This is the first time that a highly ordered crystal of bosonic particles called excitons has been created in a system of real, non-synthetic matter.

Xiong et al.  observed a correlated exciton insulator interlayered in a bilayer consisting of tungsten diselenide, which harbored holes, and tungsten disulfide, which harbored electrons.  Image credit: Xiong et al., doi: 10.1126/science.add5574.

Xiong et al. observed a correlated exciton insulator interlayered in a bilayer consisting of tungsten diselenide, which harbored holes, and tungsten disulfide, which harbored electrons. Image credit: Xiong et al., doi: 10.1126/science.add5574.

Subatomic particles come in one of two broad types: Fermions and Bosons. One of the biggest distinctions is in their behavior.

Bosons can occupy the same energy level; Fermions don’t like being together. 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, 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, said first author Richen Xiong, a graduate student at the University of California at Santa Barbara.

Fermions have semi-integer spins such as 1/2 or 3/2 etc., while bosons have integer spins (1, 2, etc.).

An exciton is a state in which a negatively charged electron (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, Xiong and colleagues 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, said senior author Dr. Chenhao Jin, a physicist at the University of California at Santa Barbara.

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 said.

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 explained.

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, until this project there hadn’t been a way to get them to interact strongly with each other.

With the teams’ approach, 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, Dr. Jin 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.

The work appears in the magazine Science.

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Richen Xiong et al. 2023. Correlated exciton insulator in WSe2/WS2 moir superlattices. Science 380 (6647): 860-864 ; doi: 10.1126/science.add5574

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