The super-special material graphene continues to surprise and fascinate scientists, this time revealing a rare electronic state termed 'ferro-valleytricity', which occurs when graphene is stacked up in a particular five-layer combination.
When in this new state, the graphene stack exhibits weird and wonderful magnetic and electronic behavior, as reported by researchers from the Massachusetts Institute of Technology (MIT), Harvard University, and the National Institute for Materials Science in Japan.
Using graphene in this way could help in the development of both classical and quantum computers, according to the team, especially in terms of creating data storage solutions that offer large capacities but that also need relatively little energy to run.
"Graphene is a fascinating material," says physicist Long Ju from MIT. "Every layer you add gives you essentially a new material."
"And now this is the first time we see ferro-valleytricity, and unconventional magnetism, in five layers of graphene. But we don't see this property in one, two, three, or four layers."
Ferroic materials show some kind of coordinated behavior in their electric, magnetic, or structural properties – like a magnet, for instance, which has electrons that all spin and point in the same direction without being directed by an external magnetic field.
In other materials, electrons might instead align in tiny whirlpools. To be multiferroic, the same material must display multiple types of coordinated behavior.
Based on their calculations, the researchers thought that graphene may turn multiferroic if five layers were arranged in a rhombohedral pattern (not dissimilar to chicken wire fences stacked on top of each other).
Importantly, this creates an environment where electrons are slowed down, and ferroic alignment starts to happen. Through a close analysis of graphene flakes that had naturally formed into five layers and in this particular pattern, the researchers did indeed see multiferroic behavior.
First, there was unconventional magnetism, where the electrons coordinated their orbital motion (rather than their spin or rotation, as in a standard magnet). Second, the electrons showed a tendency to settle in one particular electronic 'valley' (or low-energy state), whereas in standard graphene, they don't show any preference.
The team demonstrated that both these ferroic properties could be controlled by an electric field. While this all remains very high-level and technical for now, eventually it could be used to develop computer chips that can store double the data – because the material's electrons can be manipulated in two ways rather than one.
"We knew something interesting would happen in this structure, but we didn't know exactly what, until we tested it," says physicist Zhengguang Lu, from MIT.
"It's the first time we've seen ferro-valleytronics, and also the first time we've seen a coexistence of ferro-valleytronics with an unconventional ferro-magnet."
The research has been published in Nature.