National Institute Of Materials Physics - Romania

Cubic boron nitride (c-BN) crystalline superabrasive powder (Borazon** CBN 400), consisting of 200-300 microns sized amber colored crystallites prepared by HP/HT synthesis, has been examined from 2.1 K to 293 K by X-band ESR spectroscopy. The observed spectrum consists of a component line A1, visible in the whole temperature range, and two component lines A2 and A3, visible at high and low temperatures, respectively. The A1 and A3 lines originate from transitions inside S = 1/2 ground states of distinct paramagnetic species and A2 from transitions inside an excited state of another paramagnetic center. The intensity of the A1 and A3 lines changes differently during in situ low temperature illumination in the UV-VIS range. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Triglycine sulfate crystals (TGS) doped With L- and D-alanine, respectively, where grown in paraelectric phase at 52 degreesC. They show stable parameters versus pure TGS crystals, lower permittivities and losses, i.e. higher pyroelectric figure of merit. L- and D-alanine-doped crystals show a mirror asymmetry of the growth habit along the ferroelectric axis. Hysteresis loop of pure and doped samples were automatically recorded. Internal bias field of about I kV/cm, induced by the two dopants, stabilizes the polarization components P+ and P- in opposite directions. Spontaneous polarization of doped crystals shows two unequal P+/P- components, having a peculiar dependence on the ac applied electric field. Dopage simulation of pure TGS samples, using several bias fields, shows even more complicated pattern. Pyroelectric coefficients were evaluated from pyroelectric current measurements. A remarkable asymmetry of the pyroelectric coefficient peak values for the enantiomorphous-doped crystals was noticed, crossing up and down the Curie point. This fact suggests a non-equivalent L/D-alanine dopant substitution of the glycine group GI in the host lattice. It has been shown that the pyroelectric coefficient can be increased at room temperature, under an optimized dc electric field applied on the pyroelectric wafer. Doped crystals are suitable for IR detector and for more stable gas sensors applications in pollution monitoring. (C) 2004 Elsevier B.V. All rights reserved.

We have used scanning Hall probe microscopy to correlate vortex images and local magnetisation measurements in two different high temperature superconducting samples. Near the edge of a continuous YBCO thin film we have observed local hysteresis inversion and negative remanent fields, which can be semi-quantitatively explained in terms of a theoretical model of flux penetration in an infinitely long superconducting strip. In a YBCO film containing a regular square array of antidots we have further find that vortices trapped at antidots exhibit an unusual behaviour upon field sweep reversal. (C) 2004 Elsevier B.V. All rights reserved.

We study the transport properties of coherently coupled quantum dots in the quantum Hall regime within the Landauer-Buttiker formalism which captures and explains the experimentally observed features in terms of the spectral properties of the coupled dot system. The subpeak structure of the transmittance spectrum and the charging stability diagrams are obtained and discussed. The role of the intradot and interdot Coulomb interaction are pointed out. We show the subpeak evolution with the magnetic field and predict a specific oscillatory behavior of the Hall resistance in strong magnetic field which can be experimentally tested.

The third harmonics of the ac susceptibility chi(3) of MgB2 bulk samples have been measured in the frequency range 61-1070 Hz as a function of the temperature for different applied dc magnetic fields. The irreversibility line (T-irr versus H-dc) deduced from the temperature onsets of chi(3) for each field shows a frequency dependence: The onset temperature increases as the frequency increases. This frequency dependence has been analyzed using the 3D vortex-glass model. Moreover the real versus imaginary components of the chi(3) for each dc magnetic field have been interpreted with a 3D glass creep using a numerical analysis based on a non-linear diffusion equation of the magnetic field. (C) 2004 Elsevier B.V. All rights reserved.

The corrosion product films that were formed on industrial, low-carbon concentration Fe-C steel after immersion in diluted ammoniac solutions were investigated by 57 Fe Mossbauer spectroscopy. The amorphous Fe3+ oxyhydroxide was the major constituent of the corrosion layers. Layer thickness increased as NH3 concentration changed from 10(-1) to 10(-4) N. All GEMS spectra indicated a magnetic anisotropy on the surface of the samples. (C) 2004 Elsevier B.V. All rights reserved.

Surface-enhanced Raman scattering (SERS) and cyclic voltammetry (CV) were used to investigate oxidation-reduction processes of single-wall carbon nanotube (SWNT) films deposited on Au supports in 0.5 M H2SO4 solutions. In the potential range (0; +1000) and (0; +1500)mV versus saturated calomel electrode (SCE), the oxidation-reduction reactions of SWNT films are quasi-reversible and irreversible, respectively. Anodic polarization of SWNT films until +1000 mV versus SCE produced compounds similar to the bisulfate intercalated graphite. Regardless of excitation wavelength, i.e. 1064 or 676.4 nm, variation in the Raman spectra exhibited a decrease in the intensity of the bands associated with the radial breathing mode (RBM) situated in the 120-240 cm(-1) spectral range. Also an increase in the intensity of the D band is accompanied an up-shift of this band. A gradual decrease of the Breit-Wigner-Fano component was observed at lambda(exc) = 676.4 mn. Partial restoration of the Raman spectra was achieved by a subsequent alkaline solution treatment. Potentials higher than +1000 mV versus SCE resulted in SWNTs breakage and fragments of different length were formed such as closed-shell fullerene. This was observed in the SERS spectrum by: (i) the disappearance of the RBM band, (ii) the increased D-band shifted to ca. 1330 cm(-1) and (iii) the appearance of a new band at 1494 cm(-1), frequently observed also in the Raman spectrum of fullerenes on the type C-70, C-84, C-119, as well as in its derivative compounds (e.g. C60O, clathrates, etc.). Appearance and increase in the intensity of the Raman band at 1494 cm(-1) as result of an anodic polarization of the SWNT film in solution of H2SO4 0.5 M in 1-butanol is a further evidence of the nanotubes breakage. (C) 2004 Elsevier B.V. All rights reserved.

Y123 thin films were grown by pulsed laser deposition (PLD) on YSZ (001) substrate. Prior to the film deposition, a discontinuous layer of Ag was deposited on the substrate, also using PLD, in the form of separate islands. Atomic force microscopy (AFM) investigation of the Ag layer showed that its morphology consisted of self-assembled islands of nanometre size, randomly distributed on the surface of the substrate, called nano-dots. The Y123 superconducting films grown on such a surface were characterized using AFM, x-ray diffraction, secondary electron microscopy, ac susceptibility and dc magnetization. The results show that there is no significant difference in surface morphology, crystallographic orientation, phase composition or superconducting transition temperature between the Y123 films grown on YSZ (001) with an Ag nano-dots layer and a control Y123 film grown on a virgin YSZ (001) surface. On the other hand, at 77 K, the magnetic critical current density (J(c)(m)) was three times higher for the Y123 film grown on YSZ with the modified (001) surface than for the film grown on YSZ with a virgin (001) surface. At 5 K the enhancement of J(c)(m) was approximately seven times, at both low and high fields. This suggests an increase in pinning, caused presumably by point defects formed in the Y123 film above the Ag islands.

We investigate transport in nanotransistors in the Landauer-Buttiker formalism. A systematic linearization of the general expression for the current response yields the quantum version of the small signal equivalent circuit. This equivalent circuit can be compared with classical schemes so that explicit quantum mechanical expressions for the circuit elements can be extracted. Reducing our analysis to an effective Y-parameter description of the equivalent circuit we find the multi-terminal Buttiker formula except for one extra term. We show that this extra term is essential for the operation of transistors. An application of our theory to a simple transistor model yields a description of mismatch oscillations in the source-drain current experimentally observed in nano-transistors. (C) 2004 American Institute of Physics.

We review some aspects concerning the disorder effects on the spectral properties of 2D electronic systems with relevance for the transport and orbital magnetism in perpendicular magnetic field. The lattice model and random Anderson disorder are used. The discussion is carried out separately for periodic and hard wall boundary conditions. The degree of localization is evidenced by the calculation of the inverse participation number of the eigenstates of the Hamiltonian and a striking asymmetry of the localization effect of states in the Landau band is reported. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.