66 research outputs found
``X-Ray Edge'' Singularities in Nanotubes and Quantum Wires with Multiple Subbands
Band theory predicts an inverse square root van Hove singularity in the
tunneling density of states at the minimum energy of an unoccupied subband in a
one-dimensional quantum wire. With interactions, an orthogonality catastrophe
analogous to the x-ray edge effect for core levels in a metal strongly reduces
this singularity by a power B of the energy above threshold, with B
approximately 0.3 for typical carbon nanotubes. Despite the anomalous tunneling
characteristic, good quasiparticles corresponding to the unoccupied subband
states do exist.Comment: 4 page
Random walks on finite lattice tubes
Exact results are obtained for random walks on finite lattice tubes with a
single source and absorbing lattice sites at the ends. Explicit formulae are
derived for the absorption probabilities at the ends and for the expectations
that a random walk will visit a particular lattice site before being absorbed.
Results are obtained for lattice tubes of arbitrary size and each of the
regular lattice types; square, triangular and honeycomb. The results include an
adjustable parameter to model the effects of strain, such as surface curvature,
on the surface diffusion. Results for the triangular lattice tubes and the
honeycomb lattice tubes model diffusion of adatoms on single walled zig-zag
carbon nano-tubes with open ends.Comment: 22 pages, 4 figure
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
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
Superconductivity in carbon nanotube ropes
We investigate the conditions in which superconductivity may develop in ropes
of carbon nanotubes. It is shown that the interaction among a large number of
metallic nanotubes favors the appearance of a metallic phase in the ropes,
intermediate between respective phases with spin-density-wave and
superconducting correlations. These arise in samples with about 100 metallic
nanotubes or more, where the long-range Coulomb interaction is very effectively
reduced and it may be overcome by the attractive interaction from the exchange
of optical phonons within each nanotube. We estimate that the probability for
the tunneling of Cooper pairs between neighboring nanotubes is much higher than
that for single electrons in a disordered rope. The effect of pair hopping is
therefore what establishes the intertube coherence, and the tunneling amplitude
of the Cooper pairs dictates the scale of the transition to the superconducting
state.Comment: 12 page
Vacuum structure of Toroidal Carbon Nanotubes
Low energy excitations in carbon nanotubes can be described by an effective
field theory of two components spinor. It is pointed out that the chiral
anomaly in 1+1 dimensions should be observed in a metallic toroidal carbon
nanotube on a planar geometry with varying magnetic field. We propose an
experimental setup for studying this quantum effect. We also analyze the vacuum
structure of the metallic toroidal carbon nanotube including the Coulomb
interactions and discuss some effects of external charges on the vacuum.Comment: 10 pages, 11 figure
Thermoelectric effects of an Aharonov-Bohm interferometer with an embedded quantum dot in the Kondo regime
Thermoelectric effects are studied in an Aharonov-Bohm (AB) interferometer
with an embedded quantum dot in the Kondo regime. The AB flux-dependent
transmission probability has an asymmetrical shape arising from the Fano
interference between the direct tunneling path and the Kondo-resonant tunneling
path through a quantum dot. The sign and magnitude of thermopower can be
modulated by the AB flux and the direct tunneling amplitude. In addition, the
thermopower is anomalously enhanced by the Kondo correlation in the quantum dot
near the Kondo temperature (). The Kondo correlation in the quantum dot
also leads to crossover behavior in diagonal transport coefficients as a
function of temperature. The amplitude of an AB oscillation in electric and
thermal conductances is small at temperatures far above , but becomes
enhanced as the system is cooled below . The AB oscillation is strong in
the thermopower and Lorenz number within the crossover region near the Kondo
temperature.Comment: 16 pages, 10 figure
Aharonov-Bohm spectral features and coherence lengths in carbon nanotubes
The electronic properties of carbon nanotubes are investigated in the
presence of disorder and a magnetic field parallel or perpendicular to the
nanotube axis. In the parallel field geometry, the -periodic
metal-insulator transition (MIT) induced in metallic or semiconducting
nanotubes is shown to be related to a chirality-dependent shifting of the
energy of the van Hove singularities (VHSs). The effect of disorder on this
magnetic field-related mechanism is considered with a discussion of mean free
paths, localization lengths and magnetic dephasing rate in the context of
recent experiments.Comment: 22 pages, 6 Postscript figures. submitted to Phys. Rev.
W=0 Pairing in Carbon Nanotubes away from Half Filling
We use the Hubbard Hamiltonian on the honeycomb lattice to represent the
valence bands of carbon single-wall nanotubes. A detailed symmetry
analysis shows that the model allows W=0 pairs which we define as two-body
singlet eigenstates of with vanishing on-site repulsion. By means of a
non-perturbative canonical transformation we calculate the effective
interaction between the electrons of a W=0 pair added to the interacting ground
state. We show that the dressed W=0 pair is a bound state for resonable
parameter values away from half filling. Exact diagonalization results for the
(1,1) nanotube confirm the expectations. For nanotubes of length ,
the binding energy of the pair depends strongly on the filling and decreases
towards a small but nonzero value as . We observe the existence
of an optimal doping when the number of electrons per C atom is in the range
1.21.3, and the binding energy is of the order of 0.1 1 meV.Comment: 16 pages, 6 figure
Aggregate structure of hydroxyproline-rich glycoprotein (HRGP) and HRGP assisted dispersion of carbon nanotubes
Hydroxyproline-rich glycoproteins (HRGP) comprise a super-family of extracellular structural glycoproteins whose precise roles in plant cell wall assembly and functioning remain to be elucidated. However, their extended structure and repetitive block co-polymer character of HRGPs may mediate their self-assembly as wall scaffolds by like-with-like alignment of their hydrophobic peptide and hydrophilic glycopeptide modules. Intermolecular crosslinking further stabilizes the scaffold. Thus the design of HRGP-based scaffolds may have practical applications in bionanotechnology and medicine. As a first step, we have used single-molecule or single-aggregate atomic force microscopy (AFM) to visualize the structure of YK20, an amphiphilic HRGP comprised entirely of 20 tandem repeats of: Ser-Hyp4-Ser-Hyp-Ser-Hyp4-Tyr-Tyr-Tyr-Lys. YK20 formed tightly aggregated coils at low ionic strength, but networks of entangled chains with a porosity of ~0.5–3 μm at higher ionic strength. As a second step we have begun to design HRGP-carbon nanotube composites. Single-walled carbon nanotubes (SWNTs) can be considered as seamless cylinders rolled up from graphene sheets. These unique all-carbon structures have extraordinary aromatic and hydrophobic properties and form aggregated bundles due to strong inter-tube van der Waals interactions. Sonicating aggregated SWNT bundles with aqueous YK20 solubilized them presumably by interaction with the repetitive, hydrophobic, Tyr-rich peptide modules of YK20 with retention of the extended polyproline-II character. This may allow YK20 to form extended structures that could potentially be used as scaffolds for site-directed assembly of nanomaterials
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