Why infinite-layer nickelates superconduct only as thin films
In plain words
Infinite-layer nickelates superconduct when made as thin films but not, so far, as bulk crystals made by the same chemical reduction step.
Precise statement
Explain why superconductivity in $\mathrm{R}1-x\mathrm{Sr}x\mathrm{NiO}2$ ($R = \mathrm{La}, \mathrm{Pr}, \mathrm{Nd}, \mathrm{Sm}$) appears in thin films but not in bulk samples made by the same topotactic reduction. 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.
What would settle it
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 in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
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.
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