Since the electrons were discovered in 1897, they have continued to amaze scientists due to their unique properties. More than a century later, physicists have pushed the boundaries of their understanding of electrons by visualizing a strange kind of matter made entirely of these infinitesimally small particles.
(Photo: Wikimedia Commons/ Arunas)
Wigner Crystal
In 1934, physics professor Eugene Wigner from Princeton University proposed the idea of a possible interaction between electrons. He suggested these particles could be arranged into a crystal-like or closely packed lattice. According to him, this event could happen because of the mutual repulsion of the electrons and under conditions of low densities and extremely cold temperatures.
For a long time, Wigner's proposal remained in theory. Later experiments tried to transform the concept of an electron crystal from conjecture to reality. One of these studies was conducted at Bell Laboratories in New Jersey in the 1970s when scientists created a classical electron crystal by spraying electrons on the surface of helium until they responded rigidly like a crystal.
However, the electrons in such experiments were very far apart and behaved more like single particles than a cohesive structure. Instead of following the familiar laws of physics in the everyday world, an accurate Wigner crystal would follow the laws of quantum physics, where electrons would act not like single particles but more like a single wave.
Over the following decades, experiments proposed different ways to create quantum Wigner crystals. Such experiments were advanced in the 1980s and 1990s when experts discovered how to confine the motion of electrons to atomically thin layers using semiconductors.
Applying a magnetic field to a layered structure causes electrons to move in a circle, creating ideal conditions for crystallization. However, these experiments failed to observe the crystal directly. They only either suggested its existence or inferred it indirectly from the flow of electrons through the semiconductor.
Crystal of Frozen Electrons
With these considerations, researchers attempted to visualize this crystal and watch its formation. By confirming many of its properties, they were able to study it in ways previous scientists have not done.
The research team, led by physics professor Al Yazdani from Princeton University, tried to directly image the Wigner crystal using a scanning tunneling microscope (STM). This device uses quantum tunneling instead of light to view the atomic and subatomic particles.
In conducting the experiment, the experts decided to use graphene, which was made as pristine and devoid of imperfections as possible. This was vital in eliminating the possibility of any electron crystals forming due to material imperfections.
The group made unprecedentedly clean samples without any atomic imperfection in the graphene atomic lattice. The system was visualized as the scientists tuned the number of electrons per unit area. This happened because, at low densities, the electrons are far apart, situated in a disordered, disorganized fashion. As the density increases, the electrons are brought closer,, and their natural repulsive tendencies kick in. This allowed them to start forming an organized lattice structure.
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