Professor Paolo G. Radaelli OSI

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.