Directional anomalous skin effect in PdCoO₂

The delafossite metal PdCoO₂ is notable for its exceptionally long low-temperature mean free path (MFP) and simple, nearly-hexagonal Fermi surface [1]. Recent measurements of DC transport in width-restricted channels of PdCoO₂ found that in the ballistic regime—in which the MFP is longer than the channel width—the strongly-faceted Fermi surface results in a large resistivity anisotropy that is symmetry-forbidden in the bulk limit, demonstrating the notion of “directional ballistics” [2]. In AC electrodynamics, current is intrinsically confined to a narrow layer, even in an infinite sample, because of the skin effect. Analogous to the ballistic regime, the anomalous skin effect occurs when the MFP is longer than the skin depth. Here we present microwave spectroscopy measurements of PdCoO₂ in two geometries which would be symmetry-equivalent for local electrodynamics. Instead, the two measurements differ strongly in magnitude and frequency dependence. By comparing our measurements to Boltzmann calculations, we show that the strongly-faceted Fermi surface of PdCoO₂ gives rise to a directional anomalous skin effect.

[1] A.P. Mackenzie, Rep. Prog. Phys. 80 032501 (2017)

[2] Bachmann et al, arXiv:2103.01332 (2021)

[3] G.E.H. Reuter & E.H. Sondheimer, Proc. R. Soc. Lond. 195, 336 (1948)