Advancement in the understanding of the field and frequency dependent microwave surface resistance of niobium

TL;DR

This paper investigates how nitrogen doping affects the microwave surface resistance of niobium cavities across various frequencies, revealing that the anti Q-slope phenomenon can occur even in clean niobium at high frequencies, advancing understanding of superconducting RF surfaces.

Contribution

It demonstrates the frequency dependence of the anti Q-slope in niobium cavities and shows this effect can occur without nitrogen doping at high frequencies, providing new insights into the physics of surface resistance.

Findings

Anti Q-slope observed at high frequencies in clean niobium.

Frequency dependence of BCS surface resistance behavior.

Nitrogen doping reduces surface resistance and influences anti Q-slope.

Abstract

The radio-frequency surface resistance of niobium resonators is incredibly reduced when nitrogen impurities are dissolved as interstitial in the material, conferring ultra-high Q-factors at medium values of accelerating field. This effect has been observed in both high and low temperature nitrogen treatments. As a matter of fact, the peculiar anti Q-slope observed in nitrogen doped cavities, i.e. the decreasing of the Q-factor with the increasing of the radio-frequency field, come from the decreasing of the BCS surface resistance component as a function of the field. Such peculiar behavior has been considered consequence of the interstitial nitrogen present in the niobium lattice after the doping treatment. The study here presented show the field dependence of the BCS surface resistance of cavities with different resonant frequencies, such as: 650 MHz, 1.3 GHz, 2.6 GHz and 3.9 GHz, and processed with different state-of-the-art surface treatments. These findings show for the first time that the anti Q-slope might be seen at high frequency even for clean Niobium cavities, revealing useful suggestion on the physics underneath the anti Q-slope effect.