National Institute Of Materials Physics - Romania

We calculate the quantum states corresponding to the drifting and channeled classical orbits in a two-dimensional electron gas (2DEG) with strong magnetic and electric modulations along one spatial direction, x. The channeled states carry high, concentrated currents along the y-axis, and are confined in an effective potential well. The quantum and the classical states are compared. (C) 1998 Elsevier Science B.V. All rights reserved.

A hydrothermal method has been used to obtain polycrystalline substituted magnetites, Fe3-xMxO4, (M = Cr, Mn, Co, Ni, Cu; 0 less than or equal to x less than or equal to 0.375) starting with ferrous-hi hydroxide mixtures at temperatures up to 300 degrees C. Well-defined magnetite crystallites, 0.2-0.3 mu m in size have been obtained. The appearance of the intermediate phase alpha-FeOOH during hydrothermal transformation into magnetite has been evidenced and discussed. By means of Fe-57 Mossbauer spectroscopy, the distribution of different incorporated cations in the magnetite lattice has been analysed. (C) 1998 Elsevier Science B.V. All rights reserved.

The theoretical treatment of one-phonon thermal diffuse scattering (TDS) in single-crystal neutron diffraction at fixed incident wavelength is reanalysed in the light of the analysis given by Popa & Willis [Acta Cryst. (1994), A50, 57-63; Acta Cryst. (1997), A53, 537-545] for the time-of-flight method. Isotropic propagation of sound with different velocities for the longitudinal and transverse modes is assumed. As in time-of-flight diffraction. there exists, for certain scanning variables, a forbidden range in the one-phonon TDS of slower-than-sound neutrons, and this permits the determination of the sound velocity in the crystal. A fast algorithm is given for the TDS correction of neutron diffraction data collected at a fixed wavelength: this algorithm is similar to that reported earlier for the time-of-flight case.

The L alpha and L beta spectra of tungsten were measured using a high-resolution single-crystal x-ray spectrometer. The observed spectra were fitted into Lorentzians. The fit residuals of the L alpha and L beta spectra indicate the presence of satellites in the vicinity of each spectrum, which are originated from Coster-Kronig transitions. Linewidths, energies, and intensities were estimated for each diagram line and compared with previous data. The L beta(2) satellite line observed by Salgueiro er al. [J. Phys. (Paris), Colloq 48, C9-609 (1987)] was confirmed on the high energy side of the L beta(2) line. The origin of this satellite is discussed experimentally and theoretically. [S1050-2947(98)02011-3].

We measure the azimuthal anchoring energy of the nematic 5CB on sinusoidal holographic unidimensional and bidimensional gratings. We find that the anchoring strength is almost the same for all of the gratings and up to one order of magnitude stronger than expected by the topographic mechanism proposed by Berreman. We observe strong memory effects which dominate both the elastic anchoring and the easy axis gliding. We show that these memory effects limit from below the anchoring energy and are the main obstacle in the realization of substrates with weak anchoring conditions.

Employing X-ray scattering, the phase morphology and the miscibility of two mixtures formed from acrylonitrile-vinylacetate-alpha-methylstyrene copolymer with both styrene-acrylonitrile copolymer and cellulose acetate have been studied. The degrees of crystallinity, determined by X-ray diffraction, revealed the preponderant amorphous character of these mixtures. By small angle scattering of synchrotron radiation, imposing the miscibility condition for Flory-Huggins interaction energy parameter, the "miscibility window" for all mixtures were determined.

Haematite particles of four different morphologies (polyhedral, platelike, needlelike and diskshape) were synthesized by the hydrothermal method. The morphology and average particle diameter (1.4; 7.4; 0.2 and 0.12 mu m, respectively) were determined by transmission electron microscopy (TEM) combined with electron diffraction. The haematite samples were studied by transmission Mossbauer spectroscopy in the temperature range 4.2-300 K. In all cases, a weak ferromagnetic phase (WF) was present above the Morin temperature of 230 K and found to coexist with an antiferromagnetic phase (Ali) below this temperature. However, the populations of the two phases at 230 K were demonstrated to depend on the morphology of the particles. Moreover, the WF and AF phases exhibit a different dependence of the magnetic texture on temperature and particle morphology. (C) 1998 Elsevier Science S.A. All rights reserved.

The modification of the ideal lattice parameter in porous silicon has been determined by X-ray diffraction. A contraction effect has been detected in several samples. There was shown that the position of the photoluminescence band is related to the modification of the lattice parameter. The quantum theory of confinement in wires was used for the interpretation of the experimental data.

A quantum confinement model is proposed to explain the electrical transport properties in fresh and stored porous silicon (PS) films. In the present paper, the model is verified for the temperature dependence of the dark current. The studied samples are Formed mainly by a network of nanowires, suggesting the separation of the electron Hamiltonian into a longitudinal and a transversal part. The last one can be well described by a two-dimensional (2D) cylindrical infinite quantum well? whose levels determine the dark current activation energies. The storage oxidation induces modifications both in the number and the values of the activation energies, which are in excellent agreement with our model. (C) 1998 Elsevier Science Ltd. All rights reserved.

The reversible photostructural effects observed in the amorphous thin films and in chalcogenide glasses based on arsenic have been explained with the help of computer relaxed models. Two photostructural mechanisms have been established: the incorporation of the chalcogenide molecular fraction into the disordered layers and the change of the relative positions of the atoms in the layers without destroying the bonding network. In the frame of this atomic scale mechanisms it is possible to explain the reversible photodarkening and the photoanisotropy observed in the chalcogenide thin Alms and bulk glasses.