# Understanding Quality Factor Degradation in Superconducting Niobium Cavities at Low Microwave Field Amplitudes

## TL;DR

This paper investigates the low field Q slope in superconducting niobium cavities, revealing new features and suggesting two-level systems in niobium oxide as a possible cause, which impacts residual resistance understanding.

## Contribution

The study extends measurement techniques to ultralow fields, uncovering two new features of LFQS and proposing a novel explanation involving niobium oxide two-level systems.

## Key findings

- Saturation of LFQS at fields below 0.1 MV/m
- Thicker niobium pentoxide enhances degradation
- LFQS may be caused by two-level systems in niobium oxide

## Abstract

In niobium superconducting radio frequency (SRF) accelerating cavities a decrease of the quality factor at lower fields - a so called \emph{low field Q slope or LFQS} - has been a long-standing unexplained effect. By extending the high $Q$ measurement techniques to ultralow fields we discover two previously unknown features of the effect: i) saturation at rf fields lower than $E_\mathrm{acc} \sim 0.1$~MV/m; ii) strong degradation enhancement by growing thicker niobium pentoxide. Our findings suggest that the LFQS may be caused by the two level systems in the natural niobium oxide on the inner cavity surface, thereby identifying a new source of residual resistance and providing guidance for potential non-accelerator low field applications of SRF cavities.