A new study performed by an international research team under the guidance of the University of Göttingen determines the fact that atomically thin carbon produces its own magnetic field.
Scanning electron microscope image of a contacted, two-atom-thick, free-floating graphene flake with a free-floating metal bridge hovering above it. Image Credit: Fabian Geisenhof/Jakob Lenz.
Generally, the electrical resistance of a material relies greatly on its basic properties and physical dimensions. Yet, under special conditions, this resistance has the ability to adopt a determined value that is independent of the fundamental material properties and “quantized” (implying that it varies in discrete steps instead of changing constantly).
Mostly, this quantization of electrical resistance takes place within powerful magnetic fields and at very low temperatures when electrons tend to move in a two-dimensional fashion. Currently, a research group headed by the University of Göttingen has been successful in illustrating this effect at low temperatures in the total absence of a magnetic field in naturally occurring double-layer graphene, which is merely two atoms thick.
The findings of the study have been published in the journal Nature.
The research team from the University of Göttingen, Ludwig Maximilian University of Munich, and the University of Texas (Dallas) has utilized two-layer graphene in its natural form. The contact of fragile graphene flakes is done with the help of standard microfabrication methods, and the flake is placed so that it freely hangs, similar to a bridge, and connected at the edges with the help of two metal contacts.
A quantization of electrical resistance at low temperatures and nearly undetectable magnetic fields has been displayed by the highly clean double-layers of graphene. The electrical current also flows without losing energy.
The reason behind this mechanism is a form of magnetism that is not produced in the normal way as seen in traditional magnets (that is, by the arrangement 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 quantization of the electrical resistance.
Thomas Weitz, Professor, Institute of Physics, University of Göttingen
The reason why this effect is considered to be unique is not just that it only needs an electric field, but also that it takes place in eight various versions that could be regulated by applied electric and magnetic fields. This leads to a high degree of control since the effect can be enabled and disabled and the direction of movement of the charged particles can also 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.
Thomas Weitz, Professor, Institute of Physics, University of Göttingen
Weitz added, “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 popularized ‘heterostructures’, which require a complex and precise composition of different materials.”
Yet, the effect should first be further analyzed, and ways to stabilize it at higher temperatures need to be discovered. This has to be done as at present it can only occur at up to 5 degrees above absolute zero (the latter being 273 °C below 0 °C).”
Journal Reference:
Geisenhof, F. R., et al. (2021) Quantum anomalous Hall octet driven by orbital magnetism in bilayer graphene. Nature. doi.org/10.1038/s41586-021-03849-w.