Microwave loss and oxygen annealing in YBa2Cu3Ox single crystals

Reversible thermal annealing of YBa2Cu3Ox single crystals in air has been studied by microwave loss measurements. Among the advantages of this contactless technique are the following: Crystals are kept in sealed capillaries throughout cycles of annealing and low-temperature measurement; features are resolved in loss versus temperature measurements that are not revealed by other techniques. Microwave loss is interpreted in terms of the Gittleman-Rosenblum model of fluxon motion in a viscous medium. Narrow temperature ranges are found for the formation of the 60- and 90-K phases. Optimal microwave response is obtained after but a few hours of annealing, indicating the importance of c-axis diffusion.