Controlling unconventional properties of correlated materials by Fermi surface topological transitions and deformations.

Widespread electronic technologies of the last few decades have been led by perfecting control over response of electrons in materials where interactions between them are essentially weak. This can now be reliably achieved, e.g., in simple metals and semiconductors, by tuning the Fermi surface 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.

In light of very recent developments of new accurate numerical tools for correlated systems, it is extremely timely to use the new methodology to address properties of certain correlated materials of great technological potential, which are currently in the focus of extensive experimental studies. In this project, cutting-edge numerics and advanced analytical techniques will allow us to develop a definitive and quantitative theoretical picture of key effects and mechanisms associated with quantum phase transitions in correlated electron systems, thereby enabling a priori control over the corresponding material properties. Specifically, we propose a comprehensive theoretical study of effects of deformations of the Fermi surface in the correlated regime by changing external parameters and the resulting emergence of new phases with unconventional physical behaviour.
Our main goals are to:

(i) gain quantitative understanding of the mechanisms and consequences of Fermi surface reconstruction and Lifshitz topological transitions in correlated-electron model systems, especially those with spin orbit coupling, and their relation to instabilities, under changes of chemical composition or magnetic field or application of pressure;

(ii) accurately predict properties of specific benchmark materials of great technological importance, which exhibit intriguing behaviour associated with changes of the Fermi surface and are the focus of current experiments, such as strontium ruthenates, strontium iridates, and fermonic superconductors.

(iii) make specific proposals for experiments on those materials to test new theories,

(iv) ultimately, achieve reliable control over the properties of these classes of correlated materials.

This is fundamental research with direct relevance to development of technology since our choice of the benchmark materials covers a wide range of potential applications. Superconducting SrRu2O4 is expected to harbour the Majorana bound states, making it a candidate for realising qubits of topological quantum computers. Strontium iridates feature a delicate interplay between spin-orbit coupling and Mott physics, which can lead to new-generation spintronic devices, while control over properties of superconductors under pressure, will open new avenues for the superconducting industry.

The project will have an immediate impact on Knowledge and People. The substantial impact on Economy will be due to development of new technologies and devices enabled by control over unconventional properties of correlated materials achieved in this work, and will be evident in the medium and long term. The new technologies will have an essential impact on Society by improving comfort and overall quality of life. To lesser extent there will also be an immediate impact on Society through the influence on policy makers.

Impact on Knowledge is discussed in substantial detail in Academic Beneficiaries. Impact on People will be realised by the direct contribution of the high-profile research to training of PDRAs and PhD students and development of world-class skills with potential application in the broad realm of novel materials and quantum technologies, both in academia and industry. We will involve directly the two PDRAs and the LU PhD student in the planned workshops as well as the outreach activities as a part of their formative experience. The PDRAs will have a chance for a significant career boost. Our fundamental research will further impact education of next generations by shaping teaching modules (e.g., condensed matter physics, computational methods and programming) in accordance with obtained results and equipping future generations of physicists and material scientists with highly competitive skills. Thereby, our work will have an additional indirect impact on Society and Economy.

The Economic impact will be achieved mainly due to significant advancement of technological applications as a result of control of material properties enabled by this project, naturally leading to creation of wealth in the medium (3-10 years) and mostly longer (more than 10 years) term. The medium-term impact will be contributed to indirectly through stimulating creation of advanced techniques and instrumentation for testing of our accurate predictions. This constitutes an impact on metrology with numerous possible applications in everyday life from medicine to security; for instance, this path has already led to creation of a spin-off company Razorbill instruments.

Being able to accurately predict and modify at will by tuning external parameters properties of the specific benchmark materials studied here, as well as their broader classes that follow the same principles, will have a major direct impact on Economy. Accurate knowledge of how properties of superconductors (critical temperature, critical currents and magnetic fields, etc.) change under pressure is the starting point for new applications in superconducting technology, ranging from precision magnetic field detectors (e.g., superconducting quantum interference devices) for various critical healthcare, computing, and military applications, to superconducting magnets and energy storage technology. Our work on Sr2RuO4, by revealing the type of superconducting order and its dependence on external parameters, will contribute to elucidating the possibility of its suggested use for quantum bits in tomorrow's quantum computers. The work on iridates will open new avenues for spintronic devices through enabling control of the effect of spin-orbit coupling on transport properties. The particular applications will be realised as a result of additional engineering work in the longer term, based upon the fundamental knowledge of electron correlation physics revealed in this project.

For an immediate impact on Society, our results will be communicated to policy makers, to help shape the agenda on strategic issues of national importance such as advanced materials and quantum technologies, noting especially the potential for Economic impact.

The success of our project will boost the UK expertise in crucial areas of R&D and will help establish and protect the leading role of our research institutions against keen international competition.