National Institute Of Materials Physics - Romania

Starting from a mean field calculation for the static capacitance of a MIS-nanostructure with a near back gate [P.N. Racec, E. R. Racec and U. Wulf, Phys. Rev. B 65, 193314, (2002)] we develop an approach to determine its ac-admittance. Mainly because of the interaction with the near back gate the inversion electron layer which forms in the considered MIS-nanostructure assumes on open character which is taken into account in the Landauer-Bilttiker formalism. For the Coulomb interaction the Hartree approximation is applied. In quantitative agreement with experiments a characteristic step in the static C-V trace results when the inversion layer is populated from the back gate. We found that this characteristic step is dominated by a particular resonance which we call intermediate resonance. Consistent with our static calculations we determine the density-density correlation function in the random phase approximation to find the ac-admittance. As an example we demonstrate that the lifetime of the static resonance induces a characteristic turnover frequency for the ac-admittance. An equivalent small-signal circuit is proposed and the dependence of its elements (capacitance and resistance) on the working point for low and high frequencies are presented. (c) 2005 Elsevier B.V. All rights reserved.

Solid solutions of the Ba1-xSrxTiO3 (BST) type are very attractive for applications in information technology, but also in microwaves for such electrically controlled devices as phase shifters, tunable filters, steerable antennas, varactors, etc. A family of solid solutions with x = 25, 50, 75, 90% was prepared by standard powder technology-solid state reaction and sintered at 1230 degrees C and 1260 degrees C. Dielectric permittivity and loss at low (1 kHz) and high frequency (1 divided by 2 GHz) were measured on a large temperature range - 200 degrees C divided by + 200 degrees C. The influence of Sr content and of I wt.% MgO and I wt.% MnO2 doping on the complex dielectric constant was studied. The dielectric permittivity at high frequencies and room temperature considerably decreases with the increase of Sr content. Similarly, low frequency measurements showed a severe and almost linear decrease of the Curie Point with the increase of Sr fraction. At low Sr content, the measured ceramic densities slowly decrease with Sr fraction and represent about 92% of the X-ray density rho(g/cm(3)) approximate to 5.99-0.99x. At x=90% Sr concentration, very low-density ceramics were obtained with both sintering temperatures. (c) 2005 Elsevier B.V All rights reserved.

The microstructure of the Cr/CrN and Ti/TiN multilayers coatings deposited with the pulsed laser deposition (PLD) technique was studied using transmission electron microscope Philips CM20 TWIN (200 kV). It was found that the metallic buffer layers are characterized by the smallest columnar crystallite size and the highest defect density; this might rise their hardness but compromise the coating adhesion. The intermediate metallic layers showed a larger and less defected columnar structure as compared with the nitride layers, what should improve the coatings toughness. The scratch test performed with a Rockwell penetrator of 200 gm diameter loaded from 0.03 N up to 20 N indicated that the switching from a single layer to a multilayered metal/nitride coating of the same thickness improved the resistance to scratch test in case of Ti/TiN from 16.4 N for a TiN single layered to 22.9 N for a 32 layered coating and Cr/CrN from 4.4 N for a CrN single layered to 6.6 for a 32 layered ones. (c) 2005 Elsevier B.V. All rights reserved.

A review of the evolution of the amorphous chalcogenide field of research from early times up to the present state of the art is presented. On the basis of recent achievements a bright future for these materials is predicted. (c) 2006 Elsevier B.V. All rights reserved.

The intermediate, self-organized phase, in the As-Se system has been modeled on the basis of clusters with one or more selenium-selenium bridges. The X-ray diffraction patterns of the glass models at the limits of the 'floppy phase-stressed rigid phase' window have been calculated. The structural and optical anisotropy in a matrix of clusters containing Se-Se bonds is supposed to be a consequence of breaking and re-linking the broken clusters in the direction of the polarized light beam that acts upon the chalcogenide material. (c) 2006 Elsevier B.V. All rights reserved.

We investigated the superconducting parameters of iodine-intercalated BiSrCaCuO high-temperature superconducting materials. While for samples containing a significant amount of Bi2Sr2CaCu2O8+x, (Bi-2212) iodine intercalation results in the dramatic decrease of the superconducting parameters, it was found that for samples with a nearly single Bi2Sr2Ca2Cu3O10-y (Bi-2223) phase these parameters can increase. We argue that this different behavior of the superconducting parameters is the result of a different doping regime, and can be explained based on the parabolic dependence between T, and the number of holes per CuO2 layer. Iodine intercalation appears to be an efficient way to bring the usually under-doped superconductor to its optimal T-c and flux pinning properties.

Small particles of Pbl(2) embedded in transparent polymer matrix have been studied by Raman scattering, UV-VIS absorption spectroscopy, low temperature photoluminescence (PL) and Scanning Electron Microscopy (SEM). The Pbl(2) crystallites were prepared by two different methods: i) by cooling to the room temperature a boiling saturated aqueous Pbl(2) solution containing polyacrylamide and ii) by chemical reaction of Kl and Pb(NO3)(2) in aqueous polyacrylamide solution. In both cases, beside the hexagonal platelets of Pbl(2) Were identified rods whose luminescence and Raman signature is quite different than of Pbl(2) platelets. Regardless the rods nature, as distinct KPbl(3) particles or lead iodide low-dimensional particles embedded in the polymer matrix, their photoluminescence is featured by an intense green emission (550 nm) appearing at low temperature at excitation wavelengths less then 350 nm.

In-plane current-voltage (I-V) characteristics of high-temperature superconductors in a zero applied magnetic field exhibit intrinsic two-dimensional (2D) behavior at relatively high transport-current densities. This allowed us an accurate determination of the temperature variation of the zero-field I-V exponent a(T) (V proportional to I-a) at the superconducting (CuO2)(2) layer level in the case of highly anisotropic Bi2Sr2CaCu2O8+delta films in the vicinity of the Kosterlitz-Thouless-Berezinskii transition temperature T-KT. The mean-field critical temperature and T-KT were determined from the zero-field resistive transition in the framework of the 2D Coulomb gas model. It was found that a(T-KT)approximate to 3, pointing toward a dynamic critical exponent z(T-KT)=2. The a(T) dependence observed by us is in good agreement with Langevin simulations of the 2D Coulomb gas model.

Band-pass filters with dielectric resonators of (Zr-0.8, Sn-0.2)TiO4 ceramic materials (ZST) were developed for X-band applications. The ZST materials exhibit a dielectric constant of about 36.8 and low loss in microwave range. A Qxf product higher than 50,000 was achieved for the ZST samples with 0.2 % wt NiO sintering addition. The external coupling was performed using microstrip lines on a substrate of 9.22 dielectric constant and 0.635 mill height. Filters with a minimum insertion loss between 0.46 dB and 1.74 dB, the bandwidth between 40 MHz and 170 MHz and with the central frequency around 10 GHz were manufactured.