What is high temperature superconductivity?

It was March, 1987. The meeting of the Condensed Matter section of the American Physical Society. It doesn’t sound like much, but this meeting has gone down in history as the “Woodstock of Physics”. Experimental results were shown which proved that superconductivity is possible at a much higher temperature than had ever been thought possible. This result came completely out of left field and captured the imagination of physicists all over the world. It has been a huge area of research ever since.

But why is this a big deal? Superconductors can conduct electricity without any resistance, so it costs no energy and generates no heat. This is different from regular metal wires which get hot and lose energy when electricity passes through them. Imagine having superconducting power lines, or very strong magnets that don’t need to be super-cooled. This would lead to huge energy savings which would be great for the environment and make a lot of technology cost less too.

I guess it makes sense to clarify what “high temperature” means in this context. Most superconductors behave like normal metals at regular temperatures, but if they are cooled far enough (beyond the “critical temperature”, which is usually called Tc) then their properties change and they start to superconduct. Traditional superconducting materials have a Tc in the range of a few Kelvin, so only a few degrees above absolute zero. These new “high temperature” materials have their Tc at up to 120 Kelvin, so substantially warmer, but still pretty cold by everyday standards. (For what it’s worth, 120K is -153°C.)

But, if we could understand how this ‘new’ type of superconductivity works, then maybe we could design materials that superconduct at everyday temperatures and make use of the technological revolution that this would enable.

Unfortunately, the elephant in the room is that, even after thirty years of vigorous research, physicists currently still don’t really understand why and how this high Tc superconductivity happens.

BSCCOMeissner
A piece of superconducting BSCCO levitating due to the Meissner effect. (Image stolen from Wikimedia commons.)

I have written about superconductivity before, but that was the old “low temperature” version. What happens in a superconductor is that electrons pair up into new particles called “Cooper pairs”, and these particles can move through the material without experiencing collisions which slow them down. In the low temperature superconductors, the glue that holds the pairs together is made from vibrations of the crystal structure of the material itself.

But this mechanism of lattice vibrations (phonons) is not what happens in the high temperature version.

BISCO
Atomic structure of BSCCO. (Image stolen from chemtube3d.com).

To explain the possible mechanisms, it’s important to see the atomic structure of these materials. To the right is a sketch of one high Tc superconductor, called bismuth strontium calcium copper oxide, or BSCCO (pronounced “bisco”) for short. The superconducting electrons are thought to live in the copper oxide (CuO4) layers.

One likely scenario is that instead of the lattice vibrations gluing the Cooper pairs together, it is fluctuations of the spins of the electrons that does it. Of course, electrons can interact with each other because they are electrically charged (and like charges repel each other), but spins can interact too. This interaction can either be attractive or repulsive, strong or weak, depending on the details.

In this case, it is thought that the spins of the electrons in the copper atoms are all pointing in nearly the same direction. But these spins can rotate a bit due to temperature or random motion. When they do this, it changes the interactions with other nearby spins and can create ripples in the spins on the lattice. In an analogy with the phonons that describe ripples in the positions of the atoms, these spin ripples can be described as particles called magnons. It is these that provide the glue: Under the right conditions, they can cause the electrons to be attracted to each other and form the Cooper pairs.

Another possibility comes from the layered structure. If electrons in the CuO4 layers can hop to the strontium or calcium layers, and then hop back again at a different point in space, this could induce correlations between the electrons that would result in superconductivity. (I appreciate that it’s probably far from obvious why this would work, but unfortunately, the explanation is too long and technical for this post.)

In principle, these two different mechanisms should give measurable effects that are slightly different from each other because the symmetry associated with the effective interactions are different. This would allow experimentalists to tell them apart and make the conclusive statement about what is going on. Naturally, these experiments have been done but so far, there is no consensus within the results. Some experiments show symmetry properties that would suggest the magnons are important, others suggest the interlayer hopping is important. Personally, I tend to think that the magnons are more likely to be the reason, but it’s really difficult to know for sure and I could well be wrong.

So, we’re kinda stuck and the puzzle of high Tc superconductivity remains one of condensed matter’s most tantalising and most embarrassing enigmas. We know a lot more than we did thirty years ago, but we are still a very long way from having superconductors that work at everyday temperatures.

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