Publications

Surface enhanced Raman scattering with 676.4 and 1064 nm excitations was used to investigate single-walled carbon nanotubes (SWNTs) compressed non-hydro statically at 0.58 GPa, alone or dispersed into chemical reactive and non-reactive host matrices. By compression, SWNTs break in fragments of different size. Between SWNTs fragments and the host matrix various mechanico-chemical reactions take place. In matrices such KI and Ag, donor-acceptor complexes are formed. In aromatic hydrocarbons like biphenyl or p-terphenyl, an ionic and covalent functionalization of SWNTs fragments is demonstrated by new bands at 1160 and 1458 cm(-1), the latter being associated to an A(g)(2) pentagonal pinch mode. (c) 2005 Elsevier B.V. All rights reserved.

The investigation of the structure, magnetic and Mossbauer properties for the series of R-3(Fe,MO)(29) compounds has been performed, where R = Y, Nd, Sm, Gd, Tb, and Dy. The crystallographic structure of the ternary phase compounds has been investigated by Rietveld refinement of the X-ray diffraction patterns obtained at room temperature. The quality of the single-phase compounds was also checked by thermomagnetic measurements, from room temperature to above the Curie temperatures. From the magnetic isotherms for the free powder samples, measured at 4.2 K, the saturation magnetizations and the iron average magnetic moments have been derived. Fe-57 Mossbauer spectra of the R3 (Fe,MO)29 compounds have been measured at 15 K. The analysis of spectra, in a model which takes into account both the Fe atom nearest neighbor numbers and the Fe-Fe nearest neighbor bond lengths, indicates that the transferred contribution to the hyperfine field at the iron sites, due to rare earth moments, can be correlated with the rare earth effective spin. (c) 2004 Elsevier B.V. All rights reserved.

Nd-doped strontium lanthanum aluminate crystals, Nd:ASL (Sr1-xNdyLax-yMgxAl12-xO19), with an extended composition parameter x (0.05 and 0.5) have been grown by the Czochralski pulling technique. Structural and compositional properties of the as-grown crystals have been studied using X-ray diffraction, chemical microanalysis and optical spectroscopy. The results show that high crystalline perfection and large size crystals of both compositions can be grown. Low-temperature optical absorption spectra of crystals with compositions corresponding to x = 0.5 and 0.05 reveal that such crystals predominantly contain either C-1- or C-2-type Nd3+-centers, respectively. The possibility to thus differentiate between the two types of the dopant structural centers is of utmost importance for laser applications. (c) 2005 Elsevier B.V. All rights reserved.

Smooth a-axis oriented ZnO thin films are grown on (001) MgO substrates by pulsed laser deposition assisted by a very directional radio frequency oxygen plasma. The structure of the film is examined by X-ray diffraction, electron diffraction and high resolution electron microscopy. The film grows with vertical columns and is highly textured. Two preferential orientation relations between the ZnO film and the MgO substrate are observed: (10010)(ZnO) vertical bar vertical bar (002)(MgO); [0002](ZnO)vertical bar vertical bar[2 0 0](MgO) and (1010)(ZnO) vertical bar (002)(ZnO) vertical bar vertical bar [200](MgO) [1210](MgO). The film/substrate interface is flat and strained because of the rather large lattice mismatch between the (1010) plane of ZnO and the (002) plane of MgO. (c) 2005 Elsevier B.V. All rights reserved.

Crystallographic relations between different forms of boron nitride ( BN) appearing at the high pressure-high temperature structural phase transformation have been revealed by high-resolution transmission electron microscopy ( HRTEM). As starting materials, crystalline hexagonal BN ( hBN) with different degrees of crystallinity, or with defects intentionally introduced, were used. Cubic BN ( cBN) is formed only as a minor component, the rest consisting of different forms of sp(2) bonded BN: hBN, compressed, monoclinic deformed hBN, or turbostratic BN ( tBN). The small cBN crystallites ( 300-400 nm) contain many defects such as twins, stacking faults and nanoinclusions of other BN forms: tBN, rhombohedral BN ( rBN) and wurtzite BN ( wBN). The cBN phase grows epitaxially on the basal plane of hBN. The nucleation sites for cBN are revealed by HRTEM. They consist of nanoarches ( sp(3) hybridized, highly curved nanostructures), frequently observed at the edges of the hBN crystallites in the starting materials. Based on HRTEM observations of specimens not fully transformed, a nucleation and growth model for cBN is proposed which is consistent with existing theoretical and experimental models.

An option of increasing the luminosity of the Large Hadron Collider (LHC) at CERN to 1035 cm-2 s-1 has been envisaged to extend the physics reach of the machine. An efficient tracking down to a few centimetres from the interaction point will be required to exploit the physics potential of the upgraded LHC. As a consequence, the semiconductor detectors close to the interaction region will receive severe doses of fast hadron irradiation and the inner tracker detectors will need to survive fast hadron fluences of up to above 1016cm-2. The CERN-RD50 project "Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders" has been established in 2002 to explore detector materials and technologies that will allow to operate devices up to, or beyond, this limit. The strategies followed by RD50 to enhance the radiation tolerance include the development of new or defect engineered detector materials (SiC, GaN, Czochralski and epitaxial silicon, oxygen enriched Float Zone silicon), the improvement of present detector designs and the understanding of the microscopic defects causing the degradation of the irradiated detectors. The latest advancements within the RD50 collaboration on radiation hard semiconductor detectors will be reviewed and discussed in this work. 2005 Published by Elsevier B.V.

Three compositions of metallic glasses, Fe58Co24Nb3Ta1Mo1B13, Fe61.5Co20.5Nb3Ta1Mo1B13 and Fe66Co18Si1B15 were prepared by rapid quenching from the melt and subsequently annealed for 1 hour at 450, 650 and 750 degrees C. The samples were analyzed by X-ray diffraction (XRD) and Mossbauer spectroscopy. All Mossbauer spectra were fitted with a six-line pattern corresponding to the crystalline alpha-(FeCo) phase and a hyperfine magnetic field distribution representing the amorphous component. The Moss-bauer spectra of annealed Fe66CO18B15 revealed the presence of a secondary crystalline phase, namely (FeCO)(3)(BSi). Moreover, the last Mossbauer spectrum in the set was fitted with an additional sextet, corresponding to the appearance of hematite in the system. It is inferred that the addition of Nb, Ta and Mo considerably delays the onset of bulk crystallization and iron oxidation in these systems. The XRD patterns are in qualitative agreement with the Mossbauer results and are consistent with a surface layer of hematite nanoparticles in the system. The activation energy of 4.34 eV for oxidation was estimated from the Mossbauer results. (c) 2005 Published by Elsevier B.V..

NaCl and KCI single crystals were irradiated with swift heavy ions (An, Ph, Bi and U) of energy between 560 and 2640 MeV. The damage induced by the ions was studied using optical absorption spectroscopy, chemical etching and small-angle X-ray scattering. The damage in the tracks is dominated by the creation of simple color centers (F and V-3 centers). Their concentration increases as a function of fluence, saturating when single tracks overlap. The F-center containingly track zone extends up to several tens of nanometers. In addition to this large track halo, there exists also a much smaller region (1-2 nm) close to the ion trajectory in which defect aggregates are formed. Indications for this track core are given by the preferential chemical etchability of ion tracks and also by the anisotropic pattern produced by small-angle X-ray scattering. This halo-core track structure is very similar as in LiF, however given by the higher mobility of the primary hole centers in NaCl and KCl, recombination with electrons is facilitated resulting in a lower efficiency for defect creation. (c) 2005 Elsevier B.V. All rights reserved.

We study the dynamics of a one-dimensional Bloch electron subjected to a constant electric field. The periodic potential is supposed to be less singular than the delta-like potential (Dirac comb). We give a rigorous proof of Ao's result that for a large class of initial conditions (high momentum regime) there is no localization in momentum space. The proof is based on the mathematical substantiation of the two simplifying assumptions made in physical literature: the transitions between far away bands can be neglected and the transitions at the quasicrossing can be described by Landau-Zener-type formulas. Using the connection between the above model and the driven quantum ring (DQR) shown by Avron and Nemirovski, our results imply the increase of energy for weakly singular such DQR and appropiate initial conditions. (C) 2005 American Institute of Physics.