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Title: Origin of the anti-Q-slope in nitrogen-doped niobium
Plain: When niobium is lightly doped with nitrogen, its energy loss falls as the field rises, the opposite of the usual behavior.
Precise: N-doped Nb cavities show the temperature-dependent surface resistance $R_{\mathrm{BCS}}$ decreasing with RF amplitude over $H_{\mathrm{pk}} \sim 100-900\,\mathrm{G}$, with a strong dependence on frequency and electron mean free path. Determine which mechanism (field-induced broadening of the quasiparticle density of states with a nonequilibrium quasiparticle distribution, reduced two-level-system losses, or other) reproduces $R_{\mathrm{BCS}}(H, f, T, \text{mean free path})$ quantitatively.
Settled by: A single model that fits $R_{\mathrm{BCS}}(H, f, T)$ for cavities at three or more frequencies with independently measured mean free path and no per-cavity fit parameters.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: (empty)