Pressure-induced enhancement of the critical current density in superconducting YBa2Cu3Ox bicrystalline rings

The dependence of the critical current density J(c)(T) on hydrostatic He-gas pressure to 0.6 GPa is determined for nearly optimally doped and strongly underdoped melt-textured YBa2Cu3Ox bicrystalline rings containing single [001]-tilt grain boundaries (GBs) with mismatch angles theta from 0 degrees to 31 degrees. For all samples with theta > 0 degrees, J(c) is found to increase rapidly under pressure, the rate of increase lying predominantly in the range +20 to +50 % GPa(-1). Within a simple tunneling model, this rate of increase is far too large to be accounted for by a decrease in the GB width W alone. Large oxygen relaxation phenomena in the GB are observed for all rings with finite theta, particularly if they are underdoped. A diagnostic method is introduced (pressure-induced J(c) relaxation) which reveals a significant concentration of vacant oxygen sites in the GB region. A concerted effort to fill such sites with oxygen anions, as well as to chemically compress the GB itself, should lead to significant enhancements in J(c) under ambient conditions.