In this work, the optical responses of graphene superlattices, i.e. graphene subjected to a periodic scalar potential, are theoretically reported. The optical properties were studied by investigating the optical conductivity, which was calculated using the Kubo formalism. It was found that the optical conductivity becomes dependent on the photon polarization and is suppressed in the photon energy range of (0, Ub), where Ub is the potential barrier height. In the higher photon energy range, i.e. Ω > Ub, the optical conductivity is, however, almost identical to that of pristine graphene. Such behaviors of the optical conductivity are explained microscopically through the analysis of the elements of optical matrices and effectively through a simple model, which is based on the Pauli blocking mechanism.
Using an empirical atomistic approach associated with the method of random‐phase approximation, we investigated the behaviors of plasmons in heavily doped graphene. We show that, in the range of low energy and of long wavelength, a novel plasmon mode may appear below the conventional Dirac plasmon. The novel mode is strongly anisotropic and only appears in a finite range of the transfer wave number. It is found that the appearance of the novel plasmon stems from the anisotropy of the energy surfaces forming the Dirac cones and the inequivalance of the two Dirac valleys in the Brillouin zone. Interestingly, we demonstrate that these two unique electronic features of graphene act, respectively, as the necessary and sufficient factors in governing the formation of the novel plasmon mode.
The back‐cover page is designed to highlight a new understanding of the low‐energy plasmonic behaviours of graphene, including the possibility of observing a special plasmon mode beside the conventional mode of two‐dimensional fermions in the case of heavily doped graphene, and the anisotropy of the plasmon modes – cf. the article by Van Nam Do and coworkers on http://doi.wiley.com/10.1002/pssb.201552791. The background image shows a hexagonal lattice for graphene, shaded in form of a set of color rings – not in the shape of circles, but of rounded triangles. These color rings illustrate the anisotropic propagation of the electron density oscillation in the graphene lattice as waves, i.e. plasmons. The anisotropy and the formation of plasmon modes stem from the anisotropy and the inequivalence of the energy surfaces in the two Dirac valleys centered at the K‐ and K'‐points in the Brillouin zone. These points are highlighted by the picture of the distribution of the group velocity of electron states together with the energy contours (see right image). The two left images, showing the electron energy loss spectrum and the plasmonic dispersion curves, respectively, point to the main finding of the novel plasmon mode.
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