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, $f_{r}$, was measured as a function of the charge $n_{g}$ induced on the CPT island by the gate electrode, and the phase difference across the CPT, $\phi_{B}$, which was controlled by the magnetic flux in the superconducting loop containing the CPT. The observed $f_{r}(n_{g},\phi_{B})$ dependences reflect the variations of the CPT Josephson inductance with $n_{g}$ and $\phi_{B}$ 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 $n_{g}$ and $\phi_{B}$ is unexpected, because the relevant energies vary widely, from 0.1K to 3K. The observed strong dependence $f_{r}(n_{g},\phi_{B})$ 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 $\alpha'\to 0$, the string perturbative expansion coincides with `t Hooft's large N expansion. We specifically concentrate on theories with d=4 and ${\cal N}=0,1,2,4$, 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 $A({\bf k}, \omega)$ of a model two-dimensional high-temperature superconductor, at both zero and finite temperatures $T$. We assume that an areal fraction $c_{\beta}$ of the superconductor has a large gap $\Delta$ ($\beta$ regions), while the rest has a smaller $\Delta$ ($\alpha$ regions), both of which are randomly distributed in space. We find that $A({\bf k}, \omega)$ is most strongly affected by inhomogeneity near the point $\mathbf k = (\pi, 0)$ (and the symmetry-related points). For $c_\beta\simeq 0.5$, $A({\bf k}, \omega)$ exhibits two double peaks (at positive and negative energy) near this k-point if the difference between $\Delta_\alpha$ and $\Delta_\beta$ 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 $A({\bf k}, \omega)$ most strongly near $\mathbf k = (\pi,0)$. Typically, peaks that are sharp at $T = 0$ become reduced in height, broadened, and shifted toward lower energies with increasing $T$; the spectral weight near $\mathbf k = (\pi, 0)$ becomes substantial at zero energy for $T$ 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$_{2}$ 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 400$\times$400 nm$^2$ 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$_{2}$ 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