between different spin levels and the effective spin gap, which depends on the degree of spin polarization. The hysteresis of the switching is accompanied by a partially ferromagnetic memory of the spin polarization when sweeping the external magnetic field. We have confirmed this assumption by calculations applying a screened Hartree-Fock model. The calculated results are in good agreement with the data obtained experimentally for different orientations of the magnetic field. A possible conjunction of the effect with nuclear spin magnetization was excluded by investigating the response of the bistable switching to exposure to radiation at the frequencies of the corresponding nuclear magnetic resonances.

Not only (Ag-)(2) and Ag-(Na+) centres but also various Ag--Na+ aggregate centres have been produced in Ag+-,Na+- and Ca2+-codoped KCl crystals by unusual electrolytic colouration such as colouration at a relatively low temperature of 280 degreesC and a high electric field of 5000 V/cm. Absorption and magnetic circular dichroism spectra due to these centres have been investigated. (C) 2001 Elsevier Science B.V. All rights reserved.

Silver layers were grown in quartz crystals by electrodiffusion from an anodic layer of AgNO3. Different experimental arrangements were employed in order to obtain such layers in a reproducible way. We performed DC ionic conductivity measurements on Z-cut and X-cut samples as a function of temperature in the heating up phase and as a function of time at the electromigration temperature. The metal structures and the quartz samples were analyzed by electron microscopy and electron diffraction. Based on the experimental results and on the observations a model, for the metal structure formation is proposed. (C) 2001 Elsevier Science B.V. All rights reserved.

In this work, we present results of the synthesis and characterization of iron and iron oxide nanoparticles aggregated in filamentary, spider-web-like structures. The particles were produced in a flow reactor by CO2 laser pyrolysis of gaseous mixtures of iron pentacarbonyl and ethylene. Low- and high-resolution electron microscopy reveals chain-like structures of particles, most of them being composed of an a-iron core and an iron oxide shell, identified as magnetite and, to a lesser extent, hematite. These results are in good agreement with a Mossbauer analysis carried out for the same samples. The role of the reaction temperature on the synthesis of filamentary iron nanostructures by infrared laser pyrolysis of Fe(CO)(5)/C2H4 mixtures is discussed.

Intermetallics of the type NdCo9-xFexSi2Cy (x = 3; 5; 6; 7 and y = 0; 1) and Nd2Co15-xFexSi2Cy (x = 7; 9; 12 and y = 0; 1) were prepared by are melting in order to study the competing effects of Fe and Co substitutions on the magnetic hyperfine fields as well as the effects of interstitial carbon additions on the symmetry and local magnetic properties in these systems. The investigations were performed by X-ray diffraction, Mossbauer spectroscopy and hysteresis loop measurements. Both site and average hyperfine magnetic field values were analyzed in order to discriminate between the Fe and Co contributions to the local atomic environments in these systems. While a higher Fe content as well as a higher Co content can both lead to an increase in the site magnetic fields, it was found that the model based on hyperfine field distributions provides a better description of the trends observed in hyperfine magnetic fields in these compounds. (C) 2001 Elsevier Science Ltd. All rights reserved.

Spray frozen, freeze dried nitrate powder with composition Bi:Pb:Sr:Ca:Cu 1.7:0.3:2.0:2.5:3.5 has been thermally decomposed (up to 870 degreesC). In situ neutron diffraction investigations during heating and cooling of the powder have been performed at DN2 instrument from the IBR2 reactor at JINR, Dubna, Russia. Phase formation and reformation and domains of stability for different phases were observed and discussed.

Dendritic structures were grown inside quartz crystals by electrodiffusion of sodium ions from a NaCl layer at the anode. An experimental arrangement using a point cathode and a graphite plate anode was employed. X-ray diffraction experiments were carried out, the structures chemical composition was found to be a mixture of sodium silicates. The chemical composition evolution in time was observed. There were made VIS and UV optical absorption spectroscopy measurements of these structures, a colloidal band being found. Thermoluminescence glow curves were measured. Based on the experimental results and observations a growth model is proposed.

An experimental study was made of the mechanical and electromechanical nonlinearities of a PZT trimorph type actuator. The actuator consisted of two piezoceramic poled plates, sandwiched on a thin metallic sheet, forming the usual trimorph structure, with one end fixed and the other one free. The experiments were conducted in two steps. In the first step a force was applied to the free end of the trimorph and its displacement was measured as a function of the applied force either for increasing and decreasing force. The nonlinearities in this case must be pure mechanical and they give information about the structural arrangement of the piezoceramic elements, glue and metallic sheet. For the second step the actuator was electrically activated with increasing and decreasing DC voltages and the displacement of the free end was determined as a function of applied DC voltage. The nonlinearities in this case are of electromechanical nature and they were the result of the combined effect of intrinsic material nonlinearities and structural arrangement. The C, experimental results showed that pure mechanical nonlinearities are practically inexistent even for forces up to 0.7 N. The displacement of the free end follows the theoretical predictions. When the actuator was electrically activated the displacement of the free end was entirely hysteretic and the electromechanical nonlinearities were different from the theoretical predictions. The results were discussed in terms of the displacement of ferroelastic-ferroelectric domain walls.

The structure of Fe/GaN/Fe ferromagnetic electrodes is studied by high resolution transmission electron microscopy. The layers grow epitaxially on the GaAs substrate with the top Fe layer 90degrees rotated compared to the bottom one. The interfaces are quite rough. There is an indication of the possible occurrence of Fe3GaAs formation on the GaAs interface.