DOI: 10.1016/0375-9601(93)90893-5
The uniform one-dimensional and square lattice hopping models are considered, where particles can be in one of two states at each site. Transport takes place by nearest neighborjumps between upper energy states. Trapping in lower energy states introduces correlations, the effect of which on the dc conductivity is estimated in the first step beyond the mean-field approximation.
Magnetic and Mossbauer experiments were performed on Fe100-xGdx thin films and on Fe80-xGdxB20 both in thin films and ribbons for x less-than-or-equal-to 20. The ferrimagnetic order is perturbed by the presence of B and Fe-interactions are.different. For small Gd content, the low anisotropy is in the film plane, while for greater Gd the anisotropy goes out of the plane giving typical features of hysteresis loops and strong decrease of Mossbauer hyperfine fields.
DOI: 10.1016/0009-2614(93)85422-K
Laser excited site-selective spectroscopy of Nd3+ ions in Ca3Ga2Ge3O12 garnet crystals has been investigated in the F-4(3/2)-->I-4(11/2) and F-4(3/2)-->I-4(9/2) regions. Six non-equivalent sites were observed for the Nd3+ in this material. The energy levels of some relevant multiplets were obtained for each Nd3+ site. The evolution of the non-equivalent Nd3+ CryStal field sites in Ca3Ga2Ge3O12 as a function of dopant concentration has been investigated in the range up to 8 at%. The nature of the Nd3+ sites in terms of the possible charge compensating defects is discussed. The formation of Nd3+-Nd3+ pairs is suggested.
The formation of nickel oxide from Ni-Al takovite-like compounds has been investigated. Information about the stoichiometry of the NiO phase obtained by Diffuse Reflectance (DR) spectroscopy is discussed in connection with TPR and solubility data.
SiO2-Sm2O3 vitreous films 120 nm thick were obtained via the sol-gel method. Infrared, energy-dispersive analysis of X-rays, spectroscopic ellipsometry and absorption spectroscopy showed a total inclusion of samarium in the silica matrix, which remains amorphous up to 1000-degrees-C. The inclusion of the samarium ion increases the porosity. The optical properties of films were investigated by spectro-ellipsometry, and the parameters obtained were compared with those obtained by other techniques.
A class of periodic one-dimensional models with nearest-neighbor interactions is presented in which particles can hop between adjacent sites with symmetric jump rates. The dc conductivity is calculated exactly for a wide choice of the factor describing the influence of the nearest-neighbor interaction on the jump rates. By allowing the cell size to become infinite, the result is extended to several nonperiodic chains including the random ones.
The results of real-time neutron diffraction measurements during the superconducting phase formation process in the Bi(Pb)-Sr Ca-Cu-O system are reported. A Sr-Ca-Cu-O type precursor, with the same stoichiometry as the 2223 phase, was used as starting material, and the temperature range favorable to the formation of the 2223 phase was investigated. The diffraction patterns were processed by a multiphase Rietveld refinement. The formation and decomposition of the 2201 and 2212 phases were directly observed. Experimental evidence on the existence of a partially melted phase in the range 855-860-degrees-C, involved in the formation of the 2223 phase, is discussed.
The dissipation mechanism in high-T(c) superconducting ceramics was studied by analyzing the magnetic field dependence of the transport critical current and the form of the current-voltage characteristics. It was found that the actual magnetic field dependence of the transport critical current is significantly slower than that predicted by the weak-link-quenching model. The low-voltage-level current-voltage characteristics were described in terms of thermally activated flux creep at grain boundaries, taking into account collective pinning of intergranular Josephson vortices. For the investigated samples, a low-field, high-temperature mean pinning energy barrier of a few tens of meV was determined.
DOI: 10.1016/0168-583X(93)95100-J
The operational range of the gas counters used in conversion electron Mossbauer spectroscopy (CEMS) is at the limit between the proportional counter and Geiger Muller domains. Starting from the theory of phenomena in gas detectors under space charge conditions, a general expression for the multiplication factor was obtained. The theoretical results, presented graphically, are discussed in connection with experimental data.