National Institute Of Materials Physics - Romania

The crystal structure of (Bi1.82Pb0.18) (ST1.84Pb0.16) CuO6 was determined from single-crystal intensity data measured on a four-circle diffractometer. The structure is orthorhombic, space group Cccm (No. 66); the lattice parameters: a=5.392(3) Angstrom, b=24.603(5) Angstrom, c=5.300(3) Angstrom, V=703.2 (5) Angstrom(3) (the space group can be denoted as Amaa by a cyclic transformation of the cell constants). The R value was 0.048 for 418 unique reflections with I>3.00 sigma(I). One of the O atoms bonded to Bi ions is disordered at two sites with a half occupancy and B-eq=3.6(6) Angstrom(2); although the thermal parameter is not so large as that reported by Torardi et al. [Phys. Rev. B 38, 225 (1988)], the disorder suggests that the crystal has a lower symmetry. The valence state of the Cu ion in the present compound was evaluated to be trivalent based on the Cu-O distances [Asbrink and Norrby, Acta Crystallogr. B 26, 8 (1970)] and the valence of Bi was estimated in comparison with that in other compounds. The O(3) sublattice dynamics in these types of superconductors is discussed based on the structural investigations.

The improvement of intragranular critical current density, j(c), and flux pinning obtained by thermal neutron irradiation (fluences between 10(16) - 10(18) neutrons/cm(2)) in YBa2Cu3O7,(-x) doped with 8 mol% lithium fluoride (LiF) (YBCO-LIF) is explained by the occurrence of various irradiation defects (point defect clusters, dislocation loops, stacking faults) observed by transmission electron microscopy (TEM), A twin fading process is also revealed accompanied by a lowering of the orthorhombicity index. At higher irradiation fluences (similar to 5 x 10(17) 10(18) neutrons/cm(2)), a new mechanism related to the dispersion of fine Cu2O precipitates in the superconducting YBCO-LiF matrix could explain the enhancement of up to eight times obtained for j(c) values. A good agreement between the dependence of the measured j(c) values on the applied magnetic field and the calculated curves, j(c) = f(B), derived from a theoretical model, previously proposed for the pinning contribution of fine particles dispersed in YBCO matrix, is obtained. The role of thermal neutron irradiation for the flux pinning improvement by introducing of strong pinning centers (irradiation defects and Cu2O microdispersoids) is also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.

Bi4Ti3O12 (BiT) ferroelectric thin films were used as photoconductive or photovoltaic ultraviolet detectors. The maximum of the spectral distribution lies around 370 or 390 nm, depending on whether the incident light is continuous or modulated. The maximum current responsivity obtained for the films annealed at 700 degrees C is about 0.01 A/W. The specific detectivity is 5.5 x 10(8) cm W-1 Hz(1/2) at a modulation frequency of 20 Hz. The frequency characteristics of the photovoltaic signal measured in modulated light suggests a 1/f frequency dependence of this signal. (C) 1998 American Institute of Physics.

We have measured I-V and C-V characteristics, the temperature dependence of dark currents, and thermally stimulated depolarisation currents on fresh and stored samples of photoluminescent porous silicon, By storage in ambient, the low rectifying I-V curves become strong rectifying, and C-V curves become MIS-like. I-T characteristics for fresh samples have only one activation energy, in the 0.49-0.55 eV range. After storage, a slightly modified value, of about 0.50-0.60 eV is observed at low temperatures only. At about 280 K, the activation energy suddenly changes to 1.20-1.80 eV. Also, both the number and the positions of maxima in thermally stimulated depolarisation currents change by storage. The annealing at about 50 degrees C induces small reversible changes in I-T characteristics and strong irreversible ones in thermally stimulated depolarisation currents, both for fresh and stored samples. A simplified quantum confinement model is proposed to explain the main aspects of the electrical behaviour of porous silicon films. The surface and/or interface contributions are observed especially in thermally stimulated depolarisation currents. The changes induced by storage are attributed to the oxidation process of the internal surface of porous silicon films. (C) 1998 Elsevier Science S.A. All rights reserved.

Thin carbon nitride films have been deposited on silicon(100) substrates downstream of a nitrogen plasma beam generated in a combined RF (13.56 MHz, 40-50 W) and DC (voltage +/-200 V, power 1-10 W) discharge between a graphite electrode and a graphite nozzle. By combining the RF and DC sources the capability of rf field to create extended plasmas is used together with the enhanced sputtering and biasing effect of the DC source. The plasma characteristics (electron temperature, presence of molecular species) have been studied by optical emission spectroscopy. Deposition rates of 2.5-3 nm/s are obtained at the centre of the plasma beam and at a few centimetres distance from the nozzle. The films have been investigated by X-ray photoelectron spectroscopy, spectroscopic ellipsometry, scanning and transmission electron microscopy, and microhardness measurements. The films have an overall N:C ratio of 0.28 but the distribution of different nitrogen bonds depends upon the DC bias conditions. In the spectral range 0.3-0.7 mu m the refractive index increases slightly from 1.5 to 2.2. The films are amorphous, with morphology consisting of a columnar structure. The columns have a diameter of about 20 nm. A hardness of 24 GPa has been measured. (C) 1998 Elsevier Science S.A. All rights reserved.

Surface-enhanced Raman scattering (SERS) has lately proved to be an effective technique which has led to its increasing application for studying the structural properties of conducting polymer thin films. It is generally accepted that the enhancement process has an electromagnetic origin arising from the excitation of surface plasmons (SPs) in the metal support on which the polymer film is deposited. However, the electromagnetic enhancement is also accompanied by a chemical process, on which only scarce experimental data are available at present. The chemical process originates in an increased polarizability of the molecules at the metal surface under the action of an incident radiation, as a result of which new chemical bonds are formed with the atoms of the metal support. This paper is devoted to the study of the SERS spectra under 1064 nm excitation for two forms of polyaniline, one of the emeraldine base type and the other of the emeraldine salt type. Our purpose has been to identify the chemical effects at the polymer/metal interface that are involved in specific modifications of the SERS spectra. The following results concerning SERS spectrum variation with metal support type (AE, Au and Cu) have been obtained. An oxidation of emeraldine base taking place at the polymer/metal interface has been revealed. In the case of emeraldine salt, an oxidized form of polyaniline, the SERS spectra remain unchanged regardless of the changes in the metal support, which proves the stability of this form. A new result has been obtained as SERS spectroscopy revealed an electron delocalization process taking place in the emeraldine salt molecule as a result of an electron transfer from metal to the polymer molecule occurring on the level of quinoid rings. The concept of a rough Raman-active surface has been clarified by using a parameter Wa, similar to the grating groove depth/grating periodicity ratio which occurs when a diffraction grating is used as optic coupler. (C) 1998 Elsevier Science S.A. All rights reserved.

This paper describes the pulsed constant current anodization of n(+) and p(+)-GaAs in ethylene glycol-water-tartaric acid (AGW) mixture, up to 17 mA cm(-2). A 0.26 mu m thick film is obtained for a final voltage of 135V at 4.3 mA cm(-2) pulsed current density. Cyclic voltammetry showed that the initial growth of a monolayer anodic pride can be described by a charge transfer with uncompensated cell resistance model. The relationship between peak current, peak voltage and scan rate has been verified for this process, based on the above model.

The magnetic properties of the amorphous and nanocrystallized Fe-Cu-Nb-Si-B and Fe-Gd-Cu-Nb-Si-B ribbons are studied using temperature dependence of the specific magnetization. A two-phase ferromagnetic behaviour is obtained for the magnetization curves. Gd addition tends to stabilize the interfacial (grain boundary) phase by forming a metastable Gd-Fe-B phase, leading to the coexistence of two phases up to thermal treatments as high as 993K.

The high-T-c superconducting system YBa2Cu3Odelta:Li (123) was investigated by microwave absorption. The effect of neutron irradiation on the 123 phase as well as the dependence of the resulting modification both on the neutron fluence and on the doping behavior with Li are discussed. The results show a significant improvement of the superconducting characteristics: critical temperature, phase homogeneity, and critical current densities for a certain neutron fluence. We have found that the optinum neutron fluence for irradiation is 4.98 x 10(21) m(-2).

Formation conditions, optical nonlinear properties and applications of F2- color centers in LiF monocrystals are investigated. The efficiency of F-2(-) centers generation depending of irradiation conditions and content of OH- impurities of the LiF monocrystal are also presented. Thermal stability of the centers generated by electron irradiation was studied. It was measured the variation of absorption factor of the LiF: F-2(-) crystal in the -40 degrees C to 60 degrees C temperature range. The LiF: F-2(-) crystals obtained by an efficient method of irradiation with electrons generated by a linear accelerator were employed in Q-switching a Nd:YAG laser cavity with output energies of 18 mJ and fluctuations less than 5% in the -40 degrees C to 60 degrees C temperature range.