Graphene supercurrents go ballistic (article in Nature Nanotechnology of 27 July 2015 by Delft and Leiden researchers Victor Calado, Srijit Goswami, Gaurav Nanda, Mathias Diez, Anton Akhmerov, Teun Klapwijk & Lieven Vandersypen and others)


(By: Web communication TU Delft)
A team of TU Delft and Leiden University scientists have observed supercurrents in graphene that bounce back and forth between the edges of the graphene without scattering along the way. Supercurrents are electrical currents that flow even when there is no voltage applied. They can be induced in graphene by bringing it in contact with a superconducting material. The ability to create such ballistic superconductor-graphene hybrids makes it possible to study the unique properties of supercurrents carried by relativistic particles in an unexplored regime. These results have been published in Nature Nanotechnology.

Ultra-clean graphene
The use of high-quality graphene is of vital importance for the performance of these devices. Being an atomically thin material, graphene is extremely sensitive to the ruggedness of its support structure and all dirt down to the atomic scale. In particular, during typical nanofabrication processes graphene is inevitably exposed to several polymers and chemicals, which easily stick to its surface making it dirty and thus degrading its electronic quality.

Graphene sandwiched between boron nitride layers and contacted to a superconductor – molybdenum rhenium (MoRe).

In order to circumvent this, the researchers first sandwich the graphene between two thin layers of boron nitride – an atomically flat insulator. This encasing effectively preserves the graphene in its pristine state by protecting it from the outside world. Finally, this stack is cut to the desired shape and the graphene is contacted from the side to the superconducting material.

A mirror for electron waves
When light bounces back and forth between two mirrors, the principle of superposition for waves gives rise to maxima and minima in the light intensity, producing a characteristic interference pattern. In analogy, the researchers here observe a similar interference in graphene due to electron waves reflecting many times off the walls of the graphene with the superconductor. This interference can only be observed in the cleanest of samples, where it is possible for electrons to move in ballistic trajectories with very little scattering from impurities and defects. When the graphene is superconducting, these ballistic effects show up as a striking periodic modulation of the supercurrent. This is the first direct observation of ballistic mirroring of supercurrents in any two-dimensional system.

Periodic oscillations in the supercurrent due to interference of electron waves. 

Article: Ballistic Josephson junctions in edge-contacted graphene (DOI 10.1038/nnano.2015.156)