4,416 research outputs found
Electronic Transport Spectroscopy of Carbon Nanotubes in a Magnetic Field
We report magnetic field spectroscopy measurements in carbon nanotube quantum
dots exhibiting four-fold shell structure in the energy level spectrum. The
magnetic field induces a large splitting between the two orbital states of each
shell, demonstrating their opposite magnetic moment and determining transitions
in the spin and orbital configuration of the quantum dot ground state. We use
inelastic cotunneling spectroscopy to accurately resolve the spin and orbital
contributions to the magnetic moment. A small coupling is found between
orbitals with opposite magnetic moment leading to anticrossing behavior at zero
field.Comment: 7 pages, 4 figure
Evolution of SU(4) Transport Regimes in Carbon Nanotube Quantum Dots
We study the evolution of conductance regimes in carbon nanotubes with doubly
degenerate orbitals (``shells'') by controlling the contact transparency within
the same sample. For sufficiently open contacts, Kondo behavior is observed for
1, 2, and 3 electrons in the topmost shell. As the contacts are opened more,
the sample enters the ``mixed valence'' regime, where different charge states
are strongly hybridized by electron tunneling. Here, the conductance as a
function of gate voltage shows pronounced modulations with a period of four
electrons, and all single-electron features are washed away at low temperature.
We successfully describe this behavior by a simple formula with no fitting
parameters. Finally, we find a surprisingly small energy scale that controls
the temperature evolution of conductance and the tunneling density of states in
the mixed valence regime.Comment: 4 pages + supplementary info. The second part of the original
submission is now split off as a separate paper (0709.1288
Electronic excitation spectrum of metallic carbon nanotubes
We have studied the discrete electronic spectrum of closed metallic nanotube
quantum dots. At low temperatures, the stability diagrams show a very regular
four-fold pattern that allows for the determination of the electron addition
and excitation energies. The measured nanotube spectra are in excellent
agreement with theoretical predictions based on the nanotube band structure.
Our results permit the complete identification of the electron quantum states
in nanotube quantum dots.Comment: 4 pages, 3 figure
Optical conductivity of wet DNA
Motivated by recent experiments we have studied the optical conductivity of
DNA in its natural environment containing water molecules and counter ions. Our
density functional theory calculations (using SIESTA) for four base pair B-DNA
with order 250 surrounding water molecules suggest a thermally activated doping
of the DNA by water states which generically leads to an electronic
contribution to low-frequency absorption. The main contributions to the doping
result from water near DNA ends, breaks, or nicks and are thus potentially
associated with temporal or structural defects in the DNA.Comment: 4 pages, 4 figures included, final version, accepted for publication
in Phys. Rev. Let
High-Field Electrical Transport in Single-Wall Carbon Nanotubes
Using low-resistance electrical contacts, we have measured the intrinsic
high-field transport properties of metallic single-wall carbon nanotubes.
Individual nanotubes appear to be able to carry currents with a density
exceeding 10^9 A/cm^2. As the bias voltage is increased, the conductance drops
dramatically due to scattering of electrons. We show that the current-voltage
characteristics can be explained by considering optical or zone-boundary phonon
emission as the dominant scattering mechanism at high field.Comment: 4 pages, 3 eps figure
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
Form removal aspects on the waviness parameters for steel sheet in automotive applications : fourier filtering versus polynomial regression
Premium car makers attach great importance to the visual appearance of the painted car skin as an indication of product quality. The “orange peel” phenomenon constitutes a major problem here. It is not only depending on the paint’s chemical composition and application method, but also on possible waviness components in the sheet substrate. Therefore one is searching hard for a valuable waviness parameter to quantify the substrate’s fitness for purpose. A technically emerging problem is how to remove the form from the measured signal, which is indeed not significant to the orange peel phenomenon. This paper will compare two commonly used approaches: i.e. Fourier filtering versus polynomial regression and will reveal and quantify some common aspects in terms of wavelengths
Velocity quantization approach of the one-dimensional dissipative harmonic oscillator
Given a constant of motion for the one-dimensional harmonic oscillator with
linear dissipation in the velocity, the problem to get the Hamiltonian for this
system is pointed out, and the quantization up to second order in the
perturbation approach is used to determine the modification on the eigenvalues
when dissipation is taken into consideration. This quantization is realized
using the constant of motion instead of the Hamiltonian.Comment: 10 pages, 2 figure
Multiwalled carbon nanotube: Luttinger liquid or not?
We have measured IV-curves of multiwalled carbon nanotubes using end
contacts. At low voltages, the tunneling conductance obeys non-Ohmic power law,
which is predicted both by the Luttinger liquid and the
environment-quantum-fluctuation theories. However, at higher voltages we
observe a crossover to Ohm's law with a Coulomb-blockade offset, which agrees
with the environment-quantum-fluctuation theory, but cannot be explained by the
Luttinger-liquid theory. From the high-voltage tunneling conductance we
determine the transmission line parameters of the nanotubes.Comment: RevTeX, 4 pages, 2 EPS-figures, submitted to Phys. Rev. Let
Completely Positive Quantum Dissipation
A completely positive master equation describing quantum dissipation for a
Brownian particle is derived starting from microphysical collisions, exploiting
a recently introduced approach to subdynamics of a macrosystem. The obtained
equation can be cast into Lindblad form with a single generator for each
Cartesian direction. Temperature dependent friction and diffusion coefficients
for both position and momentum are expressed in terms of the collision
cross-section.Comment: 8 pages, revtex, no figure
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