This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Electrons, particles that carry a negative electric charge, typically move through materials. At low densities and temperatures, however, the electrical repulsion between them can overpower their tendency to move, prompting them to arrange themselves into ordered patterns known as Wigner crystals.
In contrast with ordinary crystals, which consist of atoms arranged in a repeating pattern, Wigner crystals are ordered arrangements of electrons in regular, crystal-like patterns inside a material. These electron crystals are valuable platforms for testing fundamental theories of particle interactions and studying quantum phase transitions. Reliable methods for controlling them could also inform the development of future electronic, optoelectronic, spintronic and quantum devices.
Researchers at the University of Maryland, ETH Zurich and other institutes recently examined a Wigner crystal in a single, atomically thin layer of tungsten diselenide (WSe2), which belongs to a family of materials called transition metal dichalcogenides. Their paper, published in Nature Physics, introduces a new approach to probe the electron lattice's vibrations and manipulate some of its internal dynamics. "We discovered Wigner crystals in molybdenum diselenide (MoSe2) a couple of years ago, while I was a postdoc at Harvard," You Zhou, senior author of the paper, told Phys.org.
"It turns out that other transition metal dichalcogenides also have similar properties. Wigner crystals have been notoriously difficult to realize, and our 2021 studies showed that they are surprisingly stable in 2D materials. So, a natural question was whether we could also observe Wigner crystals in another material, WSe2, which would potentially solve many mysteries surrounding Wigner crystals in 2D materials." Building on their earlier observation of Wigner crystals in MoSe2, Zhou and his colleagues set out to detect these crystals in another transition metal dichalcogenide.
They specifically focused on WSe2, a layered semiconducting material often used to create optoelectronic devices. The researchers created a device containing a single WSe2 layer enclosed between two insulating sheets of hexagonal boron nitride. They also added graphite gates, components that allowed them to control the number of electrons inside it.
They cooled the device to a low temperature of 5 Kelvin (−268°C, −451°F). At this temperature and when the density of electrons is sufficiently low, the electrons' motion becomes weak enough for their mutual electrical repulsion to organize them into a Wigner crystal. "When we conducted this experiment, we not only observed Wigner crystals but also additional features in the optical studies," said Zhou.
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