Emergence of novel electronic states in 5d transition metal oxides

In the fascinating search for materials which display new phenomena, including materials with potential for exploitation in technological applications, the spotlight has recently fallen on the role of the spin-orbit interaction. This is a relativistic interaction which couples the electron's intrinsic spin to its orbital angular momentum. Normally, the effect of the spin-orbit interaction on the electronic properties of materials is minimal and can be taken into account by treating it as a weak perturbation.

However, the spin-orbit interaction increases rapidly with atomic number Z (in proportional to Z to the fourth power), and for heavier atoms, especially third row transition-metal atoms with unfilled 5d orbitals, it is comparable in strength with the other interactions that determine the electronic properties of materials. The resulting competition between interactions of similar strength is predicted to result in new emergent states of matter in 5d compounds which have never been observed before.

In this project we shall synthesise and investigate the fundamental properties of several families of 5d transition-metal oxides. Intense theoretical activity over the last few years has produced a number of predictions for these materials. One predictions is of phases called topological Mott insulators which owe their stability to an interesting topological property of the electron bands caused by the strong spin-orbit interaction. Other predictions include, thermally-driven metal-insulator transitions, unconventional high temperature superconductivity, and so-called spin-liquid states which have as-yet undiscovered emergent excitations called Majorana fermions.

5d transition metal oxides have been relatively unexplored from an experimental point of view, largely because of the difficulty to make good quality single crystal samples. Our proposal is designed to integrate all of the necessary components required for a successful research programme. A concerted effort will be made to prepare materials of interest in the form of pure, high-quality, single-crystals. Complete characterisation of their electrical, thermal and magnetic properties will be undertaken using equipment in our home laboratories. Armed with the best possible samples we will exploit spectacular recent advances in synchrotron X-ray techniques to probe the novel spin-orbital states predicted to exist in 5d oxides. This work will be performed partly in collaboration with colleagues at large-scale facilities such as the Diamond Light Source in Oxfordshire, and the European Synchrotron Radiation Facility in Grenoble. Finally, we shall work closely with theorists to develop a comprehensive understanding of the novel electronic states we discover.

The research we are proposing aims to provide new insights into emergent phases of electrons in solids. Emergent phases appear in large systems with many interacting components. When large numbers of electrons act together, a small stimulus can provoke a gigantic response, and the resulting enhancements in physical properties can be used in devices. Therefore, emergent phases represent very fertile territory in which to search for materials with useful functionalities.

The proposed work is underpinning in nature, and may over time lead to the discovery and development of new electronic and magnetic materials. One of the best known examples of cooperative electronic behaviour is superconductivity - the dramatic vanishing of electrical resistance below a critical temperature. Superconductors are fascinating because in the most advanced materials the mechanism which causes them to become superconducting is still not known. They are also of practical importance, for example being widely used to provide high magnetic fields for magnetic resonance imaging and other applications.

Our work on the relatively unexplored class of 5d oxides aims to search for and investigate novel emergent phenomena which, like superconductivity, could one day find application in technological devices. Given the exploratory nature of the research, the potential economic benefits, and the timescale over which the impact will be felt, is difficult to predict.

The work could, however, benefit society in ways not directly quantifiable in pure economic terms. The general public is fascinated by fundamental physics - witness the popularity of science communicators such as Brian Cox and Jim Al-Khalili. Superconductivity and other bizarre and unexpected states of matter, which our work will investigate, are wonders of nature which have the capacity to stimulate and enthrall. Therefore, the work in this project has the potential to be of considerable educational value to society and to play a role in encouraging young people to choose science and technology as a career. In addition to conventional dissemination routes we are committed to a wider spectrum of public awareness and outreach activities through our institutions which provide a vehicle to communicate any important new discoveries to the public. For example, ATB is currently leading an STFC-funded project to develop two workshops, one on Magnetism and one on Superconductivity, to take to local secondary schools in Oxfordshire.