Electrons are amazing little things. They usually hang around the rotating atomic nucleus, but they don’t have to; The universe is full of free, buzzing electrons. Ninety years ago the theoretical physicist Eugene Wigner suggested that they need not babble either: free electrons can clump together into a special kind of matter that has no atoms; .

This is known as the Wigner crystal, and physicists finally have direct observational evidence that it may exist.

“A Wigner crystal is one of the most exciting predicted quantum phases of matter and is the subject of numerous studies that claim to have the best circumstantial evidence for its formation,” says physicist Al Yazdani of Princeton University. “Visualizing this crystal not only allows us to watch its formation, confirming its many properties, but we can also study it in ways that we couldn’t in the past.”

Crystal means the arrangement of atoms in a solid. In typical crystalline materials, atoms are bonded together to form a repeating pattern in space. Wigner’s pioneering paper of 1934 suggested that electrons could form similar mechanisms by being supported rather than hindered by the mutual repulsion produced by the negative charge carried by all electrons.

He theorized that at extremely low temperatures and low densities, the repulsive interaction between electrons dominates their potential energy need to approach, forcing them into crystal lattices.

These crystals are not according to classical physics, but according to quantum mechanics, the bound electrons will behave as a separate wave, not as separate particles. Various experiments using two-dimensional systems designed to reveal the consequences of such behavior have provided indirect evidence of Wigner crystals, but direct evidence has proven somewhat more difficult to obtain.

“There are literally hundreds of scientific papers examining these effects and claiming that the results must be due to a Wigner crystal,” Yazdani says, “but no one can be sure because none of these experiments actually see the crystal.”

Given the shortcomings of these experiments, a team led by physicists Yen-Cheng Tsui, Minghao He, and Yuwen Hu of Princeton University designed an experiment that they hope will help solve the previous problems and reveal the crystal. They used magnetic fields to create an electronic Wigner crystal in graphene, but not just any old graphene. The material had to be as pure as possible to eliminate effects that could arise from the defect of the atom.

Two sheets of graphene were prepared and arranged in a special configuration before being cooled to just slightly above absolute zero. A magnetic field was then applied to adjust the density of the electron gas trapped between the layers.

Wigner crystal has the best electron density. If the density is too low, the electrons will repel each other and move away from each other. If the density is too high, electrons will mix with the electron liquid.

At the Goldilocks point, electrons will try to repel each other, but their escape will be blocked by other electrons. So they are simply arranged in a grid, keeping as much distance between them as possible.

To measure this crystalline phase, the researchers used high-resolution scanning tunneling microscopy (STM). STM uses quantum tunneling to probe materials at atomic scales that optical microscopy cannot reach.

“In our experiment, we can map the system by adjusting the number of electrons per unit area. Just by changing the density, you can trigger this phase transition and see the electrons spontaneously transform into an ordered crystal,” Tsui explains.

“Our work provides the first direct images of this crystal. We prove that the crystal really exists and that we can see it.”

Their measurements also confirmed models describing the lattice as triangular when confined to two-dimensional space; But they found that it could remain constant because the intensity was adjusted to a large enough degree; This contradicts previous theories that the density range should be quite small. They also found that electrons do not occupy a single point in the lattice, but rather a fuzzy range of positions described as zero-point motion.

“Even if the electrons are frozen in the Wigner crystal, they should exhibit strong zero motion,” Yazdani says. “It turns out that this quantum motion spans a third of the distance between them, turning the Wigner crystal into a new quantum crystal.”