National Institute Of Materials Physics - Romania

In-plane rho(ab) and out-of-plane p(c) resistivity measurements were performed on the same antiferromagnetic YBa2Cu3O6.25 single crystal over a wide range of temperatures T. rho(ab)(T) exhibits two crossovers with decreasing T: a crossover from metallic to weak localization behavior at 175 K and a second crossover to two-dimensional chiral variable-range hopping VRH behavior at 115 K. The latter reflects the topologic excitations induced in the spin system. rho(c)(T) displays an exp(-T-2/T-c,h(2)) dependence at high T and a VRH type dependence below 115 K. The T derivative of rho(c)(T) shows a kink at 32.65 K which we attribute to the antiferromagnetic ordering of the Cu( 1) spins.

The ferric form of the wild-type mouse neuroglobin (Ngb), a newly discovered heme protein which is primarily expressed in the brain of mammals, has been characterized by electron paramagnetic resonance (EPR) spectroscopy. The study reveals the simultaneous presence of two related structural forms in a wide range of pH values. The dominant low-spin form (>90%) with g-tensor principal values 3.15, 2.16 and 1.34 can be attributed to a His-Fe-III-His configuration. The high-spin form with g(perpendicular to) = 5.97 and g(parallel to) similar to 2, can be ascribed either to a hexacoordinated His-Fe-III-H2O form or to a pentacoordinated His-Fe-III. The high-spin to low-spin ratio is found to decrease with increasing pH values. (C) 2002 Elsevier Science B.V. All rights reserved.

Electrolytic colouration has been undertaken for not only a KCl crystal containing Pb2+ ions but also KCl crystals containing alkali impurity ions (Li+, Na+ and Rb+) besides Pb2+ ions. Absorption spectra of these electrolytically coloured crystals have been studied, together with the emission spectra. Similar spectra were observed among these crystals. Eight weak absorption bands are observed in the 290-700 nm region, while five intense bands are observed in the 200-290 nm region. It is suggested-that these bands arise from Pb- ions with tetragonal crystal field symmetry. The energy-level assignment for these absorption bands is made using the energy-level diagram of the isoelectronic Bi-0 atom with C-4v crystal field symmetry. It is suggested that an off-centre position effect of the Li+ ion is observed in the kCl crystal codoped with Li+ ions.. Broad emission bands at 1530 and 1580 nm have been. observed in electrolytically coloured KCl containing Pb2+ ions and KCl containing Li+ and Pb2+ ions, irrespectively.

We studied the YBa2Cu3O7-x bulk superconductor doped with BaZrO3 up to 50 wt.%, obtained by solid-state reaction powder technology. From DC magnetization loops and low frequency AC susceptibility measurements we determined the influence of the BaZrO3 doping level on the critical temperature, critical current density, field for full penetration, and intergrain lower critical field. The results show that even high content of BaZrO3 does not lead to degradation of the superconducting properties of bulk YBa2Cu3O7-x.

A phenomenological model was developed to explain quantitatively, without free parameters, the production of primary defects in silicon after particle irradiation, the kinetics of their evolution toward equilibrium and their influence on detector parameters. The type of the projectile particle and its energy is considered in the evaluation of the concentration of primary defects. Vacancy-interstitial annihilation, interstitial migration to sinks, vacancy-impurity complexes (VP, VO, V2O), and divacancy (V-2) formation are taken into account in different irradiation conditions, for different concentrations of impurities in the semiconductor material, for 20 and 0degreesC. The model can be extended to include other vacancy and interstitial complexes. The density of the reverse current in the detector after irradiation is estimated. Comparison with experimental measurements is performed. A special application considered in the paper is the modelled case of the behaviour of silicon detectors operating in the pion field estimated for the LHC accelerator, under continuum generation and annealing.

The cohesive energy of the Al-Mn Mackay icosahedron (MI) was estimated for different variants of chemical order, using pair-bonding energies. This problem is related to the nucleation of i-AlMn phase in sequentially deposited thin films. The chemically ordered MI comes out as the thermodynamically most stable configuration. The role of kinetic factors during deposition is also discussed.

A slice of the (Cu,C)Ba2Ca3Cu4O12-y bulk superconductor grown by high-pressure synthesis was polished down to a thickness of 50 mum, and then irradiated with high-energy (240 MeV) Au15+, ions (10 11 cm(-2)). Since the length of the ion tracks was estimated to be 12.2 mum, our sample consists of two different types of grain, with and without columnar defects, having very different critical current densities. Using the third-harmonic response of the sample, we were able to separate the small dissipation in the irradiated grains from the much larger dissipation in the non-irradiated grains, in a certain experimental range of temperature, field and current density. The critical current density was estimated from critical state models. For temperatures in the range of 85-105 K, we observed a peak effect at high dc magnetic fields, suggesting the possibility of a record irreversibility field at 77.3 K.

Effect of 1-4 wt.% excess PbO, added to the calcined powder, on the structure and dielectric and piezoelectric properties of Bi-doped lead titanate ceramics was investigated. The ceramics containing 4, 6 and 8 mol% Bi were prepared by the usual ceramic technique, using high purity oxides. The calcined powder of each composition was next intimately mixed with 1, 2, 3 and 4 wt.% PbO and then sintered at 1200-1300 degreesC for 3 h. The dielectric piezoelectric and electromechanical properties were determined as a function of excess PbO. Addition of PbO up to 2 wt.% was found to improve the densification, by the presence of the PbO liquid phase during sintering. For all Bi doping levels, the dielectric constant and the thickness coupling factor reach maxima (representing an increase of 2.5-8% for dielectric constant and 7-8% for thickness coupling factor) at about 2 wt.% PbO addition. The planar coupling factor is very low, less than 0.01, regardless the PbO content. The disipation factor slightly decreases with increasing PbO content and the mechanical quality factor of the thickness mode was about 300. The experimental results were discussed in terms of the mechanisms of densification in the presence of the liquid phase and of lead vacancy concentration. (C) 2002 Elsevier Science Ltd. All rights reserved.

Interface reaction and magnetism of epitaxially-grown Fe on InAs(100) are studied by core-level photoemission (As 3d and In 4d) and Fe 2p X-ray magnetic circular dichroism using synchrotron radiation. The reactivity of Fe/InAs(100) is relatively low compared to that of other interfaces involving deposition of 3d metals on III-V semiconductors. As a consequence, we observe a magnetic signal at Fe L-2,L-3 edges for the lowest thicknesses studied (1 ML). The atomic magnetic moment reaches a value close to that of the bulk alpha-Fe (2.2 mu(B)) for Fe coverages exceeding 5 ML. A ferromagnetic compound with approximate stoichiometry of FeAs is formed at the interface. The orbital magnetism represents between 12 and 20% of the total momentum, due to 3d density of states depletion and to crystal-field modification of the electronic levels. These properties make the Fe/InAs(100) interface very promising for spin-tunneling devices.

The thermochemical behavior of the coordination compound [Fe(urea)(6)](NO3)(3) was studied by simultaneous CG-TG-DTG-DTA and mass spectrometry methods non-isothermal conditions. The compound decomposes at -200 degreesC into a mixture of spinel-type oxides and hematite. The nature and particle size of the final decomposition products are strongly associated with the conditions during the thermal treatments, in particular the heating rate and the calcination temperature. A certain fraction of the products are formed as nanometric particles; they show superparamagnetic behavior at room temperature. The comparably low temperature of the calcination treatments of this compound is a promising perspective to attain small sized magnetic powders. (C) 2002 Elsevier Science B.V. All rights reserved.