In 1934, Eugene Wigner proposed a crystal-like structure formed by electrons. We call it an electron crystal or Wigner crystal. Wigner calculated - predicting the practical world. He predicted that when the repulsion forces of electrons dominate and overcome their kinetic energy, crystal-like structures would form.
Ever since Wigner proposed it, scientists all over the world have been trying to create such a crystal, it is some evidence of experimental creation However, the new work does something different. You can't just create. In addition to creating, we have to observe the crystals. So scientists have to look for the most stable form possible. In a new study published in the journal Nature scientists are looking for just that.
Difficulty in an electron crystal
Okay, that's easy to say - but let's put into perspective a little of the difficulty of creating Wigner crystals. Electrons are very energetic. Around an atom, for example, they move crazily. You know those children, who can't stop no matter how hard their parents call them? An electron is exactly like that, with the difference that it's much harder to stop it.
This is why the Coulomb repulsion must overcome the kinetic energy. Let us imagine. A crystal is a structure where the atoms arrange themselves in static structures, that is, stationary and organized. But these two words do not exist in the world of electrons. If we get enough electrons, and suppress the kinetic energy, as they repel each other, the electrons will bind, and organize themselves by forces.
"Electrons are quantum mechanics. Even if you don't do anything with them, they spontaneously move all the time" says in a statement Kin Fai Mak, an associate professor at Cornell University. "A crystal of electrons would actually tend to fall apart, because it's very difficult to keep the electrons fixed in a periodic pattern."
"You need to achieve the right conditions to create an electron crystal, and at the same time, they're also fragile," Mak explains. He adds, "You need a good way to probe them. You really don't want to disturb them significantly by probing them."
After breaking their heads, then, the scientists found a possible solution to the problem - electrons arranged in stacked two-dimensional form. And this is considerably simple if you stack two-dimensional superconductor plates. By doing so, you create a three-dimensional structure. The scientists used tungsten disulfide (WS2) and tungsten disselenide (WSe2) plates.
(Xu et al., Nature, 2020).
With the overlapping plates, scientists have created a hexagonal pattern in those somewhat psychedelic shapes of the moiré effect. It's for some effects with that pattern that scientists trap electrons in place, as they discovered in a study a few months ago.
Then, the physicist Veit Elser, co-author of the study, calculated the various possible arrangements of electrons, forming the most diverse crystals. For this, he calculated the occupation ratio required for a crystalline structure to form automatically, only by the effect of increasing repulsion force, or decrease in kinetic energy.
The next difficulty in the work would be to observe without interfering with the electrons. To do this, they brought an optical sensor a nanometer away, with a layer of hexagonal boron nitride, made by Japanese scientists, separating the superconducting plates from the sensor. In this way, the sensor got close enough to observe, but not close enough to interfere.
Then the scientists observed several crystals of electrons. Moreover, not all crystals formed the same. Several different structures were formed, from triangular structures to lines and crystal structures called dimers. The work, besides bringing a promising observation of something before only hypothetical, brings new solutions to quantum experimentsrelated to the manipulation of electrons, in addition to applications that scientists have not yet imagined
The scientific study was published in the journal Nature. With information from Science Alert and Cornell University.