Colloquium : Unconventional fully gapped superconductivity in the heavy-fermion metal CeCu2Si2

The heavy-fermion compound CeCu${}_{2}$Si${}_{2}$ has long been known to be an unconventional superconductor with $d$-wave symmetry. Ordinarily, this would imply that the gap function has nodes on the Fermi surface. This Colloquium explains that recent experiments have shown that the gap is nonzero everywhere, if small where a single-band wave gap would vanish. The Colloquium discusses theoretical scenarios to explain these observations, as well as the implications for other unconventional superconductors.