Scientists discover the unlikely quantum property of graphite


The researchers found that bulk graphite exhibits a quantum Hall effect in a remarkable way, opening up new areas of physics research.

Observation of the quantum Hall effect outside a 2D system

Scientists from the University of Manchester, UK, led by Dr. Artem Mishchenko, Professor Volodya Fal & ko and Professor Andre Geim, have discovered the Quantum Hall Effect (QHE) in loose graphite, which is a stratified crystal made up of superimposed layers. of graphene.


Their discoveries, published in the journal Nature Physics, were not anticipated, the QHE being supposed to be limited to systems known as two-dimensional systems, where the movement of electrons is limited to a single plane and can not not move perpendicularly.

The researchers used cleaved graphite crystals protected by hexagonal boron nitride in layers. Their devices were consistent with the geometry of the Hall Bar, which allowed them to measure electron transport in graphite.

"The measurements were pretty simple." explains the member of the research team and first author of the article, Dr. Jun Yin. "We passed a small current along the Hall Bar, applied a strong magnetic field perpendicular to the Hall rod plane, and then measured the voltages generated along the device and through it to extract the longitudinal resistivity and resistance. from Hall. "

Fal & # 39; 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, which are thick enough to behave like a normal semi-metal in which QHE should be prohibited. "

Other features found

Another surprising discovery is that the number of layers of graphene contained in graphite– especially if there were an odd number of layers or an even number – affected their QHE observations.

They found that stationary waves of both types of electrons present in graphite resulted in a reduction in QHE energy deviations when there was an odd number of layers of graphene in the graphite, and this even when there were hundreds of layers of graphene.

Another surprising result was the discovery of fractional QHE (FQHE) – different from normal QHE and resulting from interactions between electrons giving rise to phenomena such as superconductivity and magnetism – in very thin layers of graphite.

"Most of the results we observed can be explained with the help of a simple one-electron model, but seeing the FQHE tells us that the picture is not so simple" said Mishchenko. "Our graphite samples exhibit numerous electron-electron interactions at high magnetic fields and at low temperatures, showing that multi-body physics is important in this material."

Take back some of the Graphene projectors

Graphite has been lagging behind graphene for years, but researchers hope their work shows that the graphite-based material as a whole deserves further study.

"Our work is a new springboard for further studies on this material, including multi-body physics, such as density waves, exciton condensation or Wigner crystallization," Mishchenko said.

"For decades, researchers have used graphite as a sort of" philosopher's stone "capable of generating all likely and unlikely phenomena, including superconductivity at room temperature," said Geim. "Our work shows what is in principle possible in this material, at least when it is in its purest form."

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