National Institute Of Materials Physics - Romania

An EPR study of SrCl2:Fe2+ crystals grown in chlorine reveals the presence after X-ray irradiation, besides the trapped-hole Fe-cub(3+) center with cubic symmetry previously observed in crystals grown under argon, of a new Fe-trig(3+) center with axial [111] symmetry. The quantitative analysis of the strongly anisotropic X-and Q-band spectra resulted in a g = 2.0141 value and zero-held-splitting parameters B-2(0) = 800.6, B-4(0) = -0.44, B-4(-3) = -10.5 and B-4(3) = 0.33 (at T = 12 K, in 10(-4) cm(-1) units). The very large B-2(0) value is attributed to the presence of a charged defect replacing one of the eight nearest neighbor Cl- ligands accompanied by an off-center displacement of the Fe3+ ion towards it. (C) 1997 Elsevier Science Ltd.

In the biological effect of ionizing radiation, a very important process is the production of free radicals and their reactions with one another and with other biomolecules. Electron Spin Resonance (ESR) spectroscopy is an appropriate method to detect the free radicals produced by irradiation and also to estimate their concentration. The samples were irradiated at a linear electron accelerator, The ESR measurements were performed using X-band spectrometers. The resonance spectra of irradiated complex biological materials are presented and discussed. Interesting applications of the ESR method in the field of radiation treatment of biological material are outlined.

An alternative technique for the synthesis and crystal growth of langasite (La3Ga5SiO14) is presented. In order to keep constant the compound stoichiometry during these processes, we have used the raw material charge (mixed oxides) in a pressed pellet shape and also we have built a thermal screening system surrounded by separately controlled RF coils. In this way, a suitable solid/liquid interface shape has been obtained.

Bulk samples obtained from two wafers of a silicon monocrystal material produced by float-zone refinement have been analysed using the four-point probe method. One of the two wafers comes from an oxygenated ingot; two sets of pure and oxygenated samples have been irradiated with 24 GeV/c protons in the fluence range from 10(13) to 1.74 x 10(14) p/cm(2). Van der Pauw resistivity and Hall coefficient have been measured before and after irradiation as a function of the temperature. A thermal treatment (30 min at 100 degrees C) has been performed to accelerate the reverse annealing effect in the irradiated silicon. The irradiated samples show the same exponential dependence of the resistivity and of the Hall coefficient on the temperature corresponding to the presence of radiation-induced deep energy levels around 0.6-0.7 eV in the silicon gap. The free carrier concentrations (n, p) have been evaluated in the investigated fluence range. The inversion of the conductivity type from n to p occurred, respectively, at 7 x 10(13) and at 4 x 10(13) p/cm(2) before and after the annealing treatment for both the two sets. Only slight differences have been detected between the pure and the oxygenated samples.

In the Gd3Co11B4-xAlx alloys the ternary 3:11:4 phase with Ce3Co11B4 structure is formed with an Al content x approximate to 0.5 substituting for boron. The fraction of this phase decreases as the boron content decreases in the samples. The Al, Co and Gd in excess form Gd(Co,Al)(5) with CaCu5-type structure, and the fraction of this phase increases with increasing x. X-ray diffraction results show that these phases are not at equilibrium. A strong preference of aluminium for the 3g site in the CaCu5-type structure is determined by Rietveld refinement of the X-ray diffraction pattern of the end-series compound (x = 4). (C) 1997 Elsevier Science S.A.

The influence of Gd addition on the structural properties of Fe-Cu-Nb-Si-B nanocrystallized and amorphous alloys is studied. The crystallization temperature increases and the microstructure of the annealed samples changes. Gd addition induces the formation of Gd-Fe-B phases. In fully crystallized Fe-Gd-Cu-Nb-Si-B alloys the alpha-Fe(Si), Fe-Nb-B, Gd3Fe62B14 and Gd1.1Fe4B4 phases are observed. The evolution of the microstructure is followed as a function of the cumulative effects of annealing time and temperature. The results suggest the transformation of the metastable Gd3Fe62B14 phase into Gd1.1Fe4B4 and alpha-Fe. The hyperfine parameters of the Gd3Fe62B14 Mossbauer contribution are reported. (C) 1997 Elsevier Science S.A.

The magnetoresistivity tensor of an interacting two-dimensional electron system with a lateral and unidirectional electric or magnetic modulation, in a perpendicular quantizing magnetic field, is calculated within the Kubo formalism. The influence of the spin splitting of the Landau bands and of the density of states (DOS) on the internal structure of the Shubnikov-de Haas oscillations is analyzed. The Coulomb electron-electron interaction is responsible for strong screening and exchange effects and is taken into account in a screened Hartree-Fock approximation, in which the exchange contribution is calculated self-consistently with the DOS at the Fermi level. This approximation describes both the exchange enhancement of the spin splitting and the formation of compressible edge strips, unlike the simpler Hartree and Hartree-Fock approximations, which yield either one or the other.

The microhardness (H) of bulk glasses and thin films of the GexSb40-xS60 family has been measured. The volume of the micro-voids (Vh), their formation energy (Eh) and the modulus of elasticity (E) have been calculated. It is assumed that the bulk glasses and thin films have different elasticity levels due to the deformation of specific chemical bonds.

The current-voltage (I-V) characteristics of long Bi2Sr2Ca2Cu3O10/Ag (Bi:2223) multifilamentary tapes with different bending strains were measured in a large range of dissipation (10(-12)-10(-3) V/cm). The experimental data were successfully interpreted in terms of current-induced unbinding of the thermally-created vortex-antivortex pairs by taking into account the size limitation of vortex fluctuations. It was found that the shape of the I-V curves can be explained by the contributions to the dissipation of superconducting intergrain contacts having different qualities (i.e. different critical currents) and by the appearance of micro-cracks during the bending process. (C) 1997 Elsevier Science B.V.

The small-signal intrinsic impedance of GaAs/AlAs based resonant tunnelling diodes with thin barriers has been measured at room temperature over the full 0-2V bias-voltage and 0.05-40.05GHz frequency ranges, on stable, non-oscillating devices. The classical Esaki and the quantum-inductance equivalent circuits were used to model the impedance for CAD purposes. Information about the quasibound-state lifetime against bias-voltage was extracted.