In certain crystals you can knock an electron out of its favorite place and leave a hole: a place with a missing electron. Sometimes these holes can move around like particles. And naturally these holes attract electrons, since they are places an electron would want to be.
Since an electron and a hole attract each other, they can orbit each other. An orbiting electron-hole pair is a bit like a hydrogen atom, where an electron orbits a proton. All of this is quantum-mechanical, of course, so you should be imagining smeared-out wavefunctions, not little dots moving around. But imagine dots if it’s easier.
An orbiting electron-hole pair is called an exciton, because while it acts like a particle in its own right, it’s really just a special kind of ‘excited’ electron—an electron with extra energy, not in its lowest energy state where it wants to be.
An exciton usually doesn’t last long: the orbiting electron and hole spiral towards each other, the electron finds the hole it’s been seeking, and it settles down.
But excitons can last long enough to do interesting things. In 1978 the Russian physicist Abrikosov wrote a short and very creative paper in which he raised the possibility that excitons could form a crystal in their own right! He called this new state of matter excitonium.
In fact his reasoning was very simple.
Just as electrons have a mass, so do holes. That sounds odd, since a hole is just a vacant spot where an electron would like to be. But such a hole can move around. It has more energy when it moves faster, and it takes force to accelerate it—so it acts just like it has a mass! The precise mass of a hole depends on the nature of the substance we’re dealing with.
Now imagine a substance with very heavy holes.
When a hole is much heavier than an electron, it will stand almost still when an electron orbits it. So, they form an exciton that’s very similar to a hydrogen atom, where we have an electron orbiting a much heavier proton.
Hydrogen comes in different forms: gas, liquid, solid… and at extreme pressures, like in the core of Jupiter, hydrogen becomes metallic. So, we should expect that excitons can come in all these different forms too!
We should be able to create an exciton gas… an exciton liquid… an exciton solid…. and under the right circumstances, a metallic crystal of excitons. Abrikosov called this metallic excitonium.
People have been trying to create this stuff for a long time. Some claim to have succeeded. But a new paper claims to have found something else: a Bose–Einstein condensate of excitons:
• Anshul Kogar, Melinda S. Rak, Sean Vig, Ali A. Husain, Felix Flicker, Young Il Joe, Luc Venema, Greg J. MacDougall, Tai C. Chiang, Eduardo Fradkin, Jasper van Wezel and Peter Abbamonte, Signatures of exciton condensation in a transition metal dichalcogenide, Science 358 (2017), 1314–1317.
A lone electron acts like a fermion, so I guess a hole does do, and if so that means an exciton acts approximately like a boson. When it’s cold, a gas of bosons will ‘condense’, with a significant fraction of them settling into the lowest energy states available. I guess excitons have been seen to do this!
There’s a fairly good simplified explanation at the University of Illinois website:
• Siv Schwink, Physicists excited by discovery of new form of matter, excitonium, 7 December 2017.
However, the picture on this page, which I used above, shows domain walls moving through crystallized excitonium. I think that’s different than a Bose-Einstein condensate!
I urge you to look at Abrikosov’s paper. It’s short and beautiful:
• Alexei Alexeyevich Abrikosov, A possible mechanism of high temperature superconductivity, Journal of the Less Common Metals
62 (1978), 451–455.
(Cool journal title. Is there a journal of the more common metals?)
In this paper, Abrikoskov points out that previous authors had the idea of metallic excitonium. Maybe his new idea was that this might be a superconductor—and that this might explain high-temperature superconductivity. The reason for his guess is that metallic hydrogen, too, is widely suspected to be a superconductor.
Later, Abrikosov won the Nobel prize for some other ideas about superconductors. I think I should read more of his papers. He seems like one of those physicists with great intuitions.
Puzzle 1. If a crystal of excitons conducts electricity, what is actually going on? That is, which electrons are moving around, and how?
This is a fun puzzle because an exciton crystal is a kind of abstract crystal created by the motion of electrons in another, ordinary, crystal. And that leads me to another puzzle, that I don’t know the answer to:
Puzzle 2. Is it possible to create a hole in excitonium? If so, it possible to create an exciton in excitonium? If so, is it possible to create meta-excitonium: an crystal of excitons in excitonium?