The Anomalous Resonant Frequency Variation of Microwave Superconducting Niobium Cavities Near Tc

Superconducting radio-frequency (SRF) niobium cavities are the modern means of particle acceleration and an enabling technology for record coherence superconducting quantum systems and ultra-sensitive searches for new physics. Here, we report a systematic effect in Nb cavities indicative of improved superconducting properties - an anomalous decrease (dip) in the resonant frequency at temperatures just below the critical temperature Tc. The frequency dip magnitude correlates with cavity quality factor, near-surface impurity distribution, and Tc. It is also a precursor of the peculiar decrease in the BCS surface impedance with increasing RF current. A first demonstration of the coherence peak in the AC conductivity in Nb SRF cavities is also presented and found to correlate with a large frequency dip.