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.
Categories: cond-mat.supr-con physics.acc-ph