Alaric Sanders

In Plain English

Imagine you and a friend are standing in opposite each other, holding ultra-strong bar magnets. The magnets will naturally want to orient themselves north-to-south. The easiest way to hold the magnets is clear - the magnets are aligned with each other, pointing in opposite directions.

Now imagine turning company into a crowd by adding a third magnet-bearing friend. Which way do they orient their magnet?

It turns out there's no good answer to this - any direction that they point their magnet will be equally good, but also equally bad. This property is called frustration, and it's surprisingly common in nature. There's a sense in which non-frustrated states are 'classical,' or rather 'not quantum mechanically interesting' - for example, the spins inside a bar magnet all point in the same direction, there's no quantum uncertainty about how they are oriented. When there are a large number of possible, equally-good classical states, we have some kind of uncertainty about where the spins will order, and so-called quantum fluctuations play a dominant role in the material's physics.

This has surprising consequences when you look at very low temperatures. Typically, a system cooled down close to absolute zero will "freeze" in place - the atoms stop vibrating and the spins stop moving, settling into the lowest-energy configuration that they can sustain. In the presence of dominant quantum fluctuations though, disorder persists all the way to absolute zero - a puzzling contradiction that may hold the key to the mechanism underlying some high-temperature superconductors.