Imagine placing two delicate sheets of fabric, each with its own intricate pattern, one slightly askew on top of the other. What you see is a mesmerizing new design, larger and more complex than either pattern alone. This effect is known as a moiré pattern—a visual phenomenon many of us have seen in daily life, from window screens to digital images.
Now, replace the fabric with atomically thin sheets of semiconducting materials. Instead of just creating beautiful visuals, these moiré patterns transform the very laws that govern the material’s behavior. They twist the fabric of quantum mechanics, giving rise to entirely new properties—new forms of light-matter interaction, unexpected electrical behavior, and even novel states of matter. These are moiré superlattices, and they are rapidly becoming one of the most exciting frontiers in modern physics.
What Exactly Are Moiré Superlattices?
Moiré superlattices form when two or more ultrathin materials—each only a few atoms thick—are stacked on top of each other with a slight mismatch in angle or lattice spacing. This tiny twist creates a periodic “superlattice” pattern that dramatically alters how electrons, atoms, and light behave within the material.
In recent years, physicists have discovered that these superlattices are not just mathematical curiosities—they are platforms for unlocking exotic physics. In twisted graphene, for example, moiré patterns can turn a simple conductor into a superconductor. In semiconducting transition metal dichalcogenides (TMDCs), they enable the formation of unique excitons—bound states of electrons and holes that behave like quasiparticles of light and matter combined.
Excitons: Quantum Fireflies
To understand the breakthroughs happening in these systems, we need to appreciate what excitons are. Imagine shining light on a semiconductor. The energy from the photon kicks an electron into a higher energy state, leaving behind a positively charged hole. Instead of drifting apart, the electron and hole can bind together, held by their opposite charges, much like a miniature atom. This electron–hole pair is what physicists call an exciton.
Excitons are fleeting, fragile, and yet deeply important. They determine how semiconductors interact with light, making them central to technologies like LEDs, solar cells, and lasers. But in moiré superlattices, excitons do something extraordinary: they rearrange themselves into entirely new configurations, as though the superlattice is a stage for exotic quantum choreography.
From Dipolar to Quadrupolar: New Exciton States
In bilayer moiré systems, scientists have already observed dipolar excitons—where the electron and hole are confined to different layers of the stacked material. This separation gives the exciton an electric dipole moment, meaning it interacts strongly with electric fields and with other excitons.
But in trilayer moiré systems, researchers recently uncovered something even more exotic: quadrupolar excitons (QX). Instead of a simple dipole with two charges, these excitons exhibit a more complex four-pole structure. Such configurations are rare in nature and carry with them the potential for entirely new physics.
The central question has been: can these unusual exciton states be controlled, manipulated, or even stabilized for use in new quantum technologies?
A Breakthrough from Carnegie Mellon
This is precisely what a team led by physicist Sufei Shi at Carnegie Mellon University set out to explore. Working with collaborators from Rensselaer Polytechnic Institute, Arizona State University, and other institutions, Shi’s group focused on trilayer moiré superlattices made from WSe₂/WS₂/WSe₂. These atomically thin layers were carefully aligned to maximize the moiré effects.
By building dual-gated heterostructures—essentially devices where electric fields and charge densities could be finely tuned—the team gained unprecedented control over the excitonic states. Using low-temperature optical spectroscopy, they could “see” how excitons formed, interacted, and transitioned between states under different experimental conditions.
The result? They demonstrated that by adjusting exciton density and electron concentration, they could drive and stabilize transitions between quadrupolar excitons (QXs) and dipolar excitons (DXs). In other words, they found a way to switch between exotic exciton states, opening the door to controlled manipulation of quantum matter.
The Role of Strong Correlations
One of the most fascinating aspects of the discovery is the role of correlation. In quantum physics, correlation refers to how strongly particles influence one another. When exciton density is low, they behave independently. But once the density reaches two excitons per moiré site, their mutual interactions become impossible to ignore. The system shifts into a new regime where collective behaviors dominate.
It was under these conditions that the researchers observed transitions from quadrupolar to dipolar excitons. The strong Coulomb interactions—the push and pull of charged particles—became the very mechanism that controlled the excitonic state.
As Shi put it, this shows that moiré trilayer superlattices are fertile grounds for correlated physics—phenomena where the whole system behaves in ways that can’t be predicted simply by looking at individual particles. This is the same domain where exotic states like superconductivity and superfluidity emerge.
Mapping Quantum Landscapes
By carefully recording their data, Shi’s team was able to construct a detailed phase diagram, a kind of map of the quantum landscape. This diagram showed the exact conditions under which quadrupolar or dipolar excitons dominate in the trilayer structure.
Such diagrams are more than just scientific charts. They are blueprints for future experiments, guiding researchers toward the conditions that produce new states of matter. They also deepen our understanding of how moiré patterns sculpt quantum interactions, revealing principles that may apply to many other materials.
Why This Matters
The implications of this work extend far beyond academic curiosity. Being able to control excitons could revolutionize the field of optoelectronics—technologies that manipulate light and electricity together. Potential applications include ultra-efficient LEDs, quantum communication devices, and novel forms of computing that go beyond the limits of silicon-based transistors.
Moreover, excitons that exhibit strong correlations could lead to entirely new phases of matter, such as excitonic superfluids or Bose-Einstein condensates of excitons. These states are not just strange—they could underpin technologies that move information with almost no energy loss, transforming the efficiency of future devices.
The Road Ahead
For Shi and his colleagues, the journey has only just begun. Their next steps involve probing even deeper into correlated excitonic physics. Could excitons in moiré superlattices form an excitonic Mott insulator, where their mutual repulsion locks them into place? Could they condense into a Bose-Einstein condensate, where all excitons act as a single, coherent quantum entity?
These are not far-fetched ideas. They are within reach, and the trilayer moiré superlattice is proving to be an ideal playground for such exploration.
A New Era of Quantum Materials
The story of moiré superlattices is a reminder that some of the most profound discoveries begin with simple shifts in perspective—in this case, literally twisting layers of atoms by just a few degrees. That tiny misalignment has opened an entire universe of quantum phenomena.
We are now standing at the threshold of a new era in materials science, where human ingenuity meets the strangeness of quantum mechanics to create matter that nature never intended. The ability to engineer and control excitons, as demonstrated in Shi’s recent work, is more than just a scientific achievement—it is an act of unlocking hidden possibilities within the universe.
And so, as these shimmering moiré patterns continue to reveal their secrets, we may soon find ourselves in a world where light and matter are no longer separate domains, but partners in technologies that reshape our future.
More information: Yuze Meng et al, Strong-interaction-driven quadrupolar-to-dipolar exciton transitions in a trilayer moiré superlattice, Nature Photonics (2025). DOI: 10.1038/s41566-025-01741-x.
