Mechanism of the high-field Q-slope and its cure by baking

Electropolished bulk Nb cavities at $1.3\,\mathrm{GHz}$ and $T = 2\,\mathrm{K}$ show a steep drop of quality factor $Q_0$ above peak surface magnetic field $H_{\mathrm{pk}} \sim 900-1000\,\mathrm{G}$, removed by in-situ baking at $120\,\mathrm{C}$ for $\sim 48\,\mathrm{h}$. Identify the mechanism (niobium nanohydride precipitation within ~1e-5 cm of the surface, oxygen or vacancy-hydrogen complexes, or other) with a model predicting the onset field and its dependence on bake temperature and duration.

Depth-resolved measurement of hydrogen, oxygen and vacancy profiles before and after baking, correlated with $Q_0(H_{\mathrm{pk}})$ of the same cavities and matched by a quantitative model.