researchers at Northeastern University have accidentally identified a new configuration where layers of electrons form a uniform lattice.
….Professor Kar says he thought these results were a measurement error at first. His research team layered two “sheets” of particles: one of bismuth selenide and one of transition metal dichalcogenide. Both are called 2D materials because they’re so nano-flat to almost be nonexistent—the closest real thing to a pure geometric plane.
The researchers layered these two sheets together and observed what they thought was a third layer between them. But like the famous grid illusion, Kar and his team thought they were seeing a visual artifact of some kind—an imprecise instrument at the nano scale. They reran the experiment to make sure their synthesis or measurement wasn’t introducing an error.
“Have you ever walked into a meadow and seen an apple tree with mangoes hanging from it? Of course we thought something was wrong. This couldn’t be happening,” Kar says in the statement. But when repeat tests showed the same third layer, the team looked closer. “At certain angles, these materials seem to form a way to share their electrons that ends up forming this geometrically periodic third lattice: A perfectly repeatable array of pure electronic puddles that resides between the two layers,” Kar explains.
Indeed, it turns out the phenomenon is made by a surprising arrangement of the electrons from both layers. The electrons were lining up and staying put in a regularly repeating stationary lattice. By observing with particle microscopes, the research team could watch the resulting scatter of light and use that to reverse-engineer the lattice pattern. These lattice electrons are the “puddles” Kar described.
Physicists may have accidentally discovered a new state of matter
“I’m tempted to say it’s almost like a new phase of matter,” Kar says. “Because it’s just purely electronic.”
The phenomenon appeared while the researchers were running experiments with crystalline materials that are only a few atoms thick, known as 2-D materials. These materials are made up of a repeating pattern of atoms, like an endless checkerboard, and are so thin that the electrons in them can only move in two dimensions.
Stacking these ultra-thin materials can create unusual effects as the layers interact at a quantum level.
Kar and his colleagues were examining two such 2-D materials, bismuth selenide and a transition metal dichalcogenide, layered on top of each other like sheets of paper. That’s when things started to get weird.
Electrons should repel one another—they’re negatively charged, and move away from other negatively charged things. But that’s not what the electrons in these layers were doing. They were forming a stationary pattern.
“At certain angles, these materials seem to form a way to share their electrons that ends up forming this geometrically periodic third lattice,” Kar says. “A perfectly repeatable array of pure electronic puddles that resides between the two layers.”
hm, here is a separate finding. are these related, though?
Researchers from the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg, the RWTH Aachen University (both in Germany) and the Flatiron institute in the U.S. have revealed that the possibilities created by stacking two sheets of atomically thin material atop each other at a twist are even greater than expected.
The four scientists examined germanium selenide (GeSe), a material with a rectangular unit cell, rather than focusing on lattices with three- or six-fold symmetries like graphene or WSe2. By combining large scale ab-initio and density matrix renormalization group calculations, the researchers showed that the Moiré interference pattern will create parallel wires of correlated one-dimensional systems. Their work has now been published in Nature Communications.
hm, yet another finding. is this also related?
Unique material could unlock new functionality in semiconductors
If new and promising semiconductor materials are to make it into our phones, computers, and other increasingly capable electronics, researchers must obtain greater control over how those materials function.
In an article published today in Science Advances, Rensselaer Polytechnic Institute researchers detailed how they designed and synthesized a unique material with controllable capabilities that make it very promising for future electronics.
The researchers synthesized the material—an organic-inorganic hybrid crystal made up of carbon, iodine, and lead—and then demonstrated that it was capable of two material properties previously unseen in a single material. It exhibited spontaneous electric polarization that can be reversed when exposed to an electric field, a property known as ferroelectricity. It simultaneously displayed a type of asymmetry known as chirality—a property that makes two distinct objects, like right and left hands, mirror images of one another but not able to be superimposed.
According to Jian Shi, an associate professor of materials science and engineering at Rensselaer, this unique combination of ferroelectricity and chirality is advantageous. When combined with the material’s conductivity, both of these characteristics can enable other electrical, magnetic, or optical properties.
Therefore, the ability to create many parallel Moiré wires with Majoranas attached at their ends reveals an intriguing future inroad for unlocking topological quantum computing in a naturally scalable way. Ángel Rubio, the director of the MPSD’s Theory department, concludes: “The present work provides valuable insights into how twisting 2-D materials can be used to create properties on demand in quantum materials.”
Plasmons are strongly affected by the geometry of their host materials, which makes them very tunable for different applications. However, it was not clear how plasmons behave in an extreme case: when materials are just a couple of atoms thick.
The international research team consisting of Felipe da Jornada and Steven Louie from the LBNL at the University of California, Berkeley, and Lede Xian and Ángel Rubio from the MPSD, which is based at the Center for Free-Electron Laser Science (CFEL), wanted to shed new light on the properties of plasmons in these novel, atomically thin materials.
Using parameter-free quantum calculations, they found that plasmons behave in a peculiar way in allatomically thin materials. This was initially a surprise to the authors: “Textbook physics says that plasmons in bulk materials behave in one way, and in strictly two-dimensional materials, in another way. But unlike these simplified models, plasmons in all real, atomically thin materials behave yet differently and tend to be much more localizable in space,” says Felipe Jornada, who is now based at Stanford University.
The reason for this difference, Steven Louie argues, is that “in real atomically thin materials, all the other electrons that are not conducting and oscillating can screen these plasmons, which leads to a fundamentally different dispersion relation for these excitations.”
Other key findings of their research are that the plasmons in systems such as monolayer TaS2 can remain stable for long times (~ 2 ps) and are virtually dispersionless for wavevectors which are commonly used in certain experiments. This indicates that plasmons in atomically thin materials are localizable in real space with available experimental techniques and could significantly enhance the intensity of light by a factor of more than 107 .
Now, researchers from Northeastern University have made a discovery that opens up a whole new field exploring materials for transistors, photodetectors, flexible electronics, and other applications.
The work—published recently in the journal Science Advances — involves 2-D crystals, which are super-thin materials only a few atoms tall. Combining two 2-D crystals forms a heterostructure. Until now, physicists thought 2-D crystals had to be very similar, with all the atoms to lining up perfectly, in order to form a new heterostructure.
“But nature always throws a curveball at you,” says Arun Bansil, University Distinguished Professor of Physics and one of the paper’s authors. They observed for the first time that two completely dissimilar 2-D crystals can be arranged one on top of the other, atom by atom, in such a way that they fit together nearly perfectly and produce completely new properties.
“It would be like making a club sandwich,” Kar said. “You can have something that tastes like bread and something that tastes like meat.”
But the key, Bansil explains, is not just to assemble a sandwich where you can taste each layer separately. “You want to have some cooking going on so you can get some new flavors.”
In the world of condensed matter physics, discovering that two very different 2-D crystals can form a heterostructure is like combining water and flour for the first time and creating dough. It gives way to virtually limitless possibilities for new 2-D materials.