The atomic origins of reduced critical currents at [001] tilt grain boundaries in YBa 2Cu 3O 7− δ thin films

Abstract

Grain boundaries have long been known to have a deleterious and irreproducible effect on the transport properties of high- T c oxide superconductors, particularly in the high-angle regime where an exponential decrease in critical current has been reported. We demonstrate, through a combination of atomic resolution Z-contrast imaging and bond valence sum analysis, that it is the atomic structure of the grain boundary that dominates this behavior. [001] tilt grain boundaries in thin-film YBa 2Cu 30 7− δ are composed of arrays of dislocations in defined sequences. The resulting strain fields seriously perturb the local electronic structure, leading to a non-superconducting zone at the grain boundary. The width of this zone increases linearly with misorientation angle, naturally explaining the observed exponential decrease in critical current. In addition, the widely varying J c measurements for a given grain boundary misorientation can be naturally explained by the facetting of the grain boundary plane.