7,918 research outputs found
Tunneling magnetoresistance in diluted magnetic semiconductor tunnel junctions
Using the spin-polarized tunneling model and taking into account the basic
physics of ferromagnetic semiconductors, we study the temperature dependence of
the tunneling magnetoresistance (TMR) in the diluted magnetic semiconductor
(DMS) trilayer heterostructure system (Ga,Mn)As/AlAs/(Ga,Mn)As. The
experimentally observed TMR ratio is in reasonable agreement with our result
based on the typical material parameters. It is also shown that the TMR ratio
has a strong dependence on both the itinerant-carrier density and the magnetic
ion density in the DMS electrodes. This can provide a potential way to achieve
larger TMR ratio by optimally adjusting the material parameters.Comment: 5 pages (RevTex), 3 figures (eps), submitted to PR
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An experimental study of the effects of lobed nozzles on installed jet noise
Abstract: Jet noise remains a significant aircraft noise contributor, and for modern high-bypass-ratio aero-engines the strong interaction between the jet and aircraft wing leads to intensified installed jet noise. An experiment is carried out in this paper to study the effects of lobed nozzles on installed jet noise. It is found that the lobed nozzles, compared to round nozzles, have similar effects on installed jet noise for all the plate positions and Mach numbers tested. On the shielded side of the plate, the use of lobed nozzles leads to a noise reduction in the intermediate- and high-frequency regimes, which is thought to be due to a combination of enhanced jet mixing and more effective shielding effects by the flat plate. On the reflected side of the plate, noise reduction is only achieved in the intermediate frequency range; the little noise reduction or a slight noise increase observed in the high-frequency regime is likely due to enhanced jet mixing. On both sides of the plates, little noise reduction is achieved for the low-frequency noise due to the scattering of jet instability waves. This is likely to be caused by the fact that lobed nozzles cause negligible change to the dominant mode 0 (axisymmetric) jet instability waves. That the jet mean flow quickly becomes axisymmetric downstream of the jet exit could also play a role. Graphic abstract
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On the acoustic optimality of leading-edge serration profiles
Leading-edge serrations are studied extensively as a way of reducing leading-edge noise and have been shown to be able to reduce leading-edge noise significantly. Previous experiments showed that different serration geometries have different noise reduction capabilities. However, the optimal serration geometry has not been known. Consequently, there are no guides that can be used at the design stage of serrations. In this paper, by performing an asymptotic analysis, we show that in order to achieve greater noise reduction in the high frequency regime (k1h ≫ 1, where k1 denotes the streamwise hydrodynamic wavenumber and h half of the root-to-tip amplitude of serrations), the serration profile cannot have stationary points. Therefore, piecewise smooth profiles free of stationary points are more desirable. Moreover, we show that greater noise can be achieved in the high frequency regime by using serrations that are sharper around the non-smooth points. The underlying physical mechanisms of these findings are discussed. Based on these findings, a new type of serration profile is proposed, and analytical model evaluations confirm its improved acoustic performance in the frequency range of interest. At low frequencies, a slight deterioration may be expected, but this is often negligible. To verify the conclusion drawn from the analysis, we perform an experimental study to investigate the acoustic performance of this new serration design. The results show that it is indeed superior than conventional sawtooth serrations. For example, a remarkable 7 dB additional noise reduction is observed in the intermediate frequency range with no perceivable noise increase elsewhere. The trends predicted by the analysis are well validated by the experiment. It is expected that these findings can serve as an essential guide for designing serrations, and lead to more acoustically optimized serration geometries
Software fault-tolerance by design diversity DEDIX: A tool for experiments
The use of multiple versions of a computer program, independently designed from a common specification, to reduce the effects of an error is discussed. If these versions are designed by independent programming teams, it is expected that a fault in one version will not have the same behavior as any fault in the other versions. Since the errors in the output of the versions are different and uncorrelated, it is possible to run the versions concurrently, cross-check their results at prespecified points, and mask errors. A DEsign DIversity eXperiments (DEDIX) testbed was implemented to study the influence of common mode errors which can result in a failure of the entire system. The layered design of DEDIX and its decision algorithm are described
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Phase transitions and magnetic domain coexistence in Nd0.5Sr0.5MnO3 thin films
We present a study of the physical properties of perovskite oxide Nd0.5Sr0.5MnO3 (NSMO) thin films grown on (110)-oriented SrTiO3 substrates. In bulk form, NSMO displays coupled magnetic and electronic transitions from paramagnetic/insulator to ferromagnetic (FM)/metal and then to antiferromagnetic (AFM)/charge-ordered insulator with decreasing temperature. In thin films, the AFM ordering only occurs when the films exist in an anisotropic strain state such as those obtained on (110)-oriented cubic substrates. In this work, resonant X-ray reflectivity, soft X-ray photoemission electron microscopy (X-PEEM), and magnetometry measurements showed that the NSMO film displays both vertical and lateral magnetic phase separation. Specifically, the film consists of three layers with different density and magnetic properties. The FM and AFM properties of the main NSMO layer were probed as a function of temperature using soft X-ray magnetic spectroscopy, and the coexistence of lateral FM and AFM domains was demonstrated at 110 K using X-PEEM
Efficient Quantum Work Reservoirs at the Nanoscale
When reformulated as a resource theory, thermodynamics can analyze system
behaviors in the single-shot regime. In this, the work required to implement
state transitions is bounded by alpha-Renyi divergences and so differs in
identifying efficient operations compared to stochastic thermodynamics. Thus, a
detailed understanding of the difference between stochastic thermodynamics and
resource-theoretic thermodynamics is needed. To this end, we study
reversibility in the single-shot regime, generalizing the two-level work
reservoirs used there to multi-level work reservoirs. This achieves
reversibility in any transition in the single-shot regime. Building on this, we
systematically explore multi-level work reservoirs in the nondissipation regime
with and without catalysts. The resource-theoretic results show that two-level
work reservoirs undershoot Landauer's bound, misleadingly implying energy
dissipation during computation. In contrast, we demonstrate that multi-level
work reservoirs achieve Landauer's bound and produce zero entropy.Comment: 17 pages, 5 figures, 6 tables;
https://csc.ucdavis.edu/~cmg/compmech/pubs/eqwratn.ht
Bijective Density-Equalizing Quasiconformal Map for Multiply-Connected Open Surfaces
This paper proposes a novel method for computing bijective density-equalizing
quasiconformal (DEQ) flattening maps for multiply-connected open surfaces. In
conventional density-equalizing maps, shape deformations are solely driven by
prescribed constraints on the density distribution, defined as the population
per unit area, while the bijectivity and local geometric distortions of the
mappings are uncontrolled. Also, prior methods have primarily focused on
simply-connected open surfaces but not surfaces with more complicated
topologies. Our proposed method overcomes these issues by formulating the
density diffusion process as a quasiconformal flow, which allows us to
effectively control the local geometric distortion and guarantee the
bijectivity of the mapping by solving an energy minimization problem involving
the Beltrami coefficient of the mapping. To achieve an optimal parameterization
of multiply-connected surfaces, we develop an iterative scheme that optimizes
both the shape of the target planar circular domain and the density-equalizing
quasiconformal map onto it. In addition, landmark constraints can be
incorporated into our proposed method for consistent feature alignment. The
method can also be naturally applied to simply-connected open surfaces. By
changing the prescribed population, a large variety of surface flattening maps
with different desired properties can be achieved. The method is tested on both
synthetic and real examples, demonstrating its efficacy in various applications
in computer graphics and medical imaging
Current status and developing recommendations of tailings dam failure
This is the final version of the article. Available from Hindawi Publishing Corporation via the DOI in this record.Tailings dam failure accidents with limited emergency response time and huge potential threats, can often lead to heavy
casualties and serious financial losses. In recent years, the decreasing trend of tailings dam failure accidents evidences the development
of modern technology and safety management. However, the frequency of major tailings dam failure accidents has increased, rather than
decreased. The 2015 Samarco Accident in Brazil and the 2014 Mount Polley Accident in Canada, along with their disastrous
consequences, once again sounded the alarm for the tailings ponds safety. China is now facing a complicated safety situation, with 8869
tailings ponds all over the country, including 1425 “Overhead Tailings Ponds” which represents the tailings ponds that located within 1
km upstream of residential area, workshops, schools or other important facilities. Based on a large amount of relevant research literatures, focusing on three main aspects of accident prevention and control which include safety monitoring, early-warning and emergency
preparation, safety management codes and standards, the current status and frontier progress were reviewed in this paper. Furthermore,
the relevant problems in China were discussed and several improvement recommendations were put forward, which could provide a
reference for the tailings pond accident prevention theoretical research and technological innovation. The result shows: (1) the safety
monitoring standards in China are relatively strict. However, the monitoring instruments are lack of the stability, reliability and
practicability. Thus the development of specific devices and new technologies is urgently needed. (2) The current early-warning method
is lack of diversity and reliability. And further interdisciplinary application of information technology is becoming the developing trend.
(3) The emergency management and decision-making should be based on sufficient scientific proof. However the relevant research is
limited by test methods and simulating algorithms. (4) China now has built a complete system of safety management codes and standards.
But with shortcomings of safety level classification, life-cycle management, change management process, accident investigation and so
on, there is still a long way to go
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