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, $1/\tau\propto (\hbar\omega)^2 + (p\pi\kB T)^2$ 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. The excess optical spectral weight in this regime provides evidence for strongly dispersing "resilient" quasiparticle excitations above the Fermi energy. 
D. Stricker, J. Mravlje, C. Berthod, R. Fittipaldi, A. Vecchione, A.   Georges, and D. van der Marel
cond-mat.str-el