Cao Y, Luo JY, Fatemi V, Fang S, Sanchez-Yamagishi JD, Watanabe K, Taniguchi T, Kaxiras E, Jarillo-Herrero P.
Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene. PHYSICAL REVIEW LETTERS. 2016;117 (11).
AbstractTwisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moire physics and interlayer hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theoretically predicted. Moreover, Shubnikov-de Haas oscillations and tight binding calculations reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moire wave vector.
Heller EJ, Yang Y, Kocia L, Chen W, Fang S, Borunda M, Kaxiras E.
Theory of Graphene Raman Scattering. ACS NANO. 2016;10 (2) :2803-2818.
AbstractRaman scattering plays a key role in unraveling the quantum dynamics of graphene, perhaps the most promising material of recent times. It is crucial to correctly interpret the meaning of the spectra. It is therefore very surprising that the widely accepted understanding of Raman scattering, i.e., Kramers Heisenberg Dirac theory, has never been applied to graphene. Doing so here, a remarkable mechanism we term''transition sliding'' is uncovered, explaining the uncommon brightness of overtones in graphene. Graphene's dispersive and fixed Raman bands, missing bands, defect density and laser frequency dependence of band intensities, widths of overtone bands, Stokes, anti -Stokes anomalies, and other known properties emerge simply and directly.