Most fundamentally, a superconductor is a material which becomes a perfect conductor with no electrical resistance when it gets cold enough. It was first discovered in 1911 when some Dutch experimentalists were playing around with a new way of cooling things down, and one of the things they tried was to measure the electrical resistance of various metals as they got colder and colder. Some metals just kept doing the same things that were expected based on how they behave at higher temperatures. But for others (like mercury) the resistance suddenly dropped to zero when the temperature was lowered to within a few degrees of absolute zero: they became perfect conductors. By perfect, I mean that the amount of energy that was lost as electricity went along the superconducting wire was zero. Nowadays, superconductors are very useful materials and are used in a variety of technologies. For example, they make the coils of the powerful magnets inside an MRI machine or a maglev train, they can allow ultra-precise measurements of magnetic fields in a device called a SQUID (superconducting quantum interference device), and in the future, there is some chance that junctions between different superconductors might be crucial for implementing a quantum computer.
So, how does this work?
Before I try to explain that, there is one crucial bit of terminology that I have to introduce. The types of particles that make up the universe can be classified into two types: One type is called fermions, the other type is called bosons. The big difference between these two types of particles is that for fermions, only one particle can ever be in a particular quantum state at any given time. For bosons, many particles can all be in the same state at the same time. The particles that carry electricity in metals are electrons, and they are a type of fermion. But when two fermions pair up and form a new particle, this new particle is a type of boson. Superconductivity happens when the electrons are able to form these boson pairs, and these pairs then all occupy the lowest possible energy state. In this state, they behave like a big soup of charge which can move without losing energy, and this gives the zero resistance for electrical current which we know as superconductivity.
This leaves a big unanswered question: How do the electrons pair up in the first place? If you remember back to high school, you probably learned that two objects with the same charge will repel each other, but that opposite charges attract. All electrons have negative charge and so should always repel, so how do they stay together close enough to make these pairs? The answer involves the fact that the metal in which the electrons are moving also contains lots of atoms. These atoms are arranged in a regular lattice pattern but they have positive charge because they have lost some of their electrons. (This is where the free electrons that can form the pairs come from.) So, as an electron moves past an atom, there is an attractive force between them, and the atom moves slightly towards the electron. Because electrons are small and light, they can move through the lattice quickly. The atoms are big and heavy so they move slowly and it takes them some time to go back to their original position in the lattice after the electron has gone by. So, as the electron moves through the lattice, it leaves a ripple behind it. A second electron some distance from the first one now feels the effect of this ripple, and because the atoms are positively charged, it is attracted to it. So, the second electron is indirectly attracted to the first, making them move together in a pair.
In the language of quantum mechanics, these ripples of the atoms are called phonons. (The name comes from the fact that these ripples are also what allows sound to travel through solids.) From this point of view, the first electron emits a phonon which is absorbed by the second electron, effectively gluing them together. But why does the metal have to get very cold before this phonon glue can be effective? The reason is that heat in a crystal lattice can also be thought of in terms of phonons. When the metal is warm, there are lots and lots of phonons flying around all over the place and it’s too chaotic for the electrons to feel the influence of just the phonons that were emitted by other electrons. As the metal cools down, the number of temperature phonons reduces, leaving only the ones that came from the other electrons, which allows the glue to work.
Two quick disclaimers before I finish.
Number one: I glossed over one inconvenient fact when I described the electrons and atoms interacting with each other. I made it sound like they were small particles moving around like billiard balls. For the atoms, this is a reasonable picture because they pretty much have to stay near their lattice positions. But the electrons are not like that at all. Perhaps you’ve heard of particle-wave duality? In quantum mechanics, small objects like electrons are simultaneously a bit like particles and a bit like waves. That’s true here for the electrons, so they are not little billiard balls but are more wave-like. This makes it more difficult to have a good mental picture of what they’re doing, but the basics of the mechanism are still true.
Secondly, this post has been about the type of superconductivity that occurs in metals. The temperature associated with this kind of superconductivity is quite low – a few degrees above absolute zero. But there are other kinds of superconductivity which can occur at much higher temperatures. (Imaginatively, this is usually called ‘high temperature superconductivity’!) This works in a very different way to what I’ve talked about here. It’s also not very well understood and is and active area of research. Perhaps I’ll write something about that another time.