Dependence of the microwave surface resistance of superconducting niobium on the magnitude of the rf field

Utilizing difference in temperature dependencies we decoupled BCS and residual components of the microwave surface resistance of superconducting niobium at all rf fields up to $B_\mathrm{rf}\sim110$ mT. We reveal that the residual resistance decreases with field at $B_\mathrm{rf} \lesssim$ 40 mT and strongly increases in chemically treated niobium at $B_\mathrm{rf} > 80$ mT. We find that BCS surface resistance is weakly dependent on field in the clean limit, whereas a strong and peculiar field dependence emerges after $120^\circ$C vacuum baking.