Springer Online Journal Archives 1860-2000
Abstract We have studied the physical properties of Y−Ba−Cu−Oxide superconducting materials by using Levitation, AC-susceptibility, macroscopic conductivity, Scanning Tunneling Microscopy (STM) local conductivity, Scanning Electron Microscope (SEM), X-rays, and Hall effect experimental techniques. Our results tend to indicate systematically that the grains formed in the synthesis do not show bulk superconductivity but rather are superconductors at the domain boundaries of the orthorhombic phase. It seems that a coexistence of semiconductor and metallic regions are formed at the twinned domain boundaries. The pellets become superconductors when the grains form clusters and are in intimate contact. This seems to suggest that the bulk of the grains is semiconducting and that a conducting percolative network of grain and domain boundaries may be responsible for the superconductivity. To understand the observed constant high transition temperature we propose a model of semiconductor-metal-semiconductor boundaries that give rise to superconductivity in a model like that of Little, Ginzburg, and Allender-Bray-Bardeen (1).
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