Spatially resolved observation of supercurrents across grain boundaries in YBaCuO films

Spatially resolved resistivity measurements of current transport across individual grain boundaries have been made on superconducting YBa ₂Cu₃O₇. These experiments were done by low-temperature scanning electron microscopy with a resolution of 1 to 2 micrometers, and they show directly the limitation of the critical current density caused by grain boundaries in YBa₂Cu₃O ₇. Furthermore, complex spatial patterns of the current transport across grain boundaries were observed. These patterns reflect self-excited resonances of the grain boundaries and are closely correlated to the unexplained "sub-gap structure" in the current-voltage characteristics of polycrystalline YBa₂Cu₃O₇.