Publications

Neuroglobin is a recently discovered member of the globin superfamily. Combined electron paramagnetic resonance and optical measurements show that, in Escherichia coli cell cultures with low O-2 concentration overexpressing wild-type mouse recombinant neuroglobin, the heme protein is mainly in a hexacoordinated deoxy ferrous form (F8His-Fe2+-E7His), whereby for a small fraction of the protein the endogenous protein ligand is replaced by NO. Analogous studies for mutated neuroglobin (mutation of E7-His to Leu, Val, or Gin) reveal the predominant presence of the nitrosyl ferrous form. After sonication of the cells wild-type neuroglobin oxidizes rapidly to the hexacoordinated ferric form, whereas NO ligation initially protects the mutants from oxidation. Flash photolysis studies of wild-type neuroglobin and its E7 mutants show high recombination rates (k(on)) and low dissociation rates (k(off)) for NO, indicating a high intrinsic affinity for this ligand similar to that of other hemoglobins. Since the rate-limiting step in ligand combination with the deoxy-hexacoordinated wild-type form involves the dissociation of the protein ligand, NO binding is slower than for the related mutants. Structural and kinetic characteristics of neuroglobin and its mutants are analyzed. NO production in rapidly growing E. coli cell cultures is discussed.

We calculate the orbital magnetization of single and double quantum dots coupled both by Coulomb interaction and by electron tunneling. The electronic states of the quantum dots are calculated in a tight-binding model, and the magnetization is discussed in relation to the energy spectrum and to the edge and bulk states. We identify effects of chirality of the electronic orbits and of the anticrossing of the energy levels when the magnetic field is varied. We also consider the effects of detuning the energy spectra of the quantum dots by an external gate potential. We compare our results with the recent experiments of Oosterkamp [Phys. Rev. Lett. 80, 4951 (1998)].

Application of active technology factors (for example, the introduction of Bi2Sr2Ca2CU3Oy, (2223) phase additive) and corresponding optimization of synthesis parameters of Bi(Pb)-Sr-Ca-Cu-O (BSCCO) ceramics have allowed reducing the time of oxide precursor sintering from 250 to 25 h. Conditions of positive influence of 2223 phase additives have been investigated. Intensification of BSCCO ceramics synthesis was the basic purpose, but the authors have deliberately conducted broader examinations of the influence of technological parameters on the properties and structure of this material. The complex of the obtained effects (similarly to the solution of an equation set with several unknowns) helps increasing the probability of exact comprehension of the system: technology-processes occurring under ceramics synthesis-phase composition and structure-properties of BSCCO. (C) 2002 Elsevier Science B.V. All rights reserved.

TlBa(2)Ca(2)Cu(3)O(y) (Tl-1223) superconducting films were prepared under identical conditions, on MgO substrates with different morphologies resulting from heat treatments at temperatures between 600 and 1350 degreesC. The superconducting films have almost the same morphology, in-plane alignment and composition, but different critical current densities J(c). Critical current density J(c) determined at 77.3 K differs for films with various annealed MgO substrates, by a factor of 5 in zero field and 10 in 1 T. The behavior of J(c) is discussed in relation with the flatness of the MgO surface, and with pinning effects induced by the Ca-segregates. For the present work, the best films were obtained for the un-annealed (as-received) substrates and for substrates treated at 1350 degreesC. In these samples, the substrate's flatness and morphology with regular steps are essential for high quality of the superconducting films. In the films grown on MgO annealed between 800 and 1200 degreesC and showing rough surface and Ca-segregates, J(c) was lower. However, J(c) was increasing with heat treatment temperature of the substrate, possibly due to the Ca-segregates inducing or acting as pinning centers. The effects of Ca-segregates are of interest for maximization of the quality of the high-temperature superconducting films. In the literature, up to now, these effects have not been considered and therefore Ca-segregates removal has been recommended. (C) 2002 Elsevier Science B.V. All rights reserved.

TlBa2Ca2Cu3Oy (Tl-1223) superconducting films were prepared under identical conditions, on MgO substrates with different morphologies resulting from heat treatments at temperatures between 600 and 1350 degreesC. The superconducting films have almost the same morphology, in-plane alignment and composition, but different critical current densities J(c). Critical current density J(c) determined at 77.3 K differs for films with various annealed MgO substrates, by a factor of 5 in zero field and 10 in 1 T. The behavior of J(c) is discussed in relation with the flatness of the MgO surface, and with pinning effects induced by the Ca-segregates. For the present work, the best films were obtained for the un-annealed (as-received) substrates and for substrates treated at 1350 degreesC. In these samples, the substrate's flatness and morphology with regular steps are essential for high quality of the superconducting films. In the films grown on MgO annealed between 800 and 1200 degreesC and showing rough surface and Ca-segregates, J(c) was lower. However, J(c) was increasing with heat treatment temperature of the substrate, possibly due to the Ca-segregates inducing or acting as pinning centers. The effects of Ca-segregates are of interest for maximization of the quality of the high-temperature superconducting films. In the literature, up to now, these effects have not been considered and therefore Ca-segregates removal has been recommended. (C) 2002 Elsevier Science B.V. All rights reserved.

Thin films of the infinite-layer compound Sr0.6Ca0.4CuO2-delta have been prepared by off-axis rf magnetron sputtering on SrTiO3 (001). For an opt mum level of oxygen vacancy superconductivity was observed (T-c onset = 42 K and T-c(rho=0) = 11 K). Structural and transport data suggesting n-type superconductivity are presented and discussed. Infinite-layer thin films of Sr0.5Ca0.4CuO2-delta, without trivalent cation doping and showing n-type superconductivity, are reported for the first time.

We report an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of the colossal magnetoresistive manganites La2/3Ca1/3Mn1-xMexO3 (Me = Al, In; x less than or equal to 0.05). The temperature dependences of the EPR linewidth and integral intensity have been analyzed in terms of the bottleneck spin relaxation and small-polaron hopping models. The exchange coupling integral between Mn3+ and Mn4+ ions and the polaron activation energy decrease with increasing doping level. A discussion is given concerning the factors which could explain the observed changes.

The simulation of interstitial void evolution in a model of crystalline silicon subjected to topological disordering revealed that the void size distribution broadens and shifts towards smaller mean diameter size, when the lattice disordering increases. The size distribution is not gaussian and the mean diameter of the voids is lower than the crystalline value. The calculation of the structure factor corresponding to void configuration evidences the absence of the short and long range order and point out to the presence of the medium range order.

The paper refers to the study of patch antennas with a high permittivity dielectric substrate ((Zr-0.8 Sn-0.2)TiO4) obtained at National Institute of Materials Physics Bucharest. Because of the high value of the permittivity of the substrate material it was necessary to perform a comparative investigation of different prototypes and arrays. It was considered two substrate thickness h(1)=1mm and h(2)=2mm and two frequencies f(1)=2.4 GHz and f(2)=1.8 GHz. For each case we presented the radiation pattern in the two principal planes (E and H) and input impedance. Also, some radiation pattern for rectangular array with n=4, 16, 36 patches are presented. In the Conclusion section a discussion on the suitability of patch antennas with high permittivity substrate is done.

Effects of high temperature, up to 1700 degreesC, annealing in vacuum of CVD diamond on its structure, optical and mechanical properties are investigated. Translucent polycrystalline diamond films of thickness 0.06-1.0 mm were grown by microwave plasma CVD method, and examined with transmission electron microscopy, optical absorption spectroscopy, and three-point bending technique to measure the fracture strength. A progressive darkening of the samples, with appearance of absorption features specific for graphite-like material, was observed upon annealing at temperatures above 1300 degreesC. The formation along grain boundaries of amorphous carbon and/or well crystallized graphite layers, 5-20 nm thick, as well as intra-granular graphite islands, was directly observed with TEM. This internal diamond-graphite transformation process can be described by two activation energies, both values being much less than those known for the surface graphitization of diamond. The fracture strength of the diamond films increases up to 50% with annealing temperature (1460-1640 degreesC), this being ascribed to a build up of compressive stress as a result of local diamond-graphite conversion. (C) 2003 Elsevier Science B.V. All rights reserved.