National Institute Of Materials Physics - Romania

Capacitance Deep Level Transient Spectroscopy (C-DLTS) measurements have been performed on standard and oxygen-doped silicon detectors manufactured from high-resistivity n-type float zone material with <111> and < 100> orientation. Three different oxygen concentrations were achieved by the so-called diffusion oxygenated float zone (DOFZ) process initiated by the CERN-RD48 (ROSE) collaboration. Before the irradiation a material characterization has been performed. In contrast to radiation damage by neutrons or high-energy charged hadrons, were the bulk damage is dominated by a mixture of clusters and point defects, the bulk damage caused by Co-60-gamma-radiation is only due to the introduction of point defects. The dominant electrically active defects which have been detected after Co-60-gamma-irradiation by C-DLTS are the electron traps VOi, CiCs, V-2(=/-), V-2(-/0) and the hole trap CiOi. The main difference between standard and oxygenated silicon at low dose values can be seen in the introduction rate of CiCs compared to CiOi. For highly oxygenated silicon the introduction of CiCs is fully suppressed, while the sum of the introduction rates g(CiCs)+g(CiOi) is independent on the oxygen concentration. (C) 2003 Elsevier B.V. All rights reserved.

We present results obtained on carbon nanotube systems by means of Raman scattering and surface enhanced Raman scattering (SERS). Such systems consist of single-walled nanotubes (SWNTs), multi-walled nanotubes (MWNTs), or polymer/nanotubes composites. In particular, we have focused our studies on the interactions taking place in these compounds which lead to an upshift of the radial breathing mode (RBM) in the case of SWNTs, either when they are arranged in bundles or embedded in polymers. Similar calculations has allowed us to interpret the low frequency Raman modes observed in purified MWNTs. We show that they come from the RBM of individual tubes. From intensity calculations performed to simulate Raman spectra, we demonstrate the importance of two experimental parameters, namely the diameter of internal tubes and the number of shells which constitute the MWNTs, such parameters being extracted experimentally from high resolution transmission electron microscopy images. (C) 2003 Elsevier Science B.V. All rights reserved.

The glassy composition As25Te35Si40 was investigated by microhardness, differential scanning calorimetry, X-ray diffraction, electrical conductivity and Raman spectroscopy measurements. A continuous random network model of the glass structure has been developed. (C) 2003 Elsevier B.V. All rights reserved.

Polymerization-depolymerization processes induced by bandgap light irradiation with alternating high and low intensity light control the nano-scale extension of disordered layer-like clusters in arsenic rich chalcogenides. These processes resulting in a fractal morphology of the clusters were simulated in the frame of a Monte-Carlo-Metropolis method. The fractal model for photo-darkening and photo-bleaching phenomena in arsenic chalcogenides is discussed. (C) 2003 Elsevier B.V. All rights reserved.

The effect of rare earth (Dy and Sm) and transition metal (Mn) luminescent impurities on the optical properties of As2S3 glasses is studied in a wide spectral region. The Raman, IR, band-to-band and edge absorption spectroscopy are employed to obtain information about the incorporation of impurity ions in the host glass structure and the corresponding changes in intrinsic optical characteristics. The effect of metal impurity on both extended and localized electronic states is revealed. (C) 2003 Elsevier B.V. All rights reserved.

From ac susceptibility measurements of the [Bi(Pb)](2)Ca2Sr2Cu3Oz (Bi-2223) samples doped with LiCl, we have found and report for the first time anomalously suppressed superconductivity (T, is anomalously decreasing) in the slightly under-doped region. For our BPSCCO/(LiCl)(y) samples this region is situated around y = 0.07. Hole concentration (p) per number of Cu-O planes of the Bi-2223 unit cell, determined from room temperature thermopower (S-300 K) measurements, is approximately 0.15 when y is within anomalous region. Literature data shows that for the La-based cuprates, similar suppression of superconductivity was observed at p similar to 0.12 (so-called "1/8 problem"). This discrepancy would appear because charge ordering phenomenon (probably induced by Li known as a pair breaker as e.g. Zn) may be accompanied by changes in oxygen content or oxygen ordering effects. (C) 2003 Elsevier B.V. All rights reserved.

Those nanocrystalline materials which consist of ferromagnetic nanograins embedded in a ferromagnetic matrix were modelled as a cubic lattice composed of a central sphere with strongly coupled spins surrounded by weakly coupled spins. The magnetic behaviour was studied by Monte Carlo simulation, especially the low temperature spin ordering and features exhibited in the temperature range between the Curie temperatures of the two phases. The magnetization and magnetic susceptibility are calculated as a function of temperature for different values of the interfacial exchange interactions. It is shown that the exchange coupling between matrix and nanograin influences the magnetic properties of the nanograin, the matrix and the interfacial regions differently. The magnetic behaviour of different regions has been explained in terms of a polarization mechanism acting on the surface and in the core, leading to magnetic correlation between the spins. Such features are quite consistent with the experimental results obtained on nanocrystalline alloys.

Thermal analysis measurements were applied to study the behavior of composite systems containing a nematic liquid crystal (LC), 4-n-octyl-4'-cyanobiphenyl (8CB) and aerosil particles. The silica density rho(s) (g of silica per 1 cm(3) of LC) was varied from 0.20 to 12 g/cm(3) in order to approach the monomolecular structure of the surface layer. Thus. thermogravimetric (TG) measurements were performed to get information on the LC content of the samples. Moreover, they allowed an unusual application, to characterize the interaction of 8CB molecules with aerosil surface because the corresponding curves show multistep processes the temperatures of which depend on the silica density. Differential thermal analysis (DTA) also indicates several processes. Some of them, mostly due to desorption, are endothermal. Other processes are exothermal, and these are assigned to chemical transformations. The sample with silica density of 12 g/cm(3) has no endothermal desorption peak at low temperatures, and has the highest shift for the DTA peaks. It was supposed that its surface layer has a monomolecular structure. For representative samples, TG-DTA Studies were supported by FTIR investigations. Thus conventional TG-DTA techniques have provided new tools to characterize the nanocomposites. Differential scanning calorimetry gives the behavior of the phase transitions. Samples with low silica density exhibit a depression of the phase transition temperatures characteristic for the bulk 8CB while those with high silica densities do not show any phase transitions. (C) 2003 Elsevier B.V. All rights reserved.

The nanocrystallization of two soft magnetic amorphous Finemet-type ribbons of the nominal composition Fe73.5Cu1Nb3Si13.5B9 and Fe73.5Cu1Gd1Nb2Si13.5B9 synthesized by a melt spinning technique is reported. The crystallization process was monitored by differential scanning calorimetry (DSC) that allows identification of the primary crystallization temperature as well as the other exothermic events occurring during crystallization process. The resulting phase structure of the properly annealed samples is investigated by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM) and is shown to consist of size-distributed body-centred cubic (bcc) alpha-Fe(Si) (rather than Fe3Si, DO3-type), nanograins, embedded into the amorphous Nb-rich residual matrix. The degree of dispersion of the nanograins in the amorphous matrix is estimated from the nanocrystalline volume fraction and the mean size of the nanograins obtained both from XRD data and TEM images. (C) 2003 Elsevier B.V. All rights reserved.