  The radio-frequency surface resistance of niobium resonators is incredibly
reduced when nitrogen impurities are dissolved as interstitial in the material,
conferring ultra-high Q-factors at medium values of accelerating field. This
effect has been observed in both high and low temperature nitrogen treatments.
As a matter of fact, the peculiar anti Q-slope observed in nitrogen doped
cavities, i.e. the decreasing of the Q-factor with the increasing of the
radio-frequency field, come from the decreasing of the BCS surface resistance
component as a function of the field. Such peculiar behavior has been
considered consequence of the interstitial nitrogen present in the niobium
lattice after the doping treatment. The study here presented show the field
dependence of the BCS surface resistance of cavities with different resonant
frequencies, such as: 650 MHz, 1.3 GHz, 2.6 GHz and 3.9 GHz, and processed with
different state-of-the-art surface treatments. These findings show for the
first time that the anti Q-slope might be seen at high frequency even for clean
Niobium cavities, revealing useful suggestion on the physics underneath the
anti Q-slope effect.
