Competitions in layered ruthenates: ferro- vs. antiferromagnetism and triplet vs. singlet pairing

TL;DR

This paper uses first principles calculations to explore the coexistence of ferro- and antiferromagnetic fluctuations in ruthenates, revealing their influence on superconducting pairing symmetries and competing magnetic states.

Contribution

It demonstrates the coexistence of magnetic fluctuations and their impact on superconducting pairing in ruthenates, highlighting the competition between p-wave and d-wave symmetries.

Findings

Ferro- and antiferromagnetic fluctuations coexist in ruthenates.

Antiferromagnetism is linked to Fermi surface nesting at Q=(2pi/3,2pi/3,0).

P-wave pairing dominates except near antiferromagnetic instability.

Abstract

Ru based perovskites demonstrate an amazing richness in their magnetic properties, including 3D and quasi-2D ferromagnetism, antiferromagnetism, and unconventional superconductivity. Tendency to ferromagnetism, stemming from the unusually large involvement of O in magnetism in ruthenates, leads to ferromagnetic spin fluctuations in Sr2RuO4 and eventually to p-wave superconductivity. A related compound Ca2RuO4 was measured to be antiferromagnetic, suggesting a possibility of antiferromagnetic fluctuations in Sr2RuO4 as well. Here we report first principles calculations that demonstrate that in both compounds the ferro- and antiferromagnetic fluctuations coexist, leading to an actual instability in Ca2RuO4 and to a close competition between p-wave and d-wave superconducting symmetries in Sr2RuO4. The antiferromagnetism in this system appears to be mostly related with the nesting, which is the strongest at Q=(2pi/3,2pi/3,0). Surprisingly, for the Fermiology of Sr2RuO4 the p-wave state wins over the d-wave one everywhere except in close vicinity of the antiferromagnetic instability. The most stable state within the d-wave channel has vanishing order parameter at one out of three Fermi surfaces in Sr2RuO4, while in the p channel its amplitude is comparable at all three of them.