Probing unconventional transport regimes in delafossite metals

This thesis describes investigations into the origins and nature of the remarkable electrical transport of the delafossite metals PtCoO₂ and PdCoO₂ using focused ion beam based microstructuring techniques. These compounds are amongst the highest conductivity materials known, but questions remain regarding the origin of their ultralow resistivity and the effects of their properties on transport in unconventional regimes such as the ballistic regime. In the initial introductory chapters, I will review the key properties of both delafossite metals and the application of focused ion beam microstructuring to transport measurements within This thesis describes investigations into the origins and nature of the remarkable electrical transport of the delafossite metals PtCoO₂ and PdCoO₂ using focused ion beam based microstructuring techniques. These compounds are amongst the highest conductivity materials known, but questions remain regarding the origin of their ultralow resistivity and the effects of their properties on transport in unconventional regimes such as the ballistic regime. In the initial introductory chapters, I will review the key properties of both delafossite metals and the application of focused ion beam microstructuring to transport measurements within low resistivity materials. The experimental findings are split into two chapters. Initially, I will describe an investigation into the origins of the high conductivity by introducing defects to PtCoO₂ and PdCoO₂ through high energy electron irradiation and observing the changes to the resistivity. These measurements demonstrate that the ultralow resistivity of the delafossite metals is the result of an extreme purity of up to 1 defect in 120,000 atoms, rather than backscattering suppression. In addition, I will report the effects of the defects on the electrical transport more broadly. Here, by examining the difference before and after irradiation, insight is gained into the origins of the unconventional magnetotransport of PtCoO₂. The other study uses PtCoO₂ and PdCoO₂ as test systems for the investigation of the effects of a non-circular Fermi surface on the transport within four terminal, square-shaped junctions inside the ballistic regime. These junction devices have been shown to be a sensitive probe of this regime in other materials, and I will demonstrate that the nearly hexagonal Fermi surface of the ultrapure delafossite metals results in not only strongly ballistic behaviour, present at a scale multiple times the mean free path, but also novel phenomena which are not seen with a circular Fermi surface.