163 research outputs found
The Effect of Structural Distortions on the Electronic Structure of Carbon Nanotubes
We calculated the effects of structural distortions on the electronic
structure of carbon nanotubes. The key modification of the electronic structure
brought about by bending a nanotube involves an increased mixing of
and -states. This mixing leads to an enhanced density-of-states in the
valence band near the Fermi energy region. While in a straight tube the states
accessible for electrical conduction are essentially pure C()-states,
they acquire significant C() character upon bending. Bending also
leads to a charge polarization of the C-C bonds in the deformed region
reminiscent of interface dipole formation. Scattering of conduction electrons
at the distorted regions may lead to electron localization at low temperatures.Comment: 11 pages and 4 figures, (figure 4 corrected
The structure relaxation of carbon nanotube
A simple macroscopic continuum elasticity theory (CET) is used to calculate
the structure relaxation of single-wall carbon nanotube (SWNT), an analytic
formula is obtained. We also expand an atomic scale three-parameter empirical
model [ T. Lenosky {\emph et al.} Nature 355, 333(1992)] in order to correctly
describe the bond-length change effects. The structure relaxation of SWNT
expected by the model is good in agreement with our CET results, and very well
consistent with the previous calculation from a first principles local density
function approximation. Using the expanded Lenosky model, we calculate the
strain energy of bending tube. The obtained results are good in agreement with
the previous theoretical expectation. It shows the model may be a good simple
replacement of some more sophisticated methods on determining carbon networks
deformations.Comment: 9 pages, 4 eps figure
Electronic states of metallic and semiconducting carbon nanotubes with bond and site disorder
Disorder effects on the density of states in carbon nanotubes are analyzed by
a tight binding model with Gaussian bond or site disorder. Metallic armchair
and semiconducting zigzag nanotubes are investigated. In the strong disorder
limit, the conduction and valence band states merge, and a finite density of
states appears at the Fermi energy in both of metallic and semiconducting
carbon nanotubes. The bond disorder gives rise to a huge density of states at
the Fermi energy differently from that of the site disorder case. Consequences
for experiments are discussed.Comment: Phys. Rev. B: Brief Reports (to be published). Related preprints can
be found at http://www.etl.go.jp/~harigaya/NEW.htm
Van Hove Singularities in disordered multichannel quantum wires and nanotubes
We present a theory for the van Hove singularity (VHS) in the tunneling
density of states (TDOS) of disordered multichannel quantum wires, in
particular multi-wall carbon nanotubes. We assume close-by gates which screen
off electron-electron interactions. Diagrammatic perturbation theory within a
non-crossing approximation yields analytical expressions governing the
disorder-induced broadening and shift of VHS's as new subbands are opened. This
problem is nontrivial because the (lowest-order) Born approximation breaks down
close to the VHS. Interestingly, compared to the bulk case, the boundary TDOS
shows drastically altered VHS, even in the clean limit.Comment: 4 pages, 2 figures, accepted with revisions in PR
Magnetoresistance Effect in Spin-Polarized Junctions of Ferromagnetically Contacting Multiple Conductive Paths: Applications to Atomic Wires and Carbon Nanotubes
For spin-polarized junctions of ferromagnetically contacting multiple
conductive paths, such as ferromagnet (FM)/atomic wires/FM and FM/carbon
nanotubes/FM junctions, we theoretically investigate spin-dependent transport
to elucidate the intrinsic relation between the number of paths and conduction,
and to enhance the magnetoresistance (MR) ratio. When many paths are randomly
located between the two FMs, electronic wave interference between the FMs
appears, and then the MR ratio increases with increasing number of paths.
Furthermore, at each number of paths, the MR ratio for carbon nanotubes becomes
larger than that for atomic wires, reflecting the characteristic shape of
points in contact with the FM.Comment: 7 pages, 3 figures, accepted for publication in Phys. Rev.
Multiple Functionality in Nanotube Transistors
Calculations of quantum transport in a carbon nanotube transistor show that
such a device offers unique functionality. It can operate as a ballistic
field-effect transistor, with excellent characteristics even when scaled to 10
nm dimensions. At larger gate voltages, channel inversion leads to resonant
tunneling through an electrostatically defined nanoscale quantum dot. Thus the
transistor becomes a gated resonant tunelling device, with negative
differential resistance at a tunable threshold. For the dimensions considered
here, the device operates in the Coulomb blockade regime, even at room
temperature.Comment: To appear in Phys. Rev. Let
Negative differential resistance in nanotube devices
Carbon nanotube junctions are predicted to exhibit negative differential
resistance, with very high peak-to-valley current ratios even at room
temperature. We treat both nanotube p-n junctions and undoped
metal-nanotube-metal junctions, calculating quantum transport through the
self-consistent potential within a tight-binding approximation. The undoped
junctions in particular may be suitable for device integration.Comment: 4 pages, 4 figures, to appear in Physical Review Letter
Charge Screening Effect in Metallic Carbon Nanotubes
Charge screening effect in metallic carbon nanotubes is investigated in a
model including the one-dimensional long-range Coulomb interaction. It is
pointed out that an external charge which is being fixed spatially is screened
by internal electrons so that the resulting object becomes electrically
neutral. We found that the screening length is given by about the diameter of a
nanotube.Comment: 11 pages, 6 figure
Oscillator Strength of Metallic Carbon Nanotubes
Based on the tight binding method with hopping integral between the
nearest-neighbor atoms, an oscillator strength \int_0^{\infty} \d \omega {\rm
Re} \sigma (\omega) is discussed for armchair and metallic zigzag carbon
nanotubes. The formulae of the oscillator strength are derived for both types
of nanotubes and are compared with the result obtained by a linear chain model.
In addition, the doping dependence is investigated in the absence of Coulomb
interaction. It is shown that the oscillator strength of each carbon nanotube
shows qualitatively the same doping dependence, but the fine structure is
different due to it's own peculiar band structure. Some relations independent
of the radius of the tube are derived, and a useful formula for determining the
amount of doping is proposed.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jpn. at June 30, 200
Universality of electron correlations in conducting carbon nanotubes
Effective low-energy Hamiltonian of interacting electrons in conducting
single-wall carbon nanotubes with arbitrary chirality is derived from the
microscopic lattice model. The parameters of the Hamiltonian show very weak
dependence on the chiral angle, which makes the low energy properties of
conducting chiral nanotubes universal. The strongest Mott-like electron
instability at half filling is investigated within the self-consistent harmonic
approximation. The energy gaps occur in all modes of elementary excitations and
estimate at eV.Comment: 4 pages, 2 figure
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