Separated transport relaxation scales and interband scattering in SrRuO$_3$, CaRuO$_3$, and Sr$_2$RuO$_4$ thin films

TL;DR

This study uses THz spectroscopy to analyze charge transport in ruthenate thin films, revealing two distinct relaxation rates and providing conventional explanations for anomalous optical conductivity without exotic quasiparticles.

Contribution

It demonstrates that the complex optical conductivity in ruthenates can be explained by conventional models involving multiple conduction channels and interband scattering, challenging the need for exotic quasiparticles.

Findings

Presence of two Drude peaks at low temperature indicating separate relaxation rates.

Crossover from two-Drude to one-Drude lineshape with increasing temperature.

Insights into interband scattering and Hund's coupling effects in ruthenates.

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 $T_{FL}$ 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 $\omega/T$ 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-Drude peak lineshape at low temperature and a crossover to a one-Drude peak lineshape at higher temperatures. The two-component low-temperature conductivity is indicative of two well-separated current relaxation rates for different conduction channels. 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; (c) the presence of multiple conduction channels that undergoes a crossover to stronger interband scattering at higher temperatures. None of these scenarios require the existence of exotic quasiparticles. 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.