From Ohmic to non-local transport in delafossite metals

Abstract

This thesis describes the results of electrical transport experiments in delafossite metals and explores how tuning the measurement conditions or the sample geometry affects the observed transport regime. The thesis covers two main experimental investigations studying rather different transport phenomena.

The initial chapters describe the micro-structuring techniques used to prepare samples for electrical transport experiments, the experimental setup allowing for high-quality electrical transport measurements and the procedure for solving the Boltzmann equation beyond the relaxation time approximation used in interpreting the results of the transport experiments.

The ultra-pure delafossite metals PdCoO₂ and PtCoO₂ exhibit intrinsically excellent crystalline quality. The first part focuses on investigating the isostructural delafossite semiconductors and exploring the nature of their metallic state, accessed through chemical doping. Magnetotransport measurements of single crystals of doped delafossite semiconductor CuRh₀.₉Mg₀.₁O₂ are used for basic characterisation and elucidating the origin of the low temperature resistivity upturn in this material. The physics is shown to be driven by disorder, which complicates the measurement and interpretation of the data.

The second part is devoted to studying the cross-over between Ohmic and non-local transport in the delafossite metals PdCoO₂ and PtCoO₂. Focused ion beam-defined microstructures are used to study the resistivity anisotropy in these materials, as the transport channels are gradually narrowed and the regime of transport is tuned from Ohmic to ballistic. By comparing the measured results with the Boltzmann equation incorporating the experimental Fermi surface a small viscous correction to the transport is identified. Studying the temperature dependence of the viscous correction allows the identification of the scattering from impurities outside the conduction layer as a microscopic mechanism leading to this correction. The implications of this somewhat non-intuitive finding are discussed.

Date of Award	10 Jun 2024
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Andrew Mackenzie (Supervisor), Andreas Rost (Supervisor) & Haijing Zhang (Supervisor)