  We report on an effort to improve the performance of superconducting
radiofrequency cavities by the use of heat treatment in a temperature range
sufficient to dissociate the natural surface oxide. We find that the residual
resistance is significantly decreased, and we find an unexpected reduction in
the BCS resistance. Together these result in extremely high quality factor
values at relatively large accelerating fields Eacc ~20 MV/m: Q0 of 3-4x10^11
at <1.5 K and Q0 ~5x10^10 at 2.0 K. In one cavity, measurements of surface
resistance versus temperature showed an extremely small residual resistance of
just 0.63+/-0.06 nOhms at 16 MV/m. SIMS measurements confirm that the oxide was
significantly dissociated, but they also show the presence of nitrogen after
heat treatment. We also present studies of surface oxidation via exposure to
air and to water, as well as the effects of very light surface removal via HF
rinse. The possibilities for applications and the planned future development
are discussed.
