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Wednesday, 25 April 2012

The quantum Hall effect and graphene nanoscrolls: mystery solved!

Publishing in Physical Review Letters, ICN Group Leader Stephan Roche and colleagues explain why observation of the quantum Hall effect in suspended graphene has proven surprisingly difficult.

According to the researchers, in low to medium intensity magnetic fields, the scrolled edges typical of free-standing and suspended graphene samples enhance short-circuiting of opposite chiral edge states that provide Hall quantisation in strong magnetic fields.

ICN Group Leader, and ICREA and UAB Professor, Dr Stephan Roche and colleagues have just published an article in Physical Review Letters, in which they propose an explanation for an enigma in graphene: the fact that when using conventional methods of electric current detection with low to medium intensity applied magnetic fields (2 to 20 Tesla), scientists have so far failed to observe the quantum Hall effect in graphene samples with scrolled edges, known as graphene nanoscrolls.

In the quantum Hall effect, a well-known phenomenon in 2D materials, application of a magnetic field forces the electric charges in the sample to segregate outwards to the samples' edges, much like a referee forcing the players of two brawling football teams to separate onto either side of the pitch.

When researchers study the quantum behaviour of electronic excitations in graphene (and other materials), they typically create a simple circuit in which current travels from a source contact at one end (or edge) of the material to a drain contact at the opposing end (or edge). This technique gives clear readings of the quantum Hall effect in completely flat graphene (such as samples that are bound to a substrate); in contrast, in free-standing graphene, which generally shows scrolled edges, the effect seems to be quenched.

By measuring the magnetic field along the length and depth of the scrolled edges as well as in the flat portion of graphene samples of this type, Prof Roche and his co-workers have deduced that these edges basically short circuit the very drains used to measure current in these types of experiment, making chiral current generation at the edges nearly impossible.

 

The article can be accessed here.