Exotic Matter Uncovered: Physicists Stumble Upon a Material Composed of Bosons

Physicists Unearth Exotic Matter: A Material Comprised of Bosons Revealed

A groundbreaking finding has emerged from the scientific community as researchers have stumbled upon a new state of matter called a “bosonic correlated insulator.” This intriguing discovery is the result of interactions among bosonic particles known as excitons, and it holds immense potential for advancing our knowledge of condensed matter physics and fostering the development of novel bosonic materials.

To comprehend this breakthrough, envision a lattice—a flat, uniform grid comprising cells—similar to a window screen or a honeycomb. Now, overlay a similar lattice atop the first one, but instead of aligning the edges or cells precisely, twist the top grid in a way that allows portions of the lower grid to be visible through it. This overlapping arrangement of lattices, called a moiré, served as the backdrop for the UC Santa Barbara physicists’ investigation into fascinating material behaviors.

Richen Xiong, a graduate student researcher in the group of Chenhao Jin, a condensed matter physicist at UCSB, and the lead author of a paper published in the journal Science, elucidated, “We have made an extraordinary finding—a bosonic correlated insulator.” According to Xiong and his colleagues from UCSB, Arizona State University, and the National Institute for Materials Science in Japan, this is the first instance of creating such a material in a “real” matter system, as opposed to a synthetic one. This exceptional material takes the form of a meticulously ordered crystal consisting of bosonic particles known as excitons.

Traditionally, most scientific endeavors have focused on understanding the behavior of fermions, the category to which subatomic particles like electrons belong. In contrast, the principal objective of this research is centered around fabricating a new material from interacting bosons.

The terms “Bosonic. Correlated. Insulator.” encapsulate the essence of this discovery.

Subatomic particles fall into two main types: fermions and bosons. Jin explained that their behavior is where a significant distinction lies. “Bosons can occupy the same energy level; fermions don’t prefer to coexist,” he said. “These behaviors collectively shape the universe as we know it.”

Fermions, such as electrons, underpin the matter we are most familiar with, as they possess stability and interact through the electrostatic force. Conversely, bosons, like photons (particles of light), tend to be more challenging to create or manipulate due to their ephemeral nature or lack of interaction with one another.

Xiong shed light on the dissimilarities in their quantum mechanical characteristics, providing a clue to their distinct behaviors. Fermions have half-integer “spins,” such as 1/2 or 3/2, while bosons possess whole integer spins (1, 2, etc.). An exciton arises when a negatively charged electron (a fermion) binds to its positively charged counterpart known as a “hole” (another fermion). The combination of these two half-integer spins forms a whole integer spin, thereby creating a bosonic particle.

To generate and identify excitons within their system, the researchers layered the two lattices and illuminated them with intense light using a technique called “pump-probe spectroscopy.” This method facilitated the interaction and formation of excitons while enabling the scientists to investigate their behaviors.

Jin explained, “When these excitons reached a particular density, they became immobile.” Thanks to strong interactions, these particles collectively transitioned into a crystalline state, resulting in an insulating effect due to their inability to move.

Xiong added, “We discovered a correlation that drove the bosons into a highly organized state.” Typically, a loose assembly of bosons under ultracold temperatures would form a condensate. However, in this system, under relatively higher temperatures with increased density and interaction, the bosons self-organized into a symmetric solid, resulting in a charge-neutral insulator.

The creation of this extraordinary state of matter showcases the potential of the researchers’ moiré platform and pump-probe spectroscopy as valuable tools for developing and investigating bosonic materials.

Xiong emphasized, “Just as there are many-body phases with fermions that lead to phenomena like superconductivity, there are also analogous many-body phenomena with bosons. Our achievement lies in providing a platform for studying bosons in real materials, as we lacked an effective method to examine them until now.” Although excitons have been extensively studied, he added that there was no prior means of inducing strong interactions among them.

Jin further asserted that their methodology opens the door not only to studying well-known bosonic particles like excitons but also to gaining further insights into the realm of condensed matter with the discovery of new bosonic materials.

“We are aware that certain materials exhibit incredibly peculiar properties,” he said. “One of the primary objectives of condensed matter physics is to comprehend the origins of these unique properties and develop means to reliably harness their effects.”

Reference: “Correlated insulator of excitons in WSe2/WS2 moiré superlattices” by Richen Xiong, Jacob H. Nie, Samuel L. Brantly, Patrick Hays, Renee Sailus, Kenji Watanabe, Takashi Taniguchi, Sefaattin Tongay, and Chenhao Jin, 11 May 2023, Science.
DOI: 10.1126/science.add5574

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More about exotic matter

• Science Daily: Exotic Matter Uncovered
• Original Paper: Correlated insulator of excitons in WSe2/WS2 moiré superlattices
• UC Santa Barbara: Physicists Discover New State of Matter
• National Institute for Materials Science: Bosonic Correlated Insulator