National Institute Of Materials Physics - Romania

Bi2Sr2CaCu2Ox, whiskers have been grown by a modified airtight two-crucible method, employing radial and vertical thermal gradients resulting from the relative position of the crucibles. A heating profile with a constant heating rate applied from 842 to 846degreesC allows formation of whiskers up to 2 cm in length for a certain level of Al impurity in the glass. A cation-rich atmosphere is found to be a key intrinsic parameter. Our data confirm the importance of the thermal gradients, but this parameter is a somewhat extrinsic one and from a practical point of view should be correlated to other intrinsic parameters that influence growth atmosphere (i.e. to total and partial pressures). As-grown whiskers have composition Bi:Sr:Ca:Cu = (2.9-3.5):2:(1.7-2.05):(2.4-3.0) (normalized to SrA zero-resistance critical temperature T-c(R=0) = 76-98 K and maximum transport critical current density above J(c) = 10(4) A cm(-2) at 77 K in self-field.

Composites were prepared from an aerosil and 4-n-alkyl-4'cyanobiphenyls with five to eight carbon atoms in the alkyl chain. Their high silica density of similar to7 g aerosil in 1 cm(3) of liquid crystal (LC) allows the observation of the behaviour of a thin cyanobiphenyl layer (having nearly a monolayer structure) on the silica particles. The systems are investigated by dielectric spectroscopy (10(-2)-10(9) Hz) in a large temperature range (220-370 K). All the composites show a (main) relaxation process at frequencies much lower than the processes observed for the bulk LC that was assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its characteristic frequencies obeys the Vogel-Fulcher-Tammann law, which is found to be typical for glass-forming liquids. The quasi two-dimensional character of the glass transition in the surface layer is discussed for the first time. At the nematic-to-isotropic transition temperature of the bulk, the composites show a continuous decrease of the characteristic frequencies as a function of the alkyl chain length, while the bulk LCs show the well known odd-even behaviour. The magnitude and temperature dependence of the slow relaxation process in the composites (molecules on an outer surface) agree with those of the same molecules confined to the nanopores of molecular sieves (internal surface).

The <001> axially symmetric Fe+(II) center observed in SrCl2:Fe2+ crystals has been studied by the electron nuclear double resonance (ENDOR) technique in the microwave X and Q bands. This center is produced only in crystals grown in chlorine atmosphere and x-ray irradiated at low temperature (below 100 K). The analysis of the ENDOR spectra unambiguously confirms the structural model earlier suggested by the electron paramagnetic resonance (EPR) study, as a Fe+ ion strongly displaced off center along an <001> axis, almost in the center of the square determined by the four nearest Cl- ligands. The unpaired spin densities f(3s) and f(3p) in the 3s and 3p chlorine orbitals have been determined from the resulting ENDOR parameters.

Lead zirconate-titanate (PZT) thin films with Zr/Ti ratio of 92/8 were deposited by sol-gel on Pt coated silicon and single crystal MgO(100) substrates. In case of the films deposited on Pt the perovskite structure is established only after oxygen annealing, while for the films deposited on MgO annealing in air is enough. The films are polycrystalline with preferred orientation that is substrate dependent ((110) on Pt and (100)/(200) on MgO). Average values of 2.10 muC/cm(2) and 78 kV/cm were obtained for remnant polarization and coercive field, respectively. The I-V characteristic at low-to-intermediate electric fields is controlled by thermionic emission, but the Schottky representation is Ln(current)-(voltage)(1/4), implying that the film behaves like a semiconductor and not like an insulator. From transmission-reflectance measurements a value of 3.85 +/- 0.05 eV was estimated for the band gap, while the refraction index in the transparency region was found to be 2.6. A pyroelectric signal was detected on as grown films, making them attractive for infrared detection. (C) 2003 Elsevier B.V. All rights reserved.

Investigations on barium neodymium titanate (BNT) prepared by solid state reaction techniques are presented in this paper. The samples were sintered at temperatures, T-s, in a range 1200-1300degreesC. High density values and adequate microwave parameters were obtained by adding 0.5 mol PbO substitutional for BaO. A decrease of the Curie temperature with the increase of the Nd2O3 concentration was confirmed. For the sample with 28 wt.% Nd2O3, 0.5 mol PbO pyroelectric data indicate Curie temperature around 75 K, therefore, at room temperatures, the materials are in paraelectric phase. BNT ceramics with high Nd2O3 concentration (28 wt.%), sintered at 1250degreesC for 2 h appear to be suitable for dielectric resonators for frequencies up to 3 GHz (epsilon(r)=80-90 and Q = 2000). (C) 2003 Elsevier B.V. All rights reserved.

Pb(Zr, Ti)O-3 (PZT) thin films with Zr/Ti ratio of 92/8 and doped with up to 4 at.% Nb were deposited by sol-gel on Pt coated Si substrates. The scope was to investigate the influence of Nb additive on the structural, optical and electrical properties of Zr-rich PZT phase. it was found that a residual pyrochlore phase stabilizes with increasing the Nb content. The remnant polarization, the coercive field, and the dielectric losses increase with increasing Nb content. The presence of the pyroelectric effect was also evidenced on as-deposited layers. The films have potential for light detection in the infrared region of the electromagnetic spectrum (pyroelectric effect). (C) 2003 Elsevier B.V. All rights reserved.

Isolated Ge islands, i.e., islands not connected by a wetting layer can be obtained by selective epitaxial growth in voids of ultrathin oxides of thickness 1-2 nm. Voids of 30-600 nm size were created before epitaxy during a high temperature anneal of the ultrathin oxide. The formation of one island per window was investigated at 700 degreesC as a function of Ge thickness and void size. Islands nucleate mainly at the edge of the void and for this reason they have an anisotropic shape. In voids smaller than 300 nm only one island is nucleated. Islands form only in voids greater than a critical size (30-80 nm) which depends on the total amount of Ge deposited. We observe that height, width, and aspect ratio of isolated islands increase with void size for a given Ge thickness. A metastable state of Ge in small windows was observed. Moreover, the Si interdiffusion is strongly reduced with decreasing island size (i.e., with void size) reaching only similar to10% in comparison with similar to50% in islands on large areas. (C) 2004 American Institute of Physics.

In this paper, electrical investigations performed on complex metal-oxide-semiconductor (MOS) structures with amorphous LaAlO3 deposited as gate oxide on a rapid thermal processed SiO2 buffer layer are presented. Current-voltage (I-V) measurements at different temperatures and high frequency room-temperature capacitance-voltage (C-V measurements indicate the presence of a non-equilibrium state in p-type Si (100) substrate under reverse polarization of the structure. The asymmetry in I-V characteristics is opposite to the expected one for the high-kappa/interfacial layer stack behavior. Different indications for the non-equilibrium MOS state are inferred from the experimental data, and the extraction of MOS physical parameters by using the classical C-V method is shown to be unreliable. For small and large applied forward voltages defect-related charge transport and tunneling currents dominate, respectively. (C) 2003 Elsevier B.V. All rights reserved.

Cadmium sulphide (CdS) thin films were deposited by chemical bath deposition (CBD) method on SiC2/Si (n-type) substrates. Approximately, 70 nm thick nano-crystalline CdS layers were obtained. Thin film field effect transistors were realised by deposition of two coplanar electrodes of Au (drain and source) on the US surface. The gate contact is aluminium deposited on the backside of the Si substrate. The drain current-drain voltage characteristics (I-d - V-d) were performed in dark. Normal field effect transistor characteristics are obtained in case of positive gate and drain voltages, the device acting as an n-channel transistor in the accumulation mode. For negative drain voltages the characteristic is dominated by space charge limited currents (SCLC). An on/off current ratio of about 10(2) is reported, this being limited in our case by geometry. (C) 2003 Elsevier B.V. All rights reserved.

The puzzling disappearance of the second magnetisation peak below a certain temperature T-0 in the case of Bi2Sr2CaCu2O8+delta single crystals is explained by considering the dynamic ordering of the vortex system caused by the current induced in the sample during standard magnetisation measurements. Another ingredient is the limitation of the second magnetisation peak to field values lower than the characteristic field B-0 for the 3D-quasi-2D dimensional crossover. It was found that due to the inherent increase of the inborn driving force with decreasing T the dynamic response of the vortex system for T less than or equal to T-0 is elastic on both sides of the limiting field B-0. Consequently, there will be no jump in the current-voltage characteristics across B-0 and the second magnetisation peak disappears. (C) 2004 Elsevier B.V. All rights reserved.