Abstract: While the possibility of viscous electron flow in solid state systems was originally proposed as early as the 1960s [1], possible experimental realizations have remained elusive until the recent availability of several ultra-high-conductivity materials. However, distinguishing between non-Ohmic ballistic and viscous effects, and understanding their origin, has proved challenging. In the quasi-two-dimensional metal PdCoO2, the strong and unanticipated effect of Fermi surface anisotropy has added an additional layer of complexity [2]. Experiments to date have typically employed DC measurements, using device fabrication to tune the experimental length scale. I will describe our work establishing the skin effect, as measured via high-resolution microwave spectroscopy, as a new method to achieve a highly-tunable experimental length scale and thus differentiate between transport regimes [3]. In applying this method to PdCoO2, we have uncovered evidence for an unconventional origin of viscous transport corrections. I will discuss the implications of this observation for the broader framework used to interpret this class of experiments, and an outlook for future experimental directions.
[1] Gurzhi, Sov. phys., Usp. 11 255 (1968)
[2] Bachmann et al, Nat. Phys. 18, 819 (2022)
[3] Baker et al, Phys. Rev. X 14, 011018 (2024)