101 research outputs found
Determination of critical current density from arbitrary flux relaxation process
The current-carrying ability of a type-II superconductor is generally
represented by its critical current density. This can be determined by
measuring a flux relaxation process starting with a testing current density
that is greater than or equal to the critical value. Here we show that a flux
relaxation process starting with an intermediate current density can be
converted into a process starting with the critical current density by
introducing a virtual time interval. Therefore, one may calculate the critical
current density from the flux relaxation process starting with a current
density below the critical value. The exact solutions of the time dependence of
current density in the flux relaxation process were also discussed.Comment: 5 page
Small Schools in a Big World: Thinking About a Wicked Problem
The position of small rural schools is precarious in much of rural Canada today. What is to be done about small schools in rural communities which are often experiencing population decline and aging, economic restructuring, and the loss of employment and services? We argue this issue is a classic "wicked" policy problem. Small schools activists have a worldview that is focused on maintaining infrastructure and even community survival, while school boards are mandated to focus on the efficient provision of educational services across wider geographies. Is it even possible to mitigate the predictable conflict and zero-sum games that arise with the decision to close small schools? That is the subject of this paper, which draws on poststructural and actor network theory. We suggest that wicked problems cannot be addressed satisfactorily through formulas and data-driven technical-rational processes. They can only be addressed through flexible, dialogical policy spaces that allow people who have radically different worldviews to create dynamic, bridging conversations. Fundamentally, we argue that what is required are new spaces and modes of governance that are sufficiently networked, open, and flexible to manage the complexity and the mutability of genuinely participatory democracy.De nos jours, la situation des petites écoles rurales est précaire dans beaucoup de milieux ruraux au Canada. Que faire des petites écoles dans les milieux ruraux souvent aux prises avec une population vieillissante et en déclin, une restructuration économique, et une perte d’emplois et de services? Nous soutenons que cette situation est un problème classique de politique « pernicieuse ». Les activistes des petites écoles ont une vision du monde axée sur le maintien de l’infrastructure, voire la survie communautaire, alors que les conseils scolaires sont chargés de miser sur la prestation efficace de services éducationnels sur de plus grandes étendues. Est-il même possible de mitiger le conflit prévisible et les jeux à somme nulle qui découlent de la décision de fermer de petites écoles? Voilà le sujet de cet article, qui puise dans la théorie post-structurale et la théorie du réseau d’acteurs. Nous proposons que les problèmes pernicieux ne peuvent être abordés de façon satisfaisante par les formules et les processus technico-rationnels axés sur les données. Ils ne peuvent être résolus que par des politiques souples et dialogiques qui permettent aux gens avec des visions du monde radicalement différentes de créer des conversations dynamiques qui appuient le rapprochement. Dans le fond, nous militons en faveur de nouveaux espaces et de nouveaux modes de gouvernance qui sont suffisamment réseautés, ouverts et souples pour gérer la complexité et la mutabilité d’une démocratie authentiquement participative.
Microwave Spectroscopy of a Cooper-Pair Transistor Coupled to a Lumped-Element Resonator
We have studied the microwave response of a single Cooper-pair transistor
(CPT) coupled to a lumped-element microwave resonator. The resonance frequency
of this circuit, , was measured as a function of the charge
induced on the CPT island by the gate electrode, and the phase difference
across the CPT, , which was controlled by the magnetic flux in the
superconducting loop containing the CPT. The observed
dependences reflect the variations of the CPT Josephson inductance with
and as well as the CPT excitation when the microwaves induce
transitions between different quantum states of the CPT. The results are in
excellent agreement with our simulations based on the numerical diagonalization
of the circuit Hamiltonian. This agreement over the whole range of and
is unexpected, because the relevant energies vary widely, from 0.1K
to 3K. The observed strong dependence near the
resonance excitation of the CPT provides a tool for sensitive charge
measurements.Comment: 10 pages, 6 figure
String Expansion as Large N Expansion of Gauge Theories
We consider string perturbative expansion in the presence of D-branes
imbedded in orbifolded space-time. In the regime where the string coupling is
weak and , the string perturbative expansion coincides with `t
Hooft's large N expansion. We specifically concentrate on theories with d=4 and
, and use world-sheet orbifold techniques to prove vanishing
theorems for the field theory beta functions to all orders in perturbation
theory in the large N limit. This is in accord with recent predictions.Comment: 18 pages, harvmac, minor corrections and additional reference
Effects of inhomogeneities and thermal fluctuations on the spectral function of a model d-wave superconductor
We compute the spectral function of a model
two-dimensional high-temperature superconductor, at both zero and finite
temperatures . We assume that an areal fraction of the
superconductor has a large gap ( regions), while the rest has a
smaller ( regions), both of which are randomly distributed in
space. We find that is most strongly affected by
inhomogeneity near the point (and the symmetry-related
points). For , exhibits two double
peaks (at positive and negative energy) near this k-point if the difference
between and is sufficiently large in comparison
to the hopping integral. The strength of the inhomogeneity required to produce
a split spectral function peak suggests that inhomogeneity is unlikely to be
the cause of a second branch in the dispersion relation. Thermal fluctuations
also affect most strongly near .
Typically, peaks that are sharp at become reduced in height, broadened,
and shifted toward lower energies with increasing ; the spectral weight near
becomes substantial at zero energy for greater than
the phase-ordering temperature.Comment: Accepted for publication in Phys. Rev. B. Scheduled Issue: 01 Jan
2008. 26 Pages and 10 figure
Intrinsic Optical Transition Energies in Carbon Nanotubes
Intrinsic optical transition energies for isolated and individual single wall
carbon nanotubes grown over trenches are measured using tunable resonant Raman
scattering. Previously measured E22_S optical transitions from nanotubes in
surfactants are blue shifted 70-90 meV with respect to our measurements of
nanotubes in air. This large shift in the exciton energy is attributed to a
larger change of the exciton binding energy than the band-gap renormalization
as the surrounding dielectric constant increases.Comment: Due to a mistake, a different paper was submitted as "revised v2".
This is a re-submission of the origional version in order to correct the
mistak
Reset dynamics and latching in niobium superconducting nanowire single-photon detectors
We study the reset dynamics of niobium (Nb) superconducting nanowire
single-photon detectors (SNSPDs) using experimental measurements and numerical
simulations. The numerical simulations of the detection dynamics agree well
with experimental measurements, using independently determined parameters in
the simulations. We find that if the photon-induced hotspot cools too slowly,
the device will latch into a dc resistive state. To avoid latching, the time
for the hotspot to cool must be short compared to the inductive time constant
that governs the resetting of the current in the device after hotspot
formation. From simulations of the energy relaxation process, we find that the
hotspot cooling time is determined primarily by the temperature-dependent
electron-phonon inelastic time. Latching prevents reset and precludes
subsequent photon detection. Fast resetting to the superconducting state is
therefore essential, and we demonstrate experimentally how this is achieved
Nonuniform and coherent motion of superconducting vortices in the picometer-per-second regime
We investigated vortex dynamics in a single-crystal sample of type-II
superconductor NbSe using scanning tunneling microscopy at 4.2 K. The
decay of the magnetic field at a few nT/s in our superconducting magnet induced
the corresponding motion of vortices at a few pm/s. Starting with an initial
magnetic field of 0.5 T, we continued to observe motion of vortices within a
field of view of 400400 nm subject to decay of the magnetic field
over a week. Vortices moved collectively, and maintained triangular lattices
due to strong vortex-vortex interactions during the motion. However, we
observed two peculiar characteristics of vortex dynamics in this
superconductor. First, the speed and direction of the vortex lattice motion
were not uniform in time. Second, despite the non-uniform motion, we also found
that there exists an energetically favored configuration of the moving vortices
in the single-crystal sample of NbSe based on the overlaid trajectories
and their suppressed speeds. We model the system with weak bulk pinning, strong
bulk pinning, and edge barrier effects.Comment: 6 pages, 4 figures, Physical Review B (R) in press (2011
Interaction between ionic lattices and superconducting condensates
The interaction of the ionic lattice with the superconducting condensate is
treated in terms of the electrostatic force in superconductors. It is shown
that this force is similar but not identical to the force suggested by the
volume difference of the normal and superconducting states. The BCS theory
shows larger deviations than the two-fluid model.Comment: 6 pages no figure
Investigating the influence of LiDAR ground surface errors on the utility of derived forest inventories
Light detection and ranging, or LiDAR, effectively produces products spatially characterizing both terrain and vegetation structure; however, development and use of those products has outpaced our understanding of the errors within them. LiDAR’s ability to capture three-dimensional structure has led to interest in conducting or augmenting forest inventories with LiDAR data. Prior to applying LiDAR in operational management, it is necessary to understand the errors in Li- DAR-derived estimates of forest inventory metrics (i.e., tree height). Most LiDAR-based forest inventory metrics require creation of digital elevation models (DEM), and because metrics are calculated relative to the DEM surface, errors within the DEMs propagate into delivered metrics. This study combines LiDAR DEMs and 54 ground survey plots to investigate how surface morphology and vegetation structure influence DEM errors. The study further compared two LiDAR classification algorithms and found no significant difference in their performance. Vegetation structure was found to have no influence, whereas increased variability in the vertical error was observed on slopes exceeding 30°, illustrating that these algorithms are not limited by high-biomass western coniferous forests, but that slope and sensor accuracy both play important roles. The observed vertical DEM error translated into ±1%–3% error range in derived timber volumes, highlighting the potential of LiDAR-derived inventories in forest management
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