Two-dimensional transition-metal dichalcogenides (TMDs) are gaining increasing attention as alternative to graphene for their very high potential in optoelectronics applications. Here we consider two prototypical metallic 2D TMDs, NbSe$_2$ and TaS$_2
$. Using a first-principles approach, we investigate the properties of the localised intraband $d$ plasmon that cannot be modelled on the basis of the homogeneous electron gas. Finally, we discuss the effects of the reduced dimensionality on the plasmon dispersion through the interplay between interband transitions and local-field effects. This result can be exploited to tune the plasmonic properties of these novel 2D materials.
For atomic thin layer insulating materials we provide an exact analytic form of the two-dimensional screened potential. In contrast to three-dimensional systems where the macroscopic screening can be described by a static dielectric constant in 2D sy
stems the macroscopic screening is non local (q-dependent) showing a logarithmic divergence for small distances and reaching the unscreened Coulomb potential for large distances. The cross-over of these two regimes is dictated by 2D layer polarizability that can be easily computed by standard first-principles techniques. The present results have strong implications for describing gap-impurity levels and also exciton binding energies. The simple model derived here captures the main physical effects and reproduces well, for the case of graphane, the full many-body GW plus Bethe-Salpeter calculations. As an additional outcome we show that the impurity hole-doping in graphane leads to strongly localized states, what hampers applications in electronic devices. In spite of the inefficient and nonlocal two-dimensional macroscopic screening we demonstrate that a simple $mathbf{k}cdotmathbf{p}$ approach is capable to describe the electronic and transport properties of confined 2D systems.
Recently, a new organic superconductor, K-intercalated Picene with high transition temperatures $T_c$ (up to 18,K) has been discovered. We have investigated the electronic properties of the undoped relative, solid picene, using a combination of exper
imental and theoretical methods. Our results provide detailed insight into the occuopied and unoccupied electronic states.
Using first principles many-body theory methods (GW+BSE) we demonstrate that optical properties of graphane are dominated by localized charge-transfer excitations governed by enhanced electron correlations in a two-dimensional dielectric medium. Stro
ng electron-hole interaction leads to the appearance of small radius bound excitons with spatially separated electron and hole, which are localized out-of-plane and in-plane, respectively. The presence of such bound excitons opens the path on excitonic Bose-Einstein condensate in graphane that can be observed experimentally.
