300 research outputs found
Size, Shape and Low Energy Electronic Structure of Carbon Nanotubes
A theory of the long wavelength low energy electronic structure of
graphite-derived nanotubules is presented. The propagating electrons are
described by wrapping a massless two dimensional Dirac Hamiltonian onto a
curved surface. The effects of the tubule size, shape and symmetry are included
through an effective vector potential which we derive for this model. The rich
gap structure for all straight single wall cylindrical tubes is obtained
analytically in this theory, and the effects of inhomogeneous shape
deformations on nominally metallic armchair tubes are analyzed.Comment: 5 pages, 3 postscript figure
Electric Polarization of Heteropolar Nanotubes as a Geometric Phase
The three-fold symmetry of planar boron nitride, the III-V analog to
graphene, prohibits an electric polarization in its ground state, but this
symmetry is broken when the sheet is wrapped to form a BN nanotube. We show
that this leads to an electric polarization along the nanotube axis which is
controlled by the quantum mechanical boundary conditions on its electronic
states around the tube circumference. Thus the macroscopic dipole moment has an
{\it intrinsically nonlocal quantum} mechanical origin from the wrapped
dimension. We formulate this novel phenomenon using the Berry's phase approach
and discuss its experimental consequences.Comment: 4 pages with 3 eps figures, updated with correction to Eqn (9
Electronic Structure of Carbon Nanotube Ropes
We present a tight binding theory to analyze the motion of electrons between
carbon nanotubes bundled into a carbon nanotube rope. The theory is developed
starting from a description of the propagating Bloch waves on ideal tubes, and
the effects of intertube motion are treated perturbatively in this basis.
Expressions for the interwall tunneling amplitudes between states on
neighboring tubes are derived which show the dependence on chiral angles and
intratube crystal momenta. We find that conservation of crystal momentum along
the tube direction suppresses interwall coherence in a carbon nanorope
containing tubes with random chiralities. Numerical calculations are presented
which indicate that electronic states in a rope are localized in the transverse
direction with a coherence length corresponding to a tube diameter.Comment: 15 pages, 10 eps figure
The Electronic Spectrum of Fullerenes from the Dirac Equation
The electronic spectrum of sheets of graphite (plane honeycomb lattice)
folded into regular polihedra is studied. A continuum limit valid for
sufficiently large molecules and based on a tight binding approximation is
derived. It is found that a Dirac equation describes the flat graphite lattice.
Curving the lattice by insertion of odd numbered rings can be mimicked by
coupling effective gauge fields. In particular the and related
molecules are well described by the Dirac equation on the surface of a sphere
coupled to a color monopole sitting at its center.Comment: 29 pages, 7 figures. IASSNS-HEP-92/5
Dimerization structures on the metallic and semiconducting fullerene tubules with half-filled electrons
Possible dimerization patterns and electronic structures in fullerene tubules
as the one-dimensional pi-conjugated systems are studied with the extended
Su-Schrieffer-Heeger model. We assume various lattice geometries, including
helical and nonhelical tubules. The model is solved for the half-filling case
of -electrons. (1) When the undimerized systems do not have a gap, the
Kekule structures prone to occur. The energy gap is of the order of the room
temperatures at most and metallic properties would be expected. (2) If the
undimerized systems have a large gap (about 1eV), the most stable structures
are the chain-like distortions where the direction of the arranged
trans-polyacetylene chains is along almost the tubular axis. The electronic
structures are ofsemiconductors due to the large gap.Comment: submitted to Phys. Rev. B, pages 15, figures 1
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
Coherent Control of Photocurrents in Graphene and Carbon Nanotubes
Coherent one photon () and two photon () electronic
excitations are studied for graphene sheets and for carbon nanotubes using a
long wavelength theory for the low energy electronic states. For graphene
sheets we find that coherent superposition of these excitations produces a
polar asymmetry in the momentum space distribution of the excited carriers with
an angular dependence which depends on the relative polarization and phases of
the incident fields. For semiconducting nanotubes we find a similar effect
which depends on the square of the semiconducting gap, and we calculate its
frequency dependence.
We find that the third order nonlinearity which controls the direction of the
photocurrent is robust for semiconducting t ubes and vanishes in the continuum
theory for conducting tubes. We calculate corrections to these results arising
from higher order crystal field effects on the band structure and briefly
discuss some applications of the theory.Comment: 12 pages in RevTex, 6 epsf figure
Electronic instabilities in 3D arrays of small-diameter (3,3) carbon nanotubes
We investigate the electronic instabilities of the small-diameter (3,3)
carbon nanotubes by studying the low-energy perturbations of the normal
Luttinger liquid regime. The bosonization approach is adopted to deal exactly
with the interactions in the forward-scattering channels, while renormalization
group methods are used to analyze the low-energy instabilities. In this
respect, we take into account the competition between the effective e-e
interaction mediated by phonons and the Coulomb interaction in backscattering
and Umklapp channels. Moreover, we apply our analysis to relevant experimental
conditions where the nanotubes are assembled into large three-dimensional
arrays, which leads to an efficient screening of the Coulomb potential at small
momentum-transfer. We find that the destabilization of the normal metallic
behavior takes place through the onset of critical behavior in some of the two
charge stiffnesses that characterize the Luttinger liquid state. From a
physical point of view, this results in either a divergent compressibility or a
vanishing renormalized velocity for current excitations at the point of the
transition. We observe anyhow that this kind of critical behavior occurs
without the development of any appreciable sign of superconducting
correlations.Comment: 10 pages, 12 figure
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