Atomic-level studies of superconducting and nonsuperconducting YBa2Cu3O7−x

Atomic-resolution images from superconducting and nonsuperconducting samples of YBa₂Cu₃O_7–x have been obtained by field ion microscopy. Both phases of the material exhibit a similar layered structure in the field ion images. Transmission electron micrographs of superconducting field ion tips show vividly the twinned structure of the orthorhombic phase. Time-of-flight (atom-probe) mass spectra from the two materials differ primarily in an enhanced molecular oxygen signal from the superconducting samples indicating that oxygen in the O(4) sites (CuO chains) is field desorbed preferentially as molecular oxygen ions.     

Chemical diffusion of water in the high-temperature proton conductor Ba3Ca1.18Nb1.82O9-δ

Ba₃Ca_1.18Nb_1.82O_9-δ (also referred to as BCN18) is a promising high-temperature proton conductor. In the dry state it contains an abundant amount of vacancies in the oxygen sublattice. Water may enter the compound occupying the vacancies and supplying protons to the lattice. The conductivity of these protons may be used in future applications of high-temperature proton conductors as electrolytes for SOFCs and in sensors. Chemical diffusion of water in this compound is described by the coupled outward flux of Ovacancies and the inward flux of protons (OH^•). The chemical diffusivity is in general a complicated function of the concentrations and the individual diffusivities. In this work, the chemical diffusivity of water is determined by studying the kinetics of a) the weight increase in TG experiments, and b) of dilatometric length changes upon subjecting the samples to humid atmospheres. The chemical diffusivities determined using techniques a) and b) are in good agreement. Likewise, the present data are compatible with those determined in a previous thermal desorption study. The present data, however, only apply to situations where the initial proton concentration is rather low and where the proton content is not changed dramatically in the experiment. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Activation energy in La0.7Ca0.3MnO3/YBa2Cu3O7-δ / La0.7Ca0.3MnO3 superconducting trilayers

Resistivity vs. temperature measurements on La_0.7Ca_0.3MnO₃/YBa₂Cu₃O_7-δ /La_0.7Ca_0.3MnO₃ (LCMO/YBCO/LCMO) trilayers with different YBCO thickness, were performed in external magnetic field H up to 8 T. By evaluating the activation energy U from the slope of the resistivity Arrhenius plot, a strong depression of U has been observed when decreasing the YBCO layer thickness and the absolute U values appear to be reduced with respect to the values reported in literature in the case of YBCO thin films and YBCO/insulating multilayers. Moreover, a logarithmic U vs. H dependence is shown both in the case of thick and thin YBCO layers indicating the formation of a two dimensional vortex lattice. The experimental data are discussed considering the strong influence of the ferromagnetic LCMO on the superconducting YBCO properties which reduces the effective YBCO thickness more than predicted by the conventional theories.