Researchers capture electrons to create a long-running elusive crystal

Cornell researchers piled up two-dimensional semiconductors to create a moiré super-grid structure that captures electrons in a repeating pattern, eventually forming a long-hypothesis Wigner crystal. Credit: Cornell University

Like disturbed children who are doing a family portrait, the electrons will not have enough time to stay in any fixed arrangement.

Now, the Cornell-led collaboration has developed a way to accumulate two-dimensional semiconductors and capture electrons in a repeating pattern that forms a precise crystal and long hypothesis.

The group’s article, “Correlated Insulating States in Fractional Fillings of Moiré Superlattices,” was published on November 11, 2020, Nature. The lead author of the paper is Yang Xu, a postdoctoral researcher.

The project originated from a laboratory shared in collaboration with Kin Fai Mak, an associate professor of physics at the University of Arts and Sciences, and Jie Shan, a professor of physics at the University of Engineering. Both researchers are members of the Kavli Institute in Cornell for Nanoscale Science; They came to Cornell through the provost’s Nanoscale Science and Microsystems Engineering (NEXT Nano) initiative.

The electron crystal was first announced in 1934 by the theoretical physicist Eugene Wigner. He proposed that the repulsion created by negatively charged electrons – called the Coulomb repulsion – would form a crystal when the kinetic energy of the electrons predominates. Scientists have tried several methods to remove this kinetic energy, such as placing electrons under a very large magnetic field, a million times more than the Earth’s magnetic field. Full crystallization remains elusive, but the Cornell team found a new method to achieve this.

“Electrons are quantum mechanics. Although they don’t do anything to them, they’re moving by themselves all the time, “Mak said.” An electron crystal would tend to melt because it’s very difficult to keep the electrons fixed in a periodic pattern. “

So the researchers ’solution was to build a real trap by stacking two monolayer monolayers, tungsten disulfide (WS2) and tungsten diselenide (WSe2), grown by the partners. Columbia University. Each monolayer has a slightly different network constant. Paired together, they create a moiré super-grid structure that basically looks like a hexagonal grid. The researchers then placed the electrons at specific sites in the models. As found in a previous project, the energy barrier between sites blocks electrons in place.

“We can control the average occupancy of electrons at a specific site,” Mak said.

Seeing the intricate pattern of a Moiré super-grid, combined with the rumor of electrons and the need to place them in a very precise arrangement, the researchers approached Veit Elser, a physics professor and author of the article. the occupation ratio in which different electron arrangements will self-crystallize.

However, in addition to creating these challenges for Wigner crystals, there is also no observation of them.

“You have to meet the right conditions to create an electron crystal and at the same time they are also fragile,” Mak said. “You need a good way to probe. You don’t really want to be upset while you’re probing. “

The group invented a new optical sensor technique, where the optical sensor is located close to the sample, and the entire structure is located between insulating layers of hexagonal boron nitride, created by collaborators of the National Institute of Materials Science of Japan. Since the sensor is about two nanometers apart from the sample, it does not worry the system.

The new technique allowed the group to observe a large number of electron crystals of different crystal symmetry, ranging from triangular lattice Wigner crystals to self-aligned crystals in stripes and dimers. By doing so, the team demonstrated how very simple components can create complex patterns – as long as the components are long enough.

Reference: Yang Xu, Song Liu, Daniel A. Rhodes, Kenji Watanabe, Takashi Taniguchi, James Hone, Veit Elser, Kin Fai Mak and Jie Shan, 11 November 2020, “Correlated insulating states in fragmented moiré superlate fillers”. Nature.
DOI: 10.1038 / s41586-020-2868-6

The authors of the article include researchers from Columbia University and the National Institute of Materials Science in Japan.

Research and device manufacturing were supported by the U.S. Department of Energy, the U.S. Bureau of Marine Research and the David and Lucille Packard Fellowship.

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