The history of scientific research has witnessed instances where two scientists arrived at the same conclusion without being aware of each other’s achievements. In the summer of 2021, we once again observed a rare event. However, this time, thanks to global connectivity, three different research teams were able to compare their results and quickly reach a common conclusion.
Three different research teams were all striving to create crystals from charged electron particles, and one of them succeeded by chance. The breakthrough occurred when researchers attempted to apply a semiconductor layer with a thickness comparable to that of an atom, cooled down to near absolute zero.
One team, led by two researchers from Harvard University, Hongkun Park and Eugene Demler, discovered that when a certain number of electrons were present in the semiconductor device, the electrons would remain still “mysteriously.” Noticing similarities between this phenomenon and the Wigner crystal (a concept that only existed on paper, theorized by physicist Eugene Wigner), the research group delved deeper into this strange occurrence. According to old texts, Professor Wigner noted that with the push from electrostatics, electrons arranged in a thin layer would combine into a lattice structure with triangular nodes.
The team of researchers Park and Demler was not alone in this arduous journey.
“One team, consisting of theoretical physicists led by Eugene Demler from Harvard University […] has theorized the effects that will appear when observing the excitation frequency of the exciton state – which is exactly what we observed in the laboratory,” said Ataç Imamoğlu, a researcher at ETH Zurich.
Imamoğlu discussed their findings in the study of excitons – the bound state of an electron with an electron hole (a concept describing a state of electron deficiency at a point where an atom or crystal structure composed of atoms can exist). Imamoğlu’s group also applied techniques described in the literature on how to form Wigner crystals.
Illustration from researchers at ETH Zurich, indicating electrons in a chaotic solution and how the Wigner crystal structure would appear if formed.
Based on the ability to push other particles, it can be said that electrons operate similarly to magnetic poles. In a solid, electrons can help create repeating crystal structures. However, the story changes when electrons exist in a liquid. This is because electrons in a liquid are very susceptible to influence; they change states with even the slightest perturbation.
To keep electrons in a stable state, the conditions influencing them must reach perfection. First, a lower number of electrons makes the experiment somewhat easier. Furthermore, when a certain number of electrons are present, scientists can easily arrange them into a perfect lattice structure.
There is still a third group conducting research on semiconductors with a thickness of one atom. With the involvement of author Feng Wang from UC Berkeley, this group also sought to arrange electrons into a crystal structure, although the distance between the electrons in this study was greater than in the previous two efforts.
Then there is the temperature factor. As temperatures drop, the movement of particles slows down. When approaching absolute zero, electrons no longer “run wild” but instead remain nearly fixed in their predetermined positions. This is when quantum effects emerge, filling in behaviors that belong to classical mechanics. Even in a liquid environment, electrons still exhibit particle-like properties. When temperatures are sufficiently low, keeping electrons in place becomes easier, and with a sufficient number of electrons, they naturally arrange themselves in an orderly manner.
Red caps indicate the Wigner crystal state of electrons existing in a layer of semiconductor material (green and gray).
Electrons are units related to electricity, from which we can infer that a cluster of electrons will form a conductive mass. However, the concept of the Wigner crystal indicates a different reality: electricity arises from the movement of electrons rather than their mere presence. When electron particles fit snugly into a lattice, they have little space to move and generate electricity. This electron structure functions like a true insulator.
This is why researchers know they have successfully created a crystal made from electrons. Almost fixed in their positions, the electron structure does not fulfill the role of a semiconductor; instead, it can insulate.
Quantum fluctuations occurring at temperatures near absolute zero have caused a quantum state transition, transforming a freely flowing solution into a quantum crystal, becoming a Wigner crystal. Scientists believe that this quantum state transfer plays a crucial role in many future quantum systems.
Physicist Eugene Wigner.
When the research team at Harvard realized they had a Wigner crystal, they attempted to melt the quantum structure (melting the structure at the microscopic scale) of the entire system to see what would happen.
All activities described in the research reports occurred at a scale so small that no optical microscope could observe them; scanning tunneling microscopy (STM), specialized for observing atomic-sized materials, would damage the crystal structure. Wang’s research team could observe most clearly by placing a one-atom-thick layer of graphene on the crystal structure. The Wigner crystal had a slight impact on the electron structure of the graphene layer, enough for the STM to observe the underlying structure.
To confirm that they had created a Wigner crystal, the team had to use photons to blast the electron particles apart, creating an exciton observable with specialized microscopy.
According to researcher Demler’s announcement, the new breakthrough “approaches the transition from quantum material to semi-classical material, which will produce many strange and interesting phenomena and properties.” Time will reveal what these phenomena and properties are, and whether they can be applied to future quantum systems.
