The super science of superconductors

What are superconductors?

All the electronic devices around you – your phone, your computer, even your bedside lamp – are based on moving electrons through materials. In most materials, there is an opposition to this movement (kind of like friction, but for electrons) called electrical resistance, which wastes some of the energy as heat. This is why your laptop heats up during use, and the same effect is used to boil water in a kettle. Superconductors are materials that carry electrical current with exactly zero electrical resistance. This means you can move electrons through it without losing any energy to heat.

Sounds amazing. What’s the catch?

The snag is you have to cool a superconductor below a critical temperature for it to work. That critical temperature depends on the material, but it’s usually below -100 °C.

A room temperature superconductor, if one could be found, could revolutionise modern technology, letting us transmit power across continents without any loss.

How was superconductivity discovered?

When you cool a metal, its electrical resistance tends to decrease. This is because the atoms in the metal jiggle around less, and so are less likely to get in an electrons way.

Around the turn of the 19th century, physicists were debating what would happen at absolute zero, when the jiggling stops altogether.

Some wondered whether the resistance would continue to decrease until it reached zero.

Others, such as Lord Kelvin (after whom the temperature scale is named), argued that the resistance would become infinite as electrons themselves would stop moving.

In April 1911, Dutch physicist Heike Kamerlingh Onnes cooled a solid mercury wire to 4.2 Kelvin and found the electrical resistance suddenly vanished – the mercury became a perfect conductor. It was a shocking discovery, both because of the abruptness of the change, and the fact it happened still a good four degrees above absolute zero.

Kamerlingh Onnes had discovered superconductivity, although it took another 40 years for his results to be fully explained.

What’s the explanation for superconductivity?

It turns out there are at least two kinds of superconductivity, and physicists can only explain one of them.

In the simplest case, when you cool a single element down below its critical temperature (as with the mercury example above) physicists can explain superconductivity pretty well: it arises from a weird quantum effect which causes the electrons to pair up within the material. When paired, the electrons gain the ability to flow through the material without getting knocked about by atoms.

But more complex materials, such as some ceramics which are superconducting at higher temperatures, can’t be explained using this theory.

Physicists don’t have a good explanation for what causes superconductivity in these “non-traditional superconductor” materials, although the answer must be another quantum effect which links up the electrons in some way.

What are high-temperature superconductors?

Physicists have a loose definition of what a “high temperature” is. In this case, it usually means anything above 70 Kelvin (or -203 °C). They choose this temperature because it means the superconductor can be cooled using liquid nitrogen, making it relatively cheap to run (liquid nitrogen only costs about 10-15 cents a litre.)

The threshold temperature for superconductivity has been increasing for decades. The current record (-70 °C) is held by hydrogen sulfide (yes, the same molecule that gives rotten eggs their distinctive smell).

The hope is that one day scientists will produce a material that superconducts at room temperature with no cooling required.

What are superconductors used for now?

Superconductors are used to make incredibly strong magnets for magnetic levitation (maglev) trains, for the magnetic resonance iImaging (MRI) machines in hospitals, and to keep particles on track as they race around the Large Hadron Collider.

The reason superconductors can make strong magnets comes down to Faraday’s law (a moving electric field creates a magnetic field). With no resistance, you can create a huge current, which makes for a correspondingly large magnetic field.

For example, maglev trains have a series of superconducting coils along each wagon. Each superconductor contains a permanent electric current of about 700,000 amperes.

The current runs round and round the coil without ever winding down, and so the magnetic field it generates is constant and incredibly strong. As the train passes over other electromagnets in the track, it levitates.

With no friction to slow them down, maglev trains can reach over 600 kilometres per hour, making them the fastest in the world.

A prototype hoverboard designed by Lexus also uses superconducting magnets for levitation.

What uses might superconductors have in the future?

About 6% of all the electricity generated by power plants is lost in transmitting and distributing it around the country along copper wires.

By replacing copper wires with superconducting wires, we could potentially transmit electrical power across entire continents without any loss. The problem, at the moment, is this would be ludicrously expensive.

In 2014, the German city of Essen installed a kilometre-long superconducting cable for transmitting electrical power. It can transmit five times more power than a conventional cable, and with hardly any loss, although it’s a complicated bit of kit.

To keep the superconductor below its critical temperature, liquid nitrogen must be pumped through the core and the whole thing is encased in several layers of insulation, a bit like a thermos flask.

For a more practical solution, we’ll need to wait for cheap superconductors that can operate closer to room temperature, an advance that can be expected to take decades.

Closer to reality, perhaps, are superconducting computers. Scientists have already developed computer chips based on superconductors, such as the Hypres Superconducting Microchip. Using such processors could lead to supercomputers requiring 1/50Oth the power of a regular supercomputer.

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