Quantum effects found in graphene
Quantum effects found in graphene

Electrons travelling though graphene do not behave like particles but like a wave, an international team of researchers has conclusively demonstrated [Terrés et al., Nature Communications 7 (2016) 11528].

Creating ‘constrictions’ – or very narrow regions – in graphene can reveal the telltale signs of the quantum behavior of electrons. Until now it has proven difficult to detect these signs because of other effects arising from the substrate and the rough edges of graphene interfering with each other.

But by fabricating extremely clean graphene layers sandwiched between layers of hexagonal boron nitride (hBN), the team of researchers from RWTH Aachen University, Forschungszentrum Jülich,Vienna University of Technology, NationalInstitute forMaterials Science in Japan, Lehigh University, and the Institute for Nuclear Research of the Hungarian Academy of Sciences were able to reduce the disorder created by these effects.

When constrictions were created using lithography in the high quality graphene, the team observed the signature of quantum effects in the form of jumps in electric current. These jumps or steps in current arise as electrons try to make their way through the constriction. When the wavelength of an electron is larger than the constriction, it does not fit through the gap and flux is very low. As the energy of the electron increases, its wavelength decreases until – at a certain point – it fits through.

“We have been able to show unambiguously for the first time quantized conductance in graphene constrictions of different widths,” says Christoph Stampfer of RWTH Aachen University.

The findings also demonstrate that the edges of graphene sheets play a crucial role in its behavior.

“As the atoms [in graphene] are arranged in a hexagonal pattern, the edge can never be a completely straight line. On an atomic scale, the edge is always jagged,” explains Florian Libisch of Vienna University of Technology.

Comparison of experimental data with atomic simulations indicates that there are trap states at the edges of graphene constrictions, which have a profound effect on the overall electronic properties. The effects of these trap states are particularly important when the density of charge carriers is low.

“What is surprising about our results is that they show simultaneously quantized conductance and the presence of localized states at the edges,” says Stampfer.

The findings mean that creating constrictions physically – rather than electrostatically as is common in semiconducting materials – is a possible way of realizing quantum electronic devices.

“Demonstrating quantized conductance in graphene shows that, when handled appropriately, this material can become an exciting playground for exploring and exploiting the quantum properties of matter,” says Stampfer.

This article was originally published in Nano Today (2016), doi:10.1016/j.nantod.2016.06.001