Fig. 3: The Fermi velocity density and directional ballistics.

a, The Fermi surface of PdCoO₂ (blue) compared with a circle (red). θ denotes the angle from the [110]-direction. At first sight the difference between the two is small. b, In contrast, the Fermi velocity direction distributions of the two Fermi surfaces are vastly different, with this difference leading to the strong directional ballistic effects that we have measured. While an isotropic Fermi surface (red) leads to a corresponding isotropic Fermi velocity direction distribution, the faceted Fermi surface of PdCoO₂ (blue) results in a Fermi velocity direction distribution which is sharply peaked along six directions. Note that the velocity direction distribution has been normalized the same for both Fermi surfaces. The sharp spikes in b emphasize that each facet of the PdCoO₂ Fermi surface contains a large number of states with the same direction of Fermi velocity.