Designing and exploring new quantum materials based on Fermi surface topological transitions

The advances in electronic technology that have been achieved over the last few decades have been enabled by perfecting control over non-interacting electrons in materials. This control can now be reliably obtained, e.g., in simple metals and semiconductors, by tuning the Fermi energy and the effective electron mass. However, this technology has reached the limit of its potential due to the fundamentally limited range of electronic properties exhibited by such materials. A dramatic breakthrough can be achieved if one establishes reliable control over collective electronic behaviour in systems where strong interactions between electrons give rise to intriguing macroscopic quantum phenomena. Multiferroics, giant magnetoresistance in spintronic materials, electron correlations in polymeric systems, and high-temperature superconductivity are just are a few examples with vast potential for novel applications. A quantum computer, expected to revolutionise the modern world, and well-envisaged in principle, can still not be realised due to the lack of reliably controlled material base. The reason, largely, is that a priori accurate theoretical underpinning of electron correlation physics, which would allow to design desired electronic properties at will, has remained a challenge and is currently missing.

To describe effects of interacting electrons in solids, Fermi liquid (FL) theory has been a powerful starting point. Notable successes of its application include the microscopic theory of conventional superconductivity and the physics of liquid Helium-3. Even in cases where FL theory has proven inadequate, its failure paved the way for new discoveries, and in many cases the results dictated the new directions. A central concept, naturally emerging in the FL context but relevant far beyond the cases described by the basic FL theory, is the notion of the Fermi Surface (FS) and the density of states (DOS) at different parts of the FS. In places with high DOS the interaction effects may become more pronounced and the properties of the system can be governed through them.

High, or even singular values of DOS are accompanied by topological changes of the FSs of different types. In this project, we will build on seed work and we will continue the classification, using advanced mathematical tools, of the singularities in DOS and to build a comprehensive understanding of the effects of interactions. This theoretical work will be accompanied by a wide search, through first principles calculations, of new quantum materials that can serve as examples of the different classes of singularities. Our experimental partners are keen to fabricate and characterise the new materials that will be identified. In parallel, existing materials, such as strontium ruthenates and the two-dimensional metallic chalcogenides with unexplained and unexplored properties which are of enormous scientific interest with potential technological applications, will provide the immediate playground to test the power of our theories. Although the ideas are very focused, the scope and the impact of the proposed work is very wide, therefore a concerted effort of several world leaders in condensed matter theory and experiments is necessary to achieve all the objectives. As a result, this collaborative project involves researchers, academic visitors and project partners from eight institutions in three different countries.

Systematic control of the unconventional properties of materials that exhibit macroscopic quantum phenomena has been the key driver of the technological breakthroughs such as semiconductors, lithium-ion batteries, light-emitting diodes, etc that have underpinned technological progress in recent decades. Now, what some are already describing as the "second quantum revolution" is based on an ongoing process of discovery, explanation, optimisation and exploitation of the properties of novel families of materials. This project focusses primarily on the first two steps of this process and lays groundwork for the third. For complex quantum materials, progress is driven by quantitative understanding of the underlying mechanisms of electron correlation. This in turn requires a combination of state-of-the-art analytical theory, as for example in the classification of singularities in the density of states and explanation of their impact upon electronic properties, with state-of-the-art computational simulation, enabling accurate bandstructures of layered and interfacial systems comprising thousands of atoms. We have identified several areas where our particular combination of skills and expertise in these areas allows us to make significant progress, namely Strontium Ruthenates, and 2D Metal Chalcogenides and their heterostructures involving other layered and 2D materials.

The research fields on which this project will have a strong impact include correlated electron systems,
quantum magnetism, superconductivity, and material science. Extending the classification of singularities in the DOS will provide a definite theoretical picture of existing experiments on the above materials, and guide future efforts by employing and developing new theoretical and computational tools to discover new quantum materials with novel properties. We will carry out exemplar ab initio studies of materials thus identified to guide future work by our experimental partners and others around the world utilising photoemission to study the bandstructure of these materials. Within materials science, the work will further advance the development of computational tools for precise calculations of the band-structures of large-scale model systems and theoretical spectroscopy in support of photoemission.

We can identify several clear pathways to impact: 1) understanding of properties of existing materials for technological applications; for example the superconductor Sr2RuO4 was considered a candidate for qubits for topological quantum computing if its order parameter symmetry and its behaviour under certain external conditions could be clarified; meanwhile the 2D metal chalcogenides have shown great potential in existing application areas including low-power electronics, opto-electronics and photovoltaics. 2) new materials displaying novel quantum phenomena: there is current widespread research effort pursuing the idea of "valleytronics" in transition metal dichalcogenide materials, the harnessing of valley degeneracy in a 2D semiconductor as a carrier of information. Layered material heterostructures open up the idea of "twistronics", namely the use of interlayer twist angles to control electronic properties.

To disseminate the knowledge generated, as well as writing high-impact papers we will hold two workshops: one at Loughborough, and a contribution to a large international with our project partners, and we will contribute to the ONETEP Masterclass to ensure training in the novel ab initio tools. This will propagate these tools to a wide audience, as will the existing relationship with Dassault Systemes BIOVIA who commercialise ONETEP as part of of the Materials Studio package. As 2D materials are increasingly incorporated into devices such as photovoltaics and low-power electronics, new functionality generated by this project will become increasingly crucial. Outreach activities and training of PhD students and PDRAs is also a strong driver of impact.