Professor Paolo G. Radaelli OSI

Abstract:

The effect of environmental factors such as moisture and temperature on the critical current of sintered YBCO was systematically investigated. The degradation rate of materials with different characteristics was also analysed. The synthesis procedure used, powder grain size, silver oxide addition, sintering conditions and final density were all considered as possible factors affecting the material's sensitivity to ageing. The transport critical current was measured using the d.c. four probe technique, with and without magnetic field, on wire specimens, and with an a.c. contactless method with no field on moulded rings. Complex susceptibility measurements were also carried out to further qualify the material's behaviour, especially in cases where the macroscopic Ic had dropped to zero. The results showed qualitatively similar Ic versus time behaviour for the various samples, but with strong quantitative differences, which were correlated to the above factors. © 1990.

Abstract:

A sintering process of Bi-Ca-Sr-Cu-O superconductor is reported. The electrical and microstructural analysis of the pellets shows the coexistence of B and C phases with transition temperatures of about 85 K and 105 K, respectively. © 1989 Società Italiana di Fisica.

Abstract:

In this work a procedure followed for the preparation of superconducting compounds in the Bi-Ca-Sr-Cu-O system using the cytrate polymer precursor route is reported. A preliminary characterization of the structure and superconducting properties of the samples so obtained is given through X-ray diffraction analysis, resistivity and a.c. susceptibility measurements. All the samples so obtained showed the presence of superconducting phases at 80 K and 110 K beyond other non superconducting ones.

Abstract:

We report resonant x-ray scattering measurements on a single crystal of the orbitally degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below T-S = 365 K. We observe a strong resonant enhancement of the supercell reflections through the Ni K edge. The empirically extracted K-edge shift between the crystallographically distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of similar to 1 eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.

Abstract:

We present the discovery and refinement by neutron powder diffraction of a new magnetic phase in the Na1-xCaxMn7O12 quadruple perovskite phase diagram, which is the incommensurate analogue of the well-known pseudo-CE phase of the simple perovskite manganites. We demonstrate that incommensurate magnetic order arises in quadruple perovskites due to the exchange interactions between A and B sites. Furthermore, by constructing a simple mean field Heisenberg exchange model that generically describes both simple and quadruple perovskite systems, we show that this new magnetic phase unifies a picture of the interplay between charge, magnetic and orbital ordering across a wide range of compounds.