Publications

Opposite to the well-known statistical limit, we study a semiconductor quantum system with only one relevant resonance which is in contact with a probe acting as a particle reservoir. We find that the quasibound state that exists in the nearly closed system develops at a transition to the open system into a separate type of resonance. In contrast to the quasibound state (i) it is localized in the space between the probe and the isolated quantum system, (ii) its energy lies in the classically allowed regime, and (iii) its line shape is strongly asymmetric. Excellent quantitative agreement shows that this transition is seen in capacitance experiments on MIS (metal-insulator-semiconductor)-type semiconductor heterostructures in which a field-induced two-dimensional electron gas is formed.

The magnetic properties of a ferromagnetic cylinder containing an antiphase boundary with antiferromagnetic interactions are studied using atomic Monte Carlo simulation. The approach to saturation shows a reduced magnetization compared with the completely ferromagnetic sample, even in huge applied fields. The magnetization profiles are studied as a function of the number of ferromagnetic planes in the cylinder with 80 atoms per plane and also as function of the ratio of the bulk-to-antiphase exchange coupling. Hysteresis appears after removing the applied field for relatively low antiphase boundary densities. It is shown that the antiferromagnetic coupling across the antiphase boundary leads to a progressive rotation of spins as the two half "domain walls" at the boundary with opposite chirality narrow with increasing applied field. (C) 2002 American Institute of Physics.

The effect of surface anisotropy on the magnetic ground state of a ferromagnetic nanoparticle is investigated using atomic Monte Carlo simulation for spheres of radius R=6a and R=15a, where a is the interatomic spacing. It is found that the competition between surface and bulk magnetocrystalline anisotropy imposes a "throttled" spin structure where the spins of outer shells tend to orient normal to the surface while the core spins remain parallel to each other. For large values of surface anisotropy, the spins in sufficiently small particles become radially oriented either inward or outward in a "hedgehog" configuration with no net magnetization. Implications for FePt nanoparticles are discussed. (C) 2002 American Institute of Physics.

From frequency-dependent ac susceptibility studies of (Cu,T1)BaSrCa2Cu3Oy superconducting thin films, with and without nanodot-induced artificial pinning centers, we estimated the activation energy of flux jumps. The result was that, in the film with nanodots, the pinning potential is several times higher, leading to a probability of thermally activated flux jumps several orders of magnitude lower than in the film without artificial pinning centers. We suggest that our no cost straightforward method for creating extended defects can be successfully employed for the reduction of thermal noise in superconducting electronic devices. (C) 2002 American Institute of Physics.

Lead sulfide (PbS) thin films were deposited from a chemical bath onto SiO2/Si (n-type) substrates. Pseudo-metal-oxide-semiconductor devices were obtained by evaporating source and drain gold electrodes on a PbS surface and aluminum gate electrode on a Si substrate. Field-effect-assisted photoconductivity in the PbS layer was investigated at room temperature, in the 800-2700-nm-wavelength domain for different values and polarities of the drain and gate voltages. The best results were obtained for a positive gate, when both semiconductors are in depletion. An enhancement of about 25% of the photoconductive signal is obtained compared with the case when the gate electrode is absent or is not used. A simple model is proposed that explains the behavior of the dark current and photoconductive signal in PbS film with changing the gate voltage. (C) 2002 American Institute of Physics.

The low-temperature behavior of the Co-doped magnetite is studied. The Fe3-xCoxO4 powders (x = 0.01 - 1) have been prepared by the hydrothermal method. In order to investigate the cobalt influence on the structure of magnetite, transmission Mossbauer spectroscopy measurements were performed at temperatures close and below the Verwey transition temperature characteristic for pure magnetite (122 and 77 K, respectively). All spectra were fitted with five components and related to nonequivalent Fe2+ and Fe3+ sites. The hyperfine parameters of these sites as well as their relative fractions in the powder samples were determined. (C) 2002 Elsevier Science B.V. All rights reserved.

Creation of Ag and Ag-0 centers by irradiation with ionizing radiations has been investigated using NaCl and KCl crystals containing various concentrations of Ag+ ions and using Ca2+-codoped NaCl:Ag+ and KCI:Ag+ crystals. From the absorption spectra, it is observed that Ag- center perturbed by Ca2+ ion is also created besides the Ag- and Ag-0 centers. It is confirmed that Ag- and Ag-0 centers increase with increasing the concentration of doped Ag- ions. It is suggested that there are two kinds of Ag(c)(0)v(a) centers (i.e. Ag-0 accompanied by vacancy at the nearest neighbor) with different site symmetries depending on the location of vacancy. (C) 2002 Published by Elsevier Science B.V.

We studied by means of optical absorption and transmission electron microscopy the formation of Ag nanoclusters from Ag--ions in three different potassium halides (KCl, KI and KBr). By stepwise UV illumination at a suitable temperature, the electrons of Ag--ions were detached and the controlled formation and growth of Ag nanoclusters from Ago-atoms take place. At the same time, the detached electrons are captured by K+-ions, proved by the appearance of K-colloid bands. The kinetics of the conversion Ag- + hv + U --> Ag nanoclusters was followed by monitoring the Ag--ions and Ag nanoclusters absorption. The temperatures of illumination are different, depending on the matrix in which Ag nanoclusters are formed (220 degreesC for KCl, 160 degreesC for KBr and 140 degreesC for KI). By optical absorption and TEM we proved that the nanoclusters of Ag obtained by our method are separated, spherical and ovoidal, with average diameters between 7.5 and 50 nm. (C) 2002 Elsevier Science B.V. All rights reserved.

We found Oxygen-doped GaAs crystals to be suitable materials for CO2 laser optical component preparation, with application at 10.6 mum. An optical transmission of 55% in the IR spectrum range, between 2 and 15 mum has been reached for such a GaAs type material. The GaAs crystals that we have analysed were grown by two procedures: Horizontal Bridgman (HB) and Liquid Encapsulated Czochralski (LEC). The FIB method has been used for obtaining pure (undoped) crystals, while the oxygen-doped GaAs ingots were grown by LEC technique. The two types of samples processed in the same manner as regards mechanical polishing and chemical etching, which were investigated by Hall measurements, optical transmission spectrometry and elastic recoil detection analysis (ERDA) technique. The GaAs:O (LEC) has near semi-insulating properties as can be observed from the results of the electrical resistivity and Hall effect measurements. The ERDA spectrum shows an intense signal of oxygen in the bulk of GaAs:O (LEC) crystals, while the oxygen signal is not present in the ERDA spectrum of the undoped GaAs (HB). We consider that these results could recommend the ERDA technique as a possible qualitative and quantitative analysis in an ion-beam accelerator for oxygen content in oxygen-doped GaAs crystals. The analysis is not sensitive to the native oxide, as could be seen by measuring GaAs (HB) undoped crystals. (C) 2002 Elsevier Science B.V. All rights reserved.