Electrons, usually known for moving at high speeds through matter, can be forced into a rare, ordered state when their environment is carefully controlled. In the 1930s, physicist Eugene Wigner proposed that electrons could become immobilized under certain conditions—specifically, at low densities and extremely cold temperatures—forming a unique type of electron lattice known as a Wigner crystal, or “electron ice.” Decades after Wigner’s prediction, researchers have pursued direct observation of these elusive electron formations. Finally, in 2021, a team led by Feng Wang and Michael Crommie, both faculty scientists at Berkeley Lab and professors of physics at UC Berkeley, managed to directly observe a Wigner crystal, providing the first real evidence of this phenomenon.
Recently, Wang, Crommie, and their teams achieved another breakthrough in this field, with their findings reported in the journal Science. They captured direct images of a new and unexpected quantum phase of electron organization, now called the Wigner molecular crystal. Unlike the simple Wigner crystal, where electrons align in a honeycomb-like structure, Wigner molecular crystals consist of a structured array of artificial “molecules” composed of two or more electrons bound together. This formation represents a distinct and more complex phase of electron behavior.
In a statement about their discovery, Wang emphasized the significance of their findings. “We are the first to directly observe this new quantum phase, which was quite unexpected,” he said. “It’s pretty exciting.”
Scientists have long sought a clear image of the Wigner molecular crystal, but previous attempts were thwarted by technical challenges, primarily due to the sensitivity of these electron structures to the scanning tunneling microscope (STM) used to visualize them. Typically, the electric field generated by the STM tip would disturb or even destroy the delicate configuration of electrons, making it impossible to capture a stable image. However, the Berkeley Lab team developed a method to minimize the STM tip’s electric field, allowing them to observe the Wigner molecular crystal’s intricate arrangement without disrupting it.
To produce the Wigner molecular crystal, the researchers worked with an advanced nanomaterial called a “twisted tungsten disulfide (tWS2) moiré superlattice.” This material was constructed by placing a bilayer of atomically thin tungsten disulfide (WS2) on a 49-nanometer-thick hexagonal boron nitride (hBN) substrate with an additional graphite back gate for precise control. The WS2 layers were arranged with a carefully calculated 58-degree twist between them, creating a superlattice pattern.
Using their STM imaging technique, the researchers then introduced electrons into the tWS2 moiré superlattice. At very low temperatures, the electrons filled each unit cell of the material’s superlattice, with only two or three electrons occupying each cell, each cell spanning roughly 10 nanometers. Surprisingly, these localized electrons didn’t just settle into a simple lattice. Instead, they paired up or grouped, forming an array of electron “molecules” across the superlattice—a structure that constituted the Wigner molecular crystal.
Wang elaborated on this process, explaining that the low temperatures, combined with the unique properties of the tWS2 moiré superlattice, created a potential landscape that confined the electrons to localized positions. “The interplay between quantum mechanics and the electron-electron interaction drives the localized electrons into Wigner molecule states,” he explained. This confinement and electron arrangement showcase how quantum effects and inter-electron repulsion can stabilize new, complex structures even within tiny areas.