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Candidates: residual apical oxygen or incorporated hydrogen in bulk, epitaxial strain, interface effects, secondary phases. An answer is a bulk sample with full diamagnetic shielding (filamentary or low-volume-fraction signals do not count), or an identification of the controlling structural or chemical variable.","problem_ref":"8b7dad2e733cfc5063fb3039fd55b19f0ae1abb46de61d73e5395f2bd78b440f","references":"","settled_by":"A bulk or freestanding-membrane sample with full shielding, or a controlled study varying the candidate variable in films that switches superconductivity on and off.","status_note":"Samarium-based infinite-layer films reached about $40\\ \\mathrm{K}$ (2025); a 2026 preprint reports low-volume-fraction, non-percolative superconductivity at ambient pressure in bulk crystals of the related square-planar trilayer nickelates (La,Pr)4Ni3O8 and (La,Y)4Ni3O8, so bulk superconductivity with full shielding is not established.","title":"Why infinite-layer nickelates superconduct only as thin 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