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Identify the mechanism (grain-boundary weak links and local hot spots, trapped flux in a granular film, poor film-substrate thermal contact, or other) and predict $R_{\\mathrm{s}}(H)$ from measured film microstructure for different deposition methods, including high-power impulse magnetron sputtering.","problem_ref":null,"references":"","settled_by":"$R_s(H)$ of cavities with controlled grain size and grain-boundary density, matched by a model fixed before the tests.","status_note":"A 2022 model (Ramiere, Antoine, Amrit, Phys. Rev. Accel. Beams 25, 022001, https://doi.org/10.1103/PhysRevAccelBeams.25.022001) attributed the slope to hot spots in granular films; competing explanations persist.","title":"Origin of the medium-field Q-slope in niobium-on-copper film 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