“When you believe of a crystal, you normally think of a tourist attraction in between atoms as a stabilizing force, but this crystal forms purely due to the fact that of the repulsion between electrons,” said Yazdani, who is the inaugural co-director of the Princeton Quantum Institute and director of the Princeton Center for Complex Materials.The video describes melting processes of an electron Wigner crystal into electron liquid phases. As the electron density (nu, a step of number of electrons in a magnetic field, is controlled by using electrical voltages) is increased, more electrons (dark blue websites) go into the field of view, and a routine structure of a triangular lattice emerges. Credit: Yen-Chen Tsui, Princeton UniversityAdvancements in Electron Crystal ResearchFor a long time, nevertheless, Wigners odd electron crystal remained in the world of theory. The very first of these was performed in the 1970s when researchers at Bell Laboratories in New Jersey produced a “classical” electron crystal by spraying electrons on the surface of helium and found that they reacted in a rigid manner like a crystal. “If there are any imperfections, or some type of periodic base in the product, it is possible to trap electrons and discover speculative signatures that are not due to the development of a self-organized bought Wigner crystal itself, however due to electrons stuck near a flaw or caught since of the materials structure,” he said.With these considerations in mind, Yazdani and his research group set about to see whether they could straight image the Wigner crystal using a scanning tunneling microscopic lense (STM), a gadget that relies on a method called “quantum tunneling” rather than light to see the subatomic and atomic world.
A picture of a triangular Wigner crystal taken by scanning tunneling microscope. Scientists have unveiled an elusive crystal that is formed simply from the repulsive nature of electrons. Each website (blue circular region) consists of a single localized electron. Credit: Yen-Chen Tsui and team, Princeton UniversityPrinceton University scientists discover an odd form of matter that has avoided direct detection for some 90 years.Electrons– these infinitesimally small particles that are understood to zip around atoms– continue to surprise researchers regardless of the more than a century that scientists have studied them. Now, physicists at Princeton University have pushed the limits of our understanding of these minute particles by picturing, for the very first time, direct proof for what is known as the Wigner crystal– an odd type of matter that is made entirely of electrons.The finding, released in the April 11th concern of the journal Nature, verifies a 90-year-old theory that electrons can put together into a crystal-like development of their own, without the need to coalesce around atoms. The research study could assist lead to the discovery of brand-new quantum stages of matter when electrons behave collectively.Theoretical Insights and Early Experiments”The Wigner crystal is among the most remarkable quantum phases of matter that has actually been anticipated and the subject of various research studies claiming to have actually discovered at finest indirect proof for its formation,” stated Al Yazdani, the James S. McDonnell Distinguished University Professor in Physics at Princeton University and the senior author of the study. “Visualizing this crystal permits us not just to view its formation, verifying numerous of its homes, however we can likewise study it in ways you could not in the past.”In the 1930s, Eugene Wigner, a Princeton professor of physics and winner of the 1963 Nobel Prize for his work in quantum balance concepts, composed a paper in which he proposed the then-revolutionary idea that interaction amongst electrons could result in their spontaneous arrangement into a crystal-like setup, or lattice, of carefully jam-packed electrons. This could only take place, he theorized, because of their mutual repulsion and under conditions of low densities and extremely cold temperature levels.”When you think about a crystal, you typically think of an attraction between atoms as a stabilizing force, but this crystal forms purely because of the repulsion between electrons,” stated Yazdani, who is the inaugural co-director of the Princeton Quantum Institute and director of the Princeton Center for Complex Materials.The video describes melting procedures of an electron Wigner crystal into electron liquid stages. As the electron density (nu, a step of variety of electrons in an electromagnetic field, is controlled by applying electric voltages) is increased, more electrons (dark blue websites) go into the field of view, and a regular structure of a triangular lattice emerges. The routine structure is first melted (near nu = 0.334) where the map reveals uniform signals. And then it reappears at higher density nu, and ultimately melts again (nu = 0.414). Credit: Yen-Chen Tsui, Princeton UniversityAdvancements in Electron Crystal ResearchFor a very long time, however, Wigners odd electron crystal remained in the world of theory. It was not until a series of much later experiments that the idea of an electron crystal changed from conjecture to reality. The first of these was conducted in the 1970s when researchers at Bell Laboratories in New Jersey developed a “classical” electron crystal by spraying electrons on the surface of helium and discovered that they reacted in a rigid way like a crystal. Nevertheless, the electrons in these experiments were very far apart and acted more like private particles than a cohesive structure. A true Wigner crystal, instead of following the familiar laws of physics in the everyday world, would follow the laws of quantum physics, in which the electrons would act not like individual particles however more like a single wave.This resulted in an entire series of experiments over the next decades that proposed numerous ways to produce quantum Wigner crystals. These experiments were greatly advanced in the 1980s and 1990s when physicists found how to confine electrons movement to atomically thin layers utilizing semiconductors. The application of a magnetic field to such layered structures also makes electrons move in a circle, developing favorable conditions for formation. These experiments were never ever able to directly observe the crystal. They were only able to recommend its presence or indirectly infer it from how electrons flow through the semiconductor.Breakthrough in Direct Imaging”There are actually numerous scientific documents that study these effects and claim that the outcomes should be due to the Wigner crystal,” Yazdani stated, “but one cant make certain, due to the fact that none of these experiments actually see the crystal.”An equally essential factor to consider, Yazdani kept in mind, is that what some researchers believe is evidence of a Wigner crystal might be the outcome of flaws or other routine structures intrinsic to the materials used in the experiments. “If there are any flaws, or some form of regular base in the material, it is possible to trap electrons and find experimental signatures that are not due to the development of a self-organized purchased Wigner crystal itself, but due to electrons stuck near an imperfection or trapped because of the materials structure,” he said.With these factors to consider in mind, Yazdani and his research study team gone about to see whether they might directly image the Wigner crystal using a scanning tunneling microscopic lense (STM), a gadget that counts on a method called “quantum tunneling” rather than light to see the subatomic and atomic world. They likewise decided to use graphene, an incredible product that was discovered in the 21st century and has been utilized in numerous experiments including unique quantum phenomena. To effectively conduct the experiment, nevertheless, the researchers had to make the graphene as beautiful and as lacking imperfections as possible. Since of material imperfections.Unveiling the Quantum NatureThe outcomes were impressive, this was essential to eliminating the possibility of any electron crystals forming. “Our group has actually had the ability to make unprecedentedly clean samples that made this work possible,” Yazdani said. “With our microscope we can confirm that the samples are without any atomic imperfection in the graphene atomic lattice or foreign atoms on its surface over regions with numerous thousands of atoms.”To make the pure graphene, the researchers exfoliated two carbon sheets of graphene in a configuration that is called Bernal-stacked bilayer graphene (BLG). They then cooled the sample to exceptionally low temperatures– simply a portion of a degree above outright no– and used a magnetic field perpendicular to the sample, which developed a two-dimensional electron gas system within the thin layers of graphene. With this, they could tune the density of the electrons in between the two layers.”In our experiment, we can image the system as we tune the number of the electrons per system location,” said Yen-Chen Tsui, a graduate trainee in physics and the very first author of the paper. “Just by altering the density, you can start this phase shift and discover electrons spontaneously form into a purchased crystal.”Exploring the Crystal Structure and Its DynamicsThis happens, Tsui discussed, since at low densities, the electrons are far apart from each other– and theyre located in a disordered, chaotic fashion. However, as you increase the density, which brings the electrons closer together, their natural repulsive tendencies kick in and they start to form an organized lattice. As you increase the density even more, the crystalline phase will melt into an electron liquid.Minhao He, a postdoctoral researcher and co-first author of the paper, discussed this process in higher detail. “There is an intrinsic repulsion in between the electrons,” he said. “They desire to push each other away, but in the meantime the electrons can not be definitely apart due to the limited density. The outcome is that they form a closely packed, regularized lattice structure, with each of the localized electron inhabiting a particular quantity of space.”When this transition formed, the researchers were able to visualize it utilizing the STM. “Our work offers the first direct pictures of this crystal. We showed the crystal is truly there and we can see it,” said Tsui.Future Directions in Wigner Crystal ResearchHowever, simply visualizing the crystal wasnt completion of the experiment. A concrete image of the crystal allowed them to identify a few of the crystals characteristics. They discovered that the crystal is triangular in configuration, and that it can be continuously tuned with the density of the particles. This led to the realization that the Wigner crystal is actually rather stable over a long variety, a conclusion that contrasts what many researchers have actually assumed.”By being able to constantly tune its lattice constant, the experiment showed that the crystal structure is the result of the pure repulsion in between the electrons,” said Yazdani.The scientists also found several other fascinating phenomena that will no doubt warrant further examination in the future. They found that the area to which each electron is localized in the lattice appears in the images with a certain quantity of “blurring,” as if the place is not specified by a point but a range position in which the electrons are restricted in the lattice. The paper explained this as the “zero-point” movement of electrons, a phenomenon associated to the Heisenberg uncertainty concept. The extent of this blurriness reflects the quantum nature of the Wigner crystal.”Electrons, even when frozen into a Wigner crystal, should show strong zero-point motion,” stated Yazdani. “It turns out this quantum movement covers a 3rd of the range between them, making the Wigner crystal an unique quantum crystal.”Yazdani and his team are also taking a look at how the Wigner crystal melts and transitions into other exotic liquid phases of communicating electrons in an electromagnetic field. The researchers wish to image these stages just as they have imaged the Wigner crystal.Reference: “Direct observation of a magnetic-field-induced Wigner crystal” by Yen-Chen Tsui, Minhao He, Yuwen Hu, Ethan Lake, Taige Wang, Kenji Watanabe, Takashi Taniguchi, Michael P. Zaletel and Ali Yazdani, 10 April 2024, Nature.DOI: 10.1038/ s41586-024-07212-7Graduate student Yen-Chen Tsui, postdoctoral research study partner Minhao He, and Yuwen Hu, who got her Ph.D. from the Princeton Department of Physics in 2023 and is now a postdoctoral fellow at Stanford, all contributed equivalent to this work. Other collaborators consist of, at the University of California-Berkeley, theoretical physicists Ethan Lake, Taige Wang, and Professor Michael Zaletel (likewise a member of the Material Science Division at Lawrence Berkeley National Laboratory), and Kenji Watanabe and Takashi Taniguchi from National Institute for Materials Science and International Center for Materials Nanoarchitectonics, respectively.The work at Princeton was mostly supported by DOE-BES grant DE-FG02-07ER46419 and the Gordon and Betty Moore Foundations EPiQS effort grants GBMF9469. Other assistance for the experimental infrastructure at Princeton was supplied by NSF-MRSEC through the Princeton Center for Complex Materials NSF6 DMR- 2011750, ARO MURI (W911NF-21-2-0147), and ONR N00012-21-1-2592. The group also acknowledges the hospitality of the Aspen Center for Physics, which is supported by National Science Foundation grant PHY-1607611, where part of this work was performed. Work at UC Berkeley was supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under Contract No. DE-AC02-05CH11231, within the van der Waals Heterostructures Program (KCWF16).
