A scanning tunneling microscope image reveals the Wigner crystal that formed within a layered structure underneath. (Image Credit: H. Li et al./Nature)
Favorable conditions for electrons within a material cause them to arrange into a honeycomb-like lattice. Now, physicists at the University of California, Berkeley managed to capture images of those Wigner crystals, first theorized 90 years ago by Eugene Wigner. Researchers have produced and measured Wigner crystals in the past, but this is the first time anyone captured images of the patterns.
To produce the Wigner crystals, the team developed a device with atom-thin layers of two identical semiconductors, tungsten disulfide and tungsten diselenide. Then, they applied an electric field, which fine-tuned the density of the electrons moving along the interface between the two layers.
Electrons normally move around too quickly in a traditional material that the repulsion between their negative charges doesn’t dramatically affect them. However, Wigner theorized that electrons moving around slowly enough would cause the repulsion to control their behavior. These electrons then form arrangements, minimizing their energy. With this theory, the team cooled down the device’s electrons a few degrees above absolute zero to slow them down.
The dissimilarity between the two layers contributed to the electrons forming Wigner crystals. The atoms in both tungsten layers are slightly spaced apart. When paired together, it gives them that honeycomb-like arrangement. This pattern then generated lower energy in certain areas, helping to settle down the electrons.
The team viewed the Wigner crystal through a scanning tunneling microscope (STM). An STM’s metal tip levitates above the sample’s surface. Then, applied voltage forces the electrons to move down from the tip, generating an electric current. While the tip moves across the surface, the current’s changing intensity shows where the electrons are located in the sample.
Earlier attempts to capture Wigner crystal images failed because the current ruined the Wigner arrangements. To overcome this issue, the team implemented a graphene layer on top. Doing so causes the Wigner crystal to alter the electron structure of the graphene, which was picked up by the STM. The images revealed the Wigner electrons’ arrangement. These electrons in the Wigner crystal are almost 100 times apart compared to the atoms in the device’s crystals.
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