Research Fellows Directory
Professor Stephen Hayden
University of Bristol
The goal of my work is to use quantum mechanics to understand why particular crystalline materials have the properties they do. In 1911, Heike Kamerlingh Onnes discovered that, at very low temperatures, mercury could carry electricity without a loss of energy. This phenomenon was named "superconductivity". Superconductivity was thought to be a very low temperature phenomenon, until, in 1986, a new class of materials known as "high temperature superconductors" was discovered. These (cuprate) materials are based on layered structures of copper oxide planes and can be superconducting up to a temperature of about 138 K. Superconductivity occurs when certain "collective excitations" are present. In most superconductors these excitations are "phonons" and are due to the motion of the atoms. However, phonons are unable to explain the superconductivity in the cuprates. I am interested in investigating other possibilities and discovering what makes the cuprates and other materials superconducting. Just as waves on the sea are the collective excitations of many water atoms, "spin waves" are collective excitations of the electron spins in a solid. Collective spin excitations could well explain the high temperature superconductivity phenomenon.
I measure the collective spin excitations in high-temperature superconductors (and other materials) by scattering neutrons from them. Neutrons are perfect for this because they also have a spin moment and so can "detect" the electron spin. However, since neutrons have no charge they pass right through solids. New neutron spectrometers and larger crystalline samples mean that we have fantastic opportunity to characterise comprehensively the collective spin excitations and perhaps solve one of the major outstanding problems in physics - What is the mechanism that is responsible for high-temperature superconductivity? If we understand the mechanism, more useful superconductors with even higher transition temperatures may be found.