Under the right circumstances, electrons can actually “freeze” into a bizarre solid form. Now, physicists at Berkeley Lab have created and taken the first ever direct images of this structure.
At low temperatures and densities, groups of electrons can crystallize into a solid form known as a Wigner crystal, named after theoretical physicist Eugene Wigner who first predicted their existence in the 1930s. It was only a few years ago that scientists first directly detected and imaged them.
Now, a team has for the first time imaged a new quantum phase of electrons – a related structure called a Wigner molecular crystal. Basically, it’s the same solid electron phase, except that groups of electrons settle in each place on a lattice instead of single electrons.
Electrons usually flow through materials more or less freely, acting somewhat like a disordered liquid. But if their motion can be slowed right down, another property should take over – their electrostatic repulsion. Since electrons all have the same charge, they naturally repel each other, so when they stop moving they push each other to a certain distance apart and lock there. This results in the Wigner crystal phase.
To make Wigner molecular crystals, the researchers needed a new framework that would hold the electrons so they would form “molecules.” They start with a 49-nanometer-thick layer of hexagonal boron nitride, then stack on two layers of tungsten disulfide, each a single atom thick. One layer is twisted to a 58-degree angle relative to the second.
The resulting “tWS2 moiré superlattice” is then doped with electrons, and sure enough, two or three electrons pooled into each unit cell of the lattice. Those small groups are essentially electron molecules, which together form the elusive Wigner molecular crystal.
Actually viewing the crystal proved to be another challenge. A scanning tunneling microscope (STM) is normally used to image things on this scale, but the electric field produced by the tip tends to disrupt the fragile configuration of the electrons in the crystal. The team found a way to minimize that electric field, allowing them to snap the first images of the phenomenon.
The researchers plan to investigate Wigner molecular crystals further in future experiments, to see what kind of applications might come out of them.
The research was published in the journal Science.
Source: Berkeley Lab