In the realm of quantum physics, researchers have made astonishing discoveries about the behavior of electrons in a unique material known as rhombohedral pentalayer graphene. This material, a form of pencil lead with five layers of graphene stacked in a specific order, has revealed new states of electrons when cooled to extremely low temperatures, opening doors to unprecedented scientific and technological advancements.

The Fractional Quantum Anomalous Hall Effect

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 temperatures around 400 millikelvin. This finding has sparked discussions about the mechanisms behind this effect and the role of the unique moiré pattern created by the layers. Researchers have suggested that new, complex states of electrons can form in this setup.

Electron Crystals and New Electronic States

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.

“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.

— Zhengguang Lu

The Role of Custom-Made Filters

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.

Rhombohedral Pentalayer Graphene: A Gold Mine of Discoveries

Long Ju, the lead researcher, describes the new material, rhombohedral pentalayer graphene, as a gold mine, with discoveries revealed at every step. 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.

The Fractional Quantum Hall Effect

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.”

New Discoveries at Extremely Low Temperatures

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.

Practical Applications and Future Research

These discoveries have significant implications for the development of quantum technologies. The ability to control electron states at such low temperatures could lead to breakthroughs in quantum computing and advanced materials science. The team observed these phenomena in both pentalayer and four-layer rhombohedral graphene, suggesting a family of materials with similar behavior.

While the current research focuses on extremely low temperatures, future work may explore ways to achieve similar effects at higher temperatures, making these discoveries more practical for real-world applications. The findings also raise questions about the fundamental nature of electron behavior in these materials, opening new avenues for theoretical and experimental research.

Counterarguments and Future Directions

Some critics argue that the practical applications of these discoveries are still far from being realized due to the extreme conditions required. However, the team’s ongoing research aims to bridge this gap by exploring new materials and methods to achieve similar effects at more accessible temperatures. Additionally, the unique properties of rhombohedral pentalayer graphene could lead to innovations in fields beyond quantum computing, such as energy storage and advanced electronics.