Super-geometric electron focusing on the hexagonal Fermi surface of PdCoO$_2$

TL;DR

This paper demonstrates that the hexagonal Fermi surface of PdCoO2 enables highly directional ballistic electron transport and super-geometric focusing effects, opening new possibilities for ballistic electronic devices.

Contribution

It reveals that a nearly perfect hexagonal Fermi surface causes super-geometric focusing in PdCoO2, a phenomenon not observed in materials with circular Fermi surfaces.

Findings

Hexagonal Fermi surface leads to directional ballistic transport.

Super-geometric focusing occurs at magnetic fields below the circular Fermi surface limit.

Focusing effects can be predicted for arbitrary device geometries.

Abstract

Geometric electron optics may be implemented in solid state when transport is ballistic on the length scale of a device. Currently, this is realized mainly in 2D materials characterized by circular Fermi surfaces. Here we demonstrate that the nearly perfectly hexagonal Fermi surface of PdCoO2 gives rise to highly directional ballistic transport. We probe this directional ballistic regime in a single crystal of PdCoO2 by use of focused ion beam (FIB) micro-machining, defining crystalline ballistic circuits with features as small as 250nm. The peculiar hexagonal Fermi surface naturally leads to electron self-focusing effects in a magnetic field, well below the geometric limit associated with a circular Fermi surface. This super-geometric focusing can be quantitatively predicted for arbitrary device geometry, based on the hexagonal cyclotron orbits appearing in this material. These results suggest a novel class of ballistic electronic devices exploiting the unique transport characteristics of strongly faceted Fermi surfaces.