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Moiré superlattices are periodic patterns formed when two or more thin semiconducting layers are stacked with a small twist angle or lattice mismatch. When 2D materials form these patterns, their electronic, mechanical, and optical properties can change significantly.

Over the past decades, moiré superlattices have emerged as a promising platform to study unconventional and unknown physical states. They also enabled the observation of unique excitonic configurations (i.e., arrangements of bound electron-hole pairs).

In bilayer moiré systems based on two-dimensional transition metal dichalcogenides (TMDCs), for instance, physicists have observed interlayer dipolar excitons. These are excitons produced when an electron and a hole are bound together across different layers in a stacked 2D semiconductor.

A research team led by Sufei Shi at Carnegie Mellon University, collaborating with researchers from Rensselaer Polytechnic Institute, Arizona State University, and other institutes, recently identified a promising approach to control excitonic states in an atomically thin semiconductor. Their proposed strategy, outlined in a paper in Nature Photonics, allowed them to drive and stabilize transitions between quadrupolar and dipolar excitons, two recently uncovered exciton configurations, in a trilayer moiré superlattice.

"Previously, we identified a new species of exciton called a quadrupolar exciton (QX) in the trilayer moiré superlattice. We also have found a signature of strong correlation between excitons in the moiré bilayer," Sufei Shi, senior author of the paper, told Â鶹ÒùÔº.

"So, I always had the question of how the strong correlation (defined as Coulomb interaction over kinetic energy) has any effect on the QX, and even better, if the correlation can be used to control QX."

As part of their study, Shi and his colleagues created dual-gated heterostructures that consisted of trilayers of WSe₂/WS₂/WSe₂ with intended alignment. The architecture of these heterostructures allowed the team to precisely control both the vertical electric fields in them and the doping of charge carriers.

Subsequently, the researchers probed the excitonic states in the devices they created using a technique known as low-temperature optical spectroscopy. To drive transitions between different exciton configurations, they used two primary methods.

"We first controlled the density of excitons via the excitation power," explained Shi. "Once the exciton density reaches two per moiré site, the correlation between excitons cannot be ignored, and it drives the QX to DX transition, with the DX being a staggered opposite dipolar exciton in the trilayer structure. In addition, we controlled the electron density in the system, utilizing strong electron and exciton interactions."

Based on the data they recorded, the researchers were able to produce a detailed phase diagram that delineates the conditions under which QX and DX configurations prevailed in the trilayer structure. This led to interesting insight into the factors contributing to the emergence of QXs in 2D semiconductor-based trilayer moiré superlattices.

"We found that the correlation plays an important role in the formation of the behavior of QX," said Shi. "This confirms moiré trilayer superlattices as fruitful platforms for the study of correlated physics, which does not provide an analytical solution but often exotic properties that we desire (such as superfluidity, where the quasiparticle can move around with minimal energy consumption). This will allow us to use QX to construct a new quantum phase as well."

The recent work by Shi and his colleagues opens new possibilities for the manipulation of excitons in multilayer moiré superlattices. In the future, the new approach could be used to realize new quantum and correlated physics states emerging from specific exciton configurations.

In addition, this recent study could eventually pave the way for the development of new quantum optoelectronic and photonic devices. Meanwhile, the team is planning to conduct further research exploring the emergence of QXs and DXs in other heterostructures and patterned substrates.

"As part of our next studies, we will continue to explore the correlated physics in this system in the near future, such as an excitonic Mott insulator or a Bose-Einstein condensate (BEC)," added Shi.