Stabilizing Even-Parity Chiral Superconductivity in Sr$_2$RuO$_4$

TL;DR

This paper proposes a new stable even-parity chiral superconducting state in Sr$_2$RuO$_4$, driven by orbital degrees of freedom and Hund's coupling, which explains experimental observations and predicts Bogoliubov Fermi surfaces.

Contribution

It introduces a realistic three-dimensional model showing how an exotic $E_g$ state can be stabilized in Sr$_2$RuO$_4$ through orbital and spin-orbit interactions.

Findings

Stabilization of an $E_g$ chiral superconducting state in Sr$_2$RuO$_4$.

Prediction of Bogoliubov Fermi surfaces in this state.

Orbital degrees of freedom enable a local orbital-antisymmetric spin-triplet pairing.

Abstract

Strontium ruthenate (Sr$_2$RuO$_4$) has long been thought to host a spin-triplet chiral $p$-wave superconducting state. However, the singletlike response observed in recent spin-susceptibility measurements casts serious doubts on this pairing state. Together with the evidence for broken time-reversal symmetry and a jump in the shear modulus $c_{66}$ at the superconducting transition temperature, the available experiments point towards an even-parity chiral superconductor with $k_z(k_x\pm ik_y)$-like $E_g$ symmetry, which has consistently been dismissed based on the quasi-two-dimensional electronic structure of Sr$_2$RuO$_4$. Here, we show how the orbital degree of freedom can encode the two-component nature of the $E_g$ order parameter, allowing for a local orbital-antisymmetric spin-triplet state that can be stabilized by on-site Hund's coupling. We find that this exotic $E_g$ state can be energetically stable once a complete, realistic three-dimensional model is considered, within which momentum-dependent spin-orbit coupling terms are key. This state naturally gives rise to Bogoliubov Fermi surfaces.