Effects of disorder on THz conductivity of PdCoO2 delafossite thin films*

In this work, the development of high-quality 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 THz spectroscopy. By increasing the film thickness from 15nm to 100nm, the residual resistivity is reduced and we observe an apparent large deviation from Matthiessen's rule (DMR) in the dc resistivity. We also find that the complex THz conductivity demonstrates excellent consistency to a single Drude term, and fit the data to extract the spectral weight and scattering rate simultaneously. The temperature dependence of the Drude scattering rate is found to be largely independent of the residual resistivity; however, the spectral weight appears to increase nearly by a factor of two between the most disordered and least disordered samples. This suggests that the DMR observed in dc resistivity is caused by large changes in the spectral weight with varying levels of disorder. In PdCoO2 thin films we find there exists disorder-enhanced electron-phonon scattering that can be systematically tuned by film thickness.