For over two decades, two-dimensional materials have captured the imagination of scientists, offering groundbreaking applications in physics, electronics, and materials science. Among these discoveries, graphene—a single layer of carbon atoms—has been at the forefront of revolutionary research. However, a recent study from MIT takes the field of “twistronics” to an entirely new level by uncovering unexpected magnetic properties in helical trilayer graphene.
The Magic of Twisted Layers
Twistronics, a field pioneered by MIT physicist Pablo Jarillo-Herrero, explores how stacking and slightly rotating layers of 2D materials can lead to novel electronic and magnetic properties. The latest research, published in Nature Physics on January 7, focuses on a system where three graphene layers are twisted at identical angles, forming a helical structure reminiscent of a DNA helix.
“Helicity is a fundamental concept in science, from basic physics to chemistry and molecular biology. With 2D materials, one can create special helical structures with novel properties, which we are just beginning to understand,” says Jarillo-Herrero.
Understanding the Moiré Hierarchy
When two or more layers of graphene are stacked with a slight twist, they form a unique interference pattern called a moiré lattice. This pattern alters the electronic energy levels, allowing for new quantum effects to emerge. In this case, the three twisted graphene layers created two moiré patterns, which combined to form a third, larger-scale supermoiré structure.
“It’s like a moiré hierarchy,” explains Li-Qiao Xia, a graduate student in MIT physics and one of the paper’s co-first authors. While the first two moiré patterns exist on a nanometer scale, the supermoiré pattern extends to hundreds of nanometers, introducing effects that were previously unobservable.
A Magnetic Surprise
The real shock came when researchers applied a magnetic field to the system. Instead of behaving as expected, the material exhibited clear signs of orbital magnetism—something previously unseen in carbon-based materials at such high temperatures (-263°C). This discovery was perplexing because the material should have maintained a specific symmetry that prevents magnetism.
MIT postdoc Aviram Uri recalls, “So the fact that we saw this was very puzzling. We didn’t really understand what was going on.”
Solving the Puzzle: Lattice Relaxation
After further investigation, the researchers found that a subtle but significant structural shift—known as lattice relaxation—was responsible for breaking the expected symmetry at a local level. This deformation allowed for the emergence of magnetism while still preserving overall symmetry when viewed on the supermoiré scale.
“What happens is that the atoms in this system aren’t very comfortable, so they move in a subtle orchestrated way,” explains Xia. “And the new structure formed by that relaxation breaks the symmetry locally, allowing for magnetism to emerge.”
Implications and Future Prospects
This breakthrough not only enhances our understanding of twisted materials but also opens new doors for quantum computing and advanced materials research. The ability to engineer electronic and magnetic properties in 2D materials could lead to more efficient, tunable devices and novel quantum phenomena.
“This work represents a new twist in the field of twistronics, and the community is very excited to see what else we can discover using this helical materials platform,” says Jarillo-Herrero.
Final Thoughts
The discovery of magnetism in helical trilayer graphene is yet another testament to the power of fundamental research. It highlights how curiosity-driven science can lead to unexpected and potentially transformative discoveries. As researchers continue to explore the moiré hierarchy and lattice relaxation effects, we can expect even more exciting revelations in the world of twistronics.
References:
Paper: “Topological bands and correlated states in helical trilayer graphene”
Funding: Supported by the Army Research Office, the National Science Foundation, the Gordon and Betty Moore Foundation, the Ross M. Brown Family Foundation, MIT Pappalardo Fellowship, and others.
