Physicists discover an exotic material made of bosons

Physicists discover an exotic material made of bosons

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Bosonic related insulator.(A) Illustration of a bosonic correlated insulator made of interlayer excitons. Magenta spheres indicate holes and cyan spheres, electrons. (Inset) WSe type II band alignment2/WS2 heterostructure. (b) Schematics of CW pump probe spectroscopy. The exciton and electron density are independently controlled by the pump light and the electrostatic gate. The red and green shading correspond to wide-field pump light and focused probe light, respectively. (c AND AND) Gate-dependent PL (C) and absorption (E) spectra of a 60-aligned WSe2/WS2 double layer of moir (device D1) at zero pump intensity. The PL peak shows a sudden blue shift upon electron filling And= 1 and 2 (yellow arrows), where the absorption spectrum shows folds and cleavages. (d AND f) Pump intensity-dependent PL (D) and absorption (F) spectra of device D1 at charge neutrality. The right-hand axes show the interlayer exciton energy shift induced by the dipolar interaction dipolewhich is approximately proportional to former. The dominant PL peak in (D) at low and high pump intensities are labeled as peak I and II, respectively. All measurements are performed at a baseline temperature of 1.65 K. Credit: Science (2023). DOI: 10.1126/science.add5574

Take a lattice: A flat section of a grid of uniform cells, such as a window screen or honeycomb, 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 moiré, 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 have discovered a new state of matter, a related bosonic insulator,” said Richen Xiong, a graduate student researcher in UCSB Chenhao Jin’s condensed matter physicist group, and lead author of a paper appearing in the journal Science.

This is the first time such a material, a highly ordered crystal of bosonic particles called excitons, has been created in a ” real” (as opposed to synthetic) system of matter.

“Conventionally, people have spent most of their efforts figuring out what happens when you put lots of fermions together,” Jin said. “The main thrust of our work is that we have essentially created a new material from the interaction of bosons.”

Bosonic, correlated, isolating

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 semi-integer “spins” like 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 (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 strong 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’ moiré 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 could be possible not only to study well-known bosonic particles such as excitons, but also to open more windows on 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.”

More information:
Richen Xiong et al, Correlated exciton insulator in WSe 2 /WS 2 moir superlattices, Science (2023). DOI: 10.1126/science.add5574

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