Topological Insulators: A study of bulk crystalline and Nanomaterials

Topological Insulators (TIs) are a class of quantum materials that exhibit topological surface states. These materials are usually small band gap semiconductors where the bulk of the material is insulating, but they exhibit special surface states that are conducting and topologically protected. The materials are usually made of heavy atoms that give rise to strong spin-orbit coupling and this leads to the formation of surface states that are not destroyed by scattering or impurities.
TIs are proving to be ideal materials for study in condensed matter physics, as the physics of these materials is novel and they offer huge scope for developing new theories and for the discovery of new materials.
Although the TIs have gapless edge or surface states that are protected and are in theory supposed to have an insulating gap in the bulk, most of the 3D TIs discovered to date are still fairly conducting in the bulk. Materials design and processing have emerged as being key to the investigation and the discovery of new TIs. The challenge for materials physicists is to create TI materials that are true insulators in the bulk, in order to facilitate the study of their exotic surface states.
In this proposal, we describe the methodology to be adopted to obtain high quality materials, for the different experiments proposed. We propose to synthesize a range of materials, some of which are already known to be Topological Insulators and other new materials such as the Topological Crystalline Insulators (TCIs) and those with emerging topological behaviour. The project will investigate both bulk crystalline materials and nanomaterials (in the form of nanoplatelets and nanorods). The study of the emergence of superconductivity in the 3D TIs and TCIs will be undertaken. The existence of a full pairing gap in the bulk in the superconducting Topological Insulators, together with the gapless surface states in these materials, makes them extremely interesting.

The physics of these materials will be investigated through detailed studies of the bulk properties of the crystals (including resistivity and Hall effect, magnetisation, heat capacity) in particular, to understand the influence that the bulk electronic and magnetic properties have on their topological behaviour. X-ray/electron diffraction and electron microscopy techniques will be used for the investigation of the structural properties of both the crystals and nanomaterials. Investigations of the surfaces of the crystalline and nanomaterials by XPS and ARPES will be carried out with our collaborators. Neutron scattering and muon spectroscopy techniques will also be employed. Valuable theoretical input from the Project Partner will be used in conjunction with the results from the experimental investigations, to inform the decisions for the design and fabrication of new materials exhibiting TI behaviour.

A wide network of experts, including both therorists and experimentalists, as collaborators will contribute to successfully deliver the work described this project.

Topological Insulators are seen as the new state of matter. Since their recent discovery there has been tremendous interest in these materials, not only because their physics is very exciting and new, but also because the materials have great potential to contribute to advances in technology.

The work described in this proposal is fundamental in nature. In the short term its impact will be most strongly felt in the academic research community (please see academic beneficiaries above). This is an exceptionally fast moving field of research in which theory and experiment have progressed side by side leading to innovative thinking.

The impact on the high technology industrial sector of these materials is only expected to be seen in the medium to long term, but this impact is potentially enormous. These materials could find uses in many applications including in the areas of spintronics, quantum information/computing, as thermoelectrics, and in junction devices in electronics.

There are many examples of materials that just a few years ago were the subject of similar fundamental research that are now routinely used in applications. Examples include the low and high temperature superconductors in high field magnets and superconducting devices, and oxide materials in fuels cells and batteries.

Our recent work on the use of pyrochlore oxides for the storage of nuclear waste and our investigation of new magnetic methods for characterising nano-particles in catalysts, demonstrates our ongoing commitment to seek to apply our research in an industrial setting for the benefit of society.

The research programme described in this proposal will have a significant impact on the education of students (both undergraduate project students and postgraduate Ph.D. students) and early career researchers by exposing them to highly collaborative, interdisciplinary research environments and international user facilities. In the medium term, the impact of the proposed work will be seen in the provision of trained research personnel to fill positions, including post-doctoral and faculty posts, available at universities and in science based industries.

Our former regional development agency (Advantage West Midlands) and the European Regional Development Fund, has funded a Science City Initiative involving the Universities of Warwick and Birmingham for research into Advanced Materials. The aim of this initiative is to establish the region as an international competitor in materials physics, undertaking world-class research in the development and characterisation of new materials for applications in a diverse range of industries. As part of this project the PI (GB) and CI (MRL) have been successful in obtaining equipment for crystal growth and low-temperature characterisation measurements. The current proposal will enhance our capabilities in this area and will stimulate research that will be beneficial to many local industries. Job creation in the Midlands region in specialised science areas is a stated aim of the Science City Initiative and this project will have an impact on this.