Theory of Condensed Matter Research Group

Scientists try to understand the world around us - both immediately around us, and at the extremes of distance, energy and time. Modern science would have got nowhere without careful, and often surprising, experimental observations. But all scientists are theorists as well, as they seek to understand their experimental results, discovering the simplest possible theory that explains all the known facts.

The best theories not only elegantly summarise what is known already, they allow predictions to be made about things that have never been seen before. So, in addition to building theories, theorists explore their consequences. This might be done to test how well the theory works in extreme or unusual circumstances, or to discover new phenomena.

Mathematics is the language of theory, certainly in the physical sciences, and increasingly in biology. It is not just a descriptive language - it is a tool that allows the theories to be manipulated, improved, or even disproved. Equations are solved - known quantities are used to discover the unknowns.

With the advent of modern powerful computers theorists have gained a new tool. Not only can computers now do many mathematical tasks, such as solving very complex equations, they can also manipulate theories that would be very difficult or impossible for a traditional mathematician to handle using a pencil and lots of paper.

The Theory of Condensed Matter (TCM) Group at the Cavendish Laboratory is a group of theorists who are interested in understanding 'condensed matter'. Most of the 'stuff' in the universe that we, as humans, are likely to be able to touch is condensed matter. This includes semiconductor crystals and liquid crystals, metals and superconductors, minerals that might be found deep in the Earth or other planets, and even molecules that keep us alive.

The fundamental theory that our research relies on is called 'Quantum Mechanics', and was developed in the early 20th century. Quantum Mechanics describes the mechanics of the very small particles that most matter is made of: electrons, protons and neutrons. The equations that we still believe explain most of the phenomena that we can see around us were written down over fifty years ago, but were impossible to solve!

Figure 1: An electron sitting on a defect in glass (SiO2). The blue blob indicates the position of the electron. The red and yellow "tubes" represent the bonds between the silicon and oxygen atoms in the glass. Experimental measurements of the behaviour of the trapped electron in a magnetic field allow a lot to be said about the arrangement of the atoms around the defect. Theoretical predictions help this interpretation.

In the TCM Group, using theoretical advances and enthusiastically making use of computers and supercomputers, we are actually solving these equations for a vast range of realistic situations, from discovering what makes diamond so strong, to understanding proteins. We have developed two state-of-the-art computer programs: CASINO and CASTEP. CASINO allows the very accurate simulation of Quantum Mechanics, but it takes a lot of computing power. CASTEP on the other hand is less accurate, but good enough for many purposes: it can be used to calculate the properties of very large collections of atoms. CASTEP was originally purely an academic computer program, used exclusively by researchers in universities, but it is now marketed commercially by Accelrys Inc. Accelrys Inc. are a scientific software company whose European headquarters is based in Cambridge on the Science Park. They now have several hundred commercial customers who are actively using theory to solve industrial problems.

Physics at Work 2004 is sponsored by EPSRC through the Outreach Programme of the Condensed Matter Theory Portfolio Partnership.

Condensed Matter Theory Portfolio Partnership Poster