It has become clear in recent years that many strongly correlated metals (e.g. heavy fermions, ruthenates, iron-based and cuprate superconductors, twisted bilayer graphene) show a linear temperature dependence to the resistivity which persists down to the lowest temperatures. This behaviour has been named "strange metal" and contrasts with the T2-behaviour of a Landau Fermi liquid where electron quasiparticles are present. In this talk, I will show that in two systems - the normal state of the overdoped cuprate superconductor La2-xSrxCuO4 and the bilayer ruthenate Sr3Ru2O7, the strange metal behaviour is correlated with the presence of low-energy spin fluctuations.
In the cuprate system, the spin fluctuations we measure exist across the overdoped part of the cuprate phase diagram. When measured at T=Tc=22 K and p=0.22 they have characteristic energies comparable (with in a factor of 2) with the temperature. Their energy scale also increases rapidly with temperature. Thus, they appear to be intimately connected with the Planckian relaxation observed in strange metals. Sr2Ru3O7 can be tuned through quantum criticality with magnetic field at low temperature and the T-linear resistivity appears to be associated with the presence of low-energy spin fluctuations.
[1] M. Zhu, D. J. Voneshen, S. Raymond, O. J. Lipscombe, C. C. Tam, and S. M. Hayden, Nat. Phys. 19, 99 (2023).
[2] C. Lester et al., Nat. Commun. 12, 5798 (2021).