New state of matter discovered: Subatomic particles squeezed into ultradense crystal

Scientists have made an intriguing discovery by passing a powerful beam of light through two chemical compounds, unveiling a previously unknown state of matter composed of particles called excitons.

Physicists have encountered an exotic form of matter, exhibiting a highly organized crystal structure comprising subatomic particles. Referred to as a “bosonic correlated insulator,” this fresh state of matter holds the potential to unlock a plethora of novel materials derived from condensed matter. The researchers, who have presented their findings in a study published on May 11 in the journal Science, elaborate on the implications of their discovery.

Two lattices stack to form a moiré, the foundational pattern of a newly discovered type of matter. (Image credit: Matt Perko)

Subatomic particles can be classified into fermions and bosons, distinguished by their spin and interactions. Fermions, like electrons and protons, constitute the fundamental constituents of matter, comprising atoms. They possess half-integer spins and are unable to occupy the same space simultaneously if identical. Conversely, bosons, which encompass photons or light packets responsible for transmitting forces, are believed to act as the cohesive agents of the universe, unifying the fundamental forces of nature. These particles possess whole-integer spins and can occupy the same position concurrently. Chenhao Jin, the lead author of the study and a condensed-matter physicist at the University of California, Santa Barbara, describes the significance of these behaviors in shaping our understanding of the universe.

In a unique scenario, two fermions can merge to form a boson, specifically when a negatively charged electron binds with a positively charged “hole” present in a different fermion. This results in the creation of a bosonic particle known as an “exciton.”

To examine the behavior of excitons when interacting with each other, the scientists arranged a lattice of tungsten disulfide atop a similar lattice of tungsten diselenide, generating an overlapping pattern known as a moiré. They subsequently directed a strong beam of light through the lattices using a technique called “pump-probe spectroscopy.” Under these conditions, the excitons were forced into close proximity until they became densely packed, rendering them immobile. This process yielded a new symmetric crystalline state possessing a neutral charge—a bosonic correlated insulator.

Traditionally, considerable efforts have been devoted to comprehending the collective behavior of fermions. However, the researchers’ work primarily focuses on fabricating a novel material through the interaction of bosons. Jin explains that this breakthrough has been accomplished for the first time in a “real” matter system, as opposed to synthetic systems, shedding new light on the characteristics of bosons. Additionally, the techniques employed in this study hold the potential to enable scientists to create further types of bosonic materials.

Jin highlights the peculiar properties exhibited by certain materials and emphasizes the goal of condensed matter physics to elucidate the underlying causes behind these distinctive traits while seeking methods to reliably manifest these behaviors.