Oxygen dependence of the Josephson weak link effect, specific heat, and the transition temperature of YBa2Cu3O x

We have measured χ(T) for oxygen-annealed YBa2Cu3Ox samples and observed a glass-like behavior at low magnetic fields which can be explained in terms of a Josephson weak link effect. The glass transition temperature Tg depends on the oxygen annealing temperature and sample pellet pressure. Tg disappears when a sample was powdered after receiving an oxygen anneal, indicating that the Josephson weak link effect occurs between intergranular boundaries. In addition, Tc shifts from 93 K for the air-sintered samples to 90 K for the oxygen-annealed materials. Under vacuum annealing, Tg disappears and Tc first shifts back to 93 K. Prolonged vacuum annealing or vacuum annealing at higher temperatures depresses Tc to below 90 K. The temperature dependence of the specific heat shows a single anomaly for samples sintered in air or annealed under high vacuum. But a double-peak specific heat anomaly occurs for the oxygen-annealed samples.