{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"77f28f78f8c059aa7c6c65f9f3796265f37fe644e85794871a945b31f3701e97","created":"2026-10-03T07:17:59Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"3f5d119db11a57bb584a2d9e84ceba74cb99b157b86eab3f08d2b5a4ea837a83","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"cm.heavy-fermion-sc.cecu2si2-gap","kind":"well-posed","literature_status":"contested","n":"1","parents":[],"plain":"CeCu2Si2, the first heavy-fermion superconductor (1979), was long believed to have a gap vanishing along lines on the Fermi surface, but measurements since 2014 show a full gap, which a simple d-wave state cannot produce.","posed_since":"2014","precise":"Determine the superconducting order parameter of CeCu2Si2 ($T_{c} \\sim 0.6\\,\\mathrm{K}$) consistent with: activated specific heat and penetration depth below $\\sim 0.1\\,\\mathrm{K}$ (Kittaka et al., PRL 2014), insensitivity of $T_{c}$ to electron-irradiation disorder (Takenaka et al., PRL 2017), the absence of a Hebel-Slichter peak in NMR $1/T_{1}$, and a neutron spin resonance. Candidates: $s\\pm$ between heavy and light bands, $d+d$ band-mixing pairing, and sign-preserving s++. An answer is one state fitting all these data, plus a phase-sensitive or impurity-state test that separates sign-changing from sign-preserving gaps.","problem_ref":null,"references":"","settled_by":"A phase-sensitive measurement (Josephson or quasiparticle-interference sign test) combined with disorder series that discriminates $s\\pm$, $d + d$ and $s++$ in the same crystals.","status_note":"A 2023 Reviews of Modern Physics colloquium (Smidman et al.) summarizes the evidence for a full gap; whether the gap changes sign is disputed.","title":"Order parameter of the fully gapped heavy-fermion superconductor CeCu2Si2","topic_ref":"42e2b4fc8c37f8d74e57ec83f9086a24e04ebe52927fdc7a9b95f3d2fc3088c5"},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"357458292fec08c2485037a95a31c1cc8de14915f95dbea8fe406bf0742f67bb","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"27696e430906039d06e0c84be805b74770761f485c4b2c9f2eb32767565bc81d","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"5My9EH4JiXSsZF8BcQmWvzn6RFrSbTih9gnDhWvZfxDXKShxLnSNJhUpxBD1qdyTJvq0sZXs7GWWtwiJUnLjCw"},"schema":"pubphys.envelope/1"},"record_hash":"357458292fec08c2485037a95a31c1cc8de14915f95dbea8fe406bf0742f67bb","leaf_index":1009}