Fermi surface instabilities and quantum order at high pressure

One of the biggest scientific surprises of the twentieth century was the discovery of superconductivity, whereby some metals can carry electrical currents with absolutely no energy loss. Frictionless flow of electrons appears impossible in classical physics, but does in fact occur in quantum mechanical systems such as atoms or molecules. By taking phenomena we normally associate with the unusual micro-world of quantum physics into the practical macro-world of cables and switches, the discovery of superconductivity has paved the way for new devices and applications, in magnetic resonance imaging (MRI) scanners, high current fault limiting switches, high frequency filters and ultrasensitive measurement devices based on the Josephson effect. High-technology industries and the associated need for skilled labour are germinated by fundamental discoveries such as superconductivity. Future solutions for pressing problems, particularly in the fields of energy and sustainability, demand new materials with unusual electronic properties.

Real materials contain electronic quantum liquids. Because electrons have a low mass and are present at high density, the effects of quantum physics persist up to high temperatures, in many cases far exceeding room temperature. Interactions between the electrons cause them to correlate their motion and can induce new ordered states, of which an increasing variety - including various forms of superconductivity - have been discovered in recent years. The effective interactions depend on details of the specific material and thereby become highly tunable: they can be varied by changing material composition, by applying magnetic or electric fields, or by changing the lattice spacing through applied pressure. The transition into a new ordered phase as a function of this form of quantum tuning is called a quantum phase transition. The vicinity of quantum phase transitions is a fertile ground for unexpected and often spectacular discoveries. Examples include high temperature superconductivity in the iron-pnictide materials, the quantum nematic state in Sr3Ru2O7, and unconventional superconductivity in ferromagnets.

To pave the way for future discoveries, we need to know more about the mechanisms operating near such electronic instabilities. In this project, we will examine the electronic structure of selected materials close to quantum phase transitions, which can best be accessed under pressure. In some ways this is similar to deducing a crystal structure, but because the electrons are always in motion, we do not determine their position but rather their velocity, energy and effective mass. This is achieved by observing oscillations in the magnetic field dependence of the electrical resistivity, the magnetic susceptibility or other properties. These quantum oscillation measurements are a powerful tool for examining the electronic structure of a wide range of materials of current interest. To achieve the required ultra-sensitive measurements in a high pressure environment of more than 100,000 atmospheres is challenging, but recent technical developments in our group and elsewhere suggest that such experiments are now possible and will be justified by the resulting benefits.

We will investigate the correlated metallic state on approaching metal-insulator transitions, the transition from density wave order to the normal metallic state, the local moment to itinerant electron cross-over in heavy fermion systems, and other topics which are timely and of particular theoretical and practical interest. We will also use high precision heat capacity measurements under pressure to examine the electronic density of states near quantum phase transitions and to identify thermodynamic signatures of Fermi liquid breakdown in certain high-profile cases. Our electronic structure measurements will be complemented by high pressure lattice structure determination in the new Diamond Light Source synchrotron facility.

There are already important applications for some of the correlated electron states related to this project. Magnetism, the best-known manifestation of electronic order in solids, is applied throughout the engineering disciplines. For example, modern permanent magnets are essential for efficient electric motors or generators, and magnetoresistive sensors permit high storage density in computer hard drives, which themselves store information in magnetically polarised domains. Superconductivity, on the other hand, enables loss-free high field electromagnets. Such magnets operate in the many medical MRI scanners, as well as in numerous research facilities, including particle accelerators such as the LHC. It also has found important applications in power distribution, especially where space is at a premium, in microwave filters for mobile-phone base stations, and provides the basis for schemes to achieve quantum computation.

The diversity of electronic states in correlated systems, their intrinsic quantum nature, their reach into practicable temperature regions, and their tunability by varying the crystal structure, or by applying external fields or pressure, can be exploited for next generation quantum innovations. These may be novel devices, for instance for sensing, computation or storage, or new materials with potentially major impact on current engineering challenges. This programme of fundamental research helps to address key concerns of our industrialised economy, in particular with regard to energy and sustainability. Progress in this area benefits the general population as well as specialised companies poised to exploit technological advances:

* More efficient batteries and supercapacitors: key battery materials such as LiCoO2 exist on the boundary of a Mott insulator transition, one of the central topics of this project. The high electron density and strong interactions present in Mott insulators benefit energy storage applications, and the wide variety of structures available in complex materials can be exploited for efficient ion transport. Better battery and capacitor materials strongly affect the use of increasingly ubiquitous mobile devices, as well as the performance of electrical motorcars. On a larger scale, efficient energy storage is required to secure a reliable energy supply in the presence of fluctuations of supply and demand.

* Cooling and thermal energy harvesting: materials with high thermoelectric figure of merit, including the skutterudite compounds to be investigated in this project, can be used for refrigeration and, conversely, for thermal energy harvesting. Both technologies are relevant in computing and other high-technology applications, which are increasingly limited by waste heat production. Moreover, materials with strongly enhanced magnetocaloric effects, such as some of the rare earth compounds in this work, can be used for magnetic refrigeration. Present cooling and air conditioning methods account for a substantial fraction of our energy consumption, and even modest gains in efficiency would produce major savings. Electric and magnetic refrigeration methods also have a role to play in cooling sensors and other appliances which benefit from a quiet, low temperature environment, efficiently and conveniently to their ideal operating temperature.

* Power distribution: this research contributes to the search for new superconductors with high transition temperatures, which are expected near the Mott insulating state or near other quantum phase transitions. Materials which superconduct at high temperatures, potentially even at room temperature, could trigger a technological revolution, but even modest advances benefit power transmission, distribution and switching.

The development of experimental techniques associated with this work will feed innovative solutions as well as skilled problem solvers and entrepreneurs into the UK network of high technology companies.