Take a lattice a flat section of a grid of uniform cells, such as a screen door or honeycomb, and place another similar lattice on top of it. But instead of trying to line up the edges or cells of both grids, he flips the top grid so you can see parts of the bottom one through it. This new, third pattern is a moir, and it’s among this kind of overlapping arrangement of tungsten diselenide and tungsten disulfide lattices where physicists at UC Santa Barbara have found some interesting material behavior.
We’ve discovered a new state of matter, a bosonic related insulator, said Richen Xiong, a graduate student researcher in UCSB’s condensed matter physicist Chenhao Jin’s group and lead author of a paper in the journal Science. According to Xiong, Jin and collaborators at UCSB, Arizona State University and the National Institute for Materials Science in Japan, this is the first time such a material has been created in a real (and non-synthetic) matter system. The unique material is a highly ordered crystal of boson particles called excitons.
Conventionally, people have spent most of their efforts figuring out what happens when you put lots of fermions together, Jin said. The main focus of our work is that we have basically created a new material from the interaction of bosons.
bosonic. Related. Insulating.
Subatomic particles come in one of two broad types: Fermions and Bosons. One of the biggest distinctions is in their behavior, Jin said.
Bosons can occupy the same energy level; Fermions don’t like being together, he said. Together, these behaviors build the universe as we know it.
Fermions, like electrons, are the basis of the matter we are most familiar with as they are stable and interact through electrostatic force. Meanwhile, bosons, like photons (particles of light), tend to be harder to create or manipulate since they’re fleeting or don’t interact with each other.
One clue to their distinct behaviors is in their different quantum mechanical characteristics, Xiong explained. Fermions have half-integer spins such as 1/2 or 3/2, while bosons have integer spins (1, 2, etc.). An exciton is a state in which a negatively charged electron (a fermion) is bonded to its opposite positively charged hole (another fermion), with the two half-integer spins together becoming an integer, creating a boson particle.
To create and identify excitons in their system, the researchers layered up the two gratings and shone them with bright lights in a method they call pump probe spectroscopy. The combination of particles from each of the lattices (tungsten disulfide electrons and tungsten diselenide holes) and light created a favorable environment for exciton formation and interactions, allowing the researchers to probe the behaviors of these particles.
And when these excitons reached a certain density, they couldn’t move anymore, Jin said. Thanks to strong interactions, the collective behaviors of these particles at a certain density forced them into a crystalline state and created an insulating effect due to their immobility.
What happened here is that we discovered the correlation that pushed the bosons into a highly ordered state, Xiong added. Generally, a loose collection of bosons at ultracold temperatures will form a condensate, but in this system, with both light and increased density and interaction at relatively higher temperatures, they have organized into a symmetrical solid and a neutrally charged insulator.
The creation of this exotic state of matter demonstrates that the researchers’ moir platform and pump probe spectroscopy could become an important means of creating and studying bosonic materials.
There are many-body phases with fermions that result in things like superconductivity, Xiong said. There are also many-body counterparts with bosons that are also exotic phases. So what we’ve done is build a platform, because we didn’t really have a great way to study bosons in real materials. While excitons are well studied, she added, until this project there hadn’t been a way to get them to interact strongly with each other.
With their method, according to Jin, it may be possible not only to study well-known bosonic particles such as excitons, but also to open more windows into the world of condensed matter with new bosonic materials.
We know that some materials have very bizarre properties, he said. And one of the goals of condensed matter physics is to understand why they have these rich properties and to find ways to bring out these behaviors more reliably.
Reference: Correlated exciton insulator in WSe2/WS2 moir superlattices by Richen Xiong, Jacob H. Nie, Samuel L. Brantly, Patrick Hays, Renee Sailus, Kenji Watanabe, Takashi Taniguchi, Sefaattin Tongay, and Chenhao Jin, May 11, 2023, Science.
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