National Institute Of Materials Physics - Romania

The evolution of the critical current density of Li-doped YBa2Cu3O7-x polycrystalline samples submitted to neutron irradiation is investigated as function of magnetic field (0 less than or equal to B less than or equal to 6 T) temperature (5 less than or equal to T less than or equal to 85 K) and neutron fluence (0 less than or equal to Phi less than or equal to 9.98 x 10(17) cm(-2)). At fluences lower than 10(17) cm(-2), a second peak in j(s) vs. B dependence is present (fish-tail effect). Its magnitude decreases with increasing the fluence. Above 10(17) cm(-2), the second peak of current density completely disappears; instead, the logarithmic susceptibility shows a second peak at a certain field B-infi. A dependence of B-infi on fluence is proposed.

The effect of electron irradiation having the energy of 75, 100, and 200 keV on structural modifications of Bi-2212 superconducting samples has been studied. For the last energy, the irradiation time from zero to 150 min was used. At a constant energy of the electrons, the observed phenomena consist in the disappearance of the incommensurate unidimensional modulation, in the decreasing of spots intensity and their elongation along the equivalent crystallographic axis a, and even spot splitting with the occurrence of double extra spots, with the increase of the irradiation time. After electron irradiation with energy of 75 and 100 keV, the structural modifications lead to some spot patterns consisting of some planar lattices (in some cases a pseudotetragonal one) that are twisted on each other at different angles (8 degrees, 13.6 degrees, 19 degrees, etc.) around the axis of the incident electron beam. For the irradiation at increased doses of thin microcrystals having reduced lateral dimensions, the electron diffraction spots were arranged in discrete or partial continuous Debye rings or continuous concentric Debye rings characteristic for the polycrystalline state. After electron irradiation with energy of 200 keV, the effects of electron irradiation on Bi-2212 samples depend strongly on irradiation fluence rate and time and consisted in the following: disordering defects in the diffraction patterns (disappearance of some spots, spot intensity modification, streaks occurrence, spot elongation); alteration and disappearance of incommensurate structural modification; conversion of single crystal particle areas into polycryslalline material; and quasi-amorphization. A simple approach based on the evaluation of the displacement yield of in-plane oxygen atoms vs. irradiation time for the different incident energy and electron fluence rates could explain the general trend of irradiation damage in HTS materials.

The influence of the sintering time and pressing force on the pellets on the occurrence and morphology of the superconducting phases in BSCCO system were studied. Scanning electron microscopy, X-ray microanalysis and X-ray diffraction were used to identify the phases and their morphologies. The 2223 crystals had a plate-like morphology and 2212, a polyhedral-like one. The optimum parameters in order to obtain a majority of 2223 phase were identified: i.e. a longer sintering time and medium pressing force. (C) 2000 Kluwer Academic Publishers.

In a freeze-dried nitrate precursor powder with cation composition Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.5:3.5, Sc2O3 was added. The stoichiometry for the mixed precursor powder was Bi:Pb:Sr:Sc:Ca:Cu = 1.7:0.3:2.0:0.25:2.5:3.5. The two nitrate precursor powders with and without Sc were thermally decomposed, pressed, and sintered in the same conditions. They show different behavior, as revealed by thermal analysis. X-ray diffraction (XRD), Scanning electron microscopy (SEM), and chi'(T) measurements. Addition of Sc slows down synthesis processes, including 2223-phase during reactive sintering. After 70 h of sintering, 43% of 2223-phase formed in the pellet with Sc, whereas in the Sc-free pellet, 51% of the 2223-phase was attained. Moreover, addition of Sc leads to a certain morphology, resulting in lower superconducting characteristics.

We report a direct observation of quantum dots formed spontaneously in a thick InGaN epilayer by high resolution transmission electron microscopy. Investigation of a (280 nm thick) In0.22Ga0.78N single layer, emitting in the blue/green spectral region, reveals quantum dots with estimated sizes in the range of 1.5-3 nm. Such sizes are in very good agreement with calculations based on the luminescence spectra of this specimen. (C) 2000 American Institute of Physics. [S0003-6951(00)00930-X].

The dephasing and relaxation kinetics of bosonic excitons interacting with a thermal bath of acoustic phonons is studied after coherent pulse excitation. The kinetics of the induced excitonic polarization is calculated within Markovian equations both for subcritical and supercritical excitation with respect to a Bose-Einstein condensation (BEC). For excited densities n below the critical density n,, an exponential polarization decay is obtained, which is characterized by a dephasing rate Gamma = 1/T2. This dephasing rate due to phonon scattering shows a pronounced exciton-density dependence in the vicinity of the phase transition. It is well described by the power law Gamma proportional to (n - n(c))(2) that can be understood by linearization of the equations around the equilibrium solution. Above the critical density we get a non-exponential relaxation to the final condensate value p(0) with \p(t)\ - \p(0)\ proportional to 1/t that holds for all densities. Furthermore we include the full self-consistent Hartree-Fock-Bogoliubov (HFB) terms due to the exciton-exciton interaction and the kinetics of the anomalous functions f(k) = (a(k)a-k). The collision terms are analyzed and an approximation is used which is consistent with the existence of BEG. The inclusion of the coherent exciton-exciton interaction does not change the dephasing laws. The anomalous function Fk exhibits a clear threshold behaviour at the critical density.

Electrical resistivity and calorimetric measurements on Dy3Co show that below the Neel temperature (T-N = 44 K) the non-collinear antiferromagnetic structure exhibits: field-induced magnetic phase transitions of a first-order type along all principal axes, accompanied by a strongly anisotropic giant magnetoresistance and by a change of the Sommerfeld coefficient of the specific heat. Quantum tunnelling of the magnetization appears to be possible for T < 0.6 K.

Using a newly developed multiple scattering scheme, we calculate band structure and transmission properties for acoustic waves propagating in bubbly water. We prove that the multiple scattering effects are responsible for the creation of wide gaps in the transmission even in the presence of strong positional and size disorder.

The presence and concentration of nitrogen and hydrogen impurities in thick diamond films grown by microwave plasma chemical vapor deposition at various H-2 gas flow rates, keeping a constant [CH4]:[H-2]=2.5% concentration ratio, have been determined by electron spin resonance and optical absorption spectroscopy. The relative concentration of both impurities, present as paramagnetic atomic species with different relaxation properties, has been found by ESR measurements to decrease exponentially with the increase in the H-2 gas flow rate. Moreover, the resulting values were proportional to the content of substitutional nitrogen and CHx groups obtained from infrared and ultraviolet-visible optical absorption measurements, respectively. The decrease in the concentration of both impurities with an increase in the quality of the studied diamond films, early observed from high resolution electron microscopy studies on the same samples, strongly suggests that the incorporation of both impurities, as paramagnetic atomic species, is directly related to the concentration of the extended lattice defects. (C) 2000 American Institute of Physics. [S0021- 8979(00)08611-4].

The analysis of the activation energy in the magnetization relaxation of Bi2Sr2CaCu2O8 + delta (BSCCO) single crystals reveals plastic vortex creep above the second magnetization peak (SMP). Above the peak field, the magnetic field dependence of the activation energy U(B) is very close to the form U(B) alpha B-1/2 and the intrinsic variation of U with the current density is weak. The existence of a crossover from elastic to plastic vortex creep across the SMP of high-temperature superconductors (HTSC) seems to be a general behavior, and may have important consequences on the nature of the thermally induced vortex solid-vortex fluid transition at high magnetic fields. (C) 2000 Elsevier Science B.V. All rights reserved.