Super-geometric electron focusing on the hexagonal Fermi surface of PdCoO₂

The ultrapure delafossite metal PdCoO₂ exhibits a strikingly long mean free path of ∼20 µm at temperatures below 20K. Owing to its peculiar Fermi surface, resembling a nearly perfect hexagon, the electron’s ballistic trajectories are mainly restricted into three directions, rendering PdCoO₂ an ideal material to investigate unusual anisotropic ballistic effects. We fabricate ballistic structures for transverse electron focusing from as-grown single crystals using focused ion beam machining, and demonstrate magnetic focusing up to 35 µm. Compared to typically studied materials with circular Fermi surfaces, the transverse focusing amplitude is strongly enhanced due to the large parallel sections of the hexagonal Fermi surface. We demonstrate this focusing enhancement experimentally, and corroborate it by transport simulations.