Beyond <i>s</i>- and <i>d</i>-waves: <i>d + d</i> intra- and inter-band pairing in multi-band superconductors

Recent experiments in multi-band Fe-based and heavy-fermion superconductors have challenged the long-standing dichotomy between standard s- and d-waves. We discuss an alternative in the form of a gapped d + d intra- and inter-band pairing which provides a natural interpolation between standard s and d-waves. We illustrate that this is equivalent to an sτ₃ pairing initially proposed in the context of Fe-based superconductors [1]. We demonstrate that similar candidates are possible outside of the Fe-based family by constructing an analog for
the heavy-fermion superconductor CeCu₂Si₂. We compare and contrast d + d with similar pairings which occur in ³He.

[1] E. M. Nica, R. Yu, and Q. Si, npj Quantum Materials 2, 24 (2017).