Abstract Submitted to the    NANOTUBE'04 Conference:

Based carbon-tube quantum dots are shown to exhibit discrete and interface states[1] that may be modulated by changing the dot size. Moreover, metallic quantum dots studied recently [2] and composed of achiral carbon nanotubes, provided conductance gaps and localized electronic states. Interesting physical phenomena such as metal-insulator transition, may also be manipulated by just switching external fields. Here we study the particular metallic dots reported in reference [2] named as (12,0)/(6,6)N/(12,0), N being the number of rings forming the central nanometric dot. The modeled junctions present intercalated pairs of pentagon and heptagon, disposed along the circumferential tube direction and allows the connection of the dot with the metallic contacts (leads). A detailed investigation of the mechanical stability of the dot formation is addressed by using a Monte Carlo simulation and considering an interatomic potential propose by Tersof. The temperature dependence of th e total atomic energy and the formation energy of the carbon structure is calculated. The results show unambiguously that the studied dots are stable structures, composed solely by metallic tubes. Local density of states are obtained via a single-band tight binding Hamiltonian and a superposition of discrete and continuum states are found in the region of the Fermi level. The conductance is calculated within the Landauer formalism The effects of an external magnetic field, applied along the axial direction of the dot, are studied. The vector potential associated with the magnetic field is described within the Peierls approximation. By investigating the local density of states and the system conductance behavior, one finds that metal-insulating-like transitions are induced by turning on the magnetic field. The dependence of the energy spectra on the size of the quantum dot and on the intensity of the magnetic field is reported illustrating new possibilities to induce physica l properties of the studied metallic dots. The shifting of the single-electron levels and the electronic transitions are also analyzed taking into account the spin-magnetic field interaction. An important result is the fact that the associated Aharanov-Bohm oscillation periods of the new dot-carbon-tube structures strongly change when including Zeeman effects in the theoretical description.

[1] C. G. Rocha, T. G. Dargam, and A. Latgé, Phys. Rev. B 65, 165431 (2002).
[2] L. Chico and W. Jaskólski, Phy. Rev. B 69, 85406 (2004).