2,644 research outputs found
Electrothermal feedback in superconducting nanowire single-photon detectors
We investigate the role of electrothermal feedback in the operation of
superconducting nanowire single-photon detectors (SNSPDs). It is found that the
desired mode of operation for SNSPDs is only achieved if this feedback is
unstable, which happens naturally through the slow electrical response
associated with their relatively large kinetic inductance. If this response is
sped up in an effort to increase the device count rate, the electrothermal
feedback becomes stable and results in an effect known as latching, where the
device is locked in a resistive state and can no longer detect photons. We
present a set of experiments which elucidate this effect, and a simple model
which quantitatively explains the results
Gamow Shell Model Description of Neutron-Rich Nuclei
This work presents the first continuum shell-model study of weakly bound
neutron-rich nuclei involving multiconfiguration mixing. For the
single-particle basis, the complex-energy Berggren ensemble representing the
bound single-particle states, narrow resonances, and the non-resonant continuum
background is taken. Our shell-model Hamiltonian consists of a one-body finite
potential and a zero-range residual two-body interaction. The systems with two
valence neutrons are considered. The Gamow shell model, which is a
straightforward extension of the traditional shell model, is shown to be an
excellent tool for the microscopic description of weakly bound systems. It is
demonstrated that the residual interaction coupling to the particle continuum
is important; in some cases, it can give rise to the binding of a nucleus.Comment: 4 pages, More realistic s.p. energies used than in the precedent
versio
Kinetic-inductance-limited reset time of superconducting nanowire photon counters
We investigate the recovery of superconducting NbN-nanowire photon counters
after detection of an optical pulse at a wavelength of 1550 nm, and present a
model that quantitatively accounts for our observations. The reset time is
found to be limited by the large kinetic inductance of these nanowires, which
forces a tradeoff between counting rate and either detection efficiency or
active area. Devices of usable size and high detection efficiency are found to
have reset times orders of magnitude longer than their intrinsic photoresponse
time.Comment: Submitted to Applied Physics Letter
Temperature dependent BCS equations with continuum coupling
The temperature dependent BCS equations are modified in order to include the
contribution of the continuum single particle states. The influence of the
continuum upon the critical temperature corresponding to the phase transition
from a superfluid to a normal state and upon the behaviour of the excitation
energy and of the entropy is discussed.Comment: 9 pages, 3 figures, to appear in Phys. Rev.
Dynamics of Macroscopic Wave Packet Passing through Double Slits: Role of Gravity and Nonlinearity
Using the nonlinear Schroedinger equation (Gross-Pitaevskii equation), the
dynamics of a macroscopic wave packet for Bose-Einstein condensates falling
through double slits is analyzed. This problem is identified with a search for
the fate of a soliton showing a head-on collision with a hard-walled obstacle
of finite size. We explore the splitting of the wave packet and its
reorganization to form an interference pattern. Particular attention is paid to
the role of gravity (g) and repulsive nonlinearity (u_0) in the fringe pattern.
The peak-to-peak distance in the fringe pattern and the number of interference
peaks are found to be proportional to g^(-1/2) and u_0^(1/2)g^(1/4),
respectively. We suggest a way of designing an experiment under controlled
gravity and nonlinearity.Comment: 10 pages, 4 figures and 1 tabl
Distribution of nearest distances between nodal points for the Berry function in two dimensions
According to Berry a wave-chaotic state may be viewed as a superposition of
monochromatic plane waves with random phases and amplitudes. Here we consider
the distribution of nodal points associated with this state. Using the property
that both the real and imaginary parts of the wave function are random Gaussian
fields we analyze the correlation function and densities of the nodal points.
Using two approaches (the Poisson and Bernoulli) we derive the distribution of
nearest neighbor separations. Furthermore the distribution functions for nodal
points with specific chirality are found. Comparison is made with results from
from numerical calculations for the Berry wave function.Comment: 11 pages, 7 figure
Integrated stratigraphy of the Waitakian-Otaian Stage boundary stratotype, Early Miocene, New Zealand
The base of the type section of the Otaian Stage at Bluecliffs, South Canterbury, is recognised as the stratotype for the boundary between the Waitakian and Otaian Stages. Principal problems with the boundary are the restriction of existing bioevent proxies to shelf and upper slope environments and its uncertain age. These topics are addressed by a multidisplinary study of a 125 m section about the boundary, which examines its lithostratigraphy, depositional setting, biostratigraphy, correlation, and geochronology.
The lower siltstone lithofacies (0-38.5 m) was deposited at upper bathyal depths (200-600 m) in a marginal basin which was partially sheltered from fully oceanic circulation by a submarine high and islands. The site was covered by cool-temperate water and was probably adjacent to the Subtropical Convergence. This unit is succeeded by the banded lithofacies (38.5-106 m) and the upper siltstone lithofacies (basal 19 m studied). Paleodepth probably declined up-sequence, but deposition at shelf depths is not definitely indicated. A cyclic pattern of abundance spikes in benthic and planktonic foraminifera commences 9 m above base and extends to 73 m in the banded lithofacies. Oxygen isotope excursions (up to 2.08%) in Euuvigerina miozea and Cibicides novozelandicus are greatest within the interval containing the abundance spikes. The stage boundary occurs in the banded lithofacies at the highest abundance spike (73 m). Although condensed intervals might affect the completeness of the section, they are not associated with sedimentary discontinuities, and we consider that the section is suitable as a biostratigraphic reference.
Spores, pollens, dinoflagellates, calcareous nannofossils, foraminifera, bryozoans, and ostracods are preserved near the boundary, but molluscs principally occur higher, in the shallower upper siltstone lithofacies. Siliceous microfossils are rare. There is considerable scope for further biostratigraphic research.
The primary event marking the boundary at 73 m is the appearance of the benthic foraminifer Ehrenbergina marwicki. This is a distinctive and widely distributed event but is restricted to shelf and upper bathyal environments. Supplementary events in planktonic foraminifera and calcareous nannofossils were researched. Highest occurrences of Globigerina brazieri and G. euapertura are recorded at 47 and 58 m. There is a marked decline in relative abundance of Paragloborotalia spp. at 62 m. Helicosphaera carteri becomes more abundant than H. euphratis between 56 and 87 m. These events are not exact proxies for the boundary but they may usefully indicate proximity to it. They occur in the interval of prominent spikes in foraminiferal abundance.
The Waitakian-Otaian boundary is dated at 21.7 Ma by strontium isotopes. Stable primary remanence could not be determined in a pilot paleomagnetic study of Bluecliffs specimens. However, specimens trended towards reversed polarity, and remagnetisation great circle analysis will allow directions to be calculated in future collections
Optical Properties of Superconducting Nanowire Single-Photon Detectors
We measured the optical absorptance of superconducting nanowire single photon
detectors. We found that 200-nm-pitch, 50%-fill-factor devices had an average
absorptance of 21% for normally-incident front-illumination of
1.55-um-wavelength light polarized parallel to the nanowires, and only 10% for
perpendicularly-polarized light. We also measured devices with lower
fill-factors and narrower wires that were five times more sensitive to
parallel-polarized photons than perpendicular-polarized photons. We developed a
numerical model that predicts the absorptance of our structures. We also used
our measurements, coupled with measurements of device detection efficiencies,
to determine the probability of photon detection after an absorption event. We
found that, remarkably, absorbed parallel-polarized photons were more likely to
result in detection events than perpendicular-polarized photons, and we present
a hypothesis that qualitatively explains this result. Finally, we also
determined the enhancement of device detection efficiency and absorptance due
to the inclusion of an integrated optical cavity over a range of wavelengths
(700-1700 nm) on a number of devices, and found good agreement with our
numerical model.Comment: will appear in optics express with minor revision
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