Multipolar Fermi Surface Deformations in Sr$_2$RuO$_4$ Probed by Resistivity and Sound Attenuation: A Window into Electron Viscosity and the Collision Operator

Recent developments in electron hydrodynamics have demonstrated the importance of considering the full structure of the electron-electron scattering operator, which encodes a sequence of lifetimes, one for each component of the Fermi surface deformation in a multipolar expansion. In this context, the dipolar lifetime is measured by resistivity, whereas the quadrupolar component probes the viscosity and can be measured in the bulk via sound attenuation. We introduce a framework to extract the collision operator of an arbitrary metal by combining resistivity and sound attenuation measurements with a realistic calculation of the scattering operator that includes multiband and Umklapp effects. The collision operator allows for the prediction of a plethora of properties, including the non-local conductivity, and can be used to predict hydrodynamic behavior for bulk metals. As a first application, we apply this framework to Sr$_2$RuO$_4$ in a temperature range where electron-electron scattering is dominant. We find quantitative agreement between our model and the temperature dependence of both the resistivity and the sound attenuation, we find the quadrupolar (B1g) relaxation rate to be 30% higher than the dipolar one due to the presence of hot spots on the $\gamma$ band, and we predict a strongly anisotropic viscosity arising from the $\alpha$ and $\beta$ bands.