Backscattering in carbon nanotubes : role of quantum interference effects

For similar disorder, the backscattering contribution to the conductivity, irrelevant for metallic single-walled carbon nanotubes, is proved to become more significant for doped semiconducting systems, as found in experiments. In the case of multi-walled nanotubes, the intershell coupling is further shown to enhance the contribution of backscattering for "metallic" double-walled, whereas it remains insignificant for "metallic/semiconducting" double-walled systems. This supports that MWNTs are long ballistic conductors close to the charge neutrality point.Comment: 8 pages, 3 figure

Competition between magnetic field dependent band structure and coherent backscattering in multiwall carbon nanotubes

Magnetotransport measurements in large diameter multiwall carbon nanotubes (20-40 nm) demonstrate the competition of a magnetic-field dependent bandstructure and Altshuler-Aronov-Spivak oscillations. By means of an efficient capacitive coupling to a backgate electrode, the magnetoconductance oscillations are explored as a function of Fermi level shift. Changing the magnetic field orientation with respect to the tube axis and by ensemble averaging, allows to identify the contributions of different Aharonov-Bohm phases. The results are in qualitative agreement with numerical calculations of the band structure and the conductance.Comment: 4 figures, 5 page

Spectral and Diffusive Properties of Silver-Mean Quasicrystals in 1,2, and 3 Dimensions

Spectral properties and anomalous diffusion in the silver-mean (octonacci) quasicrystals in d=1,2,3 are investigated using numerical simulations of the return probability C(t) and the width of the wave packet w(t) for various values of the hopping strength v. In all dimensions we find C(t)\sim t^{-\delta}, with results suggesting a crossover from \delta<1 to \delta=1 when v is varied in d=2,3, which is compatible with the change of the spectral measure from singular continuous to absolute continuous; and we find w(t)\sim t^{\beta} with 0<\beta(v)<1 corresponding to anomalous diffusion. Results strongly suggest that \beta(v) is independent of d. The scaling of the inverse participation ratio suggests that states remain delocalized even for very small hopping amplitude v. A study of the dynamics of initially localized wavepackets in large three-dimensional quasiperiodic structures furthermore reveals that wavepackets composed of eigenstates from an interval around the band edge diffuse faster than those composed of eigenstates from an interval of the band-center states: while the former diffuse anomalously, the latter appear to diffuse slower than any power law.Comment: 11 pages, 10 figures, 1 tabl

Conduction mechanism and magnetotransport in multi-walled carbon nanotubes

We report on a numerical study of quantum diffusion over micron lengths in defect-free multi-walled nanotubes. The intershell coupling allows the electron spreading over several shells, and when their periodicities along the nanotube axis are incommensurate, which is likely in real materials, the electronic propagation is shown to be non ballistic. This results in magnetotransport properties which are exceptional for a disorder free system, and provides a new scenario to understand the experiments (A. Bachtold et al. Nature 397, 673 (1999)).Comment: 4 page

Spin transport in disordered single-wall carbon nanotubes contacted to ferromagnetic leads

Recent conductance measurements on multi-wall carbon nanotubes (CNTs) reveal an effective behavior similar to disordered single-wall CNTs. This is due to the fact that electric current flows essentially through the outermost shell and is strongly influenced by inhomogeneous electrostatic potential coming from the inner tubes. Here, we present theoretical studies of spin-dependent transport through disorder-free double-wall CNTs as well as single-wall CNTs with Anderson-type disorder. The CNTs are end-contacted to ferromagnetic electrodes modelled as fcc (111) surfaces. Our results shed additional light on the giant magnetoresistance effect in CNTs. Some reported results concern realistically long CNTs, up to several hundred nanometers.Comment: 9 pages, 5 figures, presented at the European Conference PHYSICS OF MAGNETISM 2005, Poznan, Polan

Electronic conduction in multi-walled carbon nanotubes: Role of intershell coupling and incommensurability

Geometry incommensurability between weakly coupled shells in multi-walled carbon nanotubes is shown to be the origin of unconventional electronic conduction mechanism, power-law scaling of the conductance, and remarkable magnetotransport and low temperature dependent conductivity when the dephasing mechanism is dominated by weak electron-electron coupling

Gate-dependent magnetoresistance phenomena in carbon nanotubes

We report on the first experimental study of the magnetoresistance of double-walled carbon nanotubes under magnetic field as large as 50 Tesla. By varying the field orientation with respect to the tube axis, or by gate-mediated shifting the Fermi level position, evidences for unconventional magnetoresistance are presented and interpreted by means of theoretical calculations

Atomistic Boron-Doped Graphene Field Effect Transistors: A Route towards Unipolar Characteristics

We report fully quantum simulations of realistic models of boron-doped graphene-based field effect transistors, including atomistic details based on DFT calculations. We show that the self-consistent solution of the three-dimensional (3D) Poisson and Schr\"odinger equations with a representation in terms of a tight-binding Hamiltonian manages to accurately reproduce the DFT results for an isolated boron-doped graphene nanoribbon. Using a 3D Poisson/Schr\"odinger solver within the Non-Equilibrium Green's Functions (NEGF) formalism, self-consistent calculations of the gate-screened scattering potentials induced by the boron impurities have been performed, allowing the theoretical exploration of the tunability of transistor characteristics. The boron-doped graphene transistors are found to approach unipolar behavior as the boron concentration is increased, and by tuning the density of chemical dopants the electron-hole transport asymmetry can be finely adjusted. Correspondingly, the onset of a mobility gap in the device is observed. Although the computed asymmetries are not sufficient to warrant proper device operation, our results represent an initial step in the direction of improved transfer characteristics and, in particular, the developed simulation strategy is a powerful new tool for modeling doped graphene nanostructures.Comment: 7 pages, 5 figures, published in ACS Nan

Contact-dependent effects and tunneling currents in DNA molecules

We report on theoretical results about contact-dependent effects and tunneling currents through DNA molecules. A tetranucleotide PolyGACT chain, connected in between metallic contacts, is studied as a generic case, and compared to other periodic sequences such as PolyAT or PolyGC. Remarkable resonance conditions are analytically derived, indicating that a strong coupling does not always result in a larger conductance. This result is properly illustrated by considering intrinsic features of bias-dependent tunneling currents in the coherent regime