New Quantum Materials from High Pressure Synthesis

Electronic technologies such as mobile phones, tablets and laptops have become indispensable to modern life, with improvements in performance, size, energy-consumption etc. occurring year-on-year. Within these devices are materials with particular electronic properties such as semiconductors and magnets. Their characteristics and performance are largely limited by the quantum mechanical behaviour of single or a few electrons, as elucidated in the early 20th century. Now in the 21st century there is an increasing drive towards the use of a new generation of quantum technologies based upon more sophisticated effects such as the correlation or entanglement of multiple quantum states, e.g. in a quantum computer.

Underlying these developments is the search for new quantum materials where electron-electron correlation gives rise to entangled or correlated ground states such as long range orders of atomic spin, orbital, or charge states, or fluid-like states like superconductors and quantum spin liquids (QSLs) in which many possible paired quantum states are superimposed. Topological effects have been used to discover further types of quantum material in recent years, through effects such Kitaev coupling which depends upon the directions of pairwise magnetic interactions within a material.

This project aims to synthesise new quantum materials within the family of perovskite oxides using high pressure reaction methods. Perovskite oxides have structures based on the ABO3 arrangement of the mineral CaTiO3. They have enormous chemical and structural flexibility as well as outstanding physical and chemical properties, which are often the best in their field, e.g. ferroelectric BaTiO3, YBa2Cu3O7 high-Tc superconductor, (La,Sr)MnO3 and Sr2FeMoO6 CMR (colossal magnetoresistance) for spintronics, multiferroic BiFeO3, and mixed conductors such as doped LaCrO3 for fuel cells. We will target perovskites based on heavy transition metals with electronic states that have small spins, which amplifies quantum behaviour, and strong spin-orbit coupling that gives rise to strong anisotropy (local directionality) in properties that accentuate topological influences. Chemical ordering of cations like that of Fe/Mo in Sr2FeMoO6, known as a double perovskite derivative, will be used to create interesting network topologies that tend to frustrate simple 'up-down' magnetic orders, making quantum fluctuations more dominant.

High pressure (HP) synthesis methods will be used as these are known to be effective for stabilising perovskite type materials, and also for generating cation ordered networks within the basic ABO3 arrangement. Proof of concept experiments have shown that 1:1 or 1:2 orders of cations on the perovskite B sites, and also 1:1 ordering of transition metal and other types of cation at A sites can be generated at HP. A 'double double perovskite' arrangement we discovered even has 1:1 A and B site orders, e.g. CaMnFeReO6, and so offers many permutations for discovery of new quantum materials. Oxide materials are incompressible so GPa-scale pressures (1 GPa = 10,000 atmospheres) are needed to change their chemistry, structures and properties significantly. Our ability to reach pressures up to 22 GPa (whereas many early and even present-day groups can only access 6-8 GPa) will enable us to discover these new materials. We will also collaborate with external partner groups to make other types of oxide and nitride materials that show interesting and useful magnetic, catalytic, and energy-related properties.