Quantum Dynamics in Correlated Spin Systems

Contrary to our natural perceptions materials and devices which rely on quantum properties play an integral role in our everyday life. Even seemingly mundane phenomena such as electrical conduction relies on the band gap picture which is fundamentally quantum mechanical in nature; indeed a theoretical model which describes why, for example, copper is a metal, and silicon a semiconductor can be constructed using the quantum mechanics of electrons interacting with the crystal lattice upon which they sit. Extensions of this picture have led to the explanation of some superconductors, materials which conduct electricity without any resistance. Superconductors, although initially of fundamental interest at the time of their discovery in 1911, have more recently underpinned the function of MRI scanners in hospitals. Although the timelines from discovery, fundamental experimentation and interest to commercial applications are long, this has generally been the pathway to paradigm shifting technologies for society.

This methodology of investigating fundamental properties of matter remain highly relevant and indeed, quantum topological materials are of general interest today. Most devices are fabricated in the micro-metre regime, but a full understanding of the parent bulk material is generally required to design and control the materials used. This proposal is firmly based in the realm of frontier science, and will exploit new techniques that have been developed by the investigation team to tune and understand quantum interactions at a fundamental level. The project is based upon a material known as spin ice. The beauty of this material is that previous research has identified the basic, apparently classical, properties that are well-understood. We want to go further to investigate and characterise emergent states, where properties can be manipulated by changing experimental variables and the effect of hitherto neglected quantum dynamics that underly the observed physical behaviour. This proposal exploits our existing knowledge of the classical properties of spin ice and will investigate underlying quantum processes. In particular we will study the quantum tunneling of spin ice's so-called magnetic monopoles. This will allow us to understand how to tune quantum states in the future.

In correlated spin physics a clear goal has been to understand the crossover between classical and quantum behaviour. In this proposal we will investigate spin ice which has a very large magnetic moment and has often been described as a classical magnet, to reveal underlying quantum behaviour. We have identified several methods to explore this, and importantly all require our unique high frequency susceptometer that we have developed over the past few years. We have already identified dilute spin ice as a material to investigate tunneling of the magnetisation and propose to investigate the phase diagram as a function of magnetic field. This will allow us to understand how magnetic monopoles hop at low temperatures in this material, and how the emergent and non-equilibrium states develop. Moreover we can tune a magnetic field applied to spin ice to look at a critical point where quantum fluctuations may play a significant role. We can also look at the tunneling of monopoles in new spin ice materials at higher frequency than previously possible.

This grant will allow us to develop and retain high-level technical and scientific expertise and train a future scientific leader in developing cutting-edge science and technology.

The research outlined in this proposal is fundamental and in the short term the impact is likely to be academic, aimed at elucidating detailed dynamics in the hitherto classically described spin ice family of materials. We have identified several modes of impact, specifically these are (i) extending our understanding of the physics of quantum dynamics in these strongly correlated emergent materials. (ii) Contributing to the next generation of scientists and wealth generation by transferable skills training of the research team members. (iii) Developing high frequency susceptibility techniques to enable other researches in other fields. (iv) helping to increase scientific literacy and enthusiasm within society.

(i) Understanding the behaviour of systems which show quantum properties has allowed us to fully understand previous technologies which had been discovered through experimentation and are currently being used to develop the next generation of innovation which will define our future culture and technological advances. The key to such advancement is understanding the underlying physics of quantum processes in numerous types of materials so that our knowledge is broadened. As a society we have struggled to visualise the transition from preliminary scientific discoveries to applications and wealth generation, examples include the laser, which needs no discussion through to topological systems which are the basis of our next generation of electronic devices. In all cases it stems from a fundamental physical understanding. From our preliminary measurements and from theoretical constructs it is clear that quantum processes need to be accounted for in spin ice, specifically in the monopole model and this proposal will help to clarify our general understanding of the process of crossover from the classical to quantum regime.

(ii and iii) We are already engaged with our industrial partner to develop the measurement technology required to investigate the properties of spin ice. The goal here is to enable our technology to be available to other scientists. Moreover if our investigation of the scientific properties of spin ice develop any exploitable scientific knowledge we are primed to exploit this as exampled by our already demonstrated engagement of partners when opportunities arise.

(iv) We would like to help to maintain the public's interest in fundamental science. We will exploit our already established links with local schools and "Science Made Simple", to develop content for "Why Physics?" schools presentations to enhance the pupils' appreciation of magnets and magnetism. We believe this is beneficial to both society and importantly teaching and explaining helps to benefit the underlying physics knowledge essential for a successful future.