National Institute Of Materials Physics - Romania

Publications

894 articles found

841. Non steady-state thermoelectric conduction

Authors: Apostol, M; Nedelcu, M

Published: MAR 2001, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 3, 132, DOI:

It is shown that the figure of merit of a thermoelectric couple can be appreciably improved by using a pulsed current transferred from the thermoelectric circuit. In the frame of this theory un unusual drop of temperature in a thermoelectric circuit is possible. An ultrafast process of thermoelectric conduction is described, whose increased performance is realized by minimizing the effects of thermal dissipation.

842. Coulomb effects on the transport properties of quantum dots in a strong magnetic field

Authors: Moldoveanu, V; Aldea, A; Manolescu, A; Nita, M

Published: JAN 15 2001, PHYSICAL REVIEW B, 63, DOI: 10.1103/PhysRevB.63.045301

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.

843. Kinetics of silver structures growth by electrodiffusion in quartz crystals

Authors: Enculescu, I; Iliescu, B; Teodorescu, V

Published: JAN 2001, SOLID STATE IONICS, 138, 321, DOI: 10.1016/S0167-2738(00)00794-3

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.

844. Chemical composition of structures obtained inside quartz crystals by sodium electrodiffusion

Authors: Iliescu, B; Enculescu, I; Pera, I; Alexe, G; Polosan, S; Stanculescu, A

Published: 2001, CRYSTAL RESEARCH AND TECHNOLOGY, 36, 410, DOI: 10.1002/1521-4079(200106)36:4/5<403::AID-CRAT403>3.0.CO;2-H

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.

845. Magnetization and level statistics at the quantum Hall liquid-insulator transition in the lattice model

Authors: Nita, M; Aldea, A; Zittartz, J

Published: DEC 15 2000, PHYSICAL REVIEW B, 62, 15370, DOI: 10.1103/PhysRevB.62.15367

Statistics of level spacing and magnetization are studied for the phase diagram of the integer quantum Hall effect in a two-dimensional finite lattice model with Anderson disorder.

846. The Pb(2+)(Li) and Pb(2+)(Na) centres in KCl crystals

Authors: Tsuboi, T; Polosan, S; Topa, V

Published: FEB 2000, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 217, 980, DOI: 10.1002/(SICI)1521-3951(200002)217:2<975::AID-PSSB975>3.0.CO;2-U

Emission, absorption and magnetic circular dichroism spectra have been studied on Li(+)- and Pb(2+)-codoped KCl crystals. Two emission bands (one is the A(T)-like emission and the other the A(X)-like emission) have been observed at 350 and 453 nm at room temperature, respectively These emissions have been observed by excitation in the absorption bands with peaks at 270 and 281 nm. These emission bands are attributed to the Pb(2+)(Li) centre, where a Pb(2+) ion has a Li(+) ion at one of the six nearest neighbouring cation sites. Measurements have been also made on Na(+)- and Pb(2+)-codopcd KCl crystals. Unlike the Pb(2+)(Li) centre, only the A(T)-like emission band with a peak at 347 nm due to the Pb(2+)(Na) centre has been observed. It is suggested that such a difference is due to a difference in the position of the impurity alkali ion substituted for the K(+) ion.

847. Optical absorption of Ag- centres in KCl: line shape calculation

Authors: Zota, S; Enculescu, M

Published: FEB 2000, PHYSICA B, 275, 343, DOI: 10.1016/S0921-4526(99)00762-0

The line shapes of the absorption spectra of KCl: Ag- are calculated at several temperatures in the range 80-300 K and are compared with the experimental results. The complete 12 x 12 Hamiltonian matrix of the system, which includes the linear electron-lattice interaction, was diagonalized directly and the integration has been performed by the Gaussian quadrature method. The results explain most of the characteristic features experimentally observed. (C) 2000 Elsevier Science B.V. All rights reserved.

848. Properties of metal clusters embedded in KCl matrix

Authors: Polosan, S; Apostol, E; Topa, V

Published: 2000, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 2, 642, DOI:

The characterization of nanocrystals embedded in alkali halide crystals concerning shape and size distribution has been performed. Optical absorption, transmission electron microscopy (TEM), x-ray diffraction (XRD) and electron spin resonance (RES) methods has been used. The experimental results are in good agreement with the theoretical considerations concerning absorption and scattering of light by the metal clusters.

849. DLA type metal structures in quartz crystals

Authors: Iliescu, B; Enculescu, I; Klapper, H; Stamatin, I

Published: MAY 1999, EUROPEAN PHYSICAL JOURNAL-APPLIED PHYSICS, 6, 150, DOI: 10.1051/epjap:1999164

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.

850. Silver nanocrystals obtained by the ionization of Ag--ions in KCl

Authors: Vasile, E; Datcu, M; Polosan, S; Apostol, E; Topa, V

Published: MAR 1999, JOURNAL OF CRYSTAL GROWTH, 198, 810, DOI: 10.1016/S0022-0248(98)01112-9

This paper discusses the formation of "corals" arranged silver nanocrystals in a KCl matrix starting from Ag- ions ( similar to 10(17) ions/cm(3)) by subsequent thermal annealing at temperatures between 400-700 degrees C and 250 degrees C. In the last case only, by illumination with total radiation of a 100W Xe-lamp, spherical separated silver clusters were obtained. The stepwise annealing processes were followed by the recording of the optical absorption spectra at 20 degrees C. The sizes of silver clusters were determined through transmission electron microscopy. (C) 1999 Published by Elsevier Science B.V. All rights reserved.