{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"6c7704aa8c0e0a2141d4912df0e53f4226a0f60c8bb8eb759546aa9fa74f754f","created":"2026-10-03T07:17:59Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"7ba7d78d3a58f1fa64400847ff9ea921e2f27a34e5c1c08bd38af283b9da2467","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"classification","assisted_by":[],"external_id":"cm.dilute-superconductivity.fese-bcs-bec","kind":"well-posed","literature_status":"contested","n":"1","parents":[],"plain":"In FeSe the energy needed to break a pair is comparable to the energy of the fastest electrons, which is unusual. 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":null,"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 regime","topic_ref":"1691597bf36b5c4ae6e8e76cab311c80818c254bf6df91fc1604cdef43bb8e76"},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"aa78c38194f169fba567041943acdaaf956dbfb87e994090c197b50f50b36bcc","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"e01a26c12da7434d0c8dc00f8aa5a39e09b7e1febe8408c1c4bd2585abbe1b60","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"sdpuNq2iWo0Vwumhv879hw-8LoD5v98GRNvbueC3Id3EA_nbd8nDRODMpuItsMzilJuyLQbFAvYWjAkLDu0hCQ"},"schema":"pubphys.envelope/1"},"record_hash":"aa78c38194f169fba567041943acdaaf956dbfb87e994090c197b50f50b36bcc","leaf_index":944}