National Institute Of Materials Physics - Romania

Electron localization and optical transmission in one-dimensional systems with two components distributed according to the Rudin-Shapiro sequence are investigated. The nature of the eigenstates in the diagonal tight-binding model is studied by making use of nonlinear recurrence relations satisfied by the associated transfer matrices and their traces. It is shown that the wave functions display a wide range of features going from weak to exponential localization. Numerical computations lead to the conjecture that the localization property is generic. Nevertheless, a countable dense set of critical values of the on-site potential amplitude is found for which a class of extended states exists. Accordingly, the numerical investigation of the time evolution of the electronic wave packets displays a subdiffusive behavior when the potential amplitude becomes critical. Finally, a study of the optical properties of the Rudin-Shapiro dielectric multilayer shows strong similarities with the behavior of the disordered multilayers regarding the development of gaps in the transmission spectra.

The minimum observed in the magnetic field dependence of the transport critical current density of high-T(c) superconducting ceramics at relatively low applied field values was analysed. This anomalous behaviour results from the influence of pinning assisted demagnetisation effects at low fields and an increase of the intergranular pinning force at higher fields, through vortex-vortex interactions. The low voltage level I-V characteristics of the investigated samples can be described in terms of thermally activated flux creep at grain boundaries.

Structural models are proposed for cation distribution on substitutional and interstitial sites in the scheelite-like Bi-Fe-Mo-oxides. Experimental data derived from X-ray diffraction and Mossbauer spectrometry yield an unitary image, in which the actual distribution depends on the composition and the calcination temperature.

The low-field irreversibility line of the Bi2Sr2Ca2Cu3O10 high-T(c) superconducting compound was determined for two kinds of sample: bulk-sintered specimens with randomly oriented crystallites, and samples with preferential grain orientation, and by two different methods. In both cases, an exponential decrease of the irreversibility field with increasing temperature was observed. This behaviour was explained in terms of thermally assisted flux flow.

This paper presents the results got by slag inclusion visualization in the alloys used for the acid, lead battery grids and proposes quantitative estimation formulae for the slag content in the alloy, based on a metallographic method. The alloy structure is also presented as function of the alloy elements.

A resistance-heated furnace with a good control of temperature gradients is used for the Czochralski growth of precipitate and colour free Bi4Ge3O12 (BGO) crystals up to 30 mm in diameter and 80 mm in length from a 45 mm diameter crucible. The quality of the grown crystals is discussed in connection with the growth conditions. Spectrometric tests made on scintillators obtained from BGO crystals are presented. A Monte Carlo program is used in order to compute the full energy peak efficiency of the detector for different energies and geometries. The computed values are compared to the experimental results.

The location of the Fe3+ ions in the iron-containing scheelite structures of Bi3FeMO2O12 and Bi2Fe2Mo2O12 samples (which have been prepared under various temperature treatments) has been investigated by X-ray diffraction. Mossbauer and electron spin resonance (ESR) spectroscopies. The alteration of the crystalline structure and of the Fe3+ ion location have been followed after static reduction in propylene and reoxidation. The catalytic activity of the samples in the reaction of propylene oxidation to acrolein has been measured. The structural data have indicated for the Bi3FeMo2O12 samples tetragonal or monoclinic scheelite-type structure for the low (450-degrees-C) and high (560-degrees-C) temperature preparations, respectively, with the Fe3+ ions located in tetrahedral (4a) sites; this position is not affected by the reduction. A quantitative analysis of the structural data on Bi2Fe2Mo2O12 preparations led to the important conclusion that such a compositional phase exists only as a metastable, low-temperature one; at higher temperatures. Fe3+ ions in excess of Bi3FeMo2O12 composition are segregated on octahedral interstitial, ferric molybdate and/or alpha-Fe2O3 sites. Thus, this conclusion sheds light on the debated question of these phases as well defined compounds. Correlations between structure and catalytic activity data are discussed, taking into account the actual structures found in this study.

It is shown that X-ray powder diffraction patterns of mordenites can be used to reveal, by 2D Fourier mapping, structural changes due to chemical or thermal effects. During dealumination, a preferential localization of the structural defects at the tetrahedral groups of type 1 was observed. For different degrees of dealumination and cation contents, the localization of cations and/or water molecules inside the mordenite channels can be seen.

The maximum observed in the magnetic field dependence of the transport critical-current density of high-T(c) superconducting ceramics was analyzed. Transport critical current and magnetization measurements performed on Bi(Pb)-Sr-Ca-Cu-O bulk sintered samples allowed us to conclude that such a maximum may result from the influence of demagnetization effects at low applied field values and an increase of the intergrannular pinning at higher fields.