  We report the finding of new surface treatments that permit to manipulate the
niobium resonator nitrogen content in the first few nanometers in a controlled
way, and the resonator fundamental Mattis-Bardeen surface resistance and
residual resistance accordingly. In particular, we find surface infusion
conditions that systematically a) increase the quality factor of these 1.3 GHz
superconducting radio frequency (SRF) bulk niobium resonators, up to very high
gradients; b) increase the achievable accelerating gradient of the cavity
compared to its own baseline with state-of-the-art surface processing. Cavities
subject to the new surface process have larger than two times the state of the
art Q at 2K for accelerating fields > 35 MV/m. Moreover, very high accelerating
gradients ~ 45 MV/m are repeatedly reached, which correspond to peak magnetic
surface fields of 190 mT, among the highest measured for bulk niobium cavities.
These findings open the opportunity to tailor the surface impurity content
distribution to maximize performance in Q and gradients, and have therefore
very important implications on future performance and cost of SRF based
accelerators. They also help deepen the understanding of the physics of the RF
niobium cavity surface.
