Optical Response of Sr$_{2}$RuO$_{4}$ Reveals Universal Fermi-liquid Scaling and Quasiparticles Beyond Landau Theory*

We report optical measurements demonstrating that the low-energy relaxation rate (1/$\tau )$ of the conduction electrons in Sr$_{2}$RuO$_{4}$ obeys scaling relations for its frequency ($\omega )$ and temperature (T) dependence in accordance with Fermi-liquid theory. In the thermal relaxation regime, 1/$\tau \propto $ ($\hbar \omega )^{2} + $ (p$\pi $k$_{\mathrm{B}}$T)$^{2}$ with p $=$ 2, and $\omega $/T scaling applies. Many-body electronic structure calculations using dynamical mean-field theory confirm the low-energy Fermi-liquid scaling, and provide quantitative understanding of the deviations from Fermi-liquid behavior at higher energy and temperature. In this regime, evidence for electron-like ``resilient'' quasiparticle excitations with a scattering rate deviating from Landau's Fermi-liquid form is presented.