National Institute Of Materials Physics - Romania

The abnormal polarization offset observed in case of graded ferroelectric structures is explained assuming the presence of a nonreversible part of polarization due to the imposed polarization gradient. It is shown that an upper limit for the offset should exist, fixed by the remnant polarization of the component materials considered as independent layers. This is valid for the structures in which polarization increases or decreases in steps from one layer to the other, but should also be valid for the structures with continuous gradient. However, large values of the charge offset could be achieved in the last case. The electric field dependence of the polarization offset is predicted, together with the possibility of obtaining large nonconventional pyroelectric coefficients. The simulation developed in the case of a bilayer ferroelectric structure and using data from the lead-lanthanum-zirconate-titanate system describes well the observed features of the graded ferroelectric structures. (C) 2003 American Institute of Physics.

Composites containing 4-n-octyl-4'-cyanobiphenyl (8CB) either confined to nanopores of molecular sieves with very large pores or coating silica nanoparticles of aerosil type at high silica-to-8CB ratios are investigated by IR spectroscopy. Band shape analysis was performed in wavenumber regions in which the peaks due to CN stretching, CH stretching and CH out-of-plane vibrations appear. Some of molecules confined to molecular sieves show spectroscopic features characteristic to a bulk-like 8CB matter located in the centre of the pores or in the inter-grain space. Other features of the IR spectra are due to 8CB molecules located in the surface layer, mostly forming hydrogen bonds between their CN groups and surface OH groups. Another part of the 8CB molecules in the surface layer may also interact by pi electrons of the aromatic rings. Hydrogen bonding is less hindered for the molecules of the surface layers onto aerosil particles than inside pores of the molecular sieves. Comparison is also made with the case of composites based on molecular sieves with small pores. (C) 2003 Elsevier Science B.V. All rights reserved.

IR photoconductive properties of PbS films can be controlled using two different methods: (1) chemical, controlling the film morphology and, thus, the physical film properties by adding small amounts of impurities in the deposition bath, (2) physical, using field effect in complex PbS/SiO2/Si pseudo-MOS structures to control the photoconductivity of PbS film. (C) 2003 Elsevier Science B.V. All rights reserved.

An iterative method is presented for the characterization of lossy piezoelectric materials in the thickness resonant mode, which allows to calculate the frequencies at which the electrical impedance is to be measured. This new method considerably reduces the measurement time by separating the programs for data acquisition and for processing thus simplifying the measurement procedure. The accuracy of the method was tested for several piezoceramic materials, covering a wide range of values of the thickness coupling factor and mechanical quality factor. The new method proved to be as accurate as other iterative methods, but it is easier to apply. (C) 2002 Elsevier Science Ltd. All rights reserved.

We investigated the superconducting critical temperature, the intra- and intergranular critical current density, and the thermopower properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)(y) samples. All these properties have been compared with those of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)(y) specimens. It was found that the critical temperature determined from resistive and AC complex susceptibility measurements show a maximum and the transition width shows a minimum for the intermediate values of y. Powder X-ray diffraction studies and the AC complex susceptibility measurements reveal that in our samples the amount of Bi2Sr2Ca2Cu3O10+delta high-temperature superconducting phase is maximum for y approximate to 0: 02: The amount of LiCl in Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)(y) changes the superconducting properties of the grains as well as of the intergrain matrix. The splitting of the peak in the temperature dependence of the imaginary part of the complex susceptibility, corresponding to the dissipation inside the grains, was also observed.

Superconducting YBCO c-axis oriented thin films have been grown by MOMBE (metalorganic molecular-beam-epitaxy). Growth of the films on different substrates (100) (Y, Nd)AlO3, (100) SrTiO3 and (100) MgO has been investigated by, in situ monitoring the reflection high energy electron diffraction (RHEED). The paper discusses our results suggesting significant differences in films growth on the three presented substrates. The highest quality (high uniformity, epitaxy and low roughness) has been attained for the films prepared on SrTiO3. All films, regardless substrates, have shown values between 81 and 84 K for zero resistance critical temperature T-c0.

Orientation control of high T-c superconductors is essential for superior device performance, because the coherence length is longer along the non c-axis directions than along c-axis direction. In this study, we report on the first successful preparation of (119) oriented Bi-2223 films by MOCVD using (100) NdGaO3 Substrates. Atomic force microscopy observations of the (119) Bi-2223 films are also reported. Our films have an in-plane aligned mountain-range-shaped surface morphology. This morphology is resulting from the epitaxial relationship between the (119) Bi-2223 film and (100) NdGaO3 substrate. Films exhibited a large in-plane anisotropy as revealed by, resistivity measurements and surface morphology observations.

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of TI-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the, actiation energy for the flux jumps, which resulted to. be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T-c dc SQUID's.

We report vortex imaging and local magnetometry studies near the edge of a YBCO thin film using scanning Hall probe microscopy (SHPM). Our data show some unusual features, namely local hysteresis inversion and negative remanent fields, which are semi-quantitatively explained in terms of a theoretical model of flux penetration in an infinite-long thin superconducting strip.

Epitaxial Tl2Ba2CaCu2Oy and Tl(Ba,Sr)(2)Ca-2 Cu3Oy thin films have been prepared on CeO2 buffered sapphire substrate by an ex situ process with high reproducibility. Microstructure analysis of the surface of the TI-2212 films showed well connected and smaller sized grains of 1 mum or less. In the case of TI-1223 film, plate-like crystals with pits and pin holes could be observed. The superconducting critical temperatures of both films are around 96 K. Critical current density measured by ac susceptibility technique is as high as 1 MA/cm(2) at 77 K. The microwave surface resistance of these films was measured over a wide range of temperatures by a HTS-sapphire-HTS resonator method at 38 GHz on films with area 10 mm x 10 mm. The temperature dependence of surface resistance of TI-2212 film follows very closely to that of the best YBCO films on MgO substrate, whereas the TI-1223 exhibits slightly higher values resulting from an inferior surface morphology.