Directional ballistic transport in the two-dimensional metal PdCoO2

TL;DR

This study demonstrates how shape-induced symmetry breaking in microstructured PdCoO2 leads to observable anisotropic transport properties and transverse voltages, revealing effects of ballistic quasiparticles in finite crystals.

Contribution

It shows how shape-induced symmetry lowering in PdCoO2 microstructures causes measurable anisotropic resistivity and transverse voltages, highlighting ballistic transport effects.

Findings

Resistivity anisotropy exceeds a factor of 2 in microchannels.

Transverse voltages appear in zero magnetic field.

Symmetry breaking is confirmed by Monte Carlo simulations.

Abstract

In an idealized infinite crystal, the material properties are constrained by the symmetries of its unit cell. Naturally, the point-group symmetry is broken by the sample shape of any finite crystal, yet this is commonly unobservable in macroscopic metals. To sense the shape-induced symmetry lowering in such metals, long-lived bulk states originating from anisotropic Fermi surfaces are needed. Here we show how strongly facetted Fermi surfaces and long quasiparticle mean free paths present in microstructures of PdCoO2 yield an in-plane resistivity anisotropy that is forbidden by symmetry on an infinite hexagonal lattice. Bar shaped transport devices narrower than the mean free path are carved from single crystals using focused ion beam (FIB) milling, such that the ballistic charge carriers at low temperatures frequently collide with both sidewalls defining a channel. Two symmetry-forbidden transport signatures appear: the in-plane resistivity anisotropy exceeds a factor of 2, and transverse voltages appear in zero magnetic field. We robustly identify the channel direction as the source of symmetry breaking via ballistic Monte- Carlo simulations and numerical solution of the Boltzmann equation.