Like Michael Faraday with his magnets and prisms, it’s not common to mess around in the laboratory to produce a fundamental breakthrough. It is even rarer that two research teams discover the same thing at the same time: think of Newton and Leibniz. But every once in a while, even the rarest events will happen. The summer of 2021 is a landmark season for condensed matter physics. Three independent research teams created a crystal composed entirely of electrons-one of which was actually done by accident.

Researchers are studying single-atom thick semiconductors cooled to ultra-low temperatures. A team led by Hongkun Park and Eugene Demler of Harvard University discovered that when there are a very specific number of electrons in the layers of these semiconductor sheets, the electrons stop in their orbits and "mysteriously stand still." In the end, the colleagues remembered an old idea related to Wigner crystals, which is one of the things that existed on paper and in theory but has never been confirmed in life. Wigner calculated that due to mutual electrostatic repulsion, the electrons in a single layer will exhibit a three-grid pattern.

The Park and Demler teams are not alone. "A group of theoretical physicists led by Eugene Demler of Harvard University will move to ETH [ETH Zurich, Switzerland] this year, and they theoretically calculated how this effect should appear in observations. In the exciton excitation frequency that we have seen-and this is what we observed in the laboratory," said Ataç Imamoğlu from ETH. Imamoglu's team used the same technique to record the formation of Wigner crystals.

Electrons are like magnetic poles in a way: like repulsion. In solids, electrons help form a regular, repeating crystal lattice. But in liquid is another matter. Because liquid electrons are susceptible to interference, when they are left to their own devices, their collective waveform is chaotic, full of varying and uneven interference.

To settle them down requires a perfect blend of extreme conditions. First, it is easier when there are not many other electrons causing interference in the mode. In addition, the number of electrons that can be divided neatly can be arranged in a perfect grid. A group of independent researchers, including Wang Feng, the corresponding author at the University of California, Berkeley, is working on a semiconductor made from a thin layer of tungsten disulphide and tungsten diselenide atoms. The distances between the atoms in these compounds are slightly different, so the overlapping layers form a "honeycomb moiré pattern" in areas with very low energy, which also helps stabilize the electrons.

Electronic (red) Wigner crystals in semiconductor materials (blue/gray). Credit: ETH Zurich

Then there is temperature. When the weather is very cold, everything slows down, so keeping the temperature within absolute zero will help keep the mobile electrons in place. This is where quantum phenomena begin to replace classical electronic behavior. Electrons no longer act like wavefronts in water, but start to better match their particle properties. But once the weather gets cold and enough, it becomes easier to suddenly contain electrons. It becomes less like a grazing cat, but more like those acoustic sand table patterns. With the correct number of electrons, they will actually line up on their own.

(Author's Note: If you want a cool particle and wave self-organized A/V demo to avoid certain places and focus on others, please give this watch. Warning: turn down the volume. This is like a microphone Feedback and air raid alerts.)

Electron is the functional unit of electricity. So, if you gather a whole ton of electrons together, you may get something similar to ball lightning: a powerful, concentrated, and very powerful electricity, waiting to break through something. But this is not what happens in Wigner crystals. Electricity is produced by the movement of electrons, not just their existence. Gathering electrons in a neat three-dimensional grid of atoms means that the electrons in the material have almost no movement. This is also the definition of insulator. This is why researchers know that they created electronic crystals: they expect their semiconductors to have semiconductor properties, but this is not the case. In their pigeon cage, the electrons did not move, so the electricity did not move either. These "crystals" are 100% electrons, but they are insulators.

Quantum fluctuations close to absolute zero cause quantum phase transitions between free-flowing liquids and quantum crystals (such as Wigner crystals). These quantum transitions are thought to be important in many other quantum systems. Once they knew that they had a Wigner crystal, in order to explore its properties, the Harvard team decided to let it undergo a "quantum melting", which is obviously like conventional melting, but on such a small scale, it is quantum...

"Do you just put the word quantum in front of everything?" (Photo: Marvel's "Ant-Man and the Wasp")

All of this excitement occurs on such a small scale that scientists cannot visually image it even with the best optical microscopes. But the initial attempt to use a scanning tunneling microscope destroyed the delicate surface of the crystal. In an instant, Wang's team covered the semiconductor with a single-atom-thick graphene sheet. The Wigner crystals below change the electronic structure of graphene very slightly, which can be detected by scanning tunneling microscopes. In order to verify whether they created the Wigner crystal, physicists must ping it with a single photon, loosen the electrons and produce something called an "exciton" that they can detect.

Demler said in a statement: "This happens to be [sic] the transition from part of quantum material to part of classical material, and it has many unusual and interesting phenomena and characteristics." What exactly is it takes some time. To solve it.

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