Superconductivity in multiorbital systems: Role of Fermi surface geometry and repulsive interactions in Hund's pairing*

Unconventional superconductivity is often referred to as originating from a pairing mechanism different from electron-phonon interactions and connected to an anisotropic superconducting order parameter with sign change of the Cooper pair wavefunction. A common microscopic mechanism is the spin-fluctuation mediated pairing where the formation of Cooper pairs is possible by avoiding the repulsive Coulomb interaction in space. For multiband systems, there is the possibility of a superconducting instability even when all local bare interactions are repulsive. This mechanism is referred to as Hund’s pairing and the specific channels are dominantly off-diagonal in orbital space resulting in exotic interband pair states. This mechanism and its relation to the exchange of spin fluctuations together with Fermi surface geometry is examined for multiband models relevant for Fe-based superconductors and Sr2RuO4 [1-2]. Additional players for superconducting pairing in these systems are strong correlations [3] and electron-phonon interactions [4]. Implications of these on the dominating superconducting instability will be discussed.

[1] Rømer, et al, Phys. Rev. Research 4, 033011 (2022)

[2] Roig, et al, Phys. Rev. B 106, L100501 (2022)

[3] Kreisel, et al, Phys. Rev. Lett. 129, 077002 (2022)

[4] Biswas, et al., Phys. Rev. B 108, L020501 (2023)