4,002 research outputs found
Quench characteristics of a stabilizer-free 2G HTS conductor
The prospect of medium/high field superconducting magnets using second generation (2G) HTS tapes is approaching reality with continued enhancement in the performance of these conductors. While the cryogenic stability and quench propagation are fundamental issues for the design and safe operation of superconducting magnets, there is insufficient understanding and experimental data for 2G HTS conductors, in particular for the high field scenario at low temperature (<77 K) where the current sharing regime is much larger than in low temperature superconductors. The present work includes a systematic characterization of the relevant thermal-electrical properties used for both qualitative discussion and numerical analysis. Direct measurements of one dimensional adiabatic quench initiation and propagation of a stabilizer-free 2G conductor have been carried out with spatial-temporal recording of temperature and voltage following the deposition of varying local heat pulses to the conductor at different temperatures between 30 K and 77 K carrying different transport currents. The minimum quench energy, and the heat generation in the minimum propagation zone (MPZ) have been obtained as a function of temperature and transport current. The results show quench features unique to HTS such as an increasing MPZ with transport current and higher quench energies at lower temperatures. The experimental results are discussed in the context of current sharing over a large temperature range
A Note on Pseudo-Hermitian Systems with Point Interactions and Quantum Separability
We study the quantum entanglement and separability of Hermitian and
pseudo-Hermitian systems of identical bosonic or fermionic particles with point
interactions. The separability conditions are investigated in detail.Comment: 6 page
Calibration of a novel microstructural damage model for wire bonds
In a previous paper, a new time-domain damage-based physics model was proposed for the lifetime prediction of wire bond interconnects in power electronic modules. Unlike cycle-dependent life prediction methodologies, this model innovatively incorporates temperature- and time-dependent properties so that rate-sensitive processes associated with the bond degradation can be accurately represented. This paper presents the work on the development and calibration of the damage model by linking its core parameter, i.e., “damage,” to the strain energy density, which is a physically quantifiable materials property. Isothermal uniaxial tensile data for unbonded pure aluminum wires (99.999%) have been used to develop constitutive functions, and the model has been calibrated by the derived values of the strain energy density
Adaptive formative assessment system based on computerized adaptive testing and the learning memory cycle for personalized learning
Computerized adaptive testing (CAT) can effectively facilitate student assessment by dynamically selecting questions on the basis of learner knowledge and item difficulty. However, most CAT models are designed for one-time evaluation rather than improving learning through formative assessment. Since students cannot remember everything, encouraging them to repeatedly evaluate their knowledge state and identify their weaknesses is critical when developing an adaptive formative assessment system in real educational contexts. This study aims to achieve this goal by proposing an adaptive formative assessment system based on CAT and the learning memory cycle to enable the repeated evaluation of students' knowledge. The CAT model measures student knowledge and item difficulty, and the learning memory cycle component of the system accounts for students’ retention of information learned from each item. The proposed system was compared with an adaptive assessment system based on CAT only and a traditional nonadaptive assessment system. A 7-week experiment was conducted among students in a university programming course. The experimental results indicated that the students who used the proposed assessment system outperformed the students who used the other two systems in terms of learning performance and engagement in practice tests and reading materials. The present study provides insights for researchers who wish to develop formative assessment systems that can adaptively generate practice tests
Testing the molecular clock using mechanistic models of fossil preservation and molecular evolution
Molecular sequence data provide information about relative times only, and fossil-based age constraints are the ultimate source of information about absolute times in molecular clock dating analyses. Thus, fossil calibrations are critical to molecular clock dating, but competing methods are difficult to evaluate empirically because the true evolutionary time scale is never known. Here, we combine mechanistic models of fossil preservation and sequence evolution in simulations to evaluate different approaches to constructing fossil calibrations and their impact on Bayesian molecular clock dating, and the relative impact of fossil versus molecular sampling. We show that divergence time estimation is impacted by the model of fossil preservation, sampling intensity and tree shape. The addition of sequence data may improve molecular clock estimates, but accuracy and precision is dominated by the quality of the fossil calibrations. Posterior means and medians are poor representatives of true divergence times; posterior intervals provide a much more accurate estimate of divergence times, though they may be wide and often do not have high coverage probability. Our results highlight the importance of increased fossil sampling and improved statistical approaches to generating calibrations, which should incorporate the non-uniform nature of ecological and temporal fossil species distributions.ISSN:0962-8452ISSN:1471-295
On Integrability and Pseudo-Hermitian Systems with Spin-Coupling Point Interactions
We study the pseudo-Hermitian systems with general spin-coupling point
interactions and give a systematic description of the corresponding boundary
conditions for PT-symmetric systems. The corresponding integrability for both
bosonic and fermionic many-body systems with PT-symmetric contact interactions
is investigated.Comment: 7 page
Recent Results From the EU POF-PLUS Project: Multi-Gigabit Transmission Over 1 mm Core Diameter Plastic Optical Fibers
Recent activity to achieve multi-gigabit transmission over 1 mm core diameter graded-index and step-index plastic optical fibers for distances up to 50 meters is reported in this paper. By employing a simple intensity-modulated direct-detection system with pulse amplitude or digital multi-tone modulation techniques, low-cost transceivers and easy to install large-core POFs, it is demonstrated that multi-gigabit transmission up to 10 Gbit/s over 1-mm core diameter POF infrastructure is feasible. The results presented in this paper were obtained in the EU FP7 POF-PLUS project, which focused on applications in different scenarios, such as in next-generation in-building residential networks and in datacom applications
Coherent States and Modified de Broglie-Bohm Complex Quantum Trajectories
This paper examines the nature of classical correspondence in the case of
coherent states at the level of quantum trajectories. We first show that for a
harmonic oscillator, the coherent state complex quantum trajectories and the
complex classical trajectories are identical to each other. This congruence in
the complex plane, not restricted to high quantum numbers alone, illustrates
that the harmonic oscillator in a coherent state executes classical motion. The
quantum trajectories are those conceived in a modified de Broglie-Bohm scheme
and we note that identical classical and quantum trajectories for coherent
states are obtained only in the present approach. The study is extended to
Gazeau-Klauder and SUSY quantum mechanics-based coherent states of a particle
in an infinite potential well and that in a symmetric Poschl-Teller (PT)
potential by solving for the trajectories numerically. For the coherent state
of the infinite potential well, almost identical classical and quantum
trajectories are obtained whereas for the PT potential, though classical
trajectories are not regained, a periodic motion results as t --> \infty.Comment: More example
Electron-Phonon Dynamics in an Ensemble of Nearly Isolated Nanoparticles
We investigate the electron population dynamics in an ensemble of nearly
isolated insulating nanoparticles, each nanoparticle modeled as an electronic
two-level system coupled to a single vibrational mode. We find that at short
times the ensemble-averaged excited-state population oscillates but has a
decaying envelope. At long times, the oscillations become purely sinusoidal
about a ``plateau'' population, with a frequency determined by the
electron-phonon interaction strength, and with an envelope that decays
algebraically as t^-{1/2} We use this theory to predict electron-phonon
dynamics in an ensemble of Y_2 O_3 nanoparticles.Comment: 11 pages, 3 figure
Nondissipative Drag Conductance as a Topological Quantum Number
We show in this paper that the boundary condition averaged nondissipative
drag conductance of two coupled mesoscopic rings with no tunneling, evaluated
in a particular many-particle eigenstate, is a topological invariant
characterized by a Chern integer. Physical implications of this observation are
discussed.Comment: 4 pages, no figure. Title modified and significant revision made to
the text. Final version appeared in PR
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