4,894 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
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
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
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
Can Electric Field Induced Energy Gaps In Metallic Carbon Nanotubes?
The low-energy electronic structure of metallic single-walled carbon nanotube
(SWNT) in an external electric field perpendicular to the tube axis is
investigated. Based on tight-binding approximation, a field-induced energy gap
is found in all (n, n) SWNTs, and the gap shows strong dependence on the
electric field and the size of the tubes. We numerically find a universal
scaling that the gap is a function of the electric field and the radius of
SWNTs, and the results are testified by the second-order perturbation theory in
weak field limit. Our calculation shows the field required to induce a 0.1
gap in metallic SWNTs can be easily reached under the current
experimental conditions. It indicates a kind of possibility to apply nanotubes
to electric signal-controlled nanoscale switching devices
ASTRO Journals' Data Sharing Policy and Recommended Best Practices.
Transparency, openness, and reproducibility are important characteristics in scientific publishing. Although many researchers embrace these characteristics, data sharing has yet to become common practice. Nevertheless, data sharing is becoming an increasingly important topic among societies, publishers, researchers, patient advocates, and funders, especially as it pertains to data from clinical trials. In response, ASTRO developed a data policy and guide to best practices for authors submitting to its journals. ASTRO's data sharing policy is that authors should indicate, in data availability statements, if the data are being shared and if so, how the data may be accessed
Through-membrane electron-beam lithography for ultrathin membrane applications
We present a technique to fabricate ultrathin (down to 20 nm) uniform
electron transparent windows at dedicated locations in a SiN membrane for in
situ transmission electron microscopy experiments. An electron-beam (e-beam)
resist is spray-coated on the backside of the membrane in a KOH- etched cavity
in silicon which is patterned using through-membrane electron-beam lithography.
This is a controlled way to make transparent windows in membranes, whilst the
topside of the membrane remains undamaged and retains its flatness. Our
approach was optimized for MEMS-based heating chips but can be applied to any
chip design. We show two different applications of this technique for (1)
fabrication of a nanogap electrode by means of electromigration in thin
free-standing metal films and (2) making low-noise graphene nanopore devices
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