Spin fluctuations in Sr1.8La0.2RuO4

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 quasilocal fluctuations induced by Hund's coupling. This imposes significant constraints for the pairing mechanism involving spin fluctuations.