Novel quantum effect discovered in naturally occurring graphene
International research team finds atomically-thin carbon generates its
own magnetic field

Date:
October 6, 2021
Source:
University of Go"ttingen
Summary:
Under special circumstances, the electrical resistance of a
material can adopt a fixed value that is independent from the basic
material properties and 'quantized'. This 'quantization' normally
occurs within strong magnetic fields at very low temperatures
when electrons move in two-dimensions. Now, a research team has
succeeded in demonstrating this effect at low temperatures in the
almost complete absence of a magnetic field in naturally occurring
double-layer graphene, just two atoms thick.



FULL STORY ========================================================================== Usually, the electrical resistance of a material depends very much
on its physical dimensions and fundamental properties. Under special circumstances, however, this resistance can adopt a fixed value that is independent of the basic material properties and "quantised" (meaning that
it changes in discrete steps rather than continuously). This quantisation
of electrical resistance normally occurs within strong magnetic fields
and at very low temperatures when electrons move in a two-dimensional
fashion. Now, a research team led by the University of Go"ttingen has
succeeded in demonstrating this effect at low temperatures in the almost complete absence of a magnetic field in naturally occurring double-layer graphene, which is just two atoms thick. The results of the study have
been published in Nature.


==========================================================================
The team from the University of Go"ttingen, Ludwig Maximilian University
of Munich and the University of Texas (Dallas) used two-layer graphene
in its natural form. The delicate graphene flakes are contacted using
standard microfabrication techniques and the flake is positioned
so that it is hangs freely like a bridge, held at the edges by two
metal contacts. The extremely clean double-layers of graphene show a quantisation of electrical resistance at low temperatures and almost undetectable magnetic fields. In addition, the electrical current flows
without any loss of energy. The reason for this is a form of magnetism
that is not generated in the usual way as seen in conventional magnets
(ie by the alignment of the intrinsic magnetic moments of electrons),
but by the motion of the charged particles in the graphene double layer
itself. "In other words, the particles generate their own intrinsic
magnetic field, which leads to the quantisation of the electrical
resistance," says Professor Thomas Weitz from the University of
Go"ttingen.

The reason this effect is special, is not just that it only requires
an electric field, but also that it occurs in eight different versions
that can be controlled by applied magnetic and electric fields. This
results in a high degree of control, because the effect can be switched
on and off and the direction of movement of the charged particles can be reversed. "This makes it a really interesting candidate for potential applications, for example, in the development of innovative computer
components in the field of spintronics, which could have implications
for data storage," says Weitz. "In addition, it is an advantage
that we can show this effect in a system comprising a simple and
naturally occurring material. This is in stark contrast to the recently popularised 'heterostructures', which require a complex and precise
composition of different materials." First, however, the effect must be further investigated and ways to stabilize it at higher temperatures
need to be found, because currently it only occurs at up to five
degrees above absolute zero (the latter being 273 degrees below 0oC)." ========================================================================== Story Source: Materials provided by University_of_Go"ttingen. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Geisenhof et al. Quantum anomalous Hall octet driven by orbital
magnetism
in bilayer graphene. Nature, 2021 DOI: 10.1038/s41586-021-03849-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211006160114.htm

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