Device Applications with Topological Insulators

This PhD aims to investigate the potential applications for topological insulators (TIs) in future technologies, with an emphasis on their use in low power electronics. The topologically protected state at the edge/surface of a TI is spin-momentum locked - that is, the intrinsic angular momentum of the electron (spin) is orientated perpendicularly to its momentum. This novel property means that non-magnetic defects and perturbations (i.e. those which do not flip the spin of electrons) do not disrupt the electronic transport of the system, facilitating the development of low dissipation electronic devices. This property of spin-momentum locking can also be exploited to utilise TIs in spintronic applications - that is, use of both the charge and spin degrees of freedom - which have exciting uses in non-volatile magnetic memories and information processing. Many of these spintronic applications rely on the inclusion of magnetism into the TI, through the use of either magnetic dopants or proximate magnetic materials interfaced with the TI. Magnetic ordering opens a gap in the surface state energy dispersion and is a prerequisite for observation of the quantum anomalous Hall effect (QAHE), a state which is of extreme importance in the development of future spintronic devices and quantum information processing.
While TIs were predicted and experimentally realised over a decade ago they remain in use only in academia and have not yet reached the stage of industrialisation. Many of these issues stem from intrinsic material defects, leading to a non-zero, topologically trivial bulk contribution to electronic transport and precluding the observation of topological effects at high temperature. Similarly, inclusion of magnetic dopants is not uniform and can lead to a spatially varying magnetic gap in the topological surface state. This imposes a limiting temperature on the observation of the QAHE of only a few hundred milli-Kelvin, many orders of magnitude lower than would be expected from a uniformly doped TI. TI interfaces in heterostructures are also of fundamental interest as a playground for the investigation of novel physics, however they remain poorly understood despite intensive research effort. It has been hypothesised that the position of the topological state can be tuned across an interface to permeate into a proximate (magnetic) insulator. The incorporation of magnetic interfaces forms the basis for the realisation of multiple novel states, such as the afore mentioned QAHE state and also the Axion Insulator state. The difficultly in uncovering these effects is, in part, due to the difficulty in probing the buried TI layers, however, extrinsic parameters such as strain, interfacial roughness and atomic diffusion across the interface can also complicate the situation and preclude the observation of topological effects.
During the course of this PhD we will investigate TIs and TI heterostructures in both theoretical and experimental settings. A TI molecular beam expitaxy (MBE) chamber is currently in the final stages of redevelopment and will be used to grow high quality (BixSb1-x)2Te3 thin films as well as integrate them into heterostructures with a variety of magnetic and non-magnetic insulators. By studying these structures using electronic transport measurements at low temperature, in collaboration with Hitachi Ltd., we aim to control the growth parameters in order to produce high mobility TI thin films with a low concentration of intrinsic defects. Using an effective Hamiltonian, complex TI heterostructures have been simulated and their electronic band structures studied in one and two dimensions. The computational aspects of this project will assist in band structure engineering of the TI based heterostructures.