In recent years, single-layered ultrathin materials such as graphene have gathered researchers’ attention. Significant advances have been made since then. One such advance is stacking individual sheets of 2D materials and sometimes twisting them slightly, which can give them new properties such as superconductivity or magnetism.
In a new study, MIT physicists reported on a new ultrathin, a two-dimensional material with unusual magnetic properties. The material comprises three graphene layers, each twisted on top of the next at the same angle.
Such stacking creates a helical structure similar to the DNA helix or a hand of three cards that are fanned apart.
Jarillo-Herrero, who is also affiliated with MIT’s Materials Research Laboratory, said, “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!”
Due to the arrangement of 2D materials’ sheets in ultrathin materials, twistronics can create new properties. This results in a unique pattern called a moiré lattice, which impacts electrons’ behavior.
In this study, the helical structure forms two moiré lattices: One is created by the first two overlapping sheets, and the other is formed between the second and third sheets. These moiré patterns are only nanometers, or billionths of a meter, in scale.
What’s more, these two moiré patterns together form a third moiré called supermoiré, or “moiré of a moiré,” which appears at a scale of hundreds of nanometers superimposed over the other two. It’s like a moiré hierarchy.
When physicists observed signatures of this moiré hierarchy, they were surprised to find a huge surprise after applying and varying a magnetic field. The researchers found magnetism in a new carbon-based material due to the motion of electrons.
This magnetism remained even at -263 °C, the highest temperature for such materials. However, this type of magnetism should not have appeared because the material supposedly had a certain symmetry. This unexpected result puzzled the team.
Li-Qiao Xia, a graduate student in MIT physics and another of the three co-first authors of the Nature Physics paper, said, “It turns out that the new system did indeed break the symmetry which prohibits the orbital magnetism the team observed but in a very unusual way. What happens is that the atoms in this system aren’t very comfortable, so they move in a subtle, orchestrated way that we call lattice relaxation. And the new structure formed by that relaxation does indeed break the symmetry locally, on the moiré length scale.”
Sergio C. de la Barrera, one of three co-first authors of the recent paper, said, “This opens the possibility for the orbital magnetism the team observed. However, the symmetry is restored if you zoom out to view the system on the supermoiré scale. The moiré hierarchy supports interesting phenomena at different length scales.”
Aviram Uri, an MIT Pappalardo postdoctoral fellow in physics and the third co-first author of the new paper, said, “It’s a lot of fun when you solve a riddle, and it’s such an elegant solution. We’ve gained new insights into how electrons behave in these complex systems, insights that we couldn’t have had unless our experimental observations forced us to think about these things.”
Journal Reference:
- Xia, LQ., de la Barrera, S.C., Uri, A. et al. Topological bands and correlated states in helical trilayer graphene. Nat. Phys. (2025). DOI: 10.1038/s41567-024-02731-6