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Researchers at The University of Manchester in the UK, led by Dr Artem Mishchenko, Prof Volodya Fal'ko and Prof Andre Geim, have discovered the quantum Hall effect in bulk graphite -- a layered crystal consisting of stacked graphene layers. This is an unexpected result because the quantum Hall effect is possible only in so-called two-dimensional (2D) systems where electrons' motion is restricted to a plane and must be disallowed in the perpendicular direction. They have also found that the material behaves differently depending on whether it contains odd or even number of graphene layers -- even when the number of layers in the crystal exceeds hundreds. The work is an important step to the understanding of the fundamental properties of graphite, which have often been misunderstood, esepcially in recent years.
In their work, published in Nature Physics, Mishchenko and colleagues studied devices made from cleaved graphite crystals, which essentially contain no defects. The researchers preserved the high quality of the material also by encapsulating it in another high-quality layered material -- hexagonal boron nitride. They shaped their devices in a Hall bar geometry, which allowed them to measure electron transport in the thin graphite.
"The measurements were quite simple." explains Dr Jun Yin, the first author of the paper. "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.
Prof Fal'ko who led the theory exploration said: "We were quite surprised when we saw the quantum Hall effect 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 strictly forbidden."
[...] "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 adds with a smile. "Our work shows what is, in principle, possible in this material, at least when it is in its purest form."
Journal Reference: Jun Yin, Sergey Slizovskiy, Yang Cao, Sheng Hu, Yaping Yang, Inna Lobanova, Benjamin A. Piot, Seok-Kyun Son, Servet Ozdemir, Takashi Taniguchi, Kenji Watanabe, Kostya S. Novoselov, Francisco Guinea, A. K. Geim, Vladimir Fal'ko, Artem Mishchenko. Dimensional reduction, quantum Hall effect and layer parity in graphite films. Nature Physics, 2019; DOI: 10.1038/s41567-019-0427-6
(Score: 2) by PiMuNu on Thursday March 07 2019, @01:32PM (1 child)
Solid state physics is not my strong suit, but let me have a go:
When a current passes through a conductor sitting in a magnetic field, all of the current carriers (e.g. electrons) get pushed to one side by the force from the field. This means that you get a voltage across the wire perpendicular to the field and direction of current. This is called "the classical Hall effect".
In the "quantum Hall effect", the conductor is cooled lots and the field is made very big. Quantum mechanical effects mean the voltage goes up in discrete steps as the field is varied, rather than changing smoothly in the classical Hall effect ("quantisation"). This only works if the wire is two-dimensional (e.g. a very thin sample).
The paper indicates that due to Graphite being made up of lots of thin layers, the quantum hall effect can occur in graphite.
I don't know why this is important, but it is interesting.
ps: I once did undergraduate labs on quantum hall effect. My overriding memory was that field leakage from the 8 T magnet distorted the nearby PC CRT monitor, due to the distorted trajectories of the electrons coming from the cathode - which made the analysis more interesting than it otherwise would have been.
(Score: 2) by PiMuNu on Thursday March 07 2019, @02:03PM
I didn't say it explicitly, but I meant to write "three-dimensional graphite".