Hidden anomalous Hall effect in Sr$_2$RuO$_4$ with chiral superconductivity dominated by the Ru $d_{xy}$ orbital

TL;DR

This paper proposes that the anomalous Hall effect observed in Sr$_2$RuO$_4$ can originate from subdominant orbitals contributing to chiral superconductivity, challenging previous assumptions about the orbital origin of this effect.

Contribution

It introduces models showing that inter-orbital pairing involving subdominant orbitals can induce intrinsic AHE in Sr$_2$RuO$_4$, even with a predominantly $d_{xy}$ orbital-driven pairing.

Findings

Finite Kerr rotation angles can be explained by inter-orbital pairing.

Intrinsic Hall effect is absent in non-chiral superconducting states.

Orbital mixing enables AHE despite dominant $d_{xy}$ orbital pairing.

Abstract

The polar Kerr effect in superconducting Sr$_2$RuO$_4$ implies finite ac anomalous Hall conductivity. Since intrinsic anomalous Hall effect (AHE) is not expected for a chiral superconducting pairing developed on the single Ru $d_{xy}$ orbital, multiorbital chiral pairing actively involving the Ru $d_{xz}$ and $d_{yz}$ orbitals has been proposed as a potential mechanism. Here we propose that AHE could still arise even if the chiral superconductivity is predominantly driven by the $d_{xy}$ orbital. This is demonstrated through two separate models which take into account subdominant orbitals in the Cooper pairing, one involving the oxygen $p_x$ and $p_y$ orbitals in the RuO$_2$ plane, and another the $d_{xz}$ and $d_{yz}$ orbitals. In both models, finite orbital mixing between the dominant $d_{xy}$ and the other orbitals may induce inter-orbital pairing between them, and the resultant states support intrinsic AHE, with Kerr rotation angles that could potentially reconcile with the experimental observation. Our proposal therefore sheds new light on the microscopic pairing in Sr$_2$RuO$_4$. We also show that intrinsic Hall effect is generally absent for non-chiral states such as $\mathcal{S}+i\mathcal{D}$, $\mathcal{D}+i\mathcal{P}$ and $\mathcal{D}+i\mathcal{G}$, which provides a clear constraint on the symmetry of the superconducting order in this material.