Question: What is a superconductor and how does it work?

Nathan Langford answered on 26 Jun 2012:

Hi gameveins,

Wow, this turned into a really long answer – you’ve asked one of those really big questions. As I mention below, it took physicists a long time to work out the answer to your question, and to be honest, I don’t understand all of the details yet myself, because I’m only just learning this stuff in my new field too. But here goes, I’ve tried to give you the general idea – main point first and then some more details afterwards.

I hope it helps – good luck getting through my answer, and well done for asking such a great question!

Cheers,
Nathan.

A superconductor is what many metals become when they’re cooled down to low enough temperatures.

You probably know that metals are often called “conductors”, because they are able to conduct electricity. Conductors are the opposite of another type of material, like glass, called insulators, because they don’t conduct any electricity. In practice, what these two statements mean is that if you take a battery and connect it to two ends of a conductor, then you get a flow of electrons (called a current) flowing around the “circuit”, which is the loop that goes through the battery and through the conductor. If you do the same thing to two ends of an insulator, however, you won’t see any flow of electrons, because they can’t move freely through an insulator.

In general, however, although electrons are able to move through conductors, they still tend to gradually lose some energy along the way. For example, the electrons can lose energy by colliding with the nuclei (central bits) of the atoms that make up the solid structure of the metal. This is called resistance and it means that you need to keep putting in energy into the electrical circuit to keep the current flowing. It’s a bit like trying to run through a forest. If the forest is more dense (and maybe there are thin branches reaching out from the trees), it takes more energy to push your way through and you can probably not run so fast. But if the trees are all standing quite far apart, then it’s much easier to run through and you can move a lot faster. If a metal is like a dense forest, then we say it has a high resistivity, and if it is like an open forest, we say it has a low resistivity. The lower the resistivity, the lower the amount of energy you need to put in (from the battery voltage) to create a current of electrons. But all normal metals have *some* resistance – it always requires *some* energy to make a current flow.

The amazing thing about a superconductor is that it has absolutely no resistance whatsoever. So it doesn’t take any energy to make a current flow, and, once it is flowing, that current can flow literally forever!!

So how does this work? Well, this is actually a very complicated question. It was discovered over 100 years ago, but it took physicists almost 50 years to explain how it worked! And do you know what? The scientists that worked it out won a Nobel Prize for that work!! And even now, people are learning more and more about it.

It turns out that if you cool a metal down far enough, it can undergo what is called a “phase transition”. This is very much like what happens to water when you cool it down and make ice. The change of water from a liquid to a solid is also called a phase transition, although it’s a slightly different type. The point is, as you cool it down, the metal goes from one “phase” into another – it becomes a completely different type of material, with completely different properties – and this is called a superconductor. Above this special temperature, the metal has some resistance when you try and create a current, but below this temperature, the resistance is suddenly zero.

But exactly how does this work? Well, this is what had physicists stumped for so long. It turns out that superconductivity is a special result of quantum physics – one of the rare effects of quantum physics that can be seen in really large objects with many particles at once. It happens because of the special property of quantum physics that particles can behave like waves, just like waves can behave like particles. Because the free electrons in the superconductor can behave like waves, as you cool down the metal, the electrons stop looking like little particles and start looking like smeared out, fuzzy blobs that occupy a large amount of space. When this happens, these electrons start to make very soft contact with all the atoms in the metal that they touch – and “tickling” touch of these atoms can very slightly change the electrons’ behaviour. As the temperature gets colder, the electrons spread out more and more and the effect of the atoms becomes stronger. Eventually, when it gets cold enough, the atoms in the metal can tickle the electrons in just the right way to get two electrons to pair together – these are called Cooper pairs, named after the guy who discovered them.

When the electrons pair up like this, it turns out that all of these pairs start blending together and they lose their individuality. They form what is called a “condensate”, which is a fancy technical word, but it’s a bit like watching a huge flock of starlings flying around – they all do exactly the same thing at exactly the same time. Interestingly, this is very similar to what happens with photons (little packets of light) inside a laser.

And now we come to the final piece of the puzzle – what on earth does this have to do with a superconductor having zero resistance to a flow of electrons? Well, because these pairs of electrons are so smeared out at this low temperature and just drifting along with the crowd, they start to pass through the metal without colliding with any of the atoms that would make them lose energy. It’s kind of like if, instead of trying to run through the forest, you could turn yourself into a thin mist and just flow through the forest – all of a sudden you wouldn’t really see any of the branches – you’d just flow between and around them.