Wigner crystals, an exotic substance made entirely of electrons, were first suggested by a theorist 90 years ago. Now, a research group has announced the creation of this crystal and has taken pictures of it for the first time.
Schematic of a scanning tunneling microscope observing microscopic particles
Feng Wang, a physicist at the University of California, Berkeley, one of the study’s lead authors, says other studies have claimed to have created Wigner crystals before, but theirs is the first to show a photograph as evidence.
The team designed a semiconductor device to produce Wigner crystals. The device contains two thin layers of semiconductor atoms with similar properties: tungsten disulfide and tungsten diselenide, and an electric field is used to adjust the density of electrons moving freely between the two layers.
Within the ordinary material, the electrons move too fast to be affected much by the negative charge repulsion between the electrons. Theoretical physicist Eugene Wigner previously predicted that if the electrons slowed to a certain speed, the effect of repulsion would dominate the electron’s behavior and the electrons would automatically find an arrangement that would keep their overall energy at a minimum and stabilize, for example, in a honeycomb shape.
Wang Feng and colleagues cooled the device they built to near absolute zero, and indeed honeycomb-like Wigner crystals appeared in the middle of the two layers of material.
The study describes their idea to use a scanning tunneling microscope (STM) to photograph the formed Wigner crystals. This device has a metal probe that moves across the surface of the object being imaged, and the voltage causes electrons to leave the probe to generate a current. As the probe moves across the surface of the object, the intensity of the current changes to reflect the location of the electrons on the surface of the object.
But their first attempt failed because the current from the microscope probe damaged the fragile Wigner crystal. They came up with another solution by matting the surface of the Wigner crystal with a thin layer of graphene that is only a single atomic layer thin. The Wigner crystal, once present, would slightly change the structure of the electrons within the graphene material, and the microscope was able to probe the change.
The photos they eventually took clearly showed the morphology of the Wigner crystal, confirming the ordered lattice structure inside it. The study also found that, as expected, the distance between electrons inside a Wigner crystal is nearly 100 times greater than the distance between atoms inside a semiconductor material.
The study was published Sept. 29 in the journal Nature.