National Institute Of Materials Physics - Romania

Nanostructured Ni/C-60 thin films, produced by co-evaporation of Ni and fullerene, have been investigated by both X-ray diffraction (XRD) and X-ray magnetic circular dichroism (XMCD). X-ray diffraction has shown the co-existence of diffraction specific peaks for the fullerene face-centered cubic crystal structure and diffraction peaks of Ni nanoparticles in the face-centered cubic structure, which are broadened by nanoparticule finite dimensions. Analysis of peak widths shows that the Ni nanocrystallite size ranges from 4.7 to 19.2 nm. The XMCD signals for a sample series, preparated on relatively low-temperature substrates during the co-evaporation, can be interpreted as having distinct contributions due to both bulk and surface Ni atoms. These two kind of atoms are antiferromagnetic coupled. The Ni atoms which are at the interface with the fullerite matrix show ail enhanced orbital magnetic moment, while the orbital magnetic moment of Ni atoms in the bulk has very low values, as it can be expected for a crystalline field with a cubic symmetry.

The results of both a line-broadening study on a ceria sample and a size - strain round robin on diffraction line-broadening methods, which was sponsored by the Commission on Powder Diffraction of the International Union of Crystallography, are presented. The sample was prepared by heating hydrated ceria at 923 K for 45 h. Another ceria sample was prepared to correct for the effects of instrumental broadening by annealing commercially obtained ceria at 1573 K for 3 h and slowly cooling it in the furnace. The diffraction measurements were carried out with two laboratory and two synchrotron X-ray sources, two constant-wavelength neutron and a time-of-flight (TOF) neutron source. Diffraction measurements were analyzed by three methods: the model assuming a lognormal size distribution of spherical crystallites, Warren-Averbach analysis and Rietveld refinement. The last two methods detected a relatively small strain in the sample, as opposed to the first method. Assuming a strain-free sample, the results from all three methods agree well. The average real crystallite size, on the assumption of a spherical crystallite shape, is 191 (5) Angstrom. The scatter of results given by different instruments is relatively small, although significantly larger than the estimated standard uncertainties. The Rietveld refinement results for this ceria sample indicate that the diffraction peaks can be successfully approximated with a pseudo-Voigt function. In a common approximation used in Rietveld refinement programs, this implies that the size-broadened profile cannot be approximated by a Lorentzian but by a full Voigt or pseudo-Voigt function. In the second part of this paper, the results of the round robin on the size - strain line-broadening analysis methods are presented, which was conducted through the participation of 18 groups from 12 countries. Participants have reported results obtained by analyzing data that were collected on the two ceria samples at seven instruments. The analysis of results received in terms of coherently diffracting, both volume-weighted and area-weighted apparent domain size are reported. Although there is a reasonable agreement, the reported results on the volume-weighted domain size show significantly higher scatter than those on the area-weighted domain size. This is most likely due to a significant number of results reporting a high value of strain. Most of those results were obtained by Rietveld refinement in which the Gaussian size parameter was not refined, thus erroneously assigning size-related broadening to other effects. A comparison of results with the average of the three-way comparative analysis from the first part shows a good agreement.

A closed cluster model for the binary arsenic-chalcogen glasses seems to be attractive for the explanation of the structural and electronic properties of non-crystalline chalcogenides. In the same time the direct consequence of the model is the absence of the defects of coordination. The full implication is discussed.

Using nuclear inelastic scattering of synchrotron radiation and inelastic neutron scattering we have investigated the vibrational properties of Fe and Co impurities in NiAl intermetallics, and compared them with the density functional theory calculations. The Fe phonon spectra show two sharp peaks appearing in the frequency gap between the Ni and Al phonon bands, and originating from the local modes of Fe residing in Ni, or Al sublattices. Co, which replaces Ni, vibrates within a narrow frequency interval occurring close to the upper edge, but still within the Ni phonon band. The ab initio calculations show that these effects are expected when impurity-host force constants differ from host-host force constants causing a dynamic mismatch between guest and host vibrations.

Magnetization in dc magnetic fields and at different temperatures has been measured on the antiperovskite Li2Pd3B with a cubic structure composed of distorted Pd6B octahedrons. This material was recently found to exhibit superconductivity at 7-8 K. The critical fields H-c1(0) and H-c2(0) are determined to be 135 Oe and 4 T, respectively. Critical current density, scaling of the pinning force within the Kramer model, and irreversibility field data are presented. Several superconductivity parameters were deduced: Coherence length xi=9.1 nm, penetration depth lambda=194 nm, and Ginzburg-Landau parameter kappa=21. The material resembles other boride superconductors from the investigated points of view. (C) 2004 American Institute of Physics.

Precipitate-free thin films of multicomponent materials (e.g. superconductors) have been successfully grown (see Figure) on substrates onto whose surfaces artificial steps have been generated. The steps had widths that were equal to twice the migration length of the atomic species being deposited on the films. The resulting clean, high-quality films were due to the gathering of precipitates at the step edges, where the free energy is lowest.

Hydroxyapatite (HA) thin films has been successfully deposited by Nd:YAG laser ablation lambda = 532 nm. The morphology and microstructure of the deposited layers was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). Polycrystalline HA films were directly obtained with the substrate at 300 degreesC and without introducing water vapors in the deposition chamber. Electron paramagnetic resonance (EPR) measurements show that the oxygen stoichiometry in the HA films is also maintained. Depositions performed at lambda = 335 nm laser wavelength and 300 degreesC substrate temperature resulted in polycrystalline layers of mixed composition of HA and tricalciumphosphate (TCP). (C) 2004 Elsevier Ltd. All rights reserved.

Nonmonotonic current transients at constant temperature, following a pulsed charge injection, have been observed in silicon diodes heavily irradiated with gamma rays. The features of the transients have been studied at different temperatures and using various reverse biases. The particular shape of the measured transients is interpreted, with the help of numeric simulations, in terms of a strong time dependency of the space charge density due to the discharge of radiation induced deep traps. In particular, a nonmonotonic transient is generated if the discharge of a dominant trap determines the change of the space charge sign (type inversion). In this case the active volume of the sample, from which the current is collected, is increased during the discharge and causes the sample to become fully depleted for a short time. During this time interval the active volume reaches its maximum value and a peak is produced in the transient shape. The measurement of this peak provides a sensitive way to detect type inversion in semiconductor devices and to determine the responsible energy levels. The correlation of nonmonotonic transients with thermally stimulated current spectra and the distortion produced in deep level transient spectroscopy spectra are discussed in detail.

Coulomb correlations in the optical spectra of semiconductor quantum dots are investigated using a full-diagonalization approach. The resulting multi-exciton spectra are discussed in terms of the symmetry of the involved states. Characteristic features of the spectra like the nearly equidistantly spaced s-shell emission lines and the approximately constant p-shell transition energies are explained using simplified Hamiltonians that are derived taking into account the relative importance of various interaction contributions. Comparisons with previous results in the literature and their interpretation are made.

Our present work investigates the structural properties of bulk NiMnSb prepared by growth from the melt, using various thermal regimes. We studied the compositional behaviour of the polycrystalline material obtained from stoichiometric 1: 1:1 and respectively off-stoichiometric melts. The latter samples were grown with Mn excess up to 10% and respectively Ni deficiency between 2 and 10%. The X-ray diffraction and the Scanning electron microscopy complement each other in the characterisation of the phases occurring in the samples. The synthesis of NiMnSb starting from an initially stoichiometric mixture associated with a thermal treatment presenting plateaus gives the best results. Also, it seems that the presence of an excess of Mn is not sufficient to prevent the occurrence of the NiSb stray phase.