Superconductivity

Superconductivity describes the property shown by some materials of conducting electricity without electrical resistance. There are two types: low-temperature, or conventional, superconductivity, which generally only occurs within a few degrees of absolute zero and is well-understood by theoretical physicists; and high-temperature superconductivity, which we understand little about at all.

If we were to discover a way to create room-temperature superconductivity, it could revolutionise how we make, store and transport energy, and help reduce the greenhouse gas emissions causing global warming.

Something like 10 per cent of all electrical power generated is lost in long-distance, high-voltage cables. A room-temperature superconductor could also be used to store energy in superconducting circuits, allowing us to keep cheaply generated power from renewable sources until it is needed. In applications such as motors and generators, they would offer a significant improvement in the power-to-weight ratio, boosting the efficiency of electric cars, for example.

When was superconductivity discovered?

Superconductivity was discovered in 1911, when the Dutch physicist Heike Kamerlingh Onnes discovered that mercury wire lost all electrical resistance at an extremely frosty 4.2 kelvin, or 4.2 degrees above absolute zero (-273.15°C), the lowest temperature possible. The next year, tin and lead were discovered to become superconductors at 3.8K and 7.2K, respectively, followed by other metals, often as alloys such as niobium-tin.

In the 1950s, US physicists John Bardeen, Leon Cooper and John Robert Schrieffer provided an explanation of this low-temperature superconductivity on the basis of quantum physics. It occurs because conducting electrons team up into so-called Cooper pairs, which use quantum properties to evade the normal barriers to their free movement through a solid. This pairing is caused by the influence of phonons – vibrations in the lattice of atoms that make up a solid. These vibrations get disrupted at higher temperatures, explaining why conventional superconductors work only below 40K or so, meaning they must be cooled using expensive liquid helium. Bardeen, Cooper and Schrieffer’s “BCS” theory won them the 1972 Nobel prize in physics.

A flurry of discoveries in the late 1980s and early 1990s of copper-oxide, or “cuprate”, superconductors that worked at temperatures up to 133K (-140°C) caught physicists by surprise. These high-temperature superconductors required only relatively cheap, freely available liquid nitrogen cooling, which works down to 77K, and they raised the hope that practical room-temperature superconductors might be around the corner.

That hope has not yet been realised. Making further progress has been hampered by the fact that, unlike with conventional superconductors, we don’t know what’s going on inside high-temperature superconductors to make them lose their electrical resistance. We suspect that they form Cooper pairs directly, without phonons, but that is only an educated guess.

What is superconductivity used for?

As we don’t yet know quite how high-temperature superconductors work, the practical applications of superconductivity are strictly limited. Mostly, they are low-temperature, liquid-helium-cooled superconductors used to make very powerful superconducting magnets. These include the magnets that steer particles at the Large Hadron Collider at the CERN particle physics laboratory near Geneva, Switzerland. Hospital magnetic resonance imaging, or MRI scanners, which use the magnetic fields generated by superconducting niobium-tin alloys to look at tissue structures within the body.

In recent years there have been breakthroughs that could bring practical room-temperature superconductors closer. Powerful computing methods have allowed us to discover more groups of superconducting materials, including ones based on the two-dimensional sheets of carbon known as graphene. In 2020, physicists announced the discovery of a superconductor that did work at 14°C , or almost room temperature, by crushing a mix of carbon, sulphur and hydrogen together between two diamonds. The catch is that superconductivity only kicked in at pressures around 70 per cent of those in Earth’s core.