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This hints that its pairs are small and tightly bound, between ordinary weakly bound pairs and molecule-like bosons.","posed_since":"2014","precise":"FeSe and FeSe1-xSx have $\\Delta/E_{F}$ of order 0.3 to 1 on some pockets (Kasahara et al., PNAS 2014). Determine their place on the BCS-BEC crossover, parameterized by $1/(k_{F} \\xi)$ per pocket ($\\xi$ the pair size), by testing for preformed pairs above $T_{c}$ (pseudogap, large superconducting fluctuations), for the shape of the Bogoliubov dispersion (back-bending at $k_{F}$ or minimum at $k = 0$), and whether tetragonal FeSe1-xSx with $x > 0.17$ is BEC-like. An answer is a classification per pocket with consistent spectroscopic and thermodynamic evidence.","problem_ref":"aa78c38194f169fba567041943acdaaf956dbfb87e994090c197b50f50b36bcc","references":"","settled_by":"Photoemission of the Bogoliubov dispersion together with thermodynamic detection or exclusion of preformed pairs above $T_c$ in the same crystals.","status_note":"Photoemission around 2020 reported BEC-like dispersion in tetragonal $\\mathrm{FeSe}1-x\\mathrm{S}x$, while the expected large pair fluctuations above $T_c$ remain disputed.","title":"Is superconducting $\\mathrm{FeSe}1-x\\mathrm{S}x$ in the BCS-BEC crossover 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