Under normal circumstances, the electrons in a material behave like a disordered liquid. But in 1934, physicist Eugene Wigner made a theoretical prediction based on quantum mechanics. He proposed that when the kinetic energy and density of electrons in metals can be reduced to a sufficiently low level, electrons in orbit are "frozen", and in this strange electronic state, the mutual repulsive forces between electrons cause them to spontaneously form an orderly arrangement, forming a hard, insulated crystal structure called Wigner crystal.
Electrons in materials typically behave as disordered liquids (left), but regular Wigner crystals (right) can be formed under certain conditions. | Image credit: ETH Zurich
Since then, this prediction has been seen as the holy grail of condensed matter physics. However, Wigna crystals can only be formed under extreme conditions. This created a barrier to the realization of such crystals in reality, which was not first observed in reality until 1979. In past studies, scientists have found this structure in magnetic fields, where kinetic energy can be artificially suppressed.
Now, using more advanced technology, the two scientific groups independently observed more convincing results.
In a new study, researchers from the Federal Institute of Technology Zurich (ETH) conducted a series of studies using the semiconductor material molybdenum diselenide (MoSe₂). The molybdenum diselenide used in the experiment is a single layer of thin sheets that are only atomically thick, so electrons can only move in one of them. In the experiment, molybdenum diselenide is sandwiched between two graphene electrodes, and the researchers can vary the number of free electrons by applying a voltage to the graphene.
Based on the theory, the researchers predicted that molybdenum diselenide in the experiment should form Vigna crystals when cooled to only a few Kelvin. They also calculated that the distance between the electrons in this Vigna crystal is about 20 nanometers, which is about 1/30 of the wavelength of visible light. Therefore, even with the best microscope, it is impossible to distinguish such a structure.
To see these regularly arranged electrons, they employed a technique called exciton umklapp spectroscopy. Excitons are important quasiparticles, and in semiconductors, when negatively charged electrons are excited by photons, they jump from low to high, leaving a positively charged hole. Excitons refer to the bound state formed by electrons and holes that attract and orbit each other. (Further reading about the exciton: "Waiting for nearly a century, this is its first image")
In the experiments, they used light of a specific frequency to excite excitons in semiconductors, and the precise frequency of the light producing excitons depended both on the nature of the material itself and on the exciton's interaction with other electrons in the material—such as with Wigner crystals. If these electrons have translational invariance, then the excitons "can't see" the lattice; but if these electrons have formed a lattice, then the excitons scatter.
Vigna crystals consisting of electrons (red) within semiconductor materials (blue and gray spheres). | Image credit: ETH Zurich
In this way, the researchers confirmed that Wigner crystals form approximately when the temperature drops to 11 Kelvin. They succeeded in making this particular crystal, which is composed entirely of electrons, and was the first example to directly confirm the regular arrangement of electrons in a crystal.
Although Wigner crystals can already be observed experimentally, there are many mysteries they hide, such as Wigner crystals can be melted by heat or quantum fluctuations. This phase transition coincides with the transition of matter from part of quantum material to part of classical material, with many unusual phenomena and properties. For physicists, they have been unable to understand how crystal states become liquid due to quantum fluctuations.
In another study, a team of researchers at Harvard University tried to record this phase transition experimentally. We know that in chemistry, physics, and thermodynamics, phase transitions occur when the solid, liquid, or gaseous state of a substance changes. At near absolute zero, these changes driven by quantum fluctuations are called quantum phase transitions. This quantum phase transition is thought to play an important role in many quantum systems.
The researchers used exciton spectroscopy to capture this phase transition. In general, Wigner crystals require very low electron density, which is a major challenge in experiments. In some earlier theoretical papers, scientists suggested that a bilayer structure might help stabilize Wigner crystals. So in the experiment, the researchers observed the transition of bilayer molybdenum diselenide from solid to liquid.
Schematic of a quantum phase transition from an electron liquid to a bilayer Vigner crystal, where each sphere represents an electron. | Image credit: Ella Maru Studio in collaboration with Hongkun Park and You Zhou
They built a device that could dop the top and bottom layer of molybdenum diselenide differently. They found that when electrons were doped with the upper and lower layers at specific density ratios (e.g., 1:1, 3:1, 4:1, 7:1), they could unexpectedly form an insulating state. The discovery was largely accidental, as they discovered double-layerEd Vigna crystals without the use of magnetic fields. Compared to the single-layer Wigner crystals observed by the ETH research team, this double-layer Wigner crystal can withstand temperatures of up to 40 Kelvin and higher electron densities.
Both studies were published in the journal Nature on June 30, marking a major step toward physicists toward creating a system that studies the transitions between quantum-level states of matter. Both research groups hope to be able to observe in more detail the phase transition between normal matter and this exotic state. The ETH researchers believe that there must be other phases between these two phases, and these phases have not been explored at all. The Harvard researchers were very surprised by the results and said they would continue to use this method to study other quantum phase transitions.
#创作团队:
Text: Light rain
#原文来源:
https://ethz.ch/en/news-and-events/eth-news/news/2021/07/a-crystal-made-of-electrons.html
https://news.harvard.edu/gazette/story/2021/06/study-marks-major-step-to-creating-a-system-to-study-quantum-phase-transitions/
https://www.chemistryworld.com/news/best-sighting-yet-of-exotic-crystals-composed-entirely-of-electrons/4013942.article
#图片素材来源:
封面图：Ella Maru Studio in collaboration with Hongkun Park and You Zhou