Effect of interstitial impurities on the field dependent microwave surface resistance of niobium

TL;DR

This study systematically investigates how dissolved nitrogen impurities affect the microwave surface resistance of niobium resonators, revealing optimal treatments for maximizing Q-factor and providing new insights into underlying physics.

Contribution

It is the first comprehensive analysis comparing different nitrogen doping levels and their impact on surface resistance components and resonator performance.

Findings

Identified optimal nitrogen doping levels for highest Q-factor.

Demonstrated the relationship between nitrogen impurities and field-dependent surface resistance.

Provided new understanding of vortex-induced losses in niobium resonators.

Abstract

Previous work has demonstrated that the radio frequency surface resistance of niobium resonators is dramatically reduced when nitrogen impurities are dissolved as interstitial in the material. The origin of this effect is attributed to the lowering of the Mattis and Bardeen surface resistance contribution with increasing accelerating field. Meanwhile, an enhancement of the sensitivity to trapped magnetic field is typically observed for such cavities. In this paper we conduct the first systematic study on these different components contributing to the total surface resistance as a function of different levels of dissolved nitrogen, in comparison with standard surface treatments for niobium resonators. Adding these results together we are able to show for the first time which is the optimum surface treatment that maximizes the Q-factor of superconducting niobium resonators as a function of expected trapped magnetic field in the cavity walls. These results also provide new insights on the physics behind the change in the field dependence of the Mattis and Bardeen surface resistance, and of the trapped magnetic vortex induced losses in superconducting niobium resonators.