First direct observation of a strange crystal made only of electrons
Princeton University Yazdani Laboratory
It is difficult to direct electrons to form crystals, and this structure is even more difficult to measure. But physicists have now succeeded in directly imaging a ‘Wigner crystal’, and the image is the clearest yet.
“There are literally hundreds of papers written about finding indirect evidence for Wigner crystals,” he says. Ali Yazdani at Princeton University. “And I never thought we would make it.” [directly] Image it. It was just a little accident. ”
At room temperature, electrons can flow together in an electric current. This is because the kinetic energy of the electrons overcomes the forces that cause particles with the same charge to repel each other. However, at very low temperatures, the repulsive forces of electricity prevail and the electrons become aligned in a uniform lattice, or crystal. Physicist Eugene Wigner predicted this phenomenon in his 1934 year, but only recently have researchers begun to figure out how to create Wigner crystals in the laboratory.
Yazdani and his colleagues created Wigner crystals from electrons in two thin sheets of graphene, each just one atom thick. To reduce the kinetic energy of the electrons, they placed the graphene in a refrigerator that cooled it to just a few hundredths of a degree below absolute zero and immersed it in a strong magnetic field.
Yazdani says it was important that the graphene had few defects where electrons could get stuck. Otherwise, as Wigner predicted, the particles could form crystal-like states not because of their interactions but because of their imperfect structure.
In past experiments, researchers looked for evidence of Wigner crystals by trying to force electrons into them to form an electric current. Once the particles stopped flowing, the researchers were able to deduce that the electrons were trapped in the grid. But Yazdani’s team used a special microscope to image the crystals directly.
This microscope utilized a quantum effect called tunneling. When a very sharp metal tip is scanned across the graphene surface and passes over the electrons, the particles tunnel through the gap between the surface and the tip, creating a small electrical current. Thanks to these currents, the researchers were able to figure out where and how densely the electrons are located within the graphene, allowing them to create the most accurate image yet of a Wigner crystal.
This method was previously used in another experiment, where the grid of electrons was inside a material that was itself sandwiched between layers of other materials. This made imaging less direct and difficult to determine why the electrons formed the crystals. The electrons may have been influenced by the lattice structure of nearby materials.
Yazdani and his colleagues saw in the images that electrons were repeatedly located at the vertices of the triangle, just as Wigner had predicted. They also tracked how the structure of the crystal changed as they varied factors such as temperature, the strength of the magnetic field, and the number of electrons in the crystal. This was possible by applying a voltage to the material. Under these changing conditions, the crystals “melted” into exotic incompressible electronic fluids and fluids in which the electrons formed stripes.
The team would like to imagine these melted states next. Some of them are filled with particle-like excitations, which resemble electrons but carry only part of their charge. Yazdani hopes he and his collaborators can also image the crystallization of excitations.
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