Multiorbital singlet pairing and d + d superconductivity

Abstract

Recent experiments in multiband Fe-based and heavy-fermion superconductors have challenged the long-held dichotomy between simple s - and d -wave spin-singlet pairing states. Here, we advance several time-reversal-invariant irreducible pairings that go beyond the standard singlet functions through a matrix structure in the band/orbital space, and elucidate their naturalness in multiband systems. We consider the s τ 3 multiorbital superconducting state for Fe-chalcogenide superconductors. This state, corresponding to a d + d intra- and inter-band pairing, is shown to contrast with the more familiar d + i d state in a way analogous to how the B- triplet pairing phase of 3 He superfluid differs from its A- phase counterpart. In addition, we construct an analog of the s τ 3 pairing for the heavy-fermion superconductor CeCu 2 Si 2 , using degrees-of-freedom that incorporate spin-orbit coupling. Our results lead to the proposition that d -wave superconductors in correlated multiband systems will generically have a fully-gapped Fermi surface when they are examined at sufficiently low energies.