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Graphene with a twist

Speaker: 
Hridis Pal (Georgia Tech, USA)
Date: 
Wed, 10/06/2015 - 11:00am to 12:00pm
Location: 
Physics Conference Room (S13-M01-11)
Host: 
CA2DM
Event Type: 
Seminars

Abstract

Graphene has been at the forefront of research for almost a decade, thanks to its unusual electronic properties. It is well known that the properties of this single layer material get modified substantially with the addition of a second layer. The most commonly studied form of graphene bilayers is one where the two layers are mutually rotated by sixty degrees--the so called Bernal stacking. However, recently there has been a surge of interest in bilayer graphene structures where the layers are rotated by an arbitrary angle instead of sixty degrees. The current understanding of these systems may be summarized as follows: at large angles the layers are essentially decoupled, and the system manifests single layer properties. With decreasing angle, however, the Fermi velocity gets renormalized, and at small enough angles, bands flatten and electrons get localized. In this talk, we will show that this current understanding is incomplete: non-trivial physics such as band flattening and localization can happen at large angles as well, as long as the system is sufficiently close to some commensuration. To this end, we formulate a long-wavelength theory near commensuration valid at arbitrary angles of rotation, generalizing existing long-wavelength theories valid only at small angles. We then use our theory to show that in the stong coupling limit and at large angles, the system becomes locally gapped and mimics the properties of a Kagome-like lattice. We discuss the implications of our model.

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