An international team has discovered that stacked layers of the intriguing carbon material graphene do not behave in quite the way that theory predicts; there's a twist.

Graphene has become the focus of much research in recent years with its potential to weave the fabric of a future of molecular electronics devices. The unique properties, of what is essentially a monolayer of graphite, make graphene the ideal material for developing novel technologies. When prepared using chemical vapor deposition (CVD) graphene layers often form stacks, which researchers hoped might be exploitable in different ways. New research suggests that the behavior of the layered form is more interesting than it at first appears.
Theoretically, at least, it seemed that stacking up graphene sheets to form a kind of pure analogue of graphite, rather than working with the familiar single sheet form, might open up another avenue of investigation, because of the coupling that can occur between the layers. In this case the massless low-energy excitations of graphene would disappear. However, experiments suggest otherwise, and paradoxically the properties of the single-layer form seem to persist in stacked layers, but with a twist.
An international collaboration led by Eva Andrei of Rutgers University in Piscataway, New Jersey, and Nobel physicists Andre Geim and Konstantin Novoselev of the University of Manchester, UK, together with Jing Kong from the Massachusetts Institute of Technology, Cambridge, USA, believe they have a solution to this anomaly. The team investigated the behavior of stacked graphene layers using high magnetic field scanning tunneling microscopy (STM). This technique can measure precisely the distances between layers and any twist angle. They also used Landau level spectroscopy to identify relativistic quasiparticles and measure their speed [Luican et al., Phys Rev Lett (2011) 106, 126802].
They have now shown that stacked layers of graphene can twist with respect to each other and that this affects the band structure in such a way that the predicted effects of stacking are lost, at least at twist angles above 20 degrees. When twisted to this degree the single-layer behavior is recovered. At smaller twist angles the team found new and surprising behavior which they attribute to coupling between the layers. In particular, they discovered that the twist-induced coupling slows down the relativistic quasiparticles, known as massless Dirac fermions, present in the structure. At the smallest twist-angles the quasiparticles become very slow until eventually at about 1 degree their motion stops altogether. This work highlights the intriguing possibilities of creating qualitative changes in the electronic properties of layered materials with a mere twist.
David Bradley

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