Multiorbital singlet pairing and $d+d$ superconductivity

TL;DR

This paper explores novel multiorbital singlet pairing states in multiband superconductors, revealing that certain $d+d$ states can be fully gapped and are natural in complex systems like Fe-based and heavy-fermion superconductors.

Contribution

It introduces new time-reversal-invariant multiorbital pairing states, including the $s\tau_{3}$ state, and compares their properties to known states like $d+id$, expanding understanding of pairing symmetries.

Findings

$s\tau_{3}$ state leads to fully gapped Fermi surfaces at low energies.

$d+d$ pairing states are natural in multiband systems with complex orbital structures.

Proposes $d$-wave superconductors can be fully gapped in correlated multiband materials.

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 $s\tau_{3}$ 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 $d +\text{i}d$ state in a way analogous to how the B- triplet pairing phase of \enhe superfluid differs from its A- phase counterpart. In addition, we construct an analogue of the $s\tau_{3}$ 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.