Transport and Optical Properties of Quantized Low-Dimensional Systems

In this thesis, we present a systematic investigation of the static and dynamic response properties of low-dimensional systems, using a variety of theoretical techniques ranging from time dependent density functional theory to the recursive Green's function method.

As typical low-dimensional systems, metal nanostructures can strongly interact with an electric field to support surface plasmons, making their optical properties extremely attractive in both fundamental and applied aspects. We have investigated the energy broadening of surface plasmons in metal structures of reduced dimensionality, where Landau damping is the dominant dissipation channel and presents an intrinsic limitation to plasmonics technology. We show that for every prototype class of systems considered, including nanoshells, coaxial nanotubes, and ultrathin films, Landau damping can be drastically tuned due to energy quantization of the individual electron levels and e-h pairs. Both the generic trend and oscillatory nature of the tunability are in stark contrast with the expectations of the semiclassical surface scattering picture.

For a more realistic environment of low-dimensional systems, the effect of a dielectric substrate is considered to mimic the experimental setup. We have studied the dispersion of various plasmon excitations in metal thin films with growth substrates. Our results qualitatively reproduce the experimentally observed plasmon spectra of the Mg/Si systems. The underlying physics for the formation of various absorption peaks can be understood with a simple hybridization concept. Based on this concept, the coexistence of surface and bulk plasmons in experimental observation turns out to be a clear evidence for the existence of multiple-multipole surface plasmons due to the quantum confinement in thin films.

To step into more confined worlds, we choose the real two-dimensional material graphene as our representive system, which is a semi-metal with zero band-gap. As the first step, the static electric response of graphene is investigated by exploring its transport properties. We have studied the pseudospin valve effect in bilayer graphene nanoribbons. The pseudospin degree of freedom is associated with the electron density in two layers and can be controlled by external gate electrodes. We find that the conductance of nanoribbons shows different behaviors compared with infinite systems due to the appearance of edge states and quantum confinement. Remarkably, a large on-off ratio can be achieved in nanoribbons with zigzag edges, even when the Fermi energy lies in the bulk energy gap. The influence of possible edge vacancies and interface conditions is also discussed.

Finally, we discuss the possibility of using plasmon excitations to detach the graphene from its growth substrate, where the dynamic electric response of the graphene-metal system is expected to play a central role.