Order parameter of the fully gapped heavy-fermion superconductor CeCu2Si2
In plain words
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.
Precise statement
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.
What would settle it
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 in the literature
A 2023 Reviews of Modern Physics colloquium (Smidman et al.) summarizes the evidence for a full gap; whether the gap changes sign is disputed.