Magnetic excitations and their possible role in the superconducting pairing in Sr₂RuO₄

The magnetic excitations in the unconventional superconductor Sr₂RuO₄ consist of several contributions, an incommensurate signal arising from nesting and quasiferromagnetic fluctuations. We were able to follow the nesting signal of the quasi-one-dimensional bands across the superconducting transition down to very low energies. Even at E=0.325 meV, which lies well below the superconducting gap 2D values reported from tunneling experiments or deduced from BCS theory, there is no change in the magnetic response [1], which seems incompatible with the picture of a large gap on these Fermi-surface sheets. The quantitative analysis of the quasiferromagnetic fluctuations in Sr₂RuO₄ is hampered by the smaller amplitude and the little structure in Q space of this signal. Only by use of polarized neutron scattering we can determine the strength and characteristics of the ferromagnetic response, which agrees with reports of specific heat, magnetic susceptibility and NMR. Incorporating this ferromagnetic response into the gap equation, however, does not stabilize a triplet pairing state [2]. Furthermore, the quasiferromagnetic response in Sr₂RuO₄ does not resemble the paramagnon scattering expected for a nearly ferromagnetic material, but it seems to arise from broad instabilities at low propagation vectors. In contrast recent INS on the ferromagnetic perovskite SrRuO₃ find the typical ferromagnetic magnon and paramagnon scattering below and above the Curie temperature [3]. The magnon stiffness and gap in SrRuO₃ is found to anomalously soften upon cooling well below the Curie temperature, which can be attributed to the impact of Weyl points [3].

[1] S. Kunkemöller et al., Phys. Rev. Lett. 118, 147002 (2017). [2] P. Steffens et al., Phys. Rev. Lett. 122, 047004 (2019). [3] K. Jenni et al., Phys. Rev. Lett. 123, 017202 (2019).