Signatures of enhanced superconducting properties in niobium cavities

Superconducting radio-frequency (SRF) niobium cavities are critical for modern particle accelerators, as well as for advancing superconducting quantum systems and enabling ultrasensitive searches for new physics. In this work, we report a systematic observation of an anomalous frequency dip in $\mathrm{Nb}$ cavities, which occurs at temperatures just below the critical temperature (${T}_{c}$), indicative of enhanced superconducting properties at $T\ensuremath{\ll}{T}_{c}$. The magnitude of this dip is strongly correlated with the rf surface resistance, impurity distribution near the surface, and ${T}_{c}$. Additionally, we report measurements of the coherence peak in the ac conductivity of two $\mathrm{Nb}$ SRF cavities processed using distinct methods. By comparing recent theories developed to model this experimental data, we show that the frequency-dip feature, larger coherence peak height, and reduction in the temperature-dependent surface resistance with rf current occur at minimal but finite levels of disorder.