Graphene, everyone’s favorite magical material, always seems to have surprises. Physicists at MIT have now discovered yet another brand-new electronic state hidden in this remarkable little material—one they've given a curious name—"iron valleyness."
Graphene is essentially just an ultrathin flake of regular graphite—so thin, in fact, that it's only one atom thick. But despite its humble beginnings, graphene is super strong, superconducting, flexible and has the potential to revolutionize everything from electronics to clothing to aerospace engineering. When you start stacking graphene sheets, and even twisting them to specific angles, other extraordinary abilities become apparent, such as magnetism or super water permeability.
In new research, the MIT research team discovered another material-"multiferroic behavior", which is very rare in the materials world. A ferroic material is one whose particles have coordinated behavior -- for example, all the electrons of a magnet will point their spins in the same direction even in the absence of an external magnetic field. Multiferroics are materials that display more than one coordinated behavior, such that magnetism points in one direction and charges point in another.
The researchers calculated that, under very special circumstances, graphene should become a multiferroic material. Theoretically, multiferroicity only occurs when five layers of graphene are stacked on top of each other, with each layer slightly offset so that the three-dimensional whole forms a rhombus.
In five-layer graphene, the electrons happen to be in a crystal lattice environment where they move very slowly, allowing them to interact effectively with other electrons. This is when electron-related effects begin to dominate, and they can begin to coordinate into certain preferred ferrite orders.
Next, the team set out to confirm the theory in practice, scraping flakes of graphene from blocks of graphite and examining them with powerful microscopes to find some that naturally had the ideal rhombus shape. They then isolated several types of graphene they found and studied them at temperatures just above absolute zero, where other effects weaken so only the graphene they were looking for glows.
Sure enough, the team found that the electrons in these special flakes responded uniformly to electric fields in one direction and magnetic fields in the other, confirming multiferroic behavior. But even these individual behaviors are unusual—magnetism arises from the coordination of the electrons' orbital motions, not their spins. Electronic behavior results from electrons preferentially entering one "valley" (or lowest energy state), rather than averaging into two valleys. Therefore, the research team calls this peculiar electronic state "iron valley property".
"We knew something interesting was happening in this structure, but we didn't know what it was until we tested it," said Zhengguang Lu, co-first author of the study. "This is the first time we've seen iron valley electronics, and the first time we've seen iron valley electronics coexisting with unconventional ferromagnets."
The researchers say this peculiar behavior could eventually be exploited to effectively double a chip's data storage capacity.
The research was published in the journal Nature.