Excitons in motion

The electrical properties of semiconducting materials are usually described in terms of electrons and holes. The concept of a hole is used to represent the absence of an electron from a certain part of the material and it is convenient to think of holes as positively charged particles that can move through the specimen. Since holes are positively charged, they are attracted by the negative charge of any electrons they encounter and, under appropriate conditions, electrons and holes will orbit about each other and form a stable entity called an exciton. The situation is very similar to that of a hydrogen atom (in which an electron orbits around a proton) and the hydrogenic model is often used to describe the behaviour. Excitons are tremendously important because they can be created by incident light and transport energy through materials. They are thus central to our understanding of how light interacts with matter and in our exploitation of opto-electronic effects.Our proposal is motivated by some remarkable behaviour that we have observed when we apply a strong magnetic field. At first sight, one would not expect the properties of excitons to be changed by movement along the direction of such a field, since it is well known that individual electrons and holes are not affected by such motion. However, in recent experiments we have discovered that very large changes in the magnetism associated with the exciton do in fact occur when it moves in this way. This surprising result is, we believe, due to the fact that the electron and hole orbit each other as they move and that, in so doing, they interact with the atoms in the crystal is such a way as to cause their magnetic behaviour to be drastically altered. We find that the excitons behave in this unexpected manner in quantum wells made from three completely different materials and we think that the phenomenon may be universal amongst all semiconductors. To observe this effect, we study excitons that are confined in layers of material that are not much bigger than the size of the exciton itself. Since these layers have thicknesses greater than the exciton's diameter (typically 10 nm), but are still small enough to make it behave according to the laws of quantum physics, they are referred to as wide quantum wells. In our experiments, which are carried out at very low temperature, we study the light absorbed by excitons in a strong magnetic field. Because we are in the regime of quantum physics, the exciton behaves as a standing wave (rather like those on a violin string), with only certain values of the wavelength being allowed. These allowed wavelengths in turn mean that the exciton can possess only certain values of kinetic energy. The exact colour (or photon energy) of the light that is absorbed by the excitons depends both on their kinetic energy in the well and on their magnetic properties, so that with high resolution spectroscopy we can determine the connection between these two quantities. Our preliminary work shows that the magnetism depends very strongly indeed on the energy and it is this effect that we plan to study in the present proposal. The work should tell us a great deal about what happens to excitons as they move through crystals, a topic which is at present only partly understood. Study of this remarkable (and previously unreported) phenomenon will also tell us whether the analogy with the hydrogen atom is any good once the exciton starts to move. This is a new and exciting area of exciton physics and is of fundamental interest. The magnetic behaviour of excitons is also in itself of considerable topical importance because of the worldwide interest in spin-dependent optical phenomena.