Radiation effects on the electronic structure of bilayer graphene

We report on the effects of laser illumination on the electronic properties of bilayer graphene. By using Floquet theory combined with Green's functions we unveil the appeareance of laser-induced gaps not only at integer multiples of $\hbar \Omega /2$ but also at the Dirac point with features which are shown to depend strongly on the laser polarization. Trigonal warping corrections are shown to lead to important corrections for radiation in the THz range, reducing the size of the dynamical gaps. Furthermore, our analysis of the topological properties at low energies reveals that when irradiated with linearly polarized light, ideal bilayer graphene behaves as a trivial insulator, whereas circular polarization leads to a non-trivial insulator per valley.Comment: 5 pages 3 figure

Crafting zero-bias one-way transport of charge and spin

We explore the electronic structure and transport properties of a metal on top of a (weakly coupled) two-dimensional topological insulator. Unlike the widely studied junctions between topological non-trivial materials, the systems studied here allow for a unique bandstructure and transport steering. First, states on the topological insulator layer may coexist with the gapless bulk and, second, the edge states on one edge can be selectively switched-off, thereby leading to nearly perfect directional transport of charge and spin even in the zero bias limit. We illustrate these phenomena for Bernal stacked bilayer graphene with Haldane or intrinsic spin-orbit terms and a perpendicular bias voltage. This opens a path for realizing directed transport in materials such as van der Waals heterostructures, monolayer and ultrathin topological insulators.Comment: 7 pages, 7 figure

Electronic properties of twisted bilayer nanoribbons

We study the band structure, density of states, and spatial localization of edge states in twisted bilayer graphene nanoribbons. We devise these ribbons by cutting a stripe of commensurate twisted bilayer graphene along a direction with a maximum number of zigzag edge atoms. Due to the spatially inhomogeneous interlayer coupling, edge states stemming from regions with AB stacking are closer to the energy of the Dirac point, whereas those arising from AA-stacked edge states are split in energy due to the stronger interlayer coupling. As opposed to bulk bilayer graphene, for which states near the Dirac point are localized in AA-stacked regions, the interplay of edge and moiré localization produces a distinct spatial distribution of low-energy states in these ribbons. © 2014 American Physical Society.This work has been partially supported by MEC-Spain under Grant No. FIS2012-33521. E.S.M. acknowledges FONDECYT Grant No. 11130129. M.P. acknowledges FONDECYT Grant No. 1100672 and DGIP/USM Internal Grant No. 11.11.62.Peer Reviewe

Electronic properties of twisted bilayer nanoribbons

We study the band structure, density of states, and spatial localization of edge states in twisted bilayer graphene nanoribbons. We devise these ribbons by cutting a stripe of commensurate twisted bilayer graphene along a direction with a maximum number of zigzag edge atoms. Due to the spatially inhomogeneous interlayer coupling, edge states stemming from regions with AB stacking are closer to the energy of the Dirac point, whereas those arising from AA-stacked edge states are split in energy due to the stronger interlayer coupling. As opposed to bulk bilayer graphene, for which states near the Dirac point are localized in AA-stacked regions, the interplay of edge and moiré localization produces a distinct spatial distribution of low-energy states in these ribbons. © 2014 American Physical Society.This work has been partially supported by MEC-Spain under Grant No. FIS2012-33521. E.S.M. acknowledges FONDECYT Grant No. 11130129. M.P. acknowledges FONDECYT Grant No. 1100672 and DGIP/USM Internal Grant No. 11.11.62.Peer Reviewe

Optical-phonon resonances with saddle-point excitons in twisted-bilayer graphene

Twisted-bilayer graphene (tBLG) exhibits van Hove singularities in the density of states that can be tuned by changing the twisting angle $\theta$. A $\theta$-defined tBLG has been produced and characterized with optical reflectivity and resonance Raman scattering. The $\theta$-engineered optical response is shown to be consistent with persistent saddle-point excitons. Separate resonances with Stokes and anti-Stokes Raman scattering components can be achieved due to the sharpness of the two-dimensional saddle-point excitons, similar to what has been previously observed for one-dimensional carbon nanotubes. The excitation power dependence for the Stokes and anti-Stokes emissions indicate that the two processes are correlated and that they share the same phonon.Comment: 5 pages, 6 figure