At its core, quantum physics explores the interactions between atoms, subatomic particles, and other fundamental features of matter and energy. Those interactions become especially interesting when quantum materials are stacked together, creating new possibilities.
In a first-of-its-kind advance led by researchers at WashU, physicists have stacked two-dimensional materials to create a new type of structure — a bichromatic supermoiré lattice —that could help shed light on the mysteries of the quantum world. “This is an important advance to help us understand new physical phenomena in quantum states,” said study leader Xi Wang, an assistant professor of physics and a member of the Center for Quantum Leaps.
The study, supported in part by Art & Sciences and the Center for Quantum Leaps, was reported in Nature Communications. The lead author is Mingfeng Chen, a postdoctoral research associate in Wang’s lab. Other co-authors include Runtong Li, AB ’25, now a PhD student at the Massachusetts Institute of Technology; Haonan Wang, PhD ’25; Li Yang, the Albert Gordon Hill Professor of Physics; Chuanwei Zhang, a professor of physics; Erik Henriksen, a professor of physics; and graduate students Yuliang Yang and Yiyang Lai.
In physics, a lattice is a crystalline material that follows highly predictable patterns at the atomic level. To create the new superlattices, Wang and her team made slightly different copies of a two-dimensional crystal containing tungsten, sulfur, and selenium. They then stacked the sub-nanometer thick sheets on top of each other at a slightly offset orientation, creating a specific physical effect called a moiré pattern.
The term “moiré pattern” is borrowed from the world of printmaking and photography, where it refers to the visual effects that form when two highly regular but slightly mismatched patterns are placed atop one another. Imagine looking through one picket fence at another that’s just a bit off kilter.
Just as the slats of those fences create intriguing visual patterns, the atoms in the supermoiré lattices interact in new ways at the quantum level. Wang and her team are especially interested in the creation of new species of excitons, quantum features with potential applications in simulation, computing, and electronics.
Excitons are formed when a negatively charged electron from one lattice pairs with a positively charged electron “hole” — a vacancy left by an absent electron — from the other lattice. The electron-hole pair is held together by Coulomb attraction, creating a neutral quasiparticle. While excitons themselves do not conduct electricity, they can transport energy through the material and strongly influence its optical and electronic properties. The excitons trapped in supermoiré lattices would experience strong interactions with each other, leading to emergent quantum phenomena.
Importantly, the supermoiré lattices created by Wang and her team are highly tunable, meaning the patterns of potential arising from the mismatched materials can be precisely adjusted using electric fields. Even subtle shifts can create new interactions, resulting in new ways to produce excitonic and electronic quantum states. In the new bichromatic supermoiré lattices, Wang and her team also discovered a new excitonic species, quadrupolar moiré trions, when tuned to a specific state.
Wang said the tunable structure could be especially useful for quantum simulations, experiments designed to determine how particles react in different scenarios. “If we can tune these materials very precisely, we might be able to find undiscovered, exotic quantum phenomena,” she said.
The interference patterns in supermoiré lattices are so complex that it takes massive computing power to sort through the possibilities. Yang, a computational physicist and newly named fellow of the American Physical Society, helped the team calculate the interactions at different tuning fields.
Now that Wang’s group has established a method for creating supermoiré lattices, she plans to use the new material to conduct further experiments to expand the fundamentals of quantum interactions. Excitons and trions may one day play a role in quantum computers, electronics, or telecommunications. For now, Wang is focused on the basic science to be learned from the new superlattices.
“In the future, highly tunable quantum phenomena could help drive all sorts of new technologies,” she said. “That’s why we’re so excited about this project.”
