Physicists discover — and explain — unexpected magnetism in an atomically thin material tri-layer graphene

Amazing stuff! Superimposed, hierarchical moire lattices.

"MIT physicists have created a new ultrathin, two-dimensional material with unusual magnetic properties that initially surprised the researchers before they went on to solve the complicated puzzle behind those properties’ emergence. As a result, the work introduces a new platform for studying how materials behave at the most fundamental level — the world of quantum physics. ...

worked with three layers of graphene. Each layer was twisted on top of the next at the same angle, creating a helical structure akin to the DNA helix or a hand of three cards that are fanned apart.

“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. This work represents a new twist in the field of twistronics ..." ...

Twistronics can lead to new properties in ultrathin materials because arranging sheets of 2D materials in this way results in a unique pattern called a moiré lattice. And a moiré pattern, in turn, has an impact on the behavior of electrons. ...

In the current work, the helical structure created by the three graphene layers forms two moiré lattices. One is created by the first two overlapping sheets; the other is formed between the second and third sheets.

The two moiré patterns together form a third moiré, a supermoiré, or “moiré of a moiré,” ... “It’s like a moiré hierarchy.” While the first two moiré patterns are only nanometers, or billionths of a meter, in scale, the supermoiré appears at a scale of hundreds of nanometers superimposed over the other two. ..."

From the abstract:

"The intrinsic anomalous Hall effect (AHE) is driven by non-zero Berry curvature and spontaneous time-reversal symmetry breaking. This effect can be realized in two-dimensional moiré systems hosting flat electronic bands but is not usually seen in inversion-symmetric materials.

Here, we show that this physics is manifested in helical trilayer graphene—three graphene layers, each twisted in sequence by the same angle—although the system retains global in-plane inversion symmetry.

We uncover a phase diagram of correlated and magnetic states at a magic twist angle of 1.8∘, which is explained by a lattice relaxation that leads to the formation of large periodic domains where in-plane inversion symmetry is broken on the moiré scale.

Each domain harbours flat topological bands with opposite Chern numbers in the two valleys. We find correlated states at multiple integer and fractional electron fillings per moiré unit cell and an AHE at a subset of them. The AHE disappears above a critical electric displacement field at one electron per unit cell, indicating a topological phase transition. We establish helical trilayer graphene as a platform that presents an opportunity to engineer topology due to its emergent moiré-scale symmetries."

Physicists discover — and explain — unexpected magnetism in an atomically thin material | MIT News | Massachusetts Institute of Technology "The work introduces a new platform for studying quantum materials."

Topological bands and correlated states in helical trilayer graphene (no public access)