Dresden 2009 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 22: Focused Session: Epitaxial Graphene III
O 22.8: Vortrag
Dienstag, 24. März 2009, 12:45–13:00, SCH 251
Electronic structure of the graphene twist bilayer — •Sam Shallcross1, Sangeeta Sharma2, and Oleg Pankratov1 — 1Lehrstuhl für Theoretische Festkörperphysik, Staudtstr. 7-B2, 91058 Erlangen — 2Institut für Theoretische Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany
While the Bernal stacked AB graphene bilayer has recieved great attention, this represents only one degree of freedom between graphene layers, i.e., the translational degree of freedom. It has recently been realised that the rotational degree of freedom between such layers is in many instances of great importance, e.g. in explaining the single layer graphene type behaviour for graphene layers grown on the C-face of SiC [1,2]. We shall explore this degree of freedom between graphene layers via the simple prototype of the "graphene twist bilayer". This analysis, based on a simple application of Diophantine algebra, leads to the conclusion that such systems may either (i) become completely electronically decoupled for some θ<θ0, or, (ii) decouple only in specific regions of the Brillouin zone. Using a standard tight-binding technique, we thoroughly explore the graphene twist bilayer, and establish that this system belongs to class (ii), and decouples only in the vicinity of the Dirac point, but that this decoupling occurs for all rotation angles. We consider the small angle limit and show that in this limit the Dirac spectrum has a considerably damped Fermi velocity, and exists only in a much reduced energy window.
[1] J. Hass et al., Phys. Rev. Lett., 100:125504, 2008. [2] S. Shallcross, S. Sharma, and O. A. Pankratov, Phys. Rev. Lett., 101:056803, 2008.