A theoretical discussion of electronic and transport properties of a particular family of double-wall carbon nanotubes, named commensurate structures of the armchair type (n,n)@(2n,2n) is addressed. A single pi-band tight binding hamiltonian is considered and the magnetic field is theoretically described by following the Peierls approximation into the hopping energies. Our emphasis is put on investigating the main effects of the geometrical aspects and relative positions of the tubes on the local density of states and on the conductance of the system. By considering intershell interactions between a set of neighboring atoms on the walls of the inner and outer tubes, we study the possibility of founding Aharonov-Bohm effects in the DWCNs when a magnetic field is applied along the axial direction.

We study the optical properties of carbon nanotubes within the single-band tight-binding approximation.The great number of one-dimensional-like subbands of the nanotubes lead to typical van Hove singularities on the local density of states. When a tube is under the influence of a laser beam, optical transitions are allowed between Van Hove singularities and they can be observed experimentally in absorption spectra. External magnetic and electric fields modify the energy spectrum of carbon nanotubes inducing changes on the optical phenomena at low frequencies. Results for the inter-band absorption coefficient are shown for nanotubes under an additional external electrostatic potential for parallel light polarization. This is an important problem to understand since low energy laser beams are being used to identify the chirality and diameter of the tubes and their electronic character.

Coherent properties and Rabi oscillations in two-level donor systems, under terahertz excitation, are theoretically investigated. Here we are concerned with donor states in bulk GaAs and GaAs - (Ga, Al)As quantum dots. We study confinement effects, in the presence of an applied magnetic field, on the electronic and on-center donor states in GaAs - (Ga, Al)As dots, as compared to the situation in bulk GaAs, and estimate some of the associated decay rate parameters. Using the optical Bloch equations with damping, we study the time evolution of the 1s and 2p+ states in the presence of an applied magnetic field and of a terahertz laser. We also discuss the role played by the distinct dephasing rates on the photocurrent and calculate the electric dipole transition moment. Results indicate that the Rabi oscillations are more robust as the total dephasing rate diminishes, corresponding to a favorable coherence time.

The possibility of coupling two Y-junctions of carbon nanotubes, forming a type of nanotube ring is addressed here. Recent experimental evidences of growing carbon nanotube structures forming basically Y-kind junctions give support to this theoretical proposition. A single-band tight-binding Hamiltonian is considered and local electronic properties are investigated via the Green function formalism following renormalization techniques. Modulation on the transport properties of those nanostructured systems may be induced by considering different kind of junctions and different symmetries. Quantum interference phenomena are proven to reduce partial or completely the conductance of such nanoring structures.

Nanotubes have been proved as promising candidates for many technological applications in the nanoscale word and different physical properties have been studied and measured along the few recent years. Here we investigate the role played by external magnetic and electric fields on the electronic properties of toroidal and cylindrical straight carbon nanotubes. A single-p band tight-binding Hamiltonian is used and two types of model-calculations are adopted: real-space renormalization techniques, based on Green function formalism, and straight diagonalization calculation. Both electric and magnetic fields may be properly applied, in different configurations, to modify the energy spectra and transport properties, providing metal-insulator transitions for particular tube geometries.

We present a theoretical study on local electronic properties of quantum-dot nanotubes formed by connecting pure semiconducting and metallic nanotubes via appropriate junctions. The junctions are formed by introducing pair-defects composed by heptagon and pentagon along the axial direction of pure nanotubes. We investigate the dependence of the confined electronic states with the characteristic sizes of the quantum dot taking it account different based-nanotube heterostructures. Quantum-well like and interface states are characterized by investigating the spatial dependence of the local density of states of the discrete levels. We follow the Green function formalism and adopt real-space renormalization techniques in the derivation of local density of states.

We extend the dressed-atom approach to treat the interaction of a laser field with a semiconductor system. The semiconductor is modeled via a simple Kane band-structure scheme and the interaction with the laser field is incorporate through the renormalization of the semiconductor energy gap and conduction/valence effective masses. Far from resonances, such one-body approach allows the study of the effects of laser fields on a variety of optoelectronic phenomena in semiconductor systems for which the effective-mass approximation provides a good physical description. We calculate the effects originated by the laser-dressing on the donor and exciton peack energies in quantum-well heterostructures, and show that they may be quite considerable and observable.