National Institute Of Materials Physics - Romania

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.

Scanning Hall probe microscopy has been used to make a microscopic study of flux structures and dynamics in yttrium barium copper oxide thin-film disks containing a regular 10-mu m-period square array of 2.5-mu m-sized holes (antidots). Images obtained after field cooling the sample to 77 K in very low fields reveal that the holes can trap two flux quanta at this temperature. Scans obtained after zero-field cooling (ZFC) to 77 K and a subsequent applied field cycle clearly display preferential flux channeling along chains of antidots in the direction of maximum induction gradient. Remarkably, upon reversal of field sweep direction, we observe flux "streaming" out of the holes towards the sample edges with almost uniform density flux "stripes" bridging the holes in the exit direction. We estimate that the antidots can preferentially trap about 15 flux quanta in these ZFC experiments. Classical electrodynamics simulations of our samples appear to be in good qualitative agreement with our results, indicating that many of the observed phenomena may be geometrical effects that depend primarily on the shape and topology of the sample, and potential applications are discussed.

Single layer TiAlN and alternate TiN/TiAlN multilayered hard coatings were deposited on Si, plain carbon steel and high-speed steel substrates by the cathodic are technique. Chemical composition, phase composition, texture, Vickers microhardness, adhesion, bilayer period, sliding friction coefficient and wear resistance were investigated by XPS, EDX and XRD analyses, microhardness measurements, scratch and tribological tests.

Electronic spin-lattice relaxation measurements performed in X and Q bands on electrons irradiated polyethylene, polycarbonate and polymethylmethacrylate have shown that at liquid helium temperatures and higher, up to 12K, the spin-lattice relaxation time of radiation induced free radicals does not depend on the magnetic. field but varies inverse proportionally with the square of absolute temperature and free radicals concentration. These peculiarities have been well explained by admitting that below 77 K there is a considerable amount of stable free radicals coupled in pairs that mediate the spin-lattice relaxation process of single radicals. Consequently, we have proposed an analytical equation that, within this hypothesis, quantitatively describes all these particularities.

Experimental results concerning the pulsed laser deposition parameters of in-situ epitaxially grown high crystalline quality YBa2Cu3O7-x thin films are reported. The structural and morphological properties of the deposited films analyzed by transmission electron microscopy, selected area electron diffraction, X-ray diffraction, and Raman spectroscopy are also presented. The YBa2Cu3O7-x thin films, predominantly c-axis oriented had required the smallest oxygen pressure during the cool-down time reported to date.

30-mu m thick polycarbonate foils were irradiated with single swift heavy ions (,e.g. Au 11.4 MeV/nucleon). After etching with solutions containing NaOH and methanol, templates containing single conical pores were obtained. The size and the shape of the nanopores depend on etching conditions such as time of etching, concentration and temperature of the etching solution. By electrochemical deposition of nickel, nanowires were grown in the single pores. The nanowires were contacted by sputtering a gold layer on top of the membrane. The magnetic measurements indicated that the nanowires possess around 1% anisotropic magnetoresistance. The current-voltage characteristic of a Ni nanowire shows a linear behavior for current densities smaller than 10(8) A/cm(2). The maximum current density that a single Ni wire can withstand was found to be 3x 10(8) A/cm(2).

Microspheres and planoconvex lenses (50 divided by 400 mu m in diameter) based on glassy As2S3, an important infrared material, have been produced by a special flame melting technique. Glassy fibres of 45 mu m in diameter, with the length of 10-20 cm have been drawn from melt. The morphological properties of these optical elements have been investigated.

Amorphous thin films of langasite and YAG have been prepared from crystals targets by pulsed laser deposition, at room temperature, in vacuum, on silicon wafers. Bulges of micrometer size are formed on the film surface. Larger size bulges (10-50 micrometers in diameter) are characteristic to the annealed langasite films. Annealing at high temperatures leads to the crystallization of the films. The annealed langasite films (850 degrees C) are polycrystalline and partially oriented with the plane (001) parallel to the surface of the silicon wafer. The bulges break easily in the heat treated films. Their empty structure was remarked on the electron microscope images. The YAG films annealed at 1100 degrees C shows a crystalline YAG phase depleted in Y and traces of Y2O3. An interesting feature is the presence of silver particles spread along the macrodefects (fracture lines) of the film.

Surfaces of GaN films have been investigated by Atomic Force Microscopy (AFM) with implemented Piezoelectric Force Microscopy (PFM) technique. The GaN layers were grown by molecular beam epitaxy (MBE) on Si(111) substrates with an AlN buffer layer. Simultaneous imaging of surface morphology in the AIM and PFM response was performed. The PFM imaging is sensitively dependent on measurement conditions. By increasing the ac voltage the contrast of the PFM images is increased. Using the Voltage-Modulated Force Microscopy (VMFM) regime, with the frequency close to the first resonant frequency of the piezoelectric signal, PFM images with high contrast and high resolution on the nanometer scale were obtained. Regions of opposite piezoresponse were observed and explained by the presence of inversion domains. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The influence of thermal neutrons irradiation at different fluencies on the superconductivity properties of Li (2 at. %) doped YBa2Cu3O7-delta system has been investigated by direct microwave absorption method. We have noticed changes of the superconducting properties of the 123 phase that depend on the neutron fluency. By increasing fluency, initially, the phase structure supports a disordering, then a re-ordering accompanied by a significant increase of the average critical temperature T-c to 85.7 K and finally we observed a degradation of the 123 the phase accompanied by a diminishing of T-c. The intragranular dephasing magnetic field B-d, i.e. the direct microwave absorption (DMA) parameter associated with the magnetic field dependence of the critical current density exhibited an opposite tendency, initially decreasing and then increasing at a value close to those of unirradiated samples. We also noticed only on the irradiated samples the presence of a flatness of the DMA curves around 0.5 T, consistent with the expression of a peak effect.