The Superconductors episode was first released in October 2018.

The life of an experimental physicist is not always an easy one. While the final papers that they produce look very formal and correct, the day-to-day practice involves a lot of dubious, inexplicable, and sometimes just plain ridiculous results. Things that don’t make any sense. And, 99.9999% of the time, the experimentalist will look at their apparatus — realize that there’s some problem with the detector, or the experimental setup — correct it, and then the ridiculous result vanishes, to be replaced by something that makes a little bit more sense.

Not that the mistake is always recognised — there was the famous case of the canals on Venus. An astronomer called Percival Lowell spent years at his telescope mapping out these ‘canals’, drawing them up in intricate diagrams, and telling the world about his discovery. It turns out that Lowell had the configuration of his telescope set up all wrong, and may in fact have been mapping the blood vessels in his own eye. So, next time you feel like all your work is futile and unappreciated, just consider the sad case of Percival Lowell. And, more recently, too — the OPERA experiment for measuring neutrinos, a few years ago, reported tentatively that they might have observed neutrinos travelling faster than the speed of light. This would have violated all known laws of physics… and it turned out that, yeah, there were errors in their detector that meant the faster than light neutrinos were actually slower than light.

So experimentalists are used to things that don’t make sense, and that turn out to be experimental errors. Perhaps this was how Kamerlingh Onnes felt. He was an experimentalist working on the physics of extremely low temperatures. At low temperatures, we tend to measure things on the Kelvin scale — the absolute scale of temperature. Zero kelvin corresponds to particles that have no energy — it’s as cold as you can possibly get. This is called absolute zero. One degree Kelvin is the same as one degree Celsius.

Onnes was looking at how substances behaved at extremely low temperatures. The huge triumph of his career had been in 1908. The work wasn’t easy: the lab day started at 5am, and by 7pm, he was almost ready to give up on his work. He was trying to cool helium down to the point where the gas would become a liquid. You had to do this at the time by gradually reducing the pressure in the vat, allowing the gas to expand and expand, and cool and cool, all the while preventing heat from flowing in from outside. He had been able to liquify nitrogen, oxygen, and hydrogen — cooling them down to only a few kelvin, which is minus many hundreds of degrees in our usual systems of Fahrenheit and Celsius. But helium was an incredibly difficult nut to crack — having worked at it all day, Onnes was almost ready to give up. Then, a curious visitor from the Chemistry department noticed what Onnes hadn’t — the vat underneath had been collecting liquid helium! Helium, in its liquid form, is a transparent liquid very close to air. Unlike water, which bends light, Helium’s refractive index is very close to one; it doesn’t bend light nearly so much. Onnes hadn’t realized that he had been producing the world’s first liquid helium for hours. And he also didn’t realize that, by cooling the Helium down to around 1.5K, he had managed to defeat the Universe. The background temperature of the Universe, the void of outer space, is 2.73K. Onnes had created, in the lab, something colder than the empty void between galaxies.

In the next few decades, extremely low temperatures fell out of fashion as an area of research. After all, liquifying these gases was all very well, and physicists as much as rogue Youtube commenters love to say “First!”. But there didn’t seem to be much use for these incredibly cold temperatures. The atoms had virtually no energy — they were barely vibrating at all. Creating low temperatures was slow, and difficult, and no-one seemed able to solidify the gases that hadn’t already been solidified. By 1911, Onnes’ lab at the University of Leiden was the only group of people extensively researching these incredibly low temperatures.

But Onnes was still interested in what happened to matter at the lowest temperatures humans could produce. Specifically, he was interested in what might happen to the conductivity of the matter. In that era, they had a theory of how electric currents worked. Metals contained free electrons — charged particles that could somehow move through the metal. So, if you applied an electrical field or an electrical force to the object, it would push the free electrons along. Electrical resistance is a measure of how easily an object can conduct electricity: how much force is required to push the electrons along. Metals have lots of free electrons, so their resistance is low.

The model of resistance was that it was caused by the presence of atoms, against this flow of electrons. The idea is that, as the electrical force pushes the electrons, they’re constantly bashing into atoms, and giving them energy. This energy causes the atoms to vibrate, which is what we see as heat. This explains why, when you pass a current through a wire, the wire heats up. And it also explained thermistors — substances whose electrical resistance changes as a function of temperature.

Generally, the hotter a substance was, the more electrical resistance it had. And this made perfect sense with this model of electrical conduction. Imagine electrons flowing through a substance where the atoms are barely moving at all — collisions are rare. But if the substance is hot, and the atoms are wildly vibrating, then they’re far more likely to crash into an electron and prevent the flow of electric current.

But Lord Kelvin, famous 19th century physicist, had a theory. He thought that, if you cooled down a metal far enough, there would no longer be enough energy for there to be any ‘free electrons’ in the metal that could carry the current. The electrons would instead be bound to the atoms, without enough energy to escape from these atomic forces that pulled them in. So he predicted that, if you cooled down a substance enough, the electrical resistance would suddenly spike to infinity: all of the electrons would be bound up in their atoms again, and they couldn’t carry a current: too cold for them to move!

This was the idea that Onnes and his student, Gilles Holst, were investigating when they were cooling down mercury to incredibly low temperatures. When they got below 4.2K, they noticed something weird. Instead of spiking to infinity, the resistance had suddenly dropped to zero. Onnes assumed that there was a short-circuit in his device, and was hunting around for the source of the short-circuit. He was probably just about ready to scrap the experiment when something strange happened. A lab assistant, tasked with keeping the temperature below 4.2K, had dozed off. The temperature rose slowly above 4.2K, and suddenly, the electrical resistance of mercury had reappeared. Quite by accident, the team had discovered superconductivity.

This was a bizarre finding. When you cooled down mercury to a cold enough temperature, the electrical resistance suddenly became zero. Not just some very small value — that you might expect, if the atoms aren’t vibrating very much — but precisely zero. This meant that, if you put a current into the superconducting loop, it would continue flowing forever. Onnes tried it, and the current never seemed to diminish with time, as you’d expect it to when it lost energy to electrical resistance.

Not only was superconductivity the exact opposite of what Onnes had expected to find, but he quickly realized that it had all kinds of exciting implications. For a start, a current that could flow forever without losing any energy — could this mean transmission of power with no losses in the cables? Electrical current was already being transmitted across the Western world, but reducing those losses to zero would save a huge amount of money. What’s more, a superconducting wire where the current never, ever diminished — it would act as a perfect store of electrical energy. Unlike batteries, which degrade and become usable over time, if the resistance was truly zero, you could return to the superconductor in a billion years and find that same old current flowing through it — energy could be captured and stored indefinitely! But there were even more exciting prospects: with no resistance, you could pass a huge current through the superconducting wire. The laws of electromagnetism tell us that, when you pass a current through a coil of wire, it produces a magnetic field. With these superconductors, presumably, you could produce huge magnetic fields — magnetic fields of incredible power. You could use them to levitate trains, and produce astonishing accelerations — the transport system could be revolutionised. Perhaps in the future, these superconductors would allow people to lift incredible weights very efficiently. And every single power generator in the world operates on the following principle: you use something to spin a turbine that turns a coil of wire in a magnetic field. Turning a coil of wire in a magnetic field generates a voltage, which we use to generate power. This is the principle behind electrical generation: the difference between coal, nuclear, hydroelectric, etc. is really just how you spin this turbine. If superconductors, and these powerful magnetic fields, could be used in power plants… they could generate more electricity than ever before. This technology could change the world.

And herein lies the beauty and the challenge and the fascination with superconductors. Because, shortly afterwards, Onnes noticed two things about the effect of superconductivity. If the magnetic field that the superconductors were in got to be too big, the resistance would suddenly reappear — the superconductivity would vanish, as mysteriously as it arose. And, if the temperature rose above that critical temperature — which was incredibly cold by anyone’s standards — the resistance also returned. If, to use a superconductor, you’d need to cool it to temperatures close to absolute zero — and ensure that the magnetic field wasn’t too high — then all of these miraculous applications would be for nothing. Onnes started to search around for other materials that might exhibit this same property — but, conveniently, at a higher temperature, and for stronger magnetic fields.

And, in some ways, you could end the story of superconductivity right there. Because, despite more than a century of effort, we haven’t managed to find that miracle substance that we’ve dreamed of for so long — a room temperature superconductor, which would revolutionise so many aspects of the modern world. We’ve found superconductivity in all kinds of different materials, at a variety of different temperatures and magnetic fields, but we’ve never found that elusive superconductor that works at room temperature. So the story’s almost done there. But, as ever with physics — it’s not quite that simple. In the next part of this episode, I’ll talk about the developments in superconductivity — in the materials that we use, the technological applications that have been found, and the theoretical understanding we now have of this mysterious experimental phenomenon.


But first: this week’s physics-based chat-up line:

“You make me feel like a superconductor at 4K… I can’t resist.”

MEOW.

And, on the same theme:

A superconductor walks into a bar. The bartender says “Get out, we don’t serve your kind here.” The superconductor leaves, without resistance.

With these lines, you’re sure to be as cool as a cuprate.

Okay. Enough of this frivolity. Back to the science of superconductors.

Like many weird things in physics, it was generally considered that superconductivity was probably a consequence of quantum mechanics, and quantum weirdness. It wasn’t *just* like having a substance with zero electrical resistance — because scientists also noticed that superconductors didn’t like having magnetic fields inside them. There’s something called the Meissner effect, which means that when a material becomes superconducting, it expels almost of the magnetic field lines inside itself — magnetic forces don’t operate inside the superconductor, but only outside, and in an incredibly thin layer on the surface of the material. This can’t be explained with classical, non-quantum physics.

We’ve talked about the experimental discovery of superconductivity, and how the experimentalists began to hunt for more. It was up to the theorists to try to explain how superconductivity actually worked. And they sort of did, until the experimentalists found a new type of superconductor, just to make things really difficult. Such is life.

First, it was discovered that superconductivity was really a little bit like a phase transition. In our day-to-day lives, we’re familiar with phase transitions of some kinds — like, for example, boiling or freezing water. Both of these are processes where, at a certain temperature, the behaviour of a system totally changes. Water goes from being a liquid to a solid crystal — or a gas, depending on whether you’re boiling or freezing it. We say that the water has changed from a liquid phase into a solid, or a gaseous phase.

Superconductivity can be understood in the same way. Below the transition temperature, the phase of the material completely changes, and it becomes superconducting, with different properties. Landau and Ginsburg managed to find an equation that described the superconductors in terms of the energy of the system, which let them make some theoretical predictions about how they should behave. This was similar to how other magnetic materials, like permanent magnets, had been described in the past — you can write down equations that tell you how energy is distributed in the magnets, and this lets you make predictions about how the magnet will behave.


They had an equation. But they still didn’t have an explanation for what was physically going on. What were the electrons in a superconducting material actually doing? Lord Kelvin’s idea that at super low temperatures, they would ‘freeze’ into atoms was clearly incorrect. But if this wasn’t what was happening to the electrons, what was?
This gets quite complicated, but I’m going to try to explain it as simply as I can. It’s something called BCS theory, named after Bardeen, Cooper, and Schrieffer who came up with it. Here’s the basic idea.

We can imagine how metals work as a lattice — a grid of atoms, regularly repeated — which has electrons in it. An electron moving through a conductor will attract nearby positive charges in the lattice. This is because the electron has negative charge, and so it attracts positive charges. But this pulls on the lattice structure, and causes it to deform. This deformation of the lattice causes another electron, with opposite spin, to move into the area where the electron is — it pulls positive charges close to it, and this dense positive charge pulls in another electron. The two electrons then become correlated. This is a quantum mechanics idea — all that matters is the idea that the electrons are linked together. Because there are a lot of such electron pairs in a superconductor, these pairs overlap very strongly. In this state, where the overlap between electron pairs is so strong, you can’t change one pair without changing all of the others. Thus, the energy required to break any single pair is related to the energy required to break all of the pairs.

What this does is that it means the tiny energy that you usually get when electrons are ‘paired together’ like this actually becomes a much larger energy — the energy required to break up all of the pairs of electrons. Normally, you don’t notice this effect. This is because the temperature is quite high, and so the atoms in the conductor are vibrating — they have a lot of energy. When they have a lot of energy, they can break all of the pairs, and so you don’t notice these quantum effects. But in a superconductor, the atoms are at very low temperatures. Bashing into the electron pairs isn’t enough to break them apart. This means that the pairs stay stuck together, and resist all kicks that try to break them apart. Because they’re resisting the kicks, the electron flow as a whole (the current through the superconductor) will not experience resistance. That, broadly, is how a superconductor works.

So, maybe if you’re feeling a little bit cute, you could get a physics-based chat-up line out of Cooper pairs. The bond between us is a lot more difficult to break than you’d think…

Of course, you can probably now see why superconductivity happens in some materials and not others, and at different temperatures. It’s all to do with this lattice structure — which varies from substance to substance — and how it gets deformed.

But the BCS theory that we described above was actually pretty depressing for researchers into superconductivity. It implied that superconductors couldn’t exist above 30K, which is -243 degrees Celsius. In other words, it was unlikely that we’d ever find a truly useful room-temperature superconductor.

So there you go! You know how superconductors work.

Or, at least, you know how CONVENTIONAL superconductors work. The story’s not over yet. Did you think it would be that easy? Unconventional, high-temperature superconductors were discovered in 1986. We have found them in strange materials called cuprates, which contain copper in their lattices. Often, you need quite weird arrangements of atoms in the lattice to get high-temperature superconductivity to work — a famous example is YBCO, which is yttrium barium copper oxide. In other words, it contains two elements that most people have never heard of. Explaining high temperature superconductivity is still an unsolved problem in theoretical physics, although people have theories. A theoretical understanding is super important, because, if we have the theory, we might be able to predict what substances will make good high-temperature superconductors.

So how high-temperature is high-temperature? The best one to date becomes superconducting at around 130K, which is still -140 degrees Celsius. In other words, it’s incredibly cold — but still warmer than liquid nitrogen, which has meant that high-temperature superconductors have found some uses. For example, superconducting magnets are used to produce the big magnetic fields at the large hadron collider, which bends those particles into flying around in circles. Superconducting magnets are used in the experimental fusion reactor, JET, in Oxfordshire — and they will be used at the new one, ITER, in France, when it’s completed; whenever they get around to finishing that. An episode on nuclear fusion shortly. They’re used in MRI and NMR scanners, which you may have had some experience with in a medical environment. And superconducting magnets have been used to levitate frogs, in a display that won the Ig Nobel Prize a few years ago.

Superconductivity is still a very active area of research. Just in 2014, some German physicists discovered they could make a superconductor by bombarding a substance with lasers — no need to cool it down at all. Unfortunately, the material only remained superconducting for a few milliseconds, so the effect isn’t useful — yet — but it’s still being investigated. We all know that a room-temperature superconductor, or one that can work at a decent range of temperatures, could change the world — change the way we think about transporting and storing energy, how we think about transporting people and objects, and how we generate our power. That marvellous, mad experimental result from 1911 continues to confound, mystify, and excite physicists to this day. There is no question that superconductors are still… ridiculously cool.


Thanks for listening to this episode of Physical Attraction.

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