Superconductivity in multiorbital systems with repulsive interactions: Hund's pairing vs. spin-fluctuation pairing

TL;DR

This paper compares Hund's pairing and spin-fluctuation pairing mechanisms in multiorbital superconductors, finding Hund's pairing is unlikely in systems like Sr2RuO4 and iron-based superconductors, especially with certain electronic features.

Contribution

It provides a comparative analysis of Hund's pairing and spin-fluctuation pairing mechanisms across different multiorbital systems using realistic electronic models.

Findings

Hund's pairing aligns with fluctuation exchange results in systems without nesting.

Spin-fluctuation pairing dominates in systems with peaked particle-hole susceptibility.

Hund's pairing is unlikely in Sr2RuO4 and typical iron-based superconductors.

Abstract

Hund's pairing refers to Cooper pairing generated by onsite interactions that become attractive due to large Hund's exchange $J$. This is possible in multiorbital systems even when all local bare interactions are repulsive, since attraction in specific channels are given by certain linear combinations of interaction parameters. On the other hand, pairing processes such as the exchange of spin fluctuations, are also present. We compare these two mechanisms on an equal footing using electronic bands appropriate for different classes of multiorbital systems over a wide range of interaction parameters. We find that for systems without clear nesting features, the superconducting state generated by the Hund's mechanism agrees well with that from the full fluctuation exchange vertex when Hund's exchange and spin-orbit coupling are sufficiently large. On the other hand, for systems characterized by a peaked finite-momentum particle-hole susceptibility, spin-fluctuation pairing generally dominates over Hund's pairing. We conclude that Hund's pairing states are unlikely to be realized in systems like Sr$_2$RuO$_4$ and generic iron-based superconductors.