Disorder-enhanced effective masses and deviations from Matthiessen's rule in PdCoO2 thin films

The observation of hydrodynamic transport in the metallic delafossite PdCoO2 has increased interest in this family of highly conductive oxides, but experimental studies so far have mostly been confined to bulk crystals. In this paper, the development of high-quality epitaxial thin films of PdCoO2 has enabled a thorough study of the conductivity as a function of film thickness using both dc transport and time-domain terahertz (THz) spectroscopy. As film thickness increases from 12 to 102 nm, the residual resistivity decreases, and we observe a large deviation from Matthiessen's rule (DMR) in the temperature dependence of the resistivity. We find that the complex THz conductivity is well fit by a single Drude term. We fit the data to extract the spectral weight and scattering rate simultaneously. The temperature dependence of the Drude scattering rate is found to be nearly independent of the residual resistivity and cannot be the primary mechanism for the observed DMR. Rather, we observe large changes in the spectral weight as a function of disorder, changing by a factor of 1.5 from the most disordered to least disordered films. We believe this corresponds to a mass enhancement of >= 2 times the value of the bulk effective mass which increases with residual disorder. This suggests that the mechanism behind the DMR observed in dc resistivity is primarily driven by changes in the electron mass. We discuss the possible origins of this behavior including the possibility of disorder-enhanced electron-phonon scattering, which can be systematically tuned by film thickness.