Manipulating anisotropic transport and superconductivity by focused ion beam microstructuring

Abstract

This thesis presents the results of electrical transport experiments performed on two microstructured quantum materials, namely on the ultra-pure metal PdCoO₂ and on the heavy fermion superconductor CeIrIn₅. Throughout this work, focused ion beam (FIB) microsculpting was utilised to design the investigated devices. I begin with an introduction to the FIB instrument, with a specific focus on its application for microstructuring transport devices from quantum materials. In particular, our standard fabrication procedure, in which a thin slab of material is extracted from a bulk single crystal for further processing is described in detail, as this approach can be utilised for most metallic compounds. Furthermore, I describe a micro-fabrication process for creating transport devices from platelet-shaped single crystals. Thereafter I present ballistic transport measurements of the ultra-pure delafossite metal PdCoO₂. By investigating mesoscopic transport bars which are narrower than the electron mean free path (up to 20 μm), I demonstrate that the ballistic transport in PdCoO₂ is strongly anisotropic as a result of the underlying quasi-hexagonal Fermi surface shape. Moreover, I report on the results of transverse electron focusing (TEF) experiments, a technique which directly probes the real space ballistic trajectories of electrons in a magnetic field, which demonstrate the super-geometric focusing effect. Furthermore, by investigating microstructures of the superconducting heavy fermion compound CeIrIn₅ by means of transport measurements as well as scanning SQUID microscopy in collaboration with external groups, a route to controllably manipulate the local strain in microstructured devices was found. The presented approach is based on exploiting the substrate-induced biaxial strain due to differential thermal contraction, which is spatially tailored by defined FIB cuts. As the superconducting transition in the heavy fermion compound CeIrIn₅ is highly sensitive to strain, the local T[sub]c within the device is controlled via the spatial strain distribution.