Researchers trap electrons to create elusive crystals

Like restless children who pose for a family portrait, electrons won’t stay still long enough to stay in any kind of fixed arrangement.

Cornell researchers stacked two-dimensional semiconductors to create a super lattice moiré structure that traps electrons in a repetitive pattern, eventually forming the long-hypothesized Wigner crystal.

Now, a collaboration led by Cornell has developed a way to stack two-dimensional semiconductors and trap electrons in a repetitive pattern that forms a specific and long-assumed crystal.

The team paper, “Correlated Insulating States at Fractional Fillings of Moiré Superlattices,” published November 11 in Nature. The lead author of the paper is postdoctoral researcher Yang Xu.

The project grew out of the shared laboratory of Kin Fai Mak, associate professor of physics at the College of Arts and Sciences, and Jie Shan, professor of applied physics and engineering at the College of Engineering, senior co-authors of the paper. Both researchers are members of the Kavli Institute at Cornell for Nanoscale Science; they came to Cornell through the rector’s Nanoscale Science and Microsystems Engineering (NEXT Nano) initiative.

An electron crystal was first predicted in 1934 by theoretical physicist Eugene Wigner. He proposed that when the repulsion that results from negatively charged electrons – called Coulomb repulsions – dominates the kinetic energy of the electrons, a crystal would form. Scientists have tried various methods to suppress that kinetic energy, such as putting electrons under an extremely large magnetic field, about a million times that of the Earth’s magnetic field. Complete crystallization remains elusive, but the Cornell team has discovered a new method to achieve it.

“Electrons are quantum mechanics. Even if nothing is done for them, they move spontaneously all the time,” Mak said. “An electron crystal would actually have a tendency to melt because it is so difficult to keep electrons fixed in a periodic pattern.”

So the researchers’ solution was to build a real trap by stacking two semiconductor monolayers, tungsten disulfide (WS2) and tungsten diselenide (WSe2), grown by partners at Columbia University. Each monolayer has a slightly different lattice constant. When paired together, they create a moiré lattice structure, which essentially resembles a hexagonal grid. The researchers then placed the electrons at specific sites in the model. As they discovered in a previous project, the energy barrier between sites locks electrons into place.

“We can check the average electron occupation at a specific moiré site,” Mak said.

Given the intricate pattern of a super moiré lattice, combined with the nervous nature of electrons and the need to place them in a very specific arrangement, the researchers turned to Veit Elser, professor of physics and co-author of the paper, who calculated ratio occupancy whereby different arrangements of electrons self-crystallize.

However, the challenge of Wigner crystals is not only to create them, but also to observe them.

“It is necessary to achieve the right conditions to create a crystal of electrons and, at the same time, they are also fragile,” Mak said. “You need a good way to probe them. You don’t really want to significantly disturb them as you probe them.”

The team devised a new optical sensing technique in which an optical sensor is placed close to the sample and the entire structure is sandwiched between insulating layers of hexagonal boron nitride, created by collaborators at the National Institute for Materials Science in Japan. . Since the sensor is separated from the sample by about two nanometers, it does not disturb the system.

The new technique allowed the team to observe numerous electron crystals with different crystalline symmetries, from triangular lattice Wigner crystals to crystals that self-align into strips and dimers. In doing so, the team demonstrated how very simple ingredients can form complex patterns, as long as the ingredients stay put long enough.