Sometimes, a book has so many buzzwords it’s hard to take it seriously. Periodic crystals were first mooted in 2012, and a definite periodic crystal was observed in 2017. The crystals we are familiar with are in quasicrystal form, so quasicrystals have recently been discovered. But now, I’ve hit the jackpot with a quasicrystal that is also a time supersolid time. If that doesn’t make sense to you, don’t worry, it doesn’t make sense to me either.
Let’s unpack the word salad and see if we can extract something sensible from it.
Crystals and quasicrystals: neither cures disease
A crystal in space is a unit that repeats itself at regular intervals so that it fills a space without gaps. A repeating unit can only be translated, not rotated. So, you can get sets of atoms that make a cube, for example, to fill a space.
Quasicrystal also uses a repeating unit that will fill space without gaps. In this case, repetitive parts are translated and rotated. The application is ordered, but not the order used to see it. Instead the order appears repeated in space through two overlapping patterns that are repeated at different intervals. On top of that, the ratios of those intervals are not neat numbers.
Exchange space for time
In a discrete time crystal, instead of repeating the position of the atoms, the repeating unit is somewhat characteristic. But it’s not like the pendulum is swinging back and forth—he’s thinking of it differently. Imagine I want to create a time crystal from a pendulum. The pendulum has a natural oscillation frequency. I can push the pendulum at the oscillation frequency and it will start to swing. I can also push it at half the oscillation frequency, and those will still swing better.
But I can’t push it to double the oscillation frequency and expect it not to change. This is because the first push sets the swing in motion, while the second push ends in time to stop the swing motion. To get strong oscillations, the frequency at which the swing is driven has to match a multiple of the natural frequency of the swing.
In a discrete time crystal, the frequency at which we push the swing’s oscillation does not match the natural frequency of the swing. Instead of turning off the oscillation, though, the swing starts oscillating at a new frequency that isn’t its natural frequency and doesn’t match the drive frequency, either. Once you achieve something like this, you might at one time crystal.
In comparison with a normal quasicrystal, a quasicrystal involves swinging with two frequencies simultaneously—it’s probably better to think of two waves vibrating on a string in this case. However, neither of those frequencies is the frequency range in which the swings are driven. And just like ordinary quasicrystals, the two halves of these two frequencies are not a neat whole.
It’s all just a bit awful
To observe both time crystals and quasitime crystals, researchers use helium-3 (helium with neutrons missing) cooled to within a whisker of a perfect zero. Because of the unequal number of particles in the nucleus, helium three has a strong magnetic moment. Helium is put in a position where all the magnets point in the same direction, creating what is called a magn.
The researchers observed a periodic crystal, formed by a magnon slowly spinning around space. The magnon is activated by the magnetic field of the container that holds the helium in place, as well as the applied radio frequency field. The resulting oscillation of the magneton has two frequencies that are not even shared by either the driving field or themselves.
This is in part because the driving radio frequency field introduces random energy. And that makes it even harder to understand. Think of it like this: A child plays piano keys at random, but hears repeated measures, not random noises.
When researchers turn off the radio frequency field, the quasicrystal phase quickly separates and the system forms a discrete phase crystal. In this case, the driving frequency comes from the fact that the magnetic field of the helium modifies the shape of the container, displacing the helium by a little.
But this is the deadline. As the helium moves, the collisions cause some of the helium atoms to change orientation. When this happens, their magnetic field changes direction and they are thrown out of the trap. That means that the discrete time crystal slowly fades away.
A supersolid time crystal
So far, so weird. But it is possible to become weird. Three helium, at these low temperatures, forms a special type of superfluid, called topological superfluid. The magnon—helium with all its complementary spins—has a non-zero spin associated with it, and this has to be conserved. So the movement of the sphere through the triple helium superfluid also behaves like a superfluid—a superfluid within a superfluid if you will.
Magnon is still a time crystal, though. If we think of a crystal as a solid phase, then this particular phase crystal must be a supersolid.
What is a supersolid you ask? Well, it’s a strong thing that doesn’t show a fight. Two supersolids can slide over each other without losing energy. Now, finally I checked, the existence of non-science supersolids is rather debatable and the evidence is equivocal. It seems a little premature to declare this exactly the crystal time.
The research is very good, though, and I like a good quasitime crystal.
Physical Review Letters2018, DOI: 10.1103/PhysRevLett.120.215301