When electrons move in special flat bands, they can interact to create new states. In a five-layer structure of graphene and hexagonal boron nitride (hBN), scientists observed the fractional quantum anomalous Hall effect (FQAHE) at very low temperatures (around 400 millikelvin).
This finding has led to discussions about how this effect happens and the role of the unique moiré pattern created by the layers. Researchers have also suggested that new, complex states of electrons can form in this setup.
MIT physicists discovered that electrons can form crystal-like structures in a material only a few billionths of a meter thick. In a paper, the team explained how electrons in devices made partly of this new material can become solid or form crystals by changing the voltage when the devices are at extremely low temperatures. They also discovered two new electronic states, adding to their previous work showing that electrons can split into smaller parts.
These discoveries were possible thanks to custom-made filters that improved insulation, allowing the devices to be cooled to much lower temperatures.
The team observed these phenomena using two versions of the new material, one with five layers of atomically thin carbon and the other with four layers. This suggests a family of materials with similar behavior.
Long Ju, the lead researcher, describes the new material, rhombohedral pentalayer graphene, as a gold mine, with discoveries revealed at every step.
A novel class of quantum particles behaves in unexpected ways
Rhombohedral pentalayer graphene is a unique form of pencil lead. Pencil lead, or graphite, consists of graphene, a single layer of carbon atoms arranged in a hexagonal pattern. Rhombohedral pentalayer graphene has five layers of graphene stacked in a specific order.
Since Ju and his team discovered this material, they’ve experimented with it by adding other materials to enhance its properties or create new effects. In 2023, they made a sandwich with rhombohedral pentalayer graphene and hexagonal boron nitride. They discovered three new properties not seen in natural graphite by applying different voltages.
Last year, they reported another surprising phenomenon: Electrons split into fractions when a current was applied to a device made of rhombohedral pentalayer graphene and hexagonal boron nitride. This effect, known as the “fractional quantum Hall effect,” usually requires high magnetic fields.
Ju’s work showed that it could happen in a simple material without a magnetic field, a phenomenon called the “fractional quantum anomalous Hall effect.”
In their latest work, the Ju team discovered more unexpected phenomena in the rhombohedral graphene/boron nitride system when cooled to extremely low temperatures (30 millikelvins or -459.668 degrees Fahrenheit). Last year, they reported six fractional electron states and discovered two more.
They also found another unique phenomenon: the integer quantum anomalous Hall effect at various electron densities. The fractional quantum anomalous Hall effect was seen in an electron “liquid” phase (like water). At the same time, the new state resembles an electron “solid” phase (like ice), coexisting with the fractional states when the voltage is finely tuned at ultra-low temperatures.
Ju explains the relationship between the integer and fractional states as a “landscape” created by tuning electric voltages, with rivers representing the liquid-like states and glaciers representing the solid-like effect.
The team observed these phenomena in both pentalayer and four-layer rhombohedral graphene, suggesting a family of materials with similar behavior.
“This work shows how rich this material is in exhibiting exotic phenomena. We’ve just added more flavor to this already very interesting material,” says Zhengguang Lu, a co-first author of the paper. Lu, who conducted the work as a postdoc at MIT, is now on the faculty at Florida State University.
Journal Reference:
- Lu, Z., Han, T., Yao, Y. et al. Extended quantum anomalous Hall states in graphene/hBN moiré superlattices. Nature 637, 1090–1095 (2025). DOI: 10.1038/s41586-024-08470-1
