{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"50780ff75922747a6badfad88a1adcd2b0daf67d0c570e22010760573b194043","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["aa78c38194f169fba567041943acdaaf956dbfb87e994090c197b50f50b36bcc","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"24d9b803b385cbf5df58b2d0aae4728474466c5bda92744a3ba75348085602e7","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":"2","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":"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 regime?","topic_ref":null},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"8d6b3e6219c6f69ae2312fec719d1607473f8c0a1ce9c1b5568a9e5c8b9a5e70","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"a51d5a9154fe918616d257fb06b386a1b338816cac354b45d8b57faca46e30ff","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"jg833GCs0rtf8fXWU1TAKAWfeMfbFo4gG2rFbhpSC1Kg2TFRGb8ImNrZHPG5d8ZF5MibJiIXZJlPqkWQ4thBBA"},"schema":"pubphys.envelope/1"},"record_hash":"8d6b3e6219c6f69ae2312fec719d1607473f8c0a1ce9c1b5568a9e5c8b9a5e70","leaf_index":2744}