2,232 research outputs found
Shot noise of a quantum dot measured with GHz stub impedance matching
The demand for a fast high-frequency read-out of high impedance devices, such
as quantum dots, necessitates impedance matching. Here we use a resonant
impedance matching circuit (a stub tuner) realized by on-chip superconducting
transmission lines to measure the electronic shot noise of a carbon nanotube
quantum dot at a frequency close to 3 GHz in an efficient way. As compared to
wide-band detection without impedance matching, the signal to noise ratio can
be enhanced by as much as a factor of 800 for a device with an impedance of 100
k. The advantage of the stub resonator concept is the ease with which
the response of the circuit can be predicted, designed and fabricated. We
further demonstrate that all relevant matching circuit parameters can reliably
be deduced from power reflectance measurements and then used to predict the
power transmission function from the device through the circuit. The shot noise
of the carbon nanotube quantum dot in the Coulomb blockade regime shows an
oscillating suppression below the Schottky value of , as well an
enhancement in specific regions.Comment: 6 pages, 4 figures, supplementar
Fitting theories of nuclear binding energies
In developing theories of nuclear binding energy such as density-functional
theory, the effort required to make a fit can be daunting due to the large
number of parameters that may be in the theory and the large number of nuclei
in the mass table. For theories based on the Skyrme interaction, the effort can
be reduced considerably by using the singular value decomposition to reduce the
size of the parameter space. We find that the sensitive parameters define a
space of dimension four or so, and within this space a linear refit is adequate
for a number of Skyrme parameters sets from the literature. We do not find
marked differences in the quality of the fit between the SLy4, the Bky4 and SkP
parameter sets. The r.m.s. residual error in even-even nuclei is about 1.5 MeV,
half the value of the liquid drop model. We also discuss an alternative norm
for evaluating mass fits, the Chebyshev norm. It focuses attention on the cases
with the largest discrepancies between theory and experiment. We show how it
works with the liquid drop model and make some applications to models based on
Skyrme energy functionals. The Chebyshev norm seems to be more sensitive to new
experimental data than the root-mean-square norm. The method also has the
advantage that candidate improvements to the theories can be assessed with
computations on smaller sets of nuclei.Comment: 17 pages and 4 figures--version encorporates referee's comment
Mechanical and SEM analysis of artificial comet nucleus samples
Since 1987 experiments dealing with comet nucleus phenomena have been carried out in the DFVLR space simulation chambers. The main objective of these experiments is a better understanding of thermal behavior, surface phenomena and especially the gas dust interaction. As a function of different sample compositions and exposure to solar irradiation (xenon-bulbs) crusts of different hardness and thickness were measured. The measuring device consists of a motor driven pressure foot (5 mm diameter), which is pressed into the sample. The applied compressive force is electronically monitored. The microstructure of the crust and dust residuals is investigated by scanning electron microscopy (SEM) techniques. Stress-depth profiles of an unirradiated and an irradiated model comet are given
Climate Vulnerability and Human Migration in Global Perspective
The relationship between climate change and human migration is not homogenous and depends critically on the differential vulnerability of population and places. If places and populations are not vulnerable, or susceptible, to climate change, then the climate–migration relationship may not materialize. The key to understanding and, from a policy perspective, planning for whether and how climate change will impact future migration patterns is therefore knowledge of the link between climate vulnerability and migration. However, beyond specific case studies, little is known about this association in global perspective. We therefore provide a descriptive, country-level portrait of this relationship. We show that the negative association between climate vulnerability and international migration holds only for countries least vulnerable to climate change, which suggests the potential for trapped populations in more vulnerable countries. However, when analyzed separately by life supporting sector (food, water, health, ecosystem services, human habitat, and infrastructure) and vulnerability dimension (exposure, sensitivity, and adaptive capacity), we detect evidence of a relationship among more, but not the most, vulnerable countries. The bilateral (i.e., country-to-country) migration show that, on average, people move from countries of higher vulnerability to lower vulnerability, reducing global risk by 15%. This finding is consistent with the idea that migration is a climate adaptation strategy. Still, ~6% of bilateral migration is maladaptive with respect to climate change, with some movement toward countries with greater climate change vulnerabilit
On the Expansions in Spin Foam Cosmology
We discuss the expansions used in spin foam cosmology. We point out that
already at the one vertex level arbitrarily complicated amplitudes contribute,
and discuss the geometric asymptotics of the five simplest ones. We discuss
what type of consistency conditions would be required to control the expansion.
We show that the factorisation of the amplitude originally considered is best
interpreted in topological terms. We then consider the next higher term in the
graph expansion. We demonstrate the tension between the truncation to small
graphs and going to the homogeneous sector, and conclude that it is necessary
to truncate the dynamics as well.Comment: 17 pages, 4 figures, published versio
Electronic band structure, Fermi surface, and elastic properties of new 4.2K superconductor SrPtAs from first-principles calculations
The hexagonal phase SrPtAs (s.g. P6/mmm; #194) with a honeycomb lattice
structure very recently was declared as a new low-temperature (TC ~ 4.2K)
superconductor. Here by means of first-principles calculations the optimized
structural parameters, electronic bands, Fermi surface, total and partial
densities of states, inter-atomic bonding picture, independent elastic
constants, bulk and shear moduli for SrPtAs were obtained for the first time
and analyzed in comparison with the related layered superconductor SrPt2As2.Comment: 8 pages, 4 figure
Optimization of sample-chip design for stub-matched radio-frequency reflectometry measurements
A radio-frequency (rf) matching circuit with an in situ tunable varactor
diode used for rf reflectometry measurements in semiconductor nanostructures is
investigated and used to optimize the sample-specific chip design. The samples
are integrated in a 2-4 GHz stub-matching circuit consisting of a waveguide
stub shunted to the terminated coplanar waveguide. Several quantum point
contacts fabricated on a GaAs/AlGaAs heterostructure with different chip
designs are compared. We show that the change of the reflection coefficient for
a fixed change in the quantum point contact conductance can be enhanced by a
factor of 3 compared to conventional designs by a suitable electrode geometry
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