Hydrodynamic electron flow in PdCoO₂: out-of-plane microwave spectroscopy

Hydrodynamic electron flow—in which electronic viscosity affects transport properties—has been observed in the DC resistance of PdCoO₂ [1]. Predictions that hydrodynamic effects also influence the AC electromagnetic response of a metal exist [2,3], but confirming experimental evidence is lacking. In these proposals, the electronic viscosity affects the gradient of the induced current density, thereby influencing the diffusion of electromagnetic fields into the sample. Here we report results of microwave spectroscopy measurements of PdCoO₂ aimed at testing these predictions.

In the measurements reported here, a microwave magnetic field was applied parallel to the c axis. The induced flow is always along a high-mobility (a/b) direction, with a velocity gradient along the other high-mobility (b/a) direction. This geometry is favorable to observing the effect of viscosity. The results appear to be inconsistent with existing predictions.

[1] Moll et al., Science 351 6277 (2016)
[2] Gurzhi, Sov. Phys. Usp. 11 255 (1968)
[3] Forcella et al., Phys. Rev. B 90 035142 (2014)