Publications

In this paper we report modifications of the surface enhanced Raman scattering (SERS) spectra with film thickness of carbon single-walled nanotubes (SWNTs). The decrease of film thickness reveals an increased degree of disorder, which is interpreted in terms of degradation of the single-walled nanotubes and the formation of particles similar to highly oriented pyrolytic graphite (HOPG), C-60 and amorphous carbon. The degradation of the nanotubes, dependent on the metal substrate type and its morphology, is explained on the basis of interactions between the nanotubes and the metal substrate. The D band, which is an indicator of the degree of disorder, increases its intensity when the film thickness is decreased. SERS spectra recorded at different excitation wavelengths reveal that metallic nanotubes are primarily affected. A mutual correlation was found between the variations in the relative intensity of the D band and the Breit-Wigner-Fano component due to metallic tubes displayed by Raman bands situated in the 1500-1600 cm(-1) range. A direct interaction between C-60 and nanotubes is revealed by comparative SERS spectra on SWNTs and (SWNTs+C-60) films. At lambda(exc)=676.4 nm the narrowing in the lower energy side of the broad band associated to tangential vibration modes indicates that the metallic tubes are involved in the formation of a [(SWNT)C-+(60)-] complex type.

In the present work, the bulk degradation of SiC in hadron (pion and proton) fields, in the energy range between 100 MeV and 10 GeV. is characterised theoretically by means of the concentration of primary defects per unit fluence. The results are compared to the similar ones corresponding to diamond, silicon and GaAs. (C) 2001 Elsevier Science B.V. All rights reserved.

A diode-pumped Yb:YAG laser with a novel edge-pumped composite Yb:YAG microchip laser has been demonstrated. At room temperature, continuous-wave generation with 50% slope efficiency in absorbed pump power is demonstrated from a 0.5 mm diameter hexagonal core of 2 at% Yb3+ ion doped YAG. Under quasi-continuous wave pumping with 2.4 Hz repetition rate and 2.5% duty cycle, laser emission with 42% slope efficiency and 41 W output peak power at 220 W input pump power was obtained from a microchip with 10 at% Yb doped YAG level in the core. The highest pulse energy of 1.5 J resulted under 17.5 J pumping pulse energy.

Superconducting samples with a starting composition Cu0.5TlxBa2Ca3Cu4O12 (x = 1-0.35) were prepared in Au tubes from oxides by using different heating rates. The optimum heating rate was 0.11 degreesC min(-1) applied between 860 and 880 degreesC. These samples show the highest content of 1234 phase and the lowest of Ba-Cu-O phases as well as the highest critical temperatures. The highest values were T-c = 119.2 K and T-c0 = 115.1 K. All samples show J(c) values around 6 x 10(5) A cm(-2), at 60 K and 0.5 T. In the rest of the samples, heated by using lower or higher heating rates, a high concentration of 1223 and/or 1245 phases is detected. The content of the 1234 phase and of Ba(Ca.Tl)-Cu-O residual liquid phase, in the samples synthesized for the optimum heating rate, depends on the Tl content in the starting mixture. The influence of the oxygen content in the starting mixture on phase composition is not as strong as of Tl, but it has a major role in establishing the superconducting behaviour of the 1234 grains. Some arguments that suggest the dependence of the transport properties of the non-superconducting matrix on oxygen content will be discussed. In our technological arrangement the determined optimum heating rate can be applied to the synthesis of other single-layered superconducting phases, e.g. 1212 phase from a Cu0.25Tl0.75Ba2CaCu2O8.115 starting composition.

After a presentation of structural aspects of iron-based nanocrystalline alloys, the magnetic behaviours of those two-phase magnetic structures are conceptually discussed at high temperatures as a function of the crystalline volumetric fraction, on the basis of the magnetic correlation length compared to the distances between crystalline grains. In addition, some results predicted by an approach based on Monte-Carlo simulation are then briefly presented: they show a qualitative agreement with experimental results.

Temperature and time dependence of the phase formation in the (Cu-0.5,Tl-0.5)-1234 composition have been studied. The liquid-vapour deposition mechanism for the formation of 12(n - 1)n superconducting phases is presented. XRD, SEM, EDS and weight loss data are discussed. Samples were superconducting with maximum T-c and J(c) (at 60 K and 0.5 T), determined from M(H) loops of 118 K and 4 x 10(5) A cm(-2), respectively.

The size-broadened profile given by the lognormal and gamma size distributions of spherical crystallites is considered. An analytical approximation for the size-broadened profile is derived that can be analytically convolved with the strain-broadened and instrumental-broadened profiles. The method is tested on two CeO2 powders; one shows 'super-Lorentzian' profiles that were successfully modelled under the assumption of a broad lognormal size distribution. It is shown that the Voigt function, as a common model for a size-broadened profile, fails for both very narrow and broad size distributions. It is argued that the size-broadened line profile is not very sensitive to variations in size distribution and that an apparent domain size or even column-length distribution function can correspond to significantly different size distributions.

The dispersion curves of the phonons propagating in polycrystalline suspensions of hard-sphere colloids are measured by Brillouin scattering and calculated by band structure techniques. Acoustic-like and optical-like experimental phonon dispersion relations are attributed to the interplay of the solvent matrix mode with the single-sphere vibration eigen-modes in these lattice structures.

Thin films of Tl(Ba, Sr)(2)Ca2Cu3Oy superconductor have been grown on CeO2 buffered r-cut sapphire substrate with the use of a template TlSr2CaCu2Oy by the amorphous phase epitaxy method. By using this template containing Sr, we have overcome the problem of chemical reactions between Ba and Cc present in the superconducting and buffer layers, respectively, at the normal processing temperatures of 840-890 degreesC. Films grown by this method have an excellent c-axis orientation as well as in-plane alignment with the TlSr2CaCu2Oy template, which in turn is aligned with the CeO2 layers. These films have superconducting critical temperature values up to 104 K and critical current density values as high as 1 MA cm(-2) at 77.3 K and 0 T.

The magnetic behavior of nanocrystalline alloys has been modeled using atomic Monte Carlo simulation. The model consists of a cubic lattice composed of a ferromagnetic nanograin in a ferromagnetic matrix. The magnetizations of nanograin core, surface and interface regions and matrix were studied as a function of the exchange coupling between the nanograin and the matrix, as well as of the nanograin/matrix volume ratio, equivalent to the crystalline fraction in the nanocrystalline alloys. The mechanism of polarization of the matrix by fields penetrating from the nanograin is discussed and correlated with the matrix-nanograin exchange coupling. Competition between interface anisotropy and magnetocrystalline anisotropy produces spin-glass-like magnetic order of the interfacial regions. (C) 2002 American Institute of Physics.