Scientists Uncover Improbable Quantum Property of Graphite
Researchers have discovered that bulk graphite exhibits a quantum Hall effect in remarkable ways, opening new areas for research in physics.
Observing the Quantum Hall Effect Outside of a 2D System
Scientists at the University of Manchester, UK, led by Dr. Artem Mishchenko, Prof Volodya Fal’ko, and Prof Andre Geim, have found the quantum Hall effect (QHE) in bulk graphite, which is a layered crystal made out of stacked layers of graphene.
Their findings, published in the journal Nature Physics, were not anticipated, as the QHE is supposed to be limited to systems known as two dimensional systems, where the movement of electrons is limited to one plane and cannot move perpendicularly.
The researchers used cleaved graphite crystals protected by layered hexagonal boron nitride. Their devices conformed to Hall bar geometry, enabling them to measure the transport of electrons in the graphite.
"The measurements were quite simple." explains research team member and first author of the paper, Dr. Jun Yin. "We passed a small current along the Hall bar, applied strong magnetic field perpendicular to the Hall bar plane and then measured voltages generated along and across the device to extract longitudinal resistivity and Hall resistance.”
Fal'ko, who worked on the theoretical part of the paper, said, "we were quite surprised when we saw the quantum Hall effect (QHE)—a sequence of quantized plateaux in the Hall resistance—accompanied by zero longitudinal resistivity in our samples. These are thick enough to behave just as a normal bulk semimetal in which QHE should be forbidden."
Other Peculiarities Found
Another surprising finding was that the number of layers of graphene contained in the graphite—specifically whether there was an odd number of layers or an even number—affected their observations of the QHE.
They found that the standing waves of the two different kinds of electrons in graphite produced reduced QHE energy gaps when there were an odd number of graphene layers in the graphite and did so even when there were hundreds of layers of graphene.
Another surprising result was the discovery of the fractional QHE (FQHE)—which is different from the normal QHE and is the product of interactions between electrons that give rise to phenomena such as superconductivity and magnetism—in very thin layers of graphite.
"Most of the results we have observed can be explained using a simple single-electron model but seeing the FQHE tells us that the picture is not so simple," Mishchenko said. "There are plenty of electron-electron interactions in our graphite samples at high magnetic fields and low temperatures, which shows that many-body physics is important in this material."
Taking Back Some of Graphene’s Spotlight
Graphite has taken a backseat to Graphene for years now, but the researchers hope that their work shows that the larger graphite material is still worthy of considerable study.
"Our work is a new stepping stone to further studies on this material, including many-body physics, like density waves, excitonic condensation or Wigner crystallization," Mishchenko said.
"For decades graphite was used by researchers as a kind of 'philosopher's stone' that can deliver all probable and improbable phenomena including room-temperature superconductivity," Geim said. "Our work shows what is, in principle, possible in this material, at least when it is in its purest form."