Clean edges play dirty trick on the Quantum Hall Effect in graphene devices
The quantum Hall effect in two-dimensional electron systems is normally considered a very robust phenomenon. The circulating currents of the Hall effect evolve into quantum mechanical edge states that each carry a current that is fixed by natural constants: the electron charge e and the Planck constant h. In graphene, Quantum Hall resistance is locked within one-tenth of a billion part to the fraction h/e2 , making it the most accurate resistance measurement standard ever reported.
A team of researchers from Technical University of Denmark with collaborators from Catalan Institute of Nanoscience and Nanotechnology and Universitat Autonoma De Barcelona have looked more closely at the Hall effect in narrow graphene devices – so-called graphene nanoribbons and nanoconstrictions. They found that the conductance quantization in a high magnetic field was destroyed when reducing the edge roughness, contrary to expectations.
As it turned out, this effect was predicted by several theoretical physicists 20 years ago, but they were largely ignored since no experimental evidence was ever found.
In the experiments, Jose Caridad and co-workers compared nanoconstrictions and found that edges with edge roughness reduced to one nanometer by carefully tuned etching, led to the disruption of the quantized current. Numerical calculations of transport through nanoribbons with different edge roughness confirmed this picture.
The explanation has to do with how the carriers distribute in the narrow channel when controlled by an electrostatic gate. The team found that the distribution of charge carriers, and thus the electrostatic potential, is highly sensitive to the edge morphology. On pristine, perfect edges, a much stronger charge accumulation occurs near the edges, and this distortion of the potential is what ultimately leads to formation of new edge channels inside the graphene device. These edge channels are much more sensitive to changes in the gate potential and the magnetic field, and ultimately sabotage the quantization characteristic of the quantum Hall effect.
The figure illustrates a graphene constriction, which in real life have widths down to 50 nm. The perpendicular magnetic field causes strong edge currents (red arrows) to creep along the edges. Screening effects, however, induce non-uniform charge accumulation, which becomes particularly sharp near pristine edges. The accumulated charges ultimately distort the electrostatic potential, allowing new edge states to appear (blue arrows).
Peter Bøggild from the Technical University of Denmark believes that this is just the beginning: "We have lately seen several examples of cleaner and more perfect structures, to exhibit new, more complex electrical behavior, and this is truly fascinating. It seems like the removal of disorder unlocks unexpected behavior – and besides the obvious fundamental aspects, it may be crucial for applications relying on extreme miniaturization of low-dimensional materials in electronics, spintronics and plasmonics."
More information:
José M. Caridad et al. Conductance quantization suppression in the quantum Hall regime, Nature Communications (2018). DOI: 10.1038/s41467-018-03064-8
Provided by Technical University of Denmark