Separated transport relaxation scales and interband scattering in thin films of SrRuO3 , CaRuO3 , and Sr2RuO4

Abstract

The anomalous charge transport observed in some strongly correlated metals raises questions as to the universal applicability of Landau Fermi-liquid theory. The coherence temperature ${T}_{\text{FL}}$ for normal metals is usually taken to be the temperature below which ${T}^{2}$ is observed in the resistivity. Below this temperature, a Fermi liquid with well-defined quasiparticles is expected. However, metallic ruthenates in the Ruddlesden-Popper family frequently show non-Drude low-energy optical conductivity and unusual $\ensuremath{\omega}/T$ scaling, despite the frequent observation of ${T}^{2}$ dc resistivity. Herein we report time-domain THz spectroscopy measurements of several different high-quality metallic ruthenate thin films and show that the optical conductivity can be interpreted in more conventional terms. In all materials, the conductivity has a two Lorentzian line shape at low temperature and a crossover to a one Drude peak line shape at higher temperatures. The two component low-temperature conductivity is indicative of two well-separated current relaxation rates for different conduction channels. In ${\mathrm{SrRuO}}_{3}$ and ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, both relaxation rates scale as ${T}^{2}$, while in ${\mathrm{CaRuO}}_{3}$ the slow relaxation rate shows ${T}^{2}$, and the fast relaxation rate generates a constant background in conductivity. We discuss three particular possibilities for the separation of rates: (a) strongly energy-dependent inelastic scattering; (b) an almost conserved pseudomomentum operator that overlaps with the current, giving rise to the narrower Drude peak; and (c) the presence of multiple conduction channels that undergoes a crossover to stronger interband scattering at higher temperatures. None of these scenarios requires the existence of exotic quasiparticles. However, the interpretation in terms of multiple conduction channels in particular is consistent with the existence of multiple Fermi surfaces in these compounds and with the expected relative weakness of ${\ensuremath{\omega}}^{2}$ dependent effects in the scattering as compared to ${T}^{2}$ dependent effects in the usual Fermi-liquid treatment. The results may give insight into the possible significance of Hund's coupling in determining interband coupling in these materials. Our results also show a route towards understanding the violation of Matthiessen's rule in this class of materials and deviations from the ``Gurzhi'' scaling relations in Fermi liquids.