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To capture the clearest and most direct images of a "Wigner crystal", a structure made entirely of electrons, researchers used a special kind of microscope and two pieces of graphene unusually free of imperfections

By Karmela Padavic-Callaghan

This is the first direct look at a bizarre crystal made of only electrons

Yazdani Lab, Princeton University

It is hard to coax electrons to form a crystal, and even harder to measure this structure. But physicists have now managed to directly image a "Wigner crystal" - and their images are the clearest ones yet.

"There have been many, literally hundreds, of papers written on finding evidence for the Wigner crystal sort of indirectly," says Ali Yazdani at Princeton University. "And we never thought that we would succeed in [directly] imaging it. It was a bit of an accident."

At room temperature, electrons can flow together in electric currents because their kinetic energy overcomes the force that makes particles with the same electric charge repel each other. At very low temperatures, however, repulsive electric forces win out, and the electrons end up arranging themselves into a uniform grid, or a crystal. Physicist Eugene Wigner predicted this phenomenon in 1934, but researchers only recently started to understand how to create Wigner crystals in the lab.

Yazdani and his colleagues made their Wigner crystal from electrons inside of two thin sheets of graphene, each only one atom thick. To diminish the electrons' kinetic energy, they put the graphene inside a fridge that cooled it to only a few hundredths of a degree above absolute zero and immersed it in a strong magnetic field.

Yazdani says that it was crucial that their graphene had very few imperfections where electrons could get stuck. Otherwise, the particles could form a crystal-like state because of the structure of those imperfections, rather than because of the interactions with each other, as Wigner predicted.

In past experiments, researchers would look for evidence of a Wigner crystal by trying to nudge electrons into forming currents: once the particles failed to flow, researchers could infer that the electrons were locked into a grid. But Yazdani's team directly imaged its crystal with a special microscope.

This microscope used a quantum effect called tunnelling. It scanned an extremely sharp metallic tip across the surface of the graphene, and when it passed over an electron, the particle would tunnel through the gap between the surface and the tip, creating a small electric current. Because of these currents, the researchers knew where and how densely the electrons were positioned inside the graphene, letting them create the most precise images of a Wigner crystal yet.

One other experiment used this method previously, but in that case, the grid of electrons was inside of a material that was itself sandwiched between layers of other materials. This made the imaging less direct, and it made it harder to determine why the electrons formed a crystal - they could have been influenced by the grid-like structure of the nearby materials.

In their images, Yazdani and his colleagues saw electrons sitting at vertices of repeating triangles, just as Wigner predicted. Additionally, they tracked how the crystal's structure changed as they shifted factors such as temperature, the strength of the magnetic field and how many electrons it contained, which they could do by applying an electric voltage to the material. Under these changing conditions, the crystal "melted" into an exotic, incompressible electron fluid, as well as a fluid where electrons formed stripes.

These melted states are what the team wants to image next. Some of them are full of particle-like excitations, which are like electrons but only carry a fraction of their charge. Yazdani hopes he and his collaborators could image the excitations' crystallisation too.

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Bizarre crystal made only of electrons revealed in astonishing detail

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To capture the clearest and most direct images of a "Wigner crystal", a structure made entirely of electrons, researchers used a special kind of microscope and two pieces of graphene unusually free of imperfections

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Move over, graphene. Your blinged-up cousin, goldene, has entered the scene.

Researchers have accidentally created a single-atom layer of gold.

The new material has been dubbed "goldene" because it replicates the two-dimensionality of the carbon-based graphene.

Graphene was discovered in 2004 after being theoretically suggested as far back as 1985.

In the 20 years since its first production, graphene has taken the world by storm, showing its usefulness in applications from semiconductors in ultra-fast computers to making tiny carbon nanotubes stronger than diamond.

The researchers who created goldene believe its properties could make it useful in applications such as carbon dioxide conversion, hydrogen production and the manufacture of other chemicals.

The goldene breakthrough is detailed in a paper published in Nature Synthesis.

"If you make a material extremely thin, something extraordinary happens - as with graphene," says first author Shun Kashiwaya, researcher at Linköping University, Sweden. "The same thing happens with gold. As you know, gold is usually a metal, but if single-atom-layer thick, the gold can become a semiconductor instead."

Like graphene, scientists have long tried to make goldene, but failed. This is because the metal tends to clump together.

A 100-year-old Japanese smithing method allowed the breakthrough at Linköping. And it was all a bit of an accident.

Using a 3D base material, the researchers embedded gold between layers of titanium and carbon.

"We had created the base material with completely different applications in mind," says senior author Lars Hultman, a professor at Linköping. "We started with an electrically conductive ceramic called titanium silicon carbide, where silicon is in thin layers. Then the idea was to coat the material with gold to make a contact. But when we exposed the component to high temperature, the silicon layer was replaced by gold inside the base material."

Hultman found a Japanese forging method called Murakami's reagent used for etching away carbon residue. Modifying the method, the team was able to get the gold out of the structure.

The final step in the process is to make gold sheets which are stable.

"The goldene sheets are in a solution, a bit like cornflakes in milk. Using a type of 'sieve,' we can collect the gold and examine it using an electron microscope to confirm that we have succeeded. Which we have," says Kashiwaya.

Gold atoms in goldene are arranged in a triangular pattern, unlike graphene in which the carbon atoms form hexagons.

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Researchers have created goldene - a single-atom layer of gold

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Move over, graphene. Your blinged-up cousin, goldene, has entered the scene. Researchers have accidentally created a single-atom layer of gold.

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Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master's degree in science journalism from Boston University.

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