Invited Paper GR+EM+NS+SP+TF-MoA3
Electronic Dispersion in Two Overlapping Graphene Sheets: Impacts of Long-Range Atomic Ordering, Periodic Potentials, and Disorder

Monday, October 28, 2013, 2:40 pm, Room 104 B

A worldwide effort is underway to build devices that take advantage of the remarkable electronic properties of graphene and other two-dimensional crystals. An outstanding question is how stacking two or a few such crystals affects their joint electronic behavior. This talk concerns “twisted bilayer graphene (TBG),” that is, two graphene layers azimuthally misoriented. Applying angle-resolved photoemission spectroscopy and density functional theory, we have found van Hove singularities (vHs) and associated mini-gaps in the TBG electronic spectrum, which represent unambiguous proof that the layers interact. Of particular interest is that the measured and calculated electronic dispersions reflect the periodicity of the moiré superlattice formed by the twist. Thus, there are vHs not just where the Dirac cones of the two layers overlap, but also at the boundaries of the moiré superlattice Brillouin zone. Such changes in the electronic dispersion also manifest themselves in TBG’s optical properties. The result is that the material’s color varies depending on the twist angle, and can be seen under optical microscope. Moirés, ubiquitous in hybrid solids based on two-dimensional crystals, accordingly present themselves as tools for manipulating the electronic behavior.

This work is carried out in collaboration with J. T. Robinson, S. W. Schmucker, J. P. Long, J. C. Culbertson, A. L. Friedman at Naval Research Laboratory, P. J. Feibelman, T. E. Beechem, B. Diaconescu, G. L. Kellogg at Sandia National Laboratories, and A. Bostwick, E. Rotenberg at Lawrence Berkeley National Laboratory. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.