Eugene Wigner in the year 1934 theorized about a crystal solely made up of electrons. Nearly, eighty seven years later, two mutually exclusive groups of physicists published their recent experimental observation regarding the Wigner crystal in the scientific journal Nature. One of the team led by Ataç Imamoğlu, professor at the Institute for Quantum Electronics at ETH Zurich and the other led by Hongkun Park at Harvard University came up with these special crystals independently.

“Wigner crystallization is such an old idea,” said Brian Skinner, a physicist at Ohio State University who was not involved with the work. “To see it so cleanly was really nice.”

To understand about Wigner crystal in simple terms, which have been beautifully explained in quanta magazine 'Physicists Create a Bizarre 'Wigner Crystal’ Made Purely of Electrons'; quoting the author of the article piece:

To make electrons form a Wigner crystal (electron that arrange themselves in regular, crystal-​like patterns because of their mutual electrical repulsion), experts would have to cool them down. Electrons repel one another, and so cooling would decrease their energy and freeze them into a lattice just as water turns to ice. Yet cold electrons obey the odd laws of quantum mechanics — they behave like waves. Instead of getting fixed into place in a neatly ordered grid, wavelike electrons tend to slosh around and crash into their neighbors. What should be a crystal turns into something more like a puddle.

How the two teams achieved these crystals?

The Harvard University group found these crystals by accident as they were actually experimenting with electron behavior in a “sandwich” of exceptionally thin sheets of a semiconductor separated by a material that electrons could not move through. The physicists proceeded with cooling this semiconductor sandwich to below −230 degrees Celsius and played around with the number of electrons in each of the layers. They observed that when there was a specific number of electrons in each layer, they all stood mysteriously still. But this behavior happened only when the number of electrons in each layer was such that the top and bottom crystal grids aligned: Smaller triangles in one layer had to exactly fill up the space inside bigger ones in the other. Eventually they theorized with the old idea of Wigner’s, Wigner had calculated that electrons in a flat two-dimensional material would assume a pattern similar to a floor perfectly covered with triangular tiles. This crystal would stop the electrons from moving entirely.  

The Harvard team made the crystal melt by forcing the electrons to embrace their quantum wave nature. Wigner crystal melting is a quantum phase transition — one that is similar to an ice cube becoming water, but without any heating involved. The researchers blasted the semiconductor layers with laser light to create a particle-like entity called an exciton. The material would then reflect or re-emit that light. By analyzing the light, researchers could tell whether the excitons had interacted with ordinary free-flowing electrons, or with electrons frozen in a Wigner crystal.

The second research team, led by Ataç Imamoğlu at the Swiss Federal Institute of Technology Zurich, also used this technique to observe the formation of a Wigner crystal.

Going forward, the Harvard team plans on using their system to answer outstanding questions about Wigner crystals and strongly correlated electrons. One open question is what happens, exactly, when the Wigner crystal melts; competing theories abound. Additionally, the team observed Wigner crystals in their semiconductor sandwich at higher temperatures and for larger numbers of electrons than theorists predicted. Investigating why this was the case could lead to new insights about strongly correlated electron behavior.