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Abstract

Rhenium oxide (ReO₃) is one of the most highly conducting metals, and has the lowest reported low-temperature residual resistivity of any oxide material. This property endows it with a remarkably long low-temperature electronic mean free path, comparable to those seen in the ultrapure delafossites. In the quasi-2D delafossite material palladium cobaltate (PdCoO₂), recent broadband microwave spectroscopy studies have observed a novel directional anomalous skin effect (ASE). These studies found that the anisotropy of the ASE in PdCoO₂ is influenced by the relative alignment of surface currents and the facets of its Fermi surface. This anisotropy even extends to the frequency-dependence of the surface resistance. Recently developed nonlocal Boltzmann transport models are able to describe its skin effect response, indicating that it falls between the ballistic and viscous-like transport regimes. The success of this new model motivated us to investigate the anomalous skin effect response in ReO₃, and in doing so, test the predictions of the generalized theory of the skin effect in a material distinct from PdCoO₂ in two main ways: it has a more complex fully three-dimensional electronic structure with multiple Fermi surface sheets of different character, and has potentially different defect scattering behaviours. As an additional test to these models, we study samples of ReO₃ that have been irradiated with high-energy electrons to introduce homogeneous point-like defects. Our experimental results reveal a rich anisotropy in the anomalous skin effect response in ultrapure ReO₃ that depends not only on the direction of surface currents with respect to crystallographic direction, but also on the direction of electromagnetic propagation. The novel skin effect models accurately predict the response for some situations, but fail in others. We suggest possible reasons for this discrepancy, by considering the Fermi surface geometry, and referring to de Haas-van Alphen measurements reported in the literature. Furthermore, our measurements of high-energy electron-irradiated ReO₃ samples found a skin effect response in the diffusive regime, confirming that a low residual resistivity is a necessary condition of the phenomenology observed in the pristine ReO₃ samples.