Question: what are "super solids"

Sam Carr answered on 27 May 2020:

Thanks for asking!

To understand a supersolid, we first need to understand the two things that make it up.

First, lets think about a solid. We can use water as an example. Water is a liquid – but if you cool it down enough, then it becomes ice – which is a solid. To do this, the molecules of water all become stuck in place, in a crystal structure. Because its formed this very structured lattice, the molecules can no longer move about very much, and instead of being a liquid which can easily change its shape, it becomes a solid which doesn’t change its shape (unless it breaks). The key thing here from the microscopic point of view is that the molecules are ordered in a nice array. The same thing happens to atoms in e.g. a lump of iron – the iron atoms are all nicely ordered, so it keeps its shape. If it gets hot enough, it will become a liquid just like water — but the temperature it does this at is much much higher, which is why you need special fire to forge iron tools. But the basic physics is the same.

Now, lets think about another much less common phase of matter – the superfluid — and for this, we will think about what happens when we cool helium. At normal temperatures, helium is a gas. But as you cool down to -269 degrees centigrade (which is 4 Kelvin – i.e. only 4 degrees above the absolute zero of temperature), helium becomes a liquid (like water – only water does this at 100 degrees). In many ways, liquid helium is just a typical liquid very similar to water. But now cool it another two degrees (to just above 2 Kelvin) and something very strange happens – it starts flowing *up* the walls of the container it is in! This is because its undergone another phase transition, from liquid to something called a superfluid. The characteristic of a superfluid is that its viscosity becomes zero, so it can flow without losing any energy at all (the better known analogue is the superconductor). Microscopically, it is an inherently quantum state that can’t happen in classical mechanics, when all of the helium atoms act coherently – i.e. they all do exactly the same thing. It takes years to fully understand this – it is just weird!

So when you cool a liquid, it could become a solid, or it could become a superfluid. But could it do both? This means that microscopically, the atoms have this quantum coherence, but they also form an ordered lattice. Until only a few years ago, this was a hypothetical state of matter — theoretical physicists predicted it could exist but nobody had ever seen it. But in 2017, a group of experimentalists working with atom traps finally made one! There is still a lot of work to do in understanding it fully though.