Physicists stumble upon matter made of bosons

Take a grid—a flat section of a grid of regular cells, such as a window screen or honeycomb—and place another similar grid on top of it. But instead of trying to align the edges or cells of each of the gratings, give the top grid a twist so that you can see parts of the bottom grid through. This new third pattern is moiré, and it’s between this kind of nested arrangement of tungsten diselenide and tungsten disulfide lattices that UC Santa Barbara physicists have found some interesting material behaviour.

“We discovered a new state of matter — a coherent bosonic insulator,” said Richen Xiong, a graduate student researcher in Chenhao Jin’s group, a condensed matter physicist at UCLA and lead author of a paper in the journal Science. According to Xiong, Jin, and collaborators from UCSB, Arizona State University, and the National Institute of Materials Science in Japan, this is the first time such materials have been created in a “real” (as opposed to synthetic) material system. The unique substance is a highly ordered crystal of boson particles called excitons.

“Traditionally, people have put most of their effort into understanding what happens when you put many fermions together,” said Jin. “The main driver of our work is that we have essentially made a new material from interacting bosons.”

Two stacked slightly offset create a new pattern called moire. Credit: Matt Bierko

Bosonic. threaded. insulator.

Subatomic particles come in one of two broad types: fermions and bosons. One of the biggest differences is in their behaviour, Jin said.

“Bosons can occupy the same energy level; fermions don’t like staying together,” he said. Together, these behaviors build the universe as we know it.”

Fermions, like electrons, underlie the matter we are most familiar with because they are stable and interact through the electrostatic force. Meanwhile, bosons, like photons (particles of light), tend to be more difficult to create or manipulate because they are either transient or do not interact with each other.

The evidence for their distinctive behavior, Xiong explained, lies in their different quantum mechanical properties. Fermions have half integer “spins” like 1/2 or 3/2, while bosons have whole integer spins (1, 2, etc). An exciton is a state in which a negatively charged electron (a fermion) is bound to its corresponding positively charged “hole” (another fermion), spinning half an integer together to become an integer, resulting in a boson.

Jin’s lab, left to right: Tian Shi, Riqin Xiong, Chenhao Jin, Samuel L Brantley. credit
Sonia Fernandez

To create and identify the excitons in their system, the researchers layered the two lattices and shone strong lights on them with a method they call “pump spectroscopy.” The collection of particles from both lattices (electrons from tungsten disulfide and holes from tungsten dioxide) and light created an environment conducive to the formation of excitons and the interactions between them while allowing researchers to probe the behavior of these particles.

“And when these excitons reached a certain density, they could no longer move,” Jin said. Thanks to the 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 stability.

“What happened here is that we discovered the relationship that drove the bosons into a higher-order state,” Xiong added. In general, a loose collection of bosons under very cold temperatures would form a condenser, but in this regime, with both light and increased density and interaction at relatively higher temperatures, they organized themselves into a symmetrically charged, neutrally solid insulator.

The creation of this exotic state of matter proves that the researchers’ platform for moiré and pump spectroscopy can become an important means for creating and investigating bosonic materials.

“There are many body phases with fermions that lead to things like superconductivity,” Xiong said. There are also many bodies similar to bosons that are also exotic phases. So what we did was create a platform, because we really didn’t have a great way to study bosons in real materials.” He added that while excitons were well studied, not even this project had a way to convince them to interact strongly with each other.

Through their method, according to Jain, it may be possible not only to study well-known bosonic particles such as excitons, but also to open more windows into the condensed matter world with new bosonic materials.

“We know that some materials have very strange properties,” he said. “One of the goals of condensed matter physics is to understand why it has such rich properties and to find ways to make these behaviors appear more reliably.”

Reference: “Coherent insulator of excitons in WSe₂/ W.S₂ Super Moire” by Richen Xiong, Jacob H.Ni, Samuel L. Brantley, Patrick Hayes, Renee Silos, Kenji Watanabe, Takashi Taniguchi, Svaten Tongai, and Chenhao Jin May 11, 2023, Available Here. Sciences.
DOI: 10.1126/science.add5574