{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"97eda9bbda974cd237d832120085ebee359f1391b1411e968cdc1da2a91640f0","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["357458292fec08c2485037a95a31c1cc8de14915f95dbea8fe406bf0742f67bb","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"538774fdaea6587d7e12d9cfef96c5f082f5ed3197be1d94ccc6e0cbfe89ed43","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":"2","parents":[],"plain":"$\\mathrm{CeCu_2Si_2}$, 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 $\\mathrm{CeCu_2Si_2}$ ($T_{c} \\sim 0.6\\,\\mathrm{K}$) 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":"357458292fec08c2485037a95a31c1cc8de14915f95dbea8fe406bf0742f67bb","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 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