Physicists Develop an Unusual ‘Wigner Crystal’ Made Simply of Electrons
In 1934, Eugene Wigner, a pioneer of quantum mechanics, theorized of an odd sort of matter– a crystal made from electrons. The idea was very straightforward, proving it had not been. With limited success, physicists tried many tricks over eighty years to nudge electrons right into forming these so-called Wigner crystals. However, in June, two independent teams of physicists reported in Nature one of the most straight experimental monitorings of Wigner crystals yet.
” Wigner crystallization is quite an old idea,” stated Brian, a physicist at Ohio State University that was not involved with the study. “To see it so clearly was truly good.”
To drive electrons to form a Wigner crystal, it could appear that a physicist would need to cool them down. Electrons repel each other; therefore, cooling down would lower their energy and freeze them into a lattice similar to when water turns to ice. However, chilly electrons obey the strange laws of quantum mechanics-they behave just like waves. As opposed to getting arranged into place in a neatly ordered grid, wavelike electrons often tend to swash around and crash into their neighbors. What was supposed to be a crystal becomes something a lot more like a puddle.
By accident, one of the teams responsible for the new work nearly found a Wigner crystal. In a group led by Hongkun Park at Harvard University, researchers were experimenting with electron behavior in a “sandwich” of extremely thin sheets of a semiconductor separated by a product that electrons can not move through. The physicists cooled this semiconductor sandwich to below − 230 degrees Celsius and experimented with the number of electrons in each layer.
The group observed that when there was a particular number of electrons in each layer, they all stood strangely still. “Somehow, electrons inside the semiconductors stayed stagnate. This was an unexpected finding,” claimed You Zhou, lead author on the brand-new study.
Zhou shared his findings with theorist colleagues, who at some point recalled an old concept of Wigner’s. Wigner calculated that electrons in a flat two-dimensional material would undoubtedly assume a pattern comparable to a floor entirely covered with triangular tiles. This crystal would completely stop the electrons from moving.
In Zhou’s crystal, repulsive forces between electrons in each layer and between the layers interacted to organize electrons into Wigner’s triangular grid. These forces were powerful enough to stop the electron from spilling and sloshing, predicted by quantum technicians. However, this behavior happened only when the amount of electrons in each layer was such that the top and lower crystal grids lined up: Smaller triangles in one layer needed to specifically fill up the space within larger ones in the other. Park named the electron ratios that resulted in these conditions the “dead signs of bilayer Wigner crystals.”
After they recognized that they had a Wigner crystal on their hands, the Harvard group made it melt by leading the electrons to accept their quantum wave nature forcibly. Wigner crystal melting is a quantum phase transition – one that is similar to ice becoming water, however, with no heating involved. Theorists previously predicted the requirements essential for the process, yet the new experiment is the first to validate it through direct measurements. “It was really, truly exciting to see what we learned from books and documents in experimental data,” Park claimed.
Past experiments found tips of Wigner crystallization; however, the brand-new studies provide the most direct proof as a result of a new experimental technique. The researchers showered the semiconductor layers with laser light to produce a particle-like entity called an exciton. The product would certainly then reflect or re-emit that light. By analyzing the light, scientists can determine whether the excitons had interacted with ordinary free-flowing electrons or electrons frozen in a Wigner crystal. “We have direct proof of a Wigner crystal,” Park said. “You can see that it is a crystal that has this triangular structure.”
The second research group, led by Ataç Imamoğlu at the Swiss Federal Institute of Innovation Zurich, additionally utilized this method to observe the formation of a Wigner crystal.
The new work shines a light on the well-known problem of many interacting electrons. When you place a lot of electrons into a small space, they all push on each other, and also, it becomes impossible to keep up with all the mutually intertwined forces.
According to Philip Phillips, a physicist at the University of Illinois, Urbana-Champaign that was not involved with the experiment, the Wigner crystals are an archetype for all such systems. He remarked that the only problem involving electrons and electrical forces that physicists know how to fix with a simple pen and paper is a single electron in the hydrogen atom. In atoms with more than one electron, the problem of predicting what the interacting electrons will certainly do ends up being unbending. The issue of several interacting electrons has long been thought about one as one of the most challenging in physics.
For the future, the Harvard team plans on utilizing their system to address impressive questions concerning Wigner crystals and strongly correlated electrons. One open question is what happens, specifically, when the Wigner crystal melts; contending theories abound. In addition, the team observed Wigner crystals in their semiconductor sandwich at greater temperatures and for higher numbers of electrons than theorists anticipated. Examining why this was the case could bring about brand-new understandings regarding highly associated electron behavior.
Eugene Demler, a theorist at Harvard who added to both new studies, thinks that the work will clear up old academic debates and influence brand-new inquiries. “It is always much easier to work with a problem when you can seek out the solutions at the end of a publication,” he claimed. “And also having more experiments resembles seeking out the answer.”
Originally published on Quanta Magazine. Read the original article.
