Investigation of transport regimes in restricted geometries of ultra-pure natural heterostructures

Abstract

This thesis describes investigations into the origins of the unconventional electrical transport of the non-magnetic delafossite metals PtCoO₂ and PdCoO₂ and the magnetic delafossite metal PdCrO₂ using focused ion beam microstructuring techniques. These compounds are among the highest conductivity materials known, with an extreme purity of up to 1 defect in 120,000 atoms. This remarkable purity, together with the hexagonal Fermi surface, opens the possibility of studying novel regimes of mesoscopic physics. This work is split into two parts. In the first part, I will review the key properties of non-magnetic delafossite metals and the application of focused ion beam microstructuring to transport measurements within low resistivity materials. The related experimental chapter describes an investigation which uses the high energy electron irradiation investigation to probe the effects of a non-circular Fermi surface on the transport within bars and four-terminal, square-shaped junctions inside the ballistic regime. The other studies were concentrated on the magnetic delafossite metal PdCrO₂. I will describe a new method of microstructure preparation which was created for PdCrO₂ transport studies but is widely applicable to other materials. This material obeys the Planckian bound at a wide range of temperatures between 200 K and 500 K. The accompanying experimental chapter details an investigation by high energy electron irradiation of the origin of this behaviour. The new method of mounting microstructures also allows, for the first time, the study of studying unconventional transport regimes in PdCrO₂.