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$_{2}$Cu$_{3}$O$_{7}$. 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 YBa$_{2}$Cu$_{3}$O$_{7}$. 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 YBa$_{2}$Cu$_{3}$O$_{7}$.