Bi-epitaxial YBCO grain boundary Josephson junctions on SrTiO 3 and sapphire substrates

Abstract

This paper deals with YBCO Josephson junctions based on artificially generated in-plane bi-epitaxial grain boundaries fabricated on SrTiO 3 and buffered R-plane sapphire. The grain boundary was obtained by partly interposing a MgO seed layer between a bare or even-buffered substrate and a CeO 2 layer [1]. The devices were produced by patterning the overhanging YBCO film in a form of stripes across the grain boundary occurring at the interface between the two regions. The samples were structurally and electrically characterized. As shown by X-ray diffraction analyses and high resolution electron microscopy, the structural perfection of the YBCO film decreases as the complexity of the stacking sequence increases. The fabricated junctions behave according to the Resistively Shunted Junction model for both the samples on SrTiO 3 and on Al 2O 3. Under microwave irradiation, the devices displayed several Shapiro steps while their critical currents were deeply modulated under magnetic field. By comparing the transport properties of the devices with the structural properties of the YBCO film on the different stacking sequences, we found a strong correlation between the normalized resistance R N A and the degree of structural disorder in the superconducting film.