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

TL;DR

This study investigates how the microwave surface resistance of superconducting niobium varies with RF field strength, revealing different behaviors of BCS and residual components under various treatments and field conditions.

Contribution

It provides a detailed analysis of the field dependence of BCS and residual surface resistance components in superconducting niobium, including effects of chemical treatments and temperature baking.

Findings

Residual resistance decreases with field below 40 mT

Residual resistance increases at fields above 80 mT after chemical treatment

BCS surface resistance shows weak field dependence in the clean limit

Abstract

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.