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Superconductivity research at ISIS

A magnet floating above a superconductor due to the Meissner Effect

A magnet floating above a superconductor due to the Meissner Effect
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Superconductors have two remarkable properties: they conduct electricity without resistance and they expel magnetic fields from the bulk of their interior. Both properties lead to use in situations where large electrical currents must be transferred or large magnetic fields generated, the most ubiquitous example of which is the MRI scanner.

Superconductivity was first discovered in 1911 when mercury was cooled to 4 degrees (Kelvin) above absolute zero. Further study gradually led to an understanding of what allowed materials to become superconducting. Electrons interact with the vibrations of the crystal lattice and form into pairs that can travel freely through the crystal lattice without scattering. The mathematical description of this process is called BCS theory and was described in 1957. Considering how many electrons can pair up and how strongly they can interact with lattice vibrations suggested that no material could superconduct above 40 Kelvin. However, in 1986 a new class of materials based on copper oxide layers was discovered that was soon found to display superconductivity at up to 150 Kelvin. Understanding the mechanism that allows them to achieve this is still an active field of study, together with improving their material properties for applications and discovering other classes of materials that are superconducting.

Soon after ISIS opened it was used to determine the structure of the new copper oxide superconductor YBCO and measure the vibrational properties of superconducting carbon 60 compounds. Subsequently, the magnetic fluctuations of copper oxide materials were mapped out at ISIS leading to a common trend found in several members of this family of materials. This also showed that the magnetic fluctuations had enough energy to pair electrons as needed for superconductivity. With the recent discovery of superconductivity in iron-based materials measurements at ISIS again proved significant in determining the properties of these new materials.

Common to many of the superconducting materials discovered since 1986 is the fact that they change from magnetic insulators to superconductors when a control parameter such as chemical substitution or pressure is altered. The behaviour of magnetism, structure, and their fluctuations all change with these parameters and ISIS experiments can probe each of these properties.

The interplay between magnetism and superconductivity can also be investigated by examining devices made from alternating layers of magnetic and superconducting materials. These can either be made by depositing films of the materials or by chemical self-assembly. Such materials offer the possibility of controlling one property using the other for use in information processing, sometimes referred to as spintronics.

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