Researchers Finally Create Electron Crystals 86 Years After First Proposed

Artist's impression of electrons trapped in a moiré superlattice. Cornell University

Eighty-six years since electron crystals were first proposed, physicists have now constructed them, trapping electrons in a repeating pattern. The achievement is reported in the journal Nature.

A crystal is made of a repeating pattern of particles but electrons are difficult to keep in place. So an electron crystal is like trying to organize a large number of electrons that won’t stay still — it’s the herding cats of particle physics.
 

However, this team had an ingenious solution. They built a Wigner crystal using layers of semi-conductors just one atom thick. They then used two different tungsten materials and created a hexagonal pattern known as a moiré superlattice by placing one material on top of the other.

A moiré superlattice design created using this Python code.

The team previously discovered that you can trap a single electron in certain places in this lattice, specifically where the electromagnetic forces are much stronger than the natural motion of the electrons. By carefully placing multiple electrons in these spots, researchers built a crystal.

"Electrons are quantum mechanical. Even if you don't do anything to them, they're spontaneously jiggling around all the time," senior author Professor Kin Fai Mak, from Cornell University, said in a statement. "A crystal of electrons would actually have the tendency to just melt because it's so hard to keep the electrons fixed at a periodic pattern."

If the construction of these crystals wasn’t difficult enough, there are also many hoops to jump through in studying these constructions. The team had to create a special optical sensing system to measure the crystals without disrupting them.

"You need to hit just the right conditions to create an electron crystal, and at the same time, they're also fragile," Mak added. "You need a good way to probe them. You don't really want to perturb them significantly while probing them."

The team was able to observe several electron crystals by placing electrons in various configurations to create different crystal symmetries. The team stress that this achievement shows how complex patterns can be achieved from relatively simple elements such as electrons.   
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