6,188 research outputs found
Single wall carbon nanotube double quantum dot
We report on two top-gate defined, coupled quantum dots in a semiconducting
single wall carbon nanotube, constituting a tunable double quantum dot system.
The single wall carbon nanotubes are contacted by titanium electrodes, and
gated by three narrow top-gate electrodes as well as a back-gate. We show that
a bias spectroscopy plot on just one of the two quantum dots can be used to
extract the addition energy of both quantum dots. Furthermore, honeycomb charge
stability diagrams are analyzed by an electrostatic capacitor model that
includes cross capacitances, and we extract the coupling energy of the double
quantum dot.Comment: Published in Applied Physics Letters 4 December 2006.
http://link.aip.org/link/?APL/89/23211
Absolute dimensions of solar-type eclipsing binaries. EF Aquarii: a G0 test for stellar evolution models
Recent studies have shown that stellar chromospheric activity, and its effect
on convective energy transport in the envelope, is most likely the cause of
significant radius and temperature discrepancies between theoretical evolution
models and observations. We aim to determine absolute dimensions and abundances
for the solar-type detached eclipsing binary EF Aqr, and to perform a detailed
comparison with results from recent stellar evolutionary models. uvby-beta
standard photometry was obtained with the Stromgren Automatic Telescope. The
broadening function formalism was applied on spectra observed with HERMES at
the Mercator telescope in La Palma, to obtain radial velocity curves. Masses
and radii with a precision of 0.6% and 1.0% respectively have been established
for both components of EF Aqr. The active 0.956 M_sol secondary shows star
spots and strong Ca II H and K emission lines. The 1.224 M_sol primary shows
signs of activity as well, but at a lower level. An [Fe/H] abundance of
0.00+-0.10 is derived with similar abundances for Si, Ca, Sc, Ti, V, Cr, Co,
and Ni. Solar calibrated evolutionary models such as Yonsei-Yale,
Victoria-Regina and BaSTI isochrones and evolutionary tracks are unable to
reproduce EF Aqr, especially for the secondary, which is 9% larger and 400 K
cooler than predicted. Models adopting significantly lower mixing length
parameters l/H_p remove these discrepancies, as seen in other solar type
binaries. For the observed metallicity, Granada models with a mixing length of
l/H_p=1.30 (primary) and 1.05 (secondary) reproduce both components at a common
age of 1.5+-0.6 Gyr. Observations of EF Aqr suggests that magnetic activity,
and its effect on envelope convection, is likely to be the cause of
discrepancies in both radius and temperature, which can be removed by adjusting
the mixing length parameter of the models downwards.Comment: 11 pages, 8 figures, accepted for publication by A&
Convex optimization problem prototyping for image reconstruction in computed tomography with the Chambolle-Pock algorithm
The primal-dual optimization algorithm developed in Chambolle and Pock (CP),
2011 is applied to various convex optimization problems of interest in computed
tomography (CT) image reconstruction. This algorithm allows for rapid
prototyping of optimization problems for the purpose of designing iterative
image reconstruction algorithms for CT. The primal-dual algorithm is briefly
summarized in the article, and its potential for prototyping is demonstrated by
explicitly deriving CP algorithm instances for many optimization problems
relevant to CT. An example application modeling breast CT with low-intensity
X-ray illumination is presented.Comment: Resubmitted to Physics in Medicine and Biology. Text has been
modified according to referee comments, and typos in the equations have been
correcte
Power filtration of CMB observational data
We propose a power filter Gp for linear reconstruction of the CMB signal from
observational maps. This Gp filter preserves the power spectrum of the CMB
signal in contrast to the Wiener filter which diminishes the power spectrum of
the reconstructed CMB signal. We demonstrate how peak statistics and a cluster
analysis can be used to estimate the probability of the presence of a CMB
signal in observational records. The efficiency of the Gp filter is
demonstrated on a toy model of an observational record consisting of a CMB
signal and noise in the form of foreground point sources.Comment: 17 pages; 4 figures; submitted to International Journal of Modern
Physic
Magnetic-Field Dependence of Tunnel Couplings in Carbon Nanotube Quantum Dots
By means of sequential and cotunneling spectroscopy, we study the tunnel
couplings between metallic leads and individual levels in a carbon nanotube
quantum dot. The levels are ordered in shells consisting of two doublets with
strong- and weak-tunnel couplings, leading to gate-dependent level
renormalization. By comparison to a one- and two-shell model, this is shown to
be a consequence of disorder-induced valley mixing in the nanotube. Moreover, a
parallel magnetic field is shown to reduce this mixing and thus suppress the
effects of tunnel renormalization.Comment: 5 pages, 3 figures; revised version as publishe
Superconductivity-enhanced bias spectroscopy in carbon nanotube quantum dots
We study low-temperature transport through carbon nanotube quantum dots in
the Coulomb blockade regime coupled to niobium-based superconducting leads. We
observe pronounced conductance peaks at finite source-drain bias, which we
ascribe to elastic and inelastic cotunneling processes enhanced by the
coherence peaks in the density of states of the superconducting leads. The
inelastic cotunneling lines display a marked dependence on the applied gate
voltage which we relate to different tunneling-renormalizations of the two
subbands in the nanotube. Finally, we discuss the origin of an especially
pronounced sub-gap structure observed in every fourth Coulomb diamond
Methanol maps of low-mass protostellar systems: the Serpens Molecular Core
Observations of Serpens have been performed at the JCMT using Harp-B. Maps
over a 4.5'x5.4' region were made in a frequency window around 338 GHz,
covering the 7-6 transitions of methanol. Emission is extended over each
source, following the column density of H2 but showing up also particularly
strongly around outflows. The rotational temperature is low, 15-20 K, and does
not vary with position within each source. The abundance is typically 10^-9 -
10^-8 with respect to H2 in the outer envelope, whereas "jumps" by factors of
up to 10^2 -10^3 inside the region where the dust temperature exceeds 100 K are
not excluded. A factor of up to ~ 10^3 enhancement is seen in outflow gas. In
one object, SMM4, the ice abundance has been measured to be ~ 3x10^-5 with
respect to H2 in the outer envelope, i.e., a factor of 10^3 larger than the
gas-phase abundance. Comparison with C18O J=3-2 emission shows that strong CO
depletion leads to a high gas-phase abundance of CH3OH not just for the Serpens
sources, but for a larger sample of protostars. The observations illustrate the
large-scale, low-level desorption of CH3OH from dust grains, extending out to
and beyond 7500 AU from each source, a scenario which is consistent with
non-thermal (photo-)desorption from the ice. The observations also illustrate
the usefulness of CH3OH as a tracer of energetic input in the form of outflows,
where methanol is sputtered from the grain surfaces. Finally, the observations
provide further evidence of CH3OH formation through CO hydrogenation proceeding
on grain surfaces in low-mass envelopes.Comment: Accepted for publication in A&A
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