Publications

The morphological changes induced by the thermal treatments of YAG: Eu crystals synthesized by a sol-gel method have investigated using the Mossbauer and optical techniques. Both methods exhibit a drastic change of the spectral parameters at amorphous - garnet phase transition, followed by a slow evolution of these parameters with the increase of the annealing temperature. This slow evolution is due to the increase in size of the nanocrystals.

Accelerate ageing of lead zirconate-titanate ferroceramics (PZT) by fast neutron irradiation is investigated. The ferroelectric behavior of Pb1-wMw(Zr,Ti)O-3, where w < 0.5 wt%, M = Li, Nb, Cr, Bi, Mn, Sb, La for Zr/Ti ratio = 51.5/48.5, prepared by solid state reaction, is strongly affected by fast neutron irradiation at Phi(i) = (10(13) divided by 10(18)) n/cm(2) integrated fluxes. Structural investigations (XRD and SEM) reveal modifications of the microstructure, porosity and elementary cell parameters. High irradiation levels induce tetragonal-cubic at the same time with a ferro-paraelectric transition. The micrographs show the evolution of 90 degrees and 180 degrees walls as well as herringbone structure typical for tetragonal distortions with irradiation level. A relation between macroscopic characteristics (piezoelectric and dielectric) and microscopic properties (crystallographic parameters, porous structure and grain morphology) as function of irradiation doses can be established using a model based on microstructural parameters. These results offer opportunities to understand the fundamentals of radiation effects upon piezoelectric ceramics. (c) 2007 Elsevier Ltd. All rights reserved.

Motivated by the pionieering experiments of Buks et al. [Nature 391, 871 (1998)] we investigate the visibility of the Fano effect in a single-dot Aharonov-Bohm interferometer which is Coulomb-coupled to a nearby quantum dot. The latter acts as a 'Which Path Detector' and is coupled to two leads on which a finite bias is applied. Using the non-equilibrium Keldysh-Green function formalism we compute the currents through the detector and the interferometer. We take into account the first two contributions to the interaction self energy and emphasize the correction to the Landauer formula which appears beyond the single-particle approximation. Particular attention is given to the coherence properties of the interferometer in the presence of the electron-electron interaction between the embedded dot and the detector. We show that when the detector is subjected to a finite bias the amplitude of Aharonov-Bohm oscillations of the current through the interferometer decreases. The Fano line is in turn rather stable under interaction. Our results generalize an earlier work of Silva and Levit [Phys. Rev. B 63, 201309 (2001)] and complement the existing description of the controlled dephasing. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this paper a study of some microwave microstrip bandpass filters (BPF) using defected ground structures (DGS) is presented. This technique allows designs of tight couplings without the necessity of using very narrow coupling gaps. Based on the results of the study, four-pole cross-coupled planar microwave bandpass filters were designed, with a single of. with two ground slots. Compared to similar microstrip filters without defected ground, the simulated performances of these novel structures indicate some technological advantages.

In multilayer structures apertures in metallic sheets can be used as coupling elements between resonators located on different layers, allowing very compact filter designs. In this paper, some novel types of planar microwave bandpass filters, using aperture couplings, in structures with three metallic layers, are considered. Couplings between two miniaturized resonators are investigated, by full electromagnetic field simulation. Based on the results of this study, several models of planar microwave bandpass filters, with a single or with two apertures, are designed, verified by simulation, fabricated and measured. Compared to the classical filters, these novel structures show a good compactness and can offer some technological advantages.

This paper presents some of the more relevant results concerning the mass-loading influence on the characteristics (effective mass-loading, motional inductance, quality factor) of AT-cut quartz, Y-cut langasite resonators and AT-cut gallium phosphate.

Electrical properties of new Polymer-Iron nanocomposite materials obtained by laser pyrolysis were investigated, in order to find applications for the detection of reducing gases. The aim of this paper was to prove the necessity of complex experimental investigation in order to point out the contribution of different types of adsorption. Usually resistance measurement cannot fully describe the surface interactions. The electron affinity changes are of real interest for in-field conditions which always involve humidity of surrounding gas-atmosphere. On the other hand, the nanocomposite materials can be a solution for specific gas-sensing properties.

PbZrO3 precursor powders were prepared from lead acetate and zirconium n-butoxide in n-butanol and hydrolysed with R-w (molar ratio of water/lead acetate) = 2 and 15 at different pH values. Upon heating the organic groups were removed in two steps for the precursor prepared at R-w = 2, and in one step at R-w = 15. The perovskite phase crystallizes at approximately 700 degrees C, irrespective of the hydrolysis conditions. (c) 2007 Elsevier Ltd. All rights reserved.

This paper presents experiments to develop SAW structures with operating frequencies in the GHz range. The experiments were developed on AlN and GaN thin films with nano-metric lines for the IDT. The nanolithography was performed at IMT with the new purchased VEGA 5136 LM SEM from TESCAN and ELPHY Plus EBL from Raith GmbH. There were obtained some very good quality inerdigitated Ti/Au structures (with 30 digits and 29 interdigits 300nm wide, 220nm high and and 200 mu m long). Lift-off techniques to remove the undesired metal were used On wafer microwave measurements of the SAW structure (performed at IMT on the new purchased Anritsu vector analyzer equipped with Suss Microtec probes), have demonstrated its functionality. It was evidenced a pronounced resonance at about 2.8GHz.

A theoretical study of resonant tunneling in multilayered heterostructures is presented based on an exact solution of the Schrodinger equation under the application of a constant electric field and a uniform magnetic field. By use of the transfer matrix approach, the transmissivity of the structure is determined as a function of the incident electron energy. The results show good agreement with other existing models as well as with the bound-state energies. The solutions obtained are exact and can be generated easily by using MathCad. Based on these calculations, a new class of resonant tunneling superlattice devices can be designed.