Publications

A unifying structural scheme of all layered superconducting cuprates is proposed. The paper is a review based on a three level correlational analysis of the behaviour of one compound, of a homologous series and a third interseries analysis, revealing multiple correlation between the critical temperature value and different bond lengths via the oxygen content.

Silver nano-dots have been deposited by a special procedure on the surface of a YAG coated silicon wafer. Sizes of 40-100 nm and a self-organized distribution according to the pattern of the pattern of defects (dislocations, terraces, etc.) on the wafer's surface were revealed. The procedure is simple and can be applied for the controlled deposition of small silver clusters to be used both in medicine and in nano-electronic devices.

InN thin films were grown by reactive magnetron sputtering in a UHV system, using Ar and N-2 as working gases. Electrical and optical investigations of the films were correlated with their structural and morphological characteristics in order to obtain good quality films for THz applications.

The 4G mobile communication systems are expected to work within the 3-6 GHz range. The development, in the last years, of wide band gap (WGB) semiconductor technologies has opened the perspective of manufacturing SAW (surface acoustic wave) and FBAR (film bulk acoustic resonator) devices for application in the GHz frequency range. SAW type structures with a resonance frequency of about 2.8 GHz and 3.2 GHz have been successfully developed on a thin AlN layer sputtered on high resistivity silicon. Nanolithography was used to manufacture the interdigitated transducer. FBAR resonators have been manufactured using micromachining techniques of GaN/Si and AlN/Si wafers. FBAR structures based on self sustainable AlN and GaN membranes with a thickness lower then 0.5 mu m have been obtained. Resonance frequencies of about 4.8 GHz have been obtained for GaN based FBAR structures and about 10GHz for AlN based FBAR structures.

PbSe nanowires were prepared using the template method. Polycarbonate ion track membranes with pore sizes ranging between 50 nm and 1.5 mu m were used as templates. The deposition of PbSe in the pores of the template was performed electrochemically. The influence of the deposition potential and membrane porosity on the morphology and composition of the prepared nanowires was studied. The composition of the wires has been determined by EDX measurements for different potentials. Scanning electron microscopy revealed the morphology of the deposited nanowires. X ray diffraction has been used for structural measurements.

We observed two dissipation peaks in the field cooled AC susceptibility measurements on aligned multilayered cuprate Superconductor, (Cu,C)Ba2Ca2Cu3Oy [(Cu,C)-1223], and demonstrated that it is the direct experimental indication of the two-component vortex matter in the multilayered cuprates. We propose that, the second loss peak at lower temperature is due to the additional degree of freedom of the rotation of 'vortex molecule' composed of two fractional vortices, originated at two components, mediated by an i-soliton bond. To probe the dynamics of this vortex molecule, we measured frequency dependence of AC susceptibility response on aligned crystallites of (Cu,C)-1223 at different temperatures and DC fields. In second peak region, it gives resonance peak. The observed frequency dependence patterns of chi ''(T), for 0.5 T H-DC, are rescaled with the resonating frequencies and show tail at low frequency region indicating that the dissipation is due to rotation and twisting of vortex molecule. The temperature dependence of the average relaxation time shows that the vortex molecule rotation/twisting glass state follow critical slowing down process.

The spectral characteristics of Er3+ in Sc2O3 transparent ceramics are presented. The Er3+ visible spectra in the ceramics, prepared by solid-state synthesis, show only lines that could be assigned to C-2 sites. The Stark energy levels, cross-sections and emission kinetics of levels involved in the green upconversion emission under infrared excitation in the 800 nm range were determined. The green upconversion emission (H-2(11/2), S-4(3/2)) -> I-4(15/2) characteristics under excitation with a pulsed Ti:sapphire laser in (I-4(15/2)-> I-4(9/2)) absorption were analyzed. The emission kinetics reveal a competition of two-step (I-4(15/2)-> I-4(9/2)) + (I-4(9/2)-> H-2(9/2)) excited state absorption (ESA) and (I-4(11/2), I-4(15/2)) -> (I-4(11/2), F-4(7/2)) energy transfer upconversion (ETU) processes function on the excitation wavelength. (c) 2007 Elsevier B.V. All rights reserved.

Field-assisted-sintering technique (FAST) has been employed to obtain polycrystalline MgB2 samples, pristine and doped with 5 mol % SiC and B4C, using commercial MgB2 powder. The mass density of the samples is around 90% of the theoretical value. The response in magnetic field shows that the upper critical field and the irreversibility field are depressed by comparison with pristine MgB2. The latter has an upper critical field H-c2=14.37 T close to the values reported for the single crystals. Scaling analysis of the field dependence of the pinning force suggests that both addition of SiC and B4C stabilizes two well defined pinning regimes in FAST-processed MgB2. At low temperatures, T < 24 K (H <= 5T), the pinning occurs at the grain boundaries whereas at higher temperatures there is a mixed pinning. These effects are supposed to be the combined result of relatively low level of C diffusion into the lattice of MgB2, specific features in formation of the grain boundaries during FAST processing and the influence of the added materials on morphology. At high fields (above 8.5T) and 4.2K, doped samples are superior to the pristine one from the critical current density viewpoint.

The electric, ferroelectric and photoelectric properties of Pb(Zr,Ti)O-3-Nb:SrTiO3 (PZT-STON) junctions were investigated on a broad range of temperatures. It was found that the hysteresis loop is strongly asymmetric, due to the asymmetry in the leakage current. The capacitance-voltage characteristic has a butterfly shape although it is also asymmetric because of the different nucleation and compensation conditions at the two interfaces. The junction shows a strong photovoltaic effect in the 200500 run wavelength range as short-circuit currents Of the order of nA for an illuminated surface of 0.018 mm(2). This makes the PZT-STON junctions attractive for UV optoelectronic applications.

Materials may show anisotropic properties on different crystal directions and this is also the case of High Temperature Superconductors (HTS). To take advantage of the materials anisotropy one concept of interest is "orientation engineering" in thin films. This can be realized through the control of the film-substrate lattice relationship. Some examples in this regard are presented in this work and through comparative analysis we try to evaluate the viability of this approach and of he entcountered problems. It is expected that in the future this approach will generate new nano composite materials with new properties and effects leading to development of new devices with new or improved functionality.