Fig. 2: Ballistic electron propagation in the case of a circular and a hexagonal Fermi surface.
From: Directional ballistic transport in the two-dimensional metal PdCoO2
a, The direction of the Fermi velocity vF is always parallel to the Fermi momentum kF for a circular Fermi surface. b, The situation is drastically different in the case of a hexagonal Fermi surface. Due to the flat sides of the polygon, there are only three possible directions for the Fermi velocities (yellow, blue and red). This restriction of the electron propagation direction results in highly anisotropic, directional ballistic transport. c, The electronic conduction in a four-point transport bar fabricated from a material with an isotropic Fermi surface will not depend on the orientation of the bar. d, In contrast, for a hexagonal Fermi surface, a bar cut parallel to an electron propagation direction will show a lower resistance than a bar aligned perpendicular to an electron propagation direction.
