Harnessing disorder to tune, tailor and design classical and quantum spin liquids

Entanglement underpins many of the defining non-classical properties of quantum mechanics, and long-range entanglement engenders exotic phenomena such as fractional quantum numbers and emergent topological excitations. The next generation quantum technologies will rely on our understanding and exploitation of coherence and entanglement, and this proposal directly tackles these issues.

Exemplars of massive long-range entangled phases are quantum spin liquids -- states of quantum magnets in which electronic spins reside in macroscopic superpositions of infinitely many disordered, liquid-like microstates. Frustrated pyrochlore magnets often exhibit liquid-like short-range correlations down to the lowest temperatures and are therefore ideal candidate materials to look for classical and quantum spin liquid behaviour.

The presence of disorder in any of its forms -- fluctuations, strain, structural defects -- is usually regarded as a nuisance that has the potential to obscure or disrupt the sought-after spin liquid phase. However, it has also been recently shown that the presence of structural disorder can sometimes stabilise classical and quantum spin liquids, and it can even lead to new magnetic degrees of freedom, the formation of topological spin glasses and the formation of entirely novel quantum spin liquids.

Inspired by these results, we here take the view of disorder as a resource to tailor, tune and control spin liquid behaviour and quantum entanglement. Specifically, we propose to introduce structural disorder in pyrochlore materials in a controlled manner via doping, and to determine the defect structures using single-crystal diffuse neutron scattering. The results from these measurements will allow us to develop theoretical models and simulations to understand how the defects change the magnetic properties of the ions and their collective behaviour. In parallel to candidate materials for quantum spin liquid behaviour, we will also study related materials in the so-called `classical' regime, where the properties without disorder are better understood and where modelling and simulation capabilities are generally greater; in doing this we shall provide insight and support to the analysis of the more challenging quantum regime. In our concerted theory-experiment approach, we expect the insight from modelling to feed back into deciding which further samples to grow and which measurements to perform to test our predictions, ranging from thermodynamic measurements to dynamical structure factors using polarized neutrons.

We will investigate questions about the stability of quantum spin liquid phases; the promotion of quantum fluctuation due to effective transverse fields introduced by disorder; the scattering and trapping of emergent excitations, and in general questions about localisation and glassiness, in response to the disorder produced by structural distortions. Our overarching aim is to investigate the relationship between topology, glassiness and liquidity, and to obtain unambiguous evidence for long-range entanglement in quantum spin liquids.

The research in this proposal will result in fundamental breakthroughs in our understanding of physics and materials, and impact will initially be realised within the scientific research community. In the longer term, there are potential opportunities to enable economic and societal impacts in a wide range of fields.

QUANTUM TECHNOLOGIES: Next generation quantum technologies will rely on our understanding and exploitation of coherence and entanglement. Defects and disorder in general are omnipresent and can deeply affect this behaviour, especially on the small scales needed for materials and devices to operate at the quantum level. This proposal directly tackles the fundamental issue of characterising and controlling disorder. Disorder is not only a nuisance: It can also alter the behaviour of a system in interesting and useful ways. This is the approach chosen in this proposal, where disorder - broadly interpreted - is seen as a resource to control and even tailor-design the single-ion and many-body properties of a system.

In the specific context of magnetic materials, spin liquids - with their topological properties and fractional excitations - are one of the most fascinating and promising settings to develop quantum technologies, for example, with applications in data storage, information processing and computing - the key tenet being that the severe limitation on potential applications due to decoherence, say, in trapped quantum particles, could be prevented by storing and manipulating the information in non-local (topological) degrees of freedom. Quantum computing has the potential of a step change in current computational capabilities, allowing breakthroughs in drug discovery, the design of smarter materials, cryptography, and a whole host of other quantum applications.

Emergent magnetic monopoles are a rare experimental example of fractional spin excitations in three dimensions. The proposed studies of spin ice in both classical and quantum systems described here will be of help in understanding how to control spin ice excitations through tuning strain and disorder, and thence gain further insight into manipulating fractional excitations in spin liquids in general.

The fundamental understanding that will derive from our proposal has the potential to directly underpin the development of new quantum technologies, benefitting researchers in academia and industry, and early stage translational / spin-off companies working in this sector. Moreover, as we outline in detail in the Pathways to Impact, our work has the potential to impact several related industries that use magnetic cooling, fuel cells, thermal barrier coatings, catalysts, and nuclear waste management.

For the example of magnetic cooling, disruption to the supply of helium makes the consideration of solid-state refrigerators imperative, and the highly frustrated pyrochlore magnets studied here are promising candidate materials for increased volumetric and gravimetric cooling. Magnetic cooling is environmentally friendly since there is no possibility of refrigerant leakage and no direct carbon dioxide emissions.

PUBLIC KNOWLEDGE: Our understanding of the materials and processes, and of the physical context in general, that will develop from our work will directly feed into outreach events targeted at disseminating the knowledge at an accessible level that will benefit the general public.

CAPACITY/TRAINING: A key impact of our research will be the training of highly skilled manpower, through several pathways described in detail in the Pathways to Impact document. Beyond providing the inspiration to become tomorrow's research leaders in academia and industry, these researchers will be equipped with a skill set that allows them to develop successful careers outside of research, directly contributing to the Government vision of building a knowledge-based economy.