National Institute Of Materials Physics - Romania

Second harmonic generation from the interface between GaAs(111)-B and an aqueous electrolyte has been investigated at a long wavelength (1064 nm) and at two shorter ones (865 and 815 nm). The signal showed the symmetry of the crystal; the isotropic amplitude was found to vary substantially with the electrode potential, indicating that a major part of the signal is generated near the interface. The potential dependence was stronger at shorter wavelengths. Impedance spectroscopy revealed the existence of surface states, which may affect the magnitude of the signal. (C) 2004 Elsevier B.V. All rights reserved.

Barium-strontium titanate (BST), ternary ceramic compounds with the molar formula (Ba1-xSrx)TiO3, were prepared by solid-state reaction from raw materials. Three compositions with x = 0.25, 0.50 and 0.75, have been checked. Samples with two type of thermal treatment at 1230 and 1260degreesC have been manufactured and analyzed. Perovskite type, polycrystalline structure of the BST ceramics was revealed by the X-ray diffraction data at room temperature. Strontium ion (Sr2+) enters substitutionally on the Ba2+ site. A small decrease in the unit cell volume with the Sr content increase was observed. Dielectric measurements have been performed at low and high frequencies. The temperature dependence of low frequency permittivity shows large interval of ferroelectric transition, which appears to be merely due to dispersion of crystallites dimensions. The increasing concentration of Sr ion leads to a shift of the Curie point below room temperature. Experimental data show a linear decrease of the Curie point T-C (degreesC); approximate to 120-360x, versus x Strontium fraction. Room temperature permittivity and dielectric loss have been measured in microwaves by using the Hakki-Coleman method. BST samples prepared using water as a milling medium exhibit lower porosity and lower dielectric loss in microwaves than samples prepared using acetone as a milling medium. Dielectric loss Of 10(-3) at 1 GHz shows that this ceramic material is quite suitable for high frequency device applications. (C) 2003 Elsevier B.V. All rights reserved.

The structure and properties (crystallographic phases, hardness) of Hartmetal-type steel we re investigated. The modification of the Hartmetal surface as a consequence of a special treatment with the purpose to improve the hardness by deposition of TiC films by chemical vapour deposition (CVD) was carefully analysed.

The electric transport of the charged particles in a spin texture was investigated in a strongly underdoped YBa2Cu3O6.25 single crystal in order to identify the characteristic electrical transport mechanism. The in-plane resistivity revealed three different regimes of charge transport: a chiral 2D VRH regime up to 55 K with a characteristic temperature T(d)approximate to12,400 K, an impurity band conduction regime above 55 K, and a metallic-like regime beyond 170 K. The out-of-plane resistivity has only one crossover at 115 K, but the conduction mechanisms controlling the two regimes are not clear.

In quantum-dot laser devices containing a quasi-two-dimensional wetting layer, a pump process initially populates the wetting-layer states. The scattering of carriers from these spatially-extended quasi-two-dimensional states into the quantum-dot states as well as the relaxation of carriers between the quantum-dot levels are studied theoretically. Based on the wave functions for the coupled quantum-dot/wetting-layer system interaction matrix elements are calculated for carrier-carrier Coulomb interaction and carrier-phonon interaction. Scattering rates for various capture and relaxation processes are evaluated under quasiequilibrium conditions. For elevated carrier densities in the wetting layer, Coulomb scattering provides processes with capture (relaxation) times typically faster than 10 ps (1 ps). When energy conservation allows for interaction with LO phonons, comparable rates are obtained.

Modeling of nano-tube clusters based on As,S(Se)(3) has been performed. The structural data were calculated on a single nano-tube model of As2S3. For the first time electron microscope images of nanometer size objects in As2Se3 and AS(5.5)Se(4.5) glass have been observed and discussed.

The point contact Andreev reflection employing niobium tips was used to determine the degree of transport current spin polarization (P-t) at the free surface of bulk NiMnSb at 4.2 K. The data was analyzed within the framework of a modified version of the Blonder, Tinkham, and Klapwijk formulism taking into account the two spin polarized channels in the ferromagnet and treating the interface as a planar delta function barrier of height Z between free electron materials. We find that the measured spin polarization is rather insensitive to different surface preparations and magnetic domain structure, and the maximal value of the P-t at Z=0 is 44%, consistent with recent calculations of the surface reconstruction of NiMnSb. Saturation magnetization of the samples was found to be 3.6 mu(B) per formula unit indicative of a small amount of atomic disorder.

Tin-doped indium oxide (ITO) thin films have been deposited by sol-gel process using 'sols' of indium and tin isopropoxides. The thickness of one deposited ITO layer is approximately 50 nm. The desired thickness was obtained by 1-5 successive depositions. The XTEM cross-sectional view of an ITO sample with five depositions showed a clear delimitation of the layers with an alternating structure dense/porous ITO layers. The void fraction in porous regions varies between 20 and 25%. Cubic bixbyite In2O3 nanocrystals with size of 10-20 nm and no phases separation of tin oxide were observed. The optical properties of the films have been investigated by optical transmission and spectroscopic ellipsometry. Reliable optical constants and porosity are obtained only with the model of internal structure based on XTEM results. (C) 2003 Elsevier B.V. All rights reserved.

We report here measurements of flux lensing behaviour which have been performed on Bi2Sr2CaCu2O8+delta single crystals placed on top of linear Hall probe arrays. In the crossing lattices regime we observe significant flux focussing as the in-plane field is swept up with a small constant density of pancake vortices. We also observe a sharp discontinous lock-in feature in local magnetic induction when the net field is close to the a-b plane. (C) 2004 Elsevier B.V. All rights reserved.