National Institute Of Materials Physics - Romania

Rich phonon spectra were observed experimentally by Brillouin spectroscopy in liquid, glassy, and crystalline state of colloidal systems of low and high elastic constant contrast. The nature of these phonons was elucidated by theoretical calculations of the single sphere scattering cross section, the energy density distribution, the light scattering intensity, and determination of the band structure by the multiple scattering method. Besides the ordinary acoustic phonon, localized optic-like modes, mixed modes, and Bragg induced modes were identified. Their relation to the physical state of a colloidal suspension sensitively depends on the micromechanical mismatch between particle and surrounding medium and the coherence of the crystalline structure. Polycrystalline colloidal suspensions show distinct acoustic excitations in the high wave vector-low frequency region. (C) 2003 American Institute of Physics.

The charge-density oscillations (plasmons) of a low-density two-dimensional uniform electron gas are studied within the framework of finite temperature and frequency dependent (dynamic) version of Singwi, Tosi, Land, and Sjolander theory and compared with the recent experimental results. The use of the Hartree-Fock approximation for the static structure factor leads to a finite temperature dynamical counterpart of the static Hubbard approximation. We observe important differences between dynamic and static local-field factors as well as between the corresponding plasmon dispersion laws. Our calculated plasmon energies that include dynamic correlations are in very good agreement with the recent experimental results.

Applied field Mossbauer spectrometry is used to investigate the magnetic structure in two amorphous Sm-Fe-B ribbons with measured compositions Sm7.2Fe72.8B20 and Sm12Fe68B20. The applied field dependence of the hyperfine field agrees with the ferromagnetic coupling between the Fe and Sm magnetic moments. For the Sm7.2Fe72.8B20 ribbon, the magnetic moments are aligned in the applied field direction for fields higher than 4 T. For the Sm12Fe68B20 ribbon, the magnetic moments are not completely aligned in the direction of the applied field for fields up to 6 T. This spin canting is attributed to the influence of the samarium anisotropy, which probably dominates in the Sm12Fe68B20 ribbon, as the related composition lies in the concentration range where the local order around iron atoms is strongly influenced by the samarium atoms. (C) 2002 Elsevier Science B.V. All rights reserved.

The gamma-Fe2O3 nanoparticles with different surface states are investigated by X-ray Diffraction, Electron Microscopy, Differential Thermal Analysis and magnetic: measurements in low applied field. The influence of the adsorbed species (H2O and sulphate complexes) on the inter-particle magnetic interactions is analysed in relation to both the magnetic states of the surface and the inter-particle distance.

A series of Fe3 - xMxO4 samples with x = 0-0.6. was prepared using the hydrothermal method. The system was characterized using transmission Mossbauer spectroscopy. The dependence of the site hyperfine magnetic. fields and populations on the content x of Mn substitution was derived from the spectra. The results are consistent with Mn2+ ions substituting for Fe2+ ions in the octahedral B sublattice. In particular, the site-specific substitution of Mn ions for the B2 sites was evidenced. (C) 2002 Elsevier Science B.V. All rights reserved.

Silicon detectors in particle physics experiments at the new accelerators or in space missions for physics goals will be exposed to extreme radiation conditions. The principal obstacles to long-term operation in these environments are the changes in detector parameters, consequence of the modifications in material properties after irradiation. The phenomenological model developed in the present paper is able to explain quantitatively, without free parameters, the production of primary defects in silicon after particle irradiation and their evolution toward equilibrium, for a large range of generation rates of primary defects. Vacancy-interstitial annihilation, interstitial migration to sinks, divacancy and vacancy-impurity complex (VP, VO, V2O and CiOi, CiCs) formation are taken into account. The effects of different initial impurity concentrations of phosphorus, oxygen and carbon, as well as of irradiation conditions are systematically studied. The correlation between the rate of defect production, the temperature and the time evolution of defect concentrations is also investigated. (C) 2002 Elsevier Science B.V. All rights reserved.

The effects of the substitution for Mn with In in the colossal magnetoresistive manganites La2/3Ca1/3Mn1-xInxO3 (x less than or equal to 0.05) have been investigated by ESR. The temperature and x dependence of the resonance linewidth have been investigated and discussed in terms of the spin-spin (exchange) interaction scenario. In the paramagnetic regime, the exchange coupling integral, J, between Mn3+ and Mn4+ spins shows a decrease with increasing x. It could arise from the weakening of the double - exchange interaction upon doping, which reduces the exchange field at the Mn sites, The high temperature limit of the ESR linewidth does not depend on the doping level.

The electrical and transport properties of GaAs crystals such as resistivity (rho), mobility (mu) and carrier concentration (n) depend on the purity and defects of a particular crystal. These parameters are normally obtained by performing a simple Hall effect analysis in a bar configuration. The technical procedure is connected with the design of appropriate contacts for Hall bar configuration. In order to ensure good ohmic contacts oil GaAs, thin metal layers of Au-Ge-Ni have been deposited in high vacuum followed by heat treatment in low vacuum. Hall effect measurement is important for checking the quality of the GaAs wafers. We analyzed various GaAs wafers with (100) and (110), orientation obtained by LEC and HB methods. The resistivity of the wafers varied from low resistivity, n-type doped crystals (n = 10(18) cm(-3)), to high resistivity crystals (n = 10(14) cm(-3)). This paper presents the data of Hall characterization of wafers cut from GaAs crystals grown by different methods (LEC, HB).

Easy axis distribution in metal particle magnetic tapes for linear digital data storage were studied by Mossbauer spectroscopy and magnetometry, for comparison. Qualitative and quantitative agreement between the distribution parameters deduced at room temperature via the two different techniques was found. The proposed methodology based on the interpretation of the Mossbauer data has the advantage that it can be applied at different temperatures and for very thin films. Moreover, angular spin distribution. at low temperatures were obtained by Mossbauer spectroscopy and discussed with respect to the room temperature results. There was evidenced that the room temperature easy axis angular distribution contains both static orientation and dynamical components.

Recent Coulomb drag experiments in low-density double-layer electron systems have the power of distinguishing various many-body formulations of the effective interactions. In this work we theoretically study the correlation effects on the drag resistivity in these systems within various models. The effective inter-layer interactions are best described by the generalization to the double-layer case of the Kukkonen-Overhauser approach which differs significantly from the self-consistent field approach of Singwi et al. [Phys. Rev. 176 (1968) 589]. Following the formulation of Vignale and Singwi [Phys. Rev. B 32 (1985) 2156] we derive an expression for the effective inter-layer interaction which embodies the many-body correlations through the local-field corrections. The drag resistivity is calculated within this approach together with the Hubbard approximation for the intra-layer local-field factor and a simple model for the inter-layer correlations. Comparison with the recent measurements of Kellogg et al. [Solid State Commun. 123 (2002) 5151 yields very good agreement. Our results are also contrasted with the corresponding drag resistivities given by the Singwi et al. theory, the dynamic random-phase approximation and the Hubbard approximation. The significant differences found between these theories emphasize the strong sensitivity of the drag resistivity to the effective inter-layer interactions. (C) 2003 Elsevier Science Ltd. All rights reserved.