Bizarre new electronic state discovered in five-layer graphene
It seems there’s no end to surprises from everybody’s favorite wonder material, graphene. MIT physicists have now discovered yet another brand new electronic state hiding in this overachieving little material – something they give the bizarre name of “ferro-valleytricity.”
Graphene is essentially just a super thin sheet of plain old graphite – so thin, in fact, that it’s only one atom thick. But despite these humble beginnings, graphene is super strong, superconducting, flexible, and poised to revolutionize everything from electronics to clothing to aerospace engineering. When you start stacking sheets up and even twisting them to specific angles, other remarkable abilities emerge, like magnetism or superpermeability to water.
In the new study, the MIT team discovered yet another one – “multiferroic behavior,” which is rare in the material world. A ferroic material is one where there’s a coordinated behavior to its particles – a magnet, for example, involves all its electrons pointing their spins in the same direction even without an external magnetic field. Multiferroic materials are those that display more than one coordinated behavior – say, if the magnetism points in one direction and its electric charge in another.
The researchers calculated that under very specific circumstances, graphene should become multiferroic. Theoretically it should only occur when five sheets of graphene are stacked on top of each other, with each layer slightly offset so the 3D whole forms a rhombus shape.
“In five layers, electrons happen to be in a lattice environment where they move very slowly, so they can interact with other electrons effectively,” said Long Ju, lead author of the study. “That’s when electron correlation effects start to dominate, and they can start to coordinate into certain preferred, ferroic orders.”
Next the team set out to confirm the theory in practice, by shaving flakes of graphene off a block of graphite and checking them with powerful microscopes to find some that naturally had the desired rhombus shape. The several they found were then isolated and studied at temperatures just above absolute zero, where other effects would be reduced so only the ones they were looking for would shine through.
And sure enough, the team found that the electrons in these special flakes responded uniformly to an electric field in one direction and a magnetic field in other, confirming multiferroic behavior. But even these behaviors separately were unusual – the magnetism arose from the coordination of the electrons’ orbital motion rather than their spin. And the electronic behavior arose from electrons preferably settling into one “valley” – or lowest energy state – rather than equally into the two valleys available to them. As such, the team dubbed the strange electronic state “ferro-valleytricity.”
“We knew something interesting would happen in this structure, but we didn’t know exactly what, until we tested it,” said Zhengguang Lu, co-first author of the study. “It’s the first time we’ve seen a ferro-valleytronics, and also the first time we’ve seen a coexistence of ferro-valleytronics with unconventional ferro-magnet.”
The researchers say that this strange behavior could eventually be harnessed to effectively double the data storage capacity of a chip.
The research was published in the journal Nature.