Can multilayer coatings raise the RF field above niobium's limit
In plain words
A superconductor can carry an RF magnetic field only up to a maximum (the superheating field); thin layered coatings might push past niobium's value, but this has not been shown in a working cavity.
Precise statement
The superheating field of clean Nb is $H_{\mathrm{sh}}$ approximately 2000-2400 G at low T, and the best Nb cavities reach $H_{\mathrm{pk}}$ near 2000 G. Determine whether superconductor-insulator-superconductor multilayers (Nb3Sn, NbN or similar layers thinner than the London penetration depth on Nb) raise the attainable RF field above bulk Nb $H_{\mathrm{sh}}$ in a cavity at $f \sim 1\, \mathrm{GHz}$, and give the maximal field versus layer thickness and defect density.
What would settle it
A coated cavity sustaining $H_{pk}$ above the bulk Nb superheating field at $T \sim 2\,\mathrm{K}$, with the field verified by calibrated RF measurement.