We report optical measurements demonstrating that the low-energy relaxation rate ($1/\tau$) of the conduction electrons in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ obeys scaling relations for its frequency ($\omega$) and temperature ($T$) dependence in accordance with Fermi-liquid theory. In the thermal relaxation regime, $1/\tau \propto (\hslash \omega {)}^2+(p\pi k_{B}T{)}^2$ 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. The excess optical spectral weight in this regime provides evidence for strongly dispersing “resilient” quasiparticle excitations above the Fermi energy.

• Received 30 December 2013

DOI:https://doi.org/10.1103/PhysRevLett.113.087404