Publications

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.

Local magnetic interactions and spin configurations in alpha-Fe/Nd2Fe14B composite exchange-spring magnets have been investigated by Mossbauer spectroscopy and the magnetic behaviour was analysed by magnetometry. A new method for the evaluation of the coupling strength between soft and hard magnetic phases is proposed. (C) 2003 Elsevier B.V. All rights reserved.

Structural and morphological characteristics of (1 - x)alpha-Fe2O3-xSnO(2) (x = 0.0- 1.0) nanoparticles obtained under hydrothermal conditions have been investigated by X-ray diffraction (XRD), transmission Mossbauer spectroscopy, scanning and transmission electron microscopy as well as energy dispersive X-ray analysis. On the basis of the Rietveld structure refinements of the XRD spectra at low tin concentrations, it was found that Sn4+ ions partially substitute for Fe3+ at the octahedral sites and also occupy the interstitial octahedral sites which are vacant in alpha-Fe2O3 corundum structure. A phase separation of alpha-Fe2O3 and SnO2 was observed for x greater than or equal to 0.4 : the alpha-Fe2O3 Structure containing tin decreases simultaneously with the increase of the SnO2 phase containing substitutional iron ions. The mean particle dimension decreases from 70 to 6 DID, as the molar fraction x increases up to x = 1.0. The estimated solubility limits in the nanoparticle system (1 - x)alpha-Fe2O3-xSnO(2) synthesized under hydrothermal conditions are: x less than or equal to 0.2 for Sn4+ in alpha-Fe2O3 and x greater than or equal to 0.7 for Fe3+ in SnO2. (C) 2003 Elsevier Ltd. All rights reserved.

Al2TiO5 has been obtained by reaction sintering of precursor powders synthesized by three powder processing methods. A field activated sintering technique has been used for the process, with 100 degreesC/min heating rate and short holding plateaus (minutes) at the maximum temperature. SEM, XRD, and density measurements were performed after sintering. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The bimetallic AgCo nanoparticles synthesized by colloidal chemistry and deposited onto Si(100) substrate are investigated. Size-selective mechanisms of colloidal crystallization during the self-organization are evidenced. The magnetic behavior is studied and correlated with the nanoparticles structure and morphology. (C) 2003 Elsevier B.V. All rights reserved.