Optical Response of Sr${}_2$RuO${}_4$ Reveals Universal Fermi-liquid Scaling and Quasiparticles Beyond Landau Theory

Abstract

We report optical measurements demonstrating that the low-energy relaxation rate ($1/\ddot{I}„$) of the conduction electrons in Sr${}_2$RuO${}_4$ obeys scaling relations for its frequency ($\ddot{I}‰$) and temperature ($T$) dependence in accordance with Fermi-liquid theory. In the thermal relaxation regime, $1/τ\propto (\hbarω)^2 + (pπ\kB T)^2$ with $p=2$, and $\ddot{I}‰/T$ 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.