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Title: What sets the residual surface resistance floor of niobium cavities
Plain: Even far below the superconducting transition, cavities keep a small leftover energy loss whose sources are only partly identified.
Precise: At $T <\sim 1.5\,\mathrm{K}$, where the BCS surface resistance at 1.3 GHz is negligible, the quality factor of the best Nb cavities saturates at $Q_0$ of order 1e11 (residual resistance $\sim 1-3\,\mathrm{nOhm}$), set by a temperature-independent residual surface resistance. Determine the fractions contributed by trapped magnetic vortices, two-level systems in the surface Nb2O5 oxide, and subgap quasiparticle states, as functions of field, frequency and surface treatment.
Settled by: Measurements of residual resistance versus trapped flux, frequency and field, combined with oxide removal or regrowth, that separate the three contributions.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: open
Status note: (empty)
Posed since: (empty)