What are Superconductors?

Dutch physicist, Heike Kamerlingh Onnes, discovered superconductivity in 1911. Onnes was studying the electrical properties of mercury when he found that its electrical resistance completely vanished when it was at a temperature of 4.2 K (close to absolute zero). An electric current was applied to this supercooled mercury, then the battery was removed. The electric current remained the same in the mercury at the same value. This discovery had massive implications for the energy industry, and if utilised could solve looming energy crisis.

Although the discovery of superconductors was in 1911, the understanding of how they work was not put forward until 1957. Physicists John Bardeen, Leon N. Cooper and Robert Schrieffer developed the theory that to create electrical resistance, the electrons in a metal need to be free to move and bounce around. In super cold conditions, under the material’s critical temperature, the electrons inside the metal become less mobile, allowing them to pair up. This prevents them from moving around. These electron pairs are called Cooper pairs and are very stable at low temperatures.  As there are no electrons free and mobile, the electrical resistance disappears completely.

Superconductors experience many phenomena, one being the Meissner Effect. When a superconductor is below its critical temperature, it expels its magnetic field. When the temperature is above the critical value, a magnetic field is able to pass through the material. Below the critical temperature, the magnetic field cannot pass through the material and instead must go around it. Surface currents that flow without resistance then develop to create magnetization within the superconducting material. This magnetization is equal and opposite to the applied magnetic field, which results in the cancelling out of the magnetic field everywhere within the superconductor. This means the superconductor has a magnetic susceptibility of -1, and it exhibits perfect diamagnetism. Diamagnetic materials are repelled by by a magnetic field, hence the superconductor is repelled by the magnetic field. One way to display this phenomena is to place a magnet above a superconductor. The magnet is observed to ‘float’ above the superconducting material. This is because repelling force can be stronger than gravity, allowing the magnet to levitate above the superconductor.

The implementation of superconductors have not been so straightforward. Superconductors only operate at temperatures close to absolute zero, and the energy costs of cooling these materials to these temperatures are enormous. Despite this, it is very likely to encounter a superconductor in everyday life. Many MRI machines use superconducting magnets, as normal magnets would melt due to the heat of even a little bit of resistance. As superconductors have no electrical resistance, no melting occurs, and the electromagnets can generate the necessary magnetic fields for the safe operation of MRIs.