National Institute Of Materials Physics - Romania

The effect of a lateral electric field on the ground-state energy of an electron-hole pair confined in a quantum dot is investigated on different levels of sophistication. Two different regimes occur: a Coulomb dominated one, where the influence of the field is weak, and a regime dominated by field induced effects. Depending on the strength of the Coulomb interaction relative to the single-particle energies, which varies with the quantum dot parameters, the transition between these regimes is either smooth or rather abrupt.

The structure of the chalcogenide glass films of composition As2S3-GeS2 has been investigated bu X-ray diffraction. The relation between the quasi-distance as evidenced from the first sharp diffraction peak, and the composition ration has been estabilished. The Boolchand (intermediate) phase was found ro be characterized by large structuro-compositional fluctuations.

We present a theoretical study of the electronic transport through a many-level quantum dot driven by time-dependent signals applied at the contacts to the leads. If the barriers oscillate out of phase, the system operates like a turnstile pump under a finite constant bias, as observed in the experiments of Kouwenhoven [Phys. Rev. Lett. 67, 1626 (1991)]. The time-dependent currents and their averages over successive pumping periods are computed from the Keldysh formalism for tight-binding models. The calculation considers a sudden application of the pumping potentials at t=0, which leads to transient features of the time-dependent and averaged currents during the first pumping cycles which turn out to be important in the high-frequency regime. We show that in the transient regime, the efficiency of the system as a pump is rather poor because it mainly absorbs charge from both leads in order to fill the levels located below the bias window. Under a finite bias and a low-frequency pumping signal, the charge transferred across the system depends on the number of levels located within the bias window. The internal charge dynamics and the role of energy sidebands are investigated. The so-called satellite peaks of the averaged current are also observed in the transient regime.

In this paper, the trapping phenomena in silicon-based nanocrystalline semiconductors are studied. We propose a general and complete model for optical charging spectroscopy measurements, which takes into account the trapping-detrapping-retrapping processes and all the types of discharge currents that can occur in a nanocrystalline system. The model was applied to the measurements performed on multi-quantum well structures, (nc-Si/CaF2)(50), as well as on nanocrystalline porous silicon. This way, the trap parameters that are not directly measurable were determined. (C) 2007 Elsevier Ltd. All rights reserved.

Positron annihilation lifetime (PAL) measurements are performed for stoichiometric glassy arsenic sulphide g-AS(2)S(3) and selenide g-AS(2)Se(3) in order to probe the corresponding model of positron trapping. An ORTEC spectrometer with Na-22 source placed between two sandwiched glass samples was used in our experiments, the obtained spectra being treated with LT computer program. Two identical samples of g-A(2)S(3) and g-AS(2)Se(3) of about similar to 1.5 mm in thickness prepared from high-purity elemental constituents by conventional melt-quenching route and additionally annealed near glass transition to avoid the inner stressed regions were tested. It is shown that two-state positron trapping model is valid for both glasses, the process of positron trapping being saturated in extended free-volume defects like to atomic vacancies and their agglomerates. Despite difference in the character of nanovoid radius distribution estimated theoretically for g-AS(2)S(3) and g-AS(2)Se(3), they have very close average free volumes resulting in the observed similarity of PAL data. But, in contrast to g-AS(2)Se(3), which has at least three sets of nanovoids centered near similar to 1.5 angstrom, similar to 2.3 angstrom and similar to 2.9 angstrom, only two sets of nanovoids centered near similar to 1.3 angstrom and at similar to 2.8 angstrom are proper to g-AS(2)S(3) giving only one variant of fitting.

In this contribution, the correlation between fundamental interaction processes induced by radiation in silicon and observable effects which limit the use of silicon detectors in high-energy physics experiments is investigated in the frame of a phenomenological model which includes: generation of primary defects at irradiation, starting from elementary interactions in silicon; kinetics of defects, effects at the p-n junction detector level. The effects due to irradiating particles (pions, protons, neutrons), to their flux, to the anisotropy of the threshold energy in silicon, to the impurity concentrations and resistivity of the starting material are investigated as time, fluence and temperature dependences of detector characteristics. The expected degradation of the electrical parameters of detectors in the complex hadron background fields at LHC & SLHC is predicted. (C) 2007 Elsevier B.V. All rights reserved.

Epitaxial antiferroelectric/ferroelectric PbZrO3/PbZr0.8Ti0.2O3 multilayers were grown on SrRuO3-electroded SrTiO3(100) substrates by pulsed laser deposition. Polarization-field and switching current-voltage curves show a mixed antiferroelectric-ferroelectric behavior of the multilayers with an individual layer thickness above 10 nm, whereas below 10 nm the multilayers show only ferroelectric behavior. Clearly the PbZrO3 layers thinner than 10 nm experienced a transition into the ferroelectric state. X-ray diffraction reciprocal space mapping showed a corresponding orthorhombic-to-rhombohedral transition of the PbZrO3 layers. The observations are discussed in terms of the influence of strain.

The Bloch T-3/2 law is obtained for three-dimensional magnons at low temperatures. When decreasing the dimensionality of the magnetic system, the 3/2 exponent is expected to be equal to one for two-dimensional (2D) systems. I present a theoretical sketch of 2D spin wave theory, in which a long magnon wavelength cutoff is introduced, connected to the lateral size of the 2D system investigated. I report a study by X-ray magnetic circular dichroism at the Ni L-edges on the onset of ferromagnetic, order on 1.5 atomic Ni layers deposited on Cu(1 0 0), with spin and orbital moments' dependence with temperature. A linear dependence is found for both spin and orbital moments with the same initial slope. This temperature dependence can be used to probe the size of the 2D magnetic islands, which is particularly useful for embedded 2D objects. (C) 2007 Elsevier B.V. All rights reserved.

Nitrogen doping was proposed recently as a low-cost means to shift the absorption edge of titania towards 450-500 run. Results are reported here on the preparation of nitrogen-doped 250 nm thick TiO2 films on glass substrates by RF magnetron sputtering. The nitrogen content in the films was modified by changing the N-2 partial pressure in the discharge from 0.00 to 0.27 Pa. An additional amount of oxygen was added in the deposition chamber (Po-2 = 0.11 Pa) to compensate for oxygen depletion in the films. Surface composition and structure were derived from XPS and XRD data, respectively. XRD data revealed a mixture of amorphous phase and rutile polymorph in the as-deposited films, while an anatase-rich phase occurs in the samples deposited under N2 pressure of 0.05 and 0.16 Pa. Above 0.16 Pa, a rutile-rich phase is present. Surface wettability was investigated from contact angle data of de-ionized water with solid surface. The films deposited under mid-range N2 pressure in the discharge are super-hydrophilic. The photo-induced super-hydrophilicity decays after 36 h. Film wettability is discussed in terms of the synergic effects of surface roughness (measured by AFM) and rutile-anatase TiO2 phase transition, induced by doping. The TiO2 surface super-hydrophilicity (and catalytic efficiency) are related to beneficial properties of anatase phase (E-g = 3.2 eV), associated with a low recombination rate of charge carriers in the surface. (C) 2007 Elsevier B.V. All rights reserved.

The reactive sputtering technique was used to obtain undoped and Fe-doped TiO2 thin films deposited on glass substrates. At 250 degrees C substrate temperature, undoped TiO2 films crystallize in a mixed rutile/anatase phase, while Fe-doped films exhibit the rutile phase only. Presence of Fe3+ ions into the TiO2 lattice is suggested by the intensity variation of forbidden 1s -> 3d transitions between the Ti and Fe K-edges. Ti K-edge EXAFS data are assessed to a mixture of the two kinds of surroundings, a rutile-like crystalline phase, identified also by X-ray diffraction, and a nanosized or amorphous anatase-like surrounding. The local atomic order about Fe atoms is quite different and could be related also to an amorphous phase. The Swanepoel method is used to obtain the dispersion of the refractive index below the interband absorption edge. The dispersion energy, the single-oscillator energy and the coordination number of the Ti atoms are evaluated using the single-oscillator model (Wemple-DiDomenico). (C) 2007 Elsevier B.V. All rights reserved.