99 research outputs found
Imaging Electron Wave Functions of Quantized Energy Levels in Carbon Nanotubes
Carbon nanotubes provide a unique system to study one-dimensional
quantization phenomena. Scanning tunneling microscopy is used to observe the
electronic wave functions that correspond to quantized energy levels in short
metallic carbon nanotubes. Discrete electron waves are apparent from periodic
oscillations in the differential conductance as a function of the position
along the tube axis, with a period that differs from that of the atomic
lattice. Wave functions can be observed for several electron states at adjacent
discrete energies. The measured wavelengths are in good agreement with the
calculated Fermi wavelength for armchair nanotubes.Comment: 11 pages, 4 figures in seperate PDF fil
Effects of magnetic field and disorder on electronic properties of Carbon Nanotubes
Electronic properties of metallic and semiconducting carbon nanotubes are
investigated in presence of magnetic field perpendicular to the CN-axis, and
disorder introduced through energy site randomness. The magnetic field field is
shown to induce a metal-insulator transition (MIT) in absence of disorder, and
surprisingly disorder does not affect significantly the MIT. These results may
find confirmation through tunneling experimentsComment: 4 pages, 6 figures. Phys. Rev. B (in press
Quantum states and specific heat of low-density He gas adsorbed within the carbon nanotube interstitial channels: Band structure effects and potential dependence
We calculate the energy-band structure of a He atom trapped within the
interstitial channel between close-packed nanotubes within a bundle and its
influence on the specific heat of the adsorbed gas. A robust prediction of our
calculations is that the contribution of the low-density adsorbed gas to the
specific heat of the nanotube material shows pronounced nonmonotonic variations
with temperature. These variations are shown to be closely related to the band
gaps in the adsorbate density of states
Bandgap Change of Carbon Nanotubes: Effect of Small Tensile and Torsional Strain
We use a simple picture based on the electron approximation to study
the bandgap variation of carbon nanotubes with uniaxial and torsional strain.
We find (i) that the magnitude of slope of bandgap versus strain has an almost
universal behaviour that depends on the chiral angle, (ii) that the sign of
slope depends on the value of and (iii) a novel change in sign
of the slope of bandgap versus uniaxial strain arising from a change in the
value of the quantum number corresponding to the minimum bandgap. Four orbital
calculations are also presented to show that the orbital results are
valid.Comment: Revised. Method explained in detai
Conductance of carbon nanotubes with disorder: A numerical study
We study the conductance of carbon nanotube wires in the presence of
disorder, in the limit of phase coherent transport. For this purpose, we have
developed a simple numerical procedure to compute transmission through carbon
nanotubes and related structures. Two models of disorder are considered, weak
uniform disorder and isolated strong scatterers. In the case of weak uniform
disorder, our simulations show that the conductance is not significantly
affected by disorder when the Fermi energy is close to the band center.
Further, the transmission around the band center depends on the diameter of
these zero bandgap wires. We also find that the calculated small bias
conductance as a function of the Fermi energy exhibits a dip when the Fermi
energy is close to the second subband minima. In the presence of strong
isolated disorder, our calculations show a transmission gap at the band center,
and the corresponding conductance is very small
Size Effects in Carbon Nanotubes
The inter-shell spacing of multi-walled carbon nanotubes was determined by
analyzing the high resolution transmission electron microscopy images of these
nanotubes. For the nanotubes that were studied, the inter-shell spacing
is found to range from 0.34 to 0.39 nm, increasing with
decreasing tube diameter. A model based on the results from real space image
analysis is used to explain the variation in inter-shell spacings obtained from
reciprocal space periodicity analysis. The increase in inter-shell spacing with
decreased nanotube diameter is attributed to the high curvature, resulting in
an increased repulsive force, associated with the decreased diameter of the
nanotube shells.Comment: 4 pages. RevTeX. 4 figure
Analysis of quantum conductance of carbon nanotube junctions by the effective mass approximation
The electron transport through the nanotube junctions which connect the
different metallic nanotubes by a pair of a pentagonal defect and a heptagonal
defect is investigated by Landauer's formula and the effective mass
approximation. From our previous calculations based on the tight binding model,
it has been known that the conductance is determined almost only by two
parameters,i.e., the energy in the unit of the onset energy of more than two
channels and the ratio of the radii of the two nanotubes. The conductance is
calculated again by the effective mass theory in this paper and a simple
analytical form of the conductance is obtained considering a special boundary
conditions of the envelop wavefunctions. The two scaling parameters appear
naturally in this treatment. The results by this formula coincide fairly well
with those of the tight binding model.
The physical origin of the scaling law is clarified by this approach.Comment: RevTe
ab initio modeling of open systems: charge transfer, electron conduction, and molecular switching of a C_{60} device
We present an {\it ab initio} analysis of electron conduction through a
molecular device. Charge transfer from the device electrodes to the
molecular region is found to play a crucial role in aligning the lowest
unoccupied molecular orbital (LUMO) of the to the Fermi level of the
electrodes. This alignment induces a substantial device conductance of . A gate potential can inhibit charge transfer and
introduce a conductance gap near , changing the current-voltage
characteristics from metallic to semi-conducting, thereby producing a field
effect molecular current switch
Superconductivity in Ropes of Single-Walled Carbon Nanotubes
We report measurements on ropes of Single Walled Carbon Nanotubes (SWNT) in
low-resistance contact to non-superconducting (normal) metallic pads, at low
voltage and at temperatures down to 70 mK. In one sample, we find a two order
of magnitude resistance drop below 0.55 K, which is destroyed by a magnetic
field of the order of 1T, or by a d.c. current greater than 2.5 microA. These
features strongly suggest the existence of superconductivity in ropes of SWNT.Comment: Accepted for publication in Phys. Rev. Let
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