A Novel Method to Detect Oxygen Vacancies in the Grain Boundaries of High-Tc Superconductors

A new diagnostic method is introduced (pressure-induced $J_{c}$ relaxation) which is capable of detecting oxygen vacancies in the grain boundaries of oxide superconductors. A series of bicrystalline rings of YBa$_{2}$Cu$_{3}$O$_{x}$ containing single [001]-tilt grain boundaries with mismatch angles from 0 to 31 degrees are studied. Even for samples with nearly optimal doping in the bulk, a significant oxygen deficiency in the grain boundary region is revealed. In addition, compressing the grain boundary region is found to lead to a considerable enhancement of the critical current density $J_{c}$.