Non-local microwave electrodynamics in ultra-pure PdCoO₂

The exceptionally long electronic mean free path in PdCoO₂ enables the exploration of novel transport regimes. Using a model developed for two-dimensional electron gases (2DEGs), mesoscopic DC transport experiments in PdCoO₂ were interpreted as evidence for viscous effects arising from momentum-conserving (MC) scattering [1]. However, it was recently shown that PdCoO₂ exhibits novel directional ballistic transport, beyond that observable in 2DEGs, owing to its nearly-hexagonal Fermi surface (FS) [2]. To understand the combined effects of FS anisotropy and MC scattering, we measured the microwave electrodynamics of PdCoO₂ in several sample geometries—introducing the skin effect as a novel, highly-tunable tool for studying non-Ohmic transport [3,4]. From symmetry alone, differences between geometries imply that our results are neither Ohmic nor purely viscous. In conjunction with Boltzmann calculations, we show that the qualitative behavior of the data stems from a novel form of ballistic skin effect owing to the strongly-faceted FS. A quantitative comparison of theory and experiment gives evidence for MC scattering.

[1] Moll et al, Science 351, 1061 (2016)

[2] Bachmann et al, Nat. Phys. 18, 819 (2022)

[3] Baker et al, arXiv:2204.14239 (2022)

[4] Valentinis et al, arXiv:2204:13344 (2022)