After 86 Years, Physicists Have Finally Made an Electron Crystal

 In 1934, theoretical physicist Wigner proposed a replacement style of crystal.

If the density of charged electrons may be maintained below a particular level, the subatomic particles can be held in a very repeating pattern to form a crystal of electrons; this concept came to called a Wigner crystal.

That's plenty easier said than done, though. Electrons are fidgety, and it's extremely difficult to urge them to take a seat still. Nevertheless, a team of physicists has now achieved it - by trapping the wiggly little brats between a pair of two-dimensional semiconducting tungsten layers.

Conventional crystals - like diamonds or quartz - are formed from a lattice of atoms arranged in an exceedingly fixed, three-dimensional repeating grid structure. per Wigner's idea, electrons may well be arranged in an exceedingly similar fashion to make a solid crystal phase, but as long as the electrons were stationary.

If the density of the electrons is low enough, the Coulomb repulsion between electrons of the identical charge produces mechanical energy that ought to dominate their mechanical energy, leading to the electrons sitting still. Therein lies the issue.

"Electrons are quantum mechanical. whether or not you do not do anything to them, they're spontaneously jiggling around all the time," said physicist Kin Fai Mak of Cornell University.

"A crystal of electrons would even have the tendency to simply melt because it is so hard to stay the electrons fixed at a periodic pattern."

Attempts to form Wigner crystals, therefore, depend on some type of electron trap, like powerful magnetic fields or single-electron transistors, but complete crystallization has still eluded physicists as yet. In 2018, MIT scientists attempting to make a kind of insulator may have instead produced a Wigner crystal, but their results left room for interpretation.

(UCSD Department of Physics)

MIT's trap was a graphene structure referred to as a moiré superlattice, where two two-dimensional grids are superimposed at a small twist and bigger regular patterns emerge, as seen within the example image above.

Now the Cornell team, led by physicist Yang Xu, has used a more targeted approach with their own moiré superlattice. for his or her two semiconducting layers, they used tungsten disulfide (WS2) and tungsten diselenide (WSe2) specially grown at Columbia.

When overlaid, these layers produced a hexagonal pattern, allowing the team to manage the common electron occupancy at any specific moiré site.

The next step was to carefully place electrons in specific places within the lattice, using calculations to see the occupancy ratio at which different arrangements of electrons will form crystals.

The final challenge was a way to actually see if their predictions were correct, by observing the Wigner crystals or lack thereof.

"You have to hit just the proper conditions to form an electron crystal, and at the identical time, they're also fragile," Mak said.

"You need an honest thanks to probing them. you do not actually need to perturb them significantly while probing them."

This problem was solved with insulating layers of hexagonal boron nitride. An optical sensor was placed very near (but not touching) the sample, at a distance of only one nanometre, separated by a boron nitride layer. This prevented electrical coupling between the sensor and therefore the sample while maintaining enough proximity for prime detection sensitivity.

This arrangement allowed the team to probe the sample cleanly, and that they made their detection. Within the moiré superlattice, electrons arranged into a range of crystal configurations, including triangular Wigner crystals, stripe phases, and dimers.

This achievement doesn't just have implications for studying electron crystals. The findings demonstrate the untapped potential of moiré superlattices for physics research.

"Our study," the researchers wrote in their paper, "lays the groundwork for using moiré superlattices to simulate a wealth of quantum many-body problems that are described by the two-dimensional extended Hubbard model or spin models with long-range charge-charge and exchange interactions."