University > Department > Condensed Matter Physics > Correlated Electron Systems Group > Research

Introduction

One of the triumphs of twentieth century physics was the development of quantum theories of the behaviour of electrons in solids. These theories underpin our understanding of many simple materials, and are directly responsible for the way in which silicon technology has so profoundly changed the world in which we live and work. They are reliant, however, on the key assumption that electron-electron interactions can be treated in a mean-field approximation. The challenge of the twenty-first century will be to understand and exploit the huge class of materials in which this approximation breaks down. In these compounds, the position and motion of each electron are correlated with those of all the others.

The correlated electron problem in solids is one of the most profound quantum mechanical problems faced by modern physics. It is hardly surprising, then, that correlated electrons form a wealth of subtle, many-body quantum states at low temperatures. There are superconductors with novel symmetry-breaking properties, and strange metals, magnets and insulators. It is also possible to tune from one kind of order to another at T=0, producing quantum phase transitions whose associated fluctuations can dictate the properties of the material at temperatures as high as room temperature and above. The scientific and technological promise of correlated electron materials is enormous, resulting in considerable world-wide investment in the field.

How can we approach the correlated electron problem? The number and density of electrons is so high that it will never be amenable to a ‘brute force’ solution using direct computation. Instead, work in this field is reliant on ingenuity. Highlights have been Landau’s Fermi liquid theory of metals and the discovery of heavy fermion quasiparticles, and the discovery and explanation of the Fractional Quantum Hall Effect (resulting in the award of the 1998 Nobel Prize for Physics), but these are only the tip of the iceberg.

Our approach centres on the study of oxides, which are proving to be ideal materials in which to establish and observe the spectacular consequences of electron correlation. In close collaboration with colleagues in Birmingham, Bristol, Edinburgh and Cambridge in the UK and Kyoto in Japan, we have established in St Andrews a programme involving the growth of oxide crystals with very high levels of crystalline perfection, using a new technique recently developed in Japan. We perform experiments covering nearly five orders of magnitude of temperature, from 25 mK in a dilution refrigerator to 1300K in a furnace, as described in the ‘equipment’ section of this web page.