484 research outputs found
Feasibility of intercalated graphite railgun armatures
Graphite intercalation compounds may provide an excellent material for the fabrication of electro-magnetic railgun armatures. As a pulse of power is fed into the armature the intercalate could be excited into the plasma state around the edges of the armature, while the bulk of the current would be carried through the graphite block. Such an armature would have the desirable characteristics of both diffuse plasma armatures and bulk conduction armatures. In addition, the highly anisotropic nature of these materials could enable the electrical and thermal conductivity to be tailored to meet the specific requirements of electromagnetic railgun armatures. Preliminary investigations were performed in an attempt to determine the feasibility of using graphite intercalation compounds as railgun armatures. Issues of fabrication, resistivity, stability, and electrical current spreading are addressed for the case of highly oriented pyrolytic graphite
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Using satellites to investigate the sensitivity of longwave downward radiation to water vapor at high elevations
Many studies suggest that high-elevation regions may be among the most sensitive to future climate change. However, in situ observations in these often remote locations are too sparse to determine the feedbacks responsible for enhanced warming rates. One of these feedbacks is associated with the sensitivity of longwave downward radiation (LDR) to changes in water vapor, with the sensitivity being particularly large in many high-elevation regions where the average water vapor is often low. We show that satellite retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Clouds and the Earth’s Radiant Energy System (CERES) can be used to expand the current ground-based observational database and that the monthly averaged clear-sky satellite estimates of humidity and LDR are in good agreement with the well-instrumented Center for Snow and Avalanche Studies ground-based site in the southwestern Colorado Rocky Mountains. The relationship between MODIS-retrieved precipitable water vapor and surface specific humidity across the contiguous United States was found to be similar to that previously found for the Alps. More important, we show that satellites capture the nonlinear relationship between LDR and water vapor and confirm that LDR is especially sensitive to changes in water vapor at high elevations in several midlatitude mountain ranges. Because the global population depends on adequate fresh water, much of which has its source in high mountains, it is critically important to understand how climate will change there. We demonstrate that satellites can be used to investigate these feedbacks in high-elevation regions where the coverage of surface-based observations is insufficient to do so
Understanding the saturation power of Josephson Parametric Amplifiers made from SQUIDs arrays
We report on the implementation and detailed modelling of a Josephson
Parametric Amplifier (JPA) made from an array of eighty Superconducting QUantum
Interference Devices (SQUIDs), forming a non-linear quarter-wave resonator.
This device was fabricated using a very simple single step fabrication process.
It shows a large bandwidth (45 MHz), an operating frequency tunable between 5.9
GHz and 6.8 GHz and a large input saturation power (-117 dBm) when biased to
obtain 20 dB of gain. Despite the length of the SQUID array being comparable to
the wavelength, we present a model based on an effective non-linear LC series
resonator that quantitatively describes these figures of merit without fitting
parameters. Our work illustrates the advantage of using array-based JPA since a
single-SQUID device showing the same bandwidth and resonant frequency would
display a saturation power 15 dB lower.Comment: 12 pages, 9 figures, Appendices include
Kerr non-linearity in a superconducting Josephson metamaterial
We present a detailed experimental and theoretical analysis of the dispersion
and non-linear Kerr frequency shifts of plasma modes in a one-dimensional
Josephson junction chain containing 500 SQUIDs in the regime of weak
nonlinearity. The measured low-power dispersion curve agrees perfectly with the
theoretical model if we take into account the Kerr renormalisation of the bare
frequencies and the long-range nature of the island charge screening by a
remote ground plane. We measured the self- and cross-Kerr shifts for the
frequencies of the eight lowest modes in the chain. We compare the measured
Kerr coefficients with theory and find good agreement
Electronic structure of epitaxial graphene layers on SiC: effect of the substrate
Recent transport measurements on thin graphite films grown on SiC show large
coherence lengths and anomalous integer quantum Hall effects expected for
isolated graphene sheets. This is the case eventhough the layer-substrate
epitaxy of these films implies a strong interface bond that should induce
perturbations in the graphene electronic structure. Our DFT calculations
confirm this strong substrate-graphite bond in the first adsorbed carbon layer
that prevents any graphitic electronic properties for this layer. However, the
graphitic nature of the film is recovered by the second and third absorbed
layers. This effect is seen in both the (0001)and 4H SiC
surfaces. We also present evidence of a charge transfer that depends on the
interface geometry. It causes the graphene to be doped and gives rise to a gap
opening at the Dirac point after 3 carbon layers are deposited in agreement
with recent ARPES experiments (T.Ohta et al, Science {\bf 313} (2006) 951)
Probing Spin-Charge Separation in Tunnel-Coupled Parallel Quantum Wires
Interactions in one-dimensional (1D) electron systems are expected to cause a
dynamical separation of electronic spin and charge degrees of freedom. A
promising system for experimental observation of this non-Fermi-liquid effect
consists of two quantum wires coupled via tunneling through an extended uniform
barrier. Here we consider the minimal model of an interacting 1D electron
system exhibiting spin-charge separation and calculate the differential
tunneling conductance as well as the density-density response function. Both
quantities exhibit distinct strong features arising from spin-charge
separation. Our analysis of these features within the minimal model neglects
interactions between electrons of opposite chirality and applies therefore
directly to chiral 1D electron systems realized, e.g., at the edge of integer
quantum-Hall systems. Physical insight gained from our results is useful for
interpreting current experiment in quantum wires as our main conclusions still
apply with nonchiral interactions present. In particular, we discuss the effect
of charging due to applied voltages, and the possibility to observe spin-charge
separation in a time-resolved experiment.Comment: 9 pages, 3 figures, expanded version with many detail
Alteration of superconductivity of suspended carbon nanotubes by deposition of organic molecules
We have altered the superconductivity of a suspended rope of single walled
carbon nanotubes, by coating it with organic polymers. Upon coating, the normal
state resistance of the rope changes by less than 20 percent. But
superconductivity, which on the bare rope shows up as a substantial resistance
decrease below 300 mK, is gradualy suppressed. We correlate this to the
suppression of radial breathing modes, measured with Raman Spectroscopy on
suspended Single and Double-walled carbon nanotubes. This points to the
breathing phonon modes as being responsible for superconductivity in carbon
nanotubes
Fast high fidelity quantum non-demolition qubit readout via a non-perturbative cross-Kerr coupling
Qubit readout is an indispensable element of any quantum information
processor. In this work, we experimentally demonstrate a non-perturbative
cross-Kerr coupling between a transmon and a polariton mode which enables an
improved quantum non-demolition (QND) readout for superconducting qubits. The
new mechanism uses the same experimental techniques as the standard QND qubit
readout in the dispersive approximation, but due to its non-perturbative
nature, it maximizes the speed, the single-shot fidelity and the QND properties
of the readout. In addition, it minimizes the effect of unwanted decay channels
such as the Purcell effect. We observed a single-shot readout fidelity of 97.4%
for short 50 ns pulses, and we quantified a QND-ness of 99% for long
measurement pulses with repeated single-shot readouts
Conductance oscillations in strongly correlated fractional quantum Hall line junctions
We present a detailed theory of transport through line junctions formed by
counterpropagating single-branch fractional-quantum-Hall edge channels having
different filling factors. Intriguing transport properties are exhibited when
strong Coulomb interactions between electrons from the two edges are present.
Such strongly correlated line junctions can be classified according to the
value of an effective line-junction filling factor n that is the inverse of an
even integer. Interactions turn out to affect transport most importantly for
n=1/2 and n=1/4. A particularly interesting case is n=1/4 corresponding to,
e.g., a junction of edge channels having filling factor 1 and 1/5,
respectively. We predict its differential tunneling conductance to oscillate as
a function of voltage. This behavior directly reflects the existence of novel
Majorana-fermion quasiparticle excitations in this type of line junction.
Experimental accessibility of such systems in current cleaved-edge overgrown
samples enables direct testing of our theoretical predictions.Comment: 2 figures, 10 pages, RevTex4, v2: added second figure for clarit
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