We use inelastic neutron scattering to study spin fluctuations in Sr1.8La0.2RuO4, where Lanthanum doping triggers a Lifshitz transition by pushing the van Hove singularity in the γ band to the Fermi energy. Strong spin fluctuations emerge at an incommensurate wave vector Qic=(0.3,0.3), corresponding to the nesting vector between α and β Fermi sheets. The incommensurate antiferromagnetic fluctuations shift toward (0.25,0.25) with increasing energy up to ∼110 meV. By contrast, scatterings near the ferromagnetic wave vectors Q=(1,0) and (1,1) remain featureless at all energies. This contradicts the weak-coupling perspective that suggests a sharp enhancement of ferromagnetic susceptibility due to the divergence of density of states in the associated γ band. Our findings imply that ferromagnetic fluctuations in Sr2RuO4 and related materials do not fit into the weak-coupling paradigm, but instead are quasi-local fluctuations induced by Hund's coupling. This imposes significant constraints for the pairing mechanism involving spin fluctuations.
@article{arxiv.2304.08214,
title = {Spin Fluctuations in Sr$_{1.8}$La$_{0.2}$RuO$_4$},
author = {Zheng He and Qisi Wang and Yu Feng and Chul Kim and Wonshik Kyung and Changyoung Kim and Hongliang Wo and Gaofeng Ding and Yiqing Hao and Feiyang Liu and Helen C. Walker and Devashibhai T. Adroja and Astrid Schneidewind and Wenbin Wang and Jun Zhao},
journal= {arXiv preprint arXiv:2304.08214},
year = {2023}
}