Light reveals internal motion in electron crystals and can trigger their melting

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Add as preferred source 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 (WSe 2 ), 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 (MoSe 2 ) 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, WSe 2 , which would potentially solve many mysteries surrounding Wigner crystals in 2D materials."

Building on their earlier observation of Wigner crystals in MoSe 2 , Zhou and his colleagues set out to detect these crystals in another transition metal dichalcogenide. They specifically focused on WSe 2 , a layered semiconducting material often used to create optoelectronic devices.

The researchers created a device containing a single WSe 2 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. "We then spent a fair amount of time trying to understand these additional optical features and figured out that they came from the phonons of the Wigner crystal—the vibrations of the crystal. So, our initial discovery was a bit of an accident."

To detect the Wigner crystal and study its internal vibrations, the researchers shone light onto the material and measured how strongly it reflected different wavelengths. Some of the incoming light created excitons that interacted with the Wigner crystals and locally disturbed the ordered electron lattice.

"If you imagine the Wigner crystal as a regular lattice of electrons, we are creating an electron–hole pair—called an exciton—in between," explained Zhou. "Because the exciton can interact with the electrons, it will move the electrons inside the crystal a little bit; in other words, the exciton can distort the Wigner crystal lattice.

"What we find is that in optics, we can see not only absorption from the exciton itself but also from the exciton plus the lattice distortion. In physics, we often say the exciton is 'dressed' by the distortion of the lattice (Wigner phonons)—and we call this a Wigner polaron ."

"First, we used optical methods to control the spins of the Wigner crystal," said Zhou. "You can imagine electrons as little magnets—this is an internal property of the electrons—and their magnetic moments can point up or down. Typically, when you want to manipulate the spins of electrons, you use a magnetic field.

"Here, what we can do is shine circularly polarized light on the sample, and that initializes the electron spins in one particular direction. So, instead of using a magnetic field to flip the spins, we can now use light to manipulate them."

Another way to manipulate a Wigner crystal is to melt it, or, in other words, prompt its electrons to lose their ordered, crystal-like arrangement and enter a more disordered electronic state. First, the researchers increased the optical power to generate a large population of excitons in the material.

The team's measurements indicated that the excitons destabilized the Wigner crystal and softened its vibrations. However, the precise mechanism behind this optical melting process remains unclear.

"We found that when a large population of excitons is present, the Wigner crystal becomes less stable—it's more likely to melt—and we actually see that the phonon energy becomes smaller," said Zhou.

"There are many different possibilities for why this happens, but one possible reason is that the excitons can screen the Coulomb interactions between the electrons, and this makes the Wigner crystal much softer. So now we have a method to nonthermally change the crystal phase and induce a phase transition, possibly at a very fast speed."

Using their light-based approach, the researchers were able to directly probe the internal vibrations of a Wigner crystal, manipulate its electron spins and induce its melting, all without applying an external magnetic fi...