National Institute Of Materials Physics - Romania

Theoretical studies of the coherent electronic transport in a system of coupled quantum wires show that switching on the conducting channel in one wire can be manifested in the other coupled wires. Interference processes and electronic correlations are taken into account in our studies on the same footing. The conductance changes depend on the interference conditions of a transmitted wave with that one reflected from the wires that indirectly influence the transport. We show that electronic correlations lead to a dynamical Coulomb blockade effect, which changes the conductance response quantitatively but its shape is still kept the same. Our results are discussed in correspondence with an experiment recently performed by Morimoto [Appl. Phys. Lett. 82, 3952 (2003)] on a system of coupled quantum wires.

Surface enhanced Raman scattering (SERS) performed with 676.4 and 1064 nm excitations were used to investigate single-walled carbon nanotubes (SWNTs) thin films prepared from platelets obtained by nonhydrostatic compression at 0.58 GPa. SWNTs were compressed alone or dispersed into chemical reactive and non-reactive host matrices. SERS spectra indicate that by compression, SWNTs break into fragments of different sizes, which in turn can react or not with the host matrix. In inorganic hosts (KI, Ag) donor-acceptor complexes are formed. The appearance of short fragments of SWNTs with a closed-shell fullerenes behaviour is revealed in SERS spectra. This typical signature appears in the Raman spectrum as a line at ca. 1460 cm(-1) associated with a pentagonal pinch mode. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fabrication by colloidal chemistry and an extended structural and magnetic investigation are reported for AgCo bimetallic colloidal nanoparticles using X-ray diffraction, transmission electron microscopy and superconducting quantum interference device magnetometry. The AgCo nanoparticles exhibit a core-shell structure, with a face-centred cubic Ag core and a hexagonal close-packed Co (either complete or partial) shell. A bimodal size distribution of superparamagnetic (SPNI) nanoparticles together with a small fraction of nanoparticles ferrornagnetic at room temperature has been determined and their influence on magnetic properties is discussed. The arrays of self-assembled nanoparticles exhibit a lack of saturation of magnetisation and typical SPM behaviour. The annealing of the arrays greatly enhances the ferromagnetic character of the samples. Finally, the observed magnetic behaviour of the AgCo nanoparticles is correlated with surface spin disorder induced by the particular structural features of the sample, high fraction of magnetic atoms in surface states and finite-size effects. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Using the on-set of third-harmonic susceptibility we have measured vortex melting lines of high-pressure synthesized (Cu0.6C0.4)Ba2Ca3Cu4Oy [(Cu,C):1234] and (Cu0.5C0.5)Ba2Ca2Cu3Oy [(Cu,C):1223] with preferentially oriented crystallites and having various carrier concentrations. Vortex melting lines of all (Cu,C):1223 samples and of optimum-doped (Cu,C):1234 were very well described by the commonly accepted theory of melting lines within the two-fluid model. Overdoped (Cu,C):1234 proved to have an anomalous melting line, that was phenomenologically explained by the rather special gaps evolution, that is the significant opening of a second superconducting gap due to CuO2 outer planes at a temperature lower than the critical one, where the first superconducting gap due to inner planes opens. The experimental melting lines of overdoped (Cu,C):1234 were modelled theoretically by assuming an empirical formula for the temperature dependence of anisotropy factor.

Nonalloyed ohmic contacts with lateral dimensions in the sub-100 nm range have been processed and characterized. The contacts are suitable for the fabrication of GaAs/AlGaAs nanocolumns designed in a 'top down' approach. They are realized on n-type GaAs using a thin low-temperature grown GaAs cap layer and Ti/Au metallization. For the lateral patterning of the nanoscaled ohmic contacts, electron-beam lithography based on Hydrogen Silsesquioxan (HSQ) negative resist is used, followed by sputtering and plasma etching of the metals. The I-V characteristics show ohmic contact behavior and demonstrate the scaling of the proposed contacts down to 50 nm lateral dimension. The specific contact resistance is situated in the range of 1-2 x 10(-5) Omega cm(2). (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report spectroscopic characterization of epitaxial YBCO thin films grown on LaAlO3 by pulsed laser deposition. Raman spectroscopy and spectroscopic ellipsometry were used for film characterization and the results were correlated with X-ray diffraction measurements. The mentioned techniques allowed us to analyze crystallographic, micro-structural, and morphological properties of YBCO thin films. We also demonstrated that relatively low resolution Raman spectroscopy and spectroscopic ellipsometry are reliable techniques for a rapid and non-destructive characterization of epitaxial YBCO thin films. (c) 2006 Elsevier B.V All rights reserved.

Pure and L-alanine doped Triglycine sulphate (TGS) crystals were grown in paraelectric phase (similar to 52 degrees C). Doped crystals show unequal growth rates along the ferroelectric axis. Pure TGS crystals show peculiar dielectric behavior in the ferroelectric phase, after crossing up and down the Curie point in two successive runs between room temperature and 80 degrees C. Much higher and unstable permittivity was found returning in the ferroelectric phase. At constant temperature (35 degrees C), permittivity follows a relaxation process, characterized by two relaxation times. L-Alanine doped TGS crystal shows more than one order of magnitude smaller permittivity and dielectric losses. Internal bias field of similar to 1 kV/cm, induced by the dopant, made the crystal almost monodomain and pined polarization in one direction. Pyroelectric coefficient measurements were performed at constant heating rate of the samples, using a computer controlled He cryostat and Keithley 6517 electrometer. The temperature dependence of P+ polarization component, obtained by computer integration of the pyroelectric coefficient, was measured on a large temperature interval (-20/ +80 degrees C). Pyroelectric coefficient of the doped samples was also measured by the same procedure, using a dc bias electric field, pointing in the opposite direction to the pined polarization. The polarization could be reversed, on the whole temperature range, by dc fields higher than bias or coercive field. Surprisingly, for the first time, the pyroelectric coefficient (p) was found constant on quite large temperature intervals. Doped TGS crystals show much smaller values of permittivity epsilon(r) versus the pure one and consequently, get higher figure of merit M = p/epsilon(r). The pyroelectric coefficient of this material can be tailored to become constant on a defined temperature range, under a dc field control. This characteristic makes this material valuable to be used as pyroelectric material for IR devices. (c) 2006 Elsevier B.V. All rights reserved.

The (Zr0.8Sn0.2)TiO4 material (ZST), has been prepared by solid state reaction and characterized. The samples were sintered in the temperature range of 1260-1320 degrees C for 2 h. The effects of sintering parameters like sintering temperature (T-s) and MgO addition (0.2 wt.%) on structural and dielectric properties were investigated. Bulk density increases from 4900 to 5050 kg/m(3) with the increase of sintering temperature. The effect of MgO addition is to lower the sintering temperature in order to obtain well sintered samples with high value of bulk density. The material exhibits a dielectric constant epsilon(r) similar to 37 and high values of the Q x f product, greater than 45,000, at microwave frequencies. The dielectric properties make the ZST material very attractive for microwave applications such as dielectric resonators, filters, dielectric antennas, substrates for hybrid microwave integrated circuits, etc. (c) 2006 Elsevier B.V. All rights reserved.

Solid solutions Ba-1 (-) xSrxTiO3 (BST) are of high technological importance, particularly in microwave domain. Barium titanate has "naturally" three transitions, between four stable ferroelectric phases: (C) cubic, (T) tetragonal, (0) orthorhombic, (R) rhombohedral. Jaffe et al. [B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, 1971] has given the dependence of the transition temperatures up to 30% of Sr content. We have extrapolated these temperatures and we have found that some phases might disappear at higher Sr concentrations. A family of solid solutions with x = 25, 50, 75, 90% was prepared by standard solid-state reaction and sintered at 1230 and 1260 degrees C, respectively. The permitivities and the dielectric losses were measured with a self-acting bridge (1 kHz), on a large temperature range (+/- 200 degrees C). The composition x = 25% shows three peak values of permittivity as expected, while the composition x = 50%, only two peak values, corresponding to phase transitions cubic-tetragonal-rhombohedral, phase orthorhombic being excluded. Compositions with x >= 63%, Sr shows only one peak value corresponding to a genuine transition cubic-rhombohedral. The cubic transition to several lower phases shows almost a linear decrease of the Curie point with the increase of Sr fraction. For Sr concentration x >= 80%, the Curie point appears to fall more rapidly than linear. To our best knowledge, there is for the first time, this effect is reported. (c) 2006 Elsevier B.V. All rights reserved.

To eliminate precipitates-segregates that can easily occur on the thin film surfaces of the multicomponent materials for electronics, a new approach is proposed, consisting of the following aspects: first, on the substrates, artificial steps of predefined height and width are produced, and second, films are grown on such substrates. The width of the step is taken equal to the 'double of the migration length' of the atomic species depositing on the substrate. In these conditions, precipitates migrate and gather at the step edges where the free energy is lowest and the resulting totally precipitate-free surface of the film on the step is suitable for device applications or integration purposes. The method has several other important advantages and they are discussed in the text. Using this new approach we present successful fabrication of a mesa structure showing intrinsic Josephson effect. We have used thin films of Bi-2212/Bi-2223 superstructure grown by MOCVD on (001) SrTiO3 single crystal substrates with artificial steps of about 20 mu m width. (c) 2006 Elsevier B.V. All rights reserved.