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

Utilizing difference in temperature dependencies we decoupled Bardeen-Cooper-Schrieffer (BCS) and residual components of the microwave surface resistance of superconducting niobium at all rf fields up to Brf∼115 mT. We reveal that the residual resistance decreases with field at Brf≲ 40 mT and strongly increases in chemically treated niobium at Brf>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 °C vacuum baking.