Optical Response of Sr₂RuO₄ Reveals Universal Fermi-Liquid Scaling and Quasiparticles Beyond Landau Theory

We report optical measurements demonstrating that the low-energy relaxation rate (1/τ) of the conduction electrons in Sr2RuO4 obeys scaling relations for its frequency (ω) and temperature (T) dependence in accordancewithFermi-liquidtheory.Inthethermalrelaxationregime,1/τ~(ħω)^2+(pπkT)^2 withp=2, and ω/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. The excess optical spectral weight in this regime provides evidence for strongly dispersing “resilient” quasiparticle excitations above the Fermi energy