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1.
2.
Lorenzo MalavasiPaolo Ghigna Gaetano ChiodelliGiorgio Maggi Giorgio Flor 《Journal of solid state chemistry》2002,166(1):171-176
The synthesis and structural properties of Mg1−xMn2+xO4, for 0≤x≤1 are described. Complete miscibility in the solid state exists for this system. For the material with the correct stoichiometry, i.e. MgMn2O4, the effect of temperature on the cation distribution was investigated= above 600°C the inversion degree (m) starts increasing. The electrical conductivity shows a small dependence on P(O2) which is consistent with the small oxygen non-stoichiometry determined by means of thermogravimetry. The main contribution to the transport properties arises from the inversion equilibrium. Two distinct conductivity regimes, below and above the inversion threshold, can be assumed to explain the electrical conductivity and thermoelectric power results. 相似文献
3.
Jos R. Ascenso Maria de Deus Carvalho Alberto R. Dias Carlos C. Romo Maria J. Calhorda Luis F. Veiros 《Journal of organometallic chemistry》1994,470(1-2):147-152
The metallocene thioether derivatives [Cp2M(MeSCH2CH2SMe)][PF6]2 (1, M = MO; 2, M = W), [Cp2Mo(SCH2CH2SMe)][PF6] (3) and [Cp2M(SCH2CH2S)] (4, M = Mo; 5, M = W) exhibit temperature-dependent fluxional behavior in solution, owing to the pyramidal sulfur inversion process. The activation energies for this process were determined from proton band-shape analysis in the cases of 1 (54.9 ± 2 kJ mol−1), 2 (51.2 ± 4.6 kJ mol−1) and 3 (30.0 ± 3.1 kJ mol−1). Extended Hückel calculations on related model complexes suggest that local inversion at the sulfur atoms, rather that an inversion of the complete S---C---C---S chain, is responsible for the observed fluxional behaviour. 相似文献
4.
The fabrication of a periodic domain inversion in LiTaO3 and LiNbO3 using direct ion-beam writing is presented. The polarization of these materials can be reversed at room temperature by irradiating Si2+ ions into +c faces. A first-order periodic domain inversion with a 50% duty cycle, a depth of 300μ, and an interaction length of 1 mm was realized in LiTaO3. Using this structure, 300μW of blue light was generated for a conversion efficiency of 0.25%/W. 相似文献
5.
An analysis is made of the effect of Doppler broadening on gain
without inversion (GWI) from different aspects in a closed
lambda-type three-level system with an incoherent pump. It is shown
that, regardless of the driving field being on resonance or not, for
the counter- or co-propagating of the probe and driving fields (PDF),
GWI does not monotonically decrease or increase with increasing
Doppler width. Except for the case of counter-propagating PDF with
off-resonance driving field, at a suitable Doppler width one can
obtain a gain maximum value much larger than that without Doppler
broadening; especially in the situation of the resonant driving
field, the co-propagating geometry leads to a larger GWI. In
addition, for the counter-propagating geometry, when Doppler width is
larger enough, GWI oscillation occurs, and the oscillation amplitude
and region increase with increasing Doppler width. These conclusions
are very different from that obtained in previous investigation
(Lukin {\it et al}, {\it Laser Phys.} {\bf 6} 436 (1996)). 相似文献
6.
7.
利用平均反转率迭代算法计算掺镱双包层光纤放大器分布泵浦方式下的稳态速率方程组,并采用遗传算法对分布泵浦功率的大小和每段光纤长度同时进行优化。评估函数中引入了每段光纤中最高温度的标准方差,以确保每段光纤中的最高工作温度是相同的。通过优化,7段泵浦的最高温度和标准方差分别为126.34 ℃和1.95 ℃ ,8段泵浦条件下的最高温度和标准方差分别为119.76 ℃和2.12 ℃。而未经优化的7段泵浦的最高温度和标准方差分别为147.12 ℃和21.37 ℃,8段泵浦条件下的最高温度和标准方差分别为139.95 ℃和20.83 ℃。计算结果表明:泵浦方式的优化降低了最高温度和标准方差,实现了光纤中温度分布的均匀性,并且通过增加泵浦段数可以进一步降低最高温度和平坦温度分布。 相似文献
8.
Nadjette Debbouche Shaher Momani Adel Ouannas Mohd Taib Shatnawi Giuseppe Grassi Zohir Dibi Iqbal M. Batiha 《Entropy (Basel, Switzerland)》2021,23(3)
This article investigates a non-equilibrium chaotic system in view of commensurate and incommensurate fractional orders and with only one signum function. By varying some values of the fractional-order derivative together with some parameter values of the proposed system, different dynamical behaviors of the system are explored and discussed via several numerical simulations. This system displays complex hidden dynamics such as inversion property, chaotic bursting oscillation, multistabilty, and coexisting attractors. Besides, by means of adapting certain controlled constants, it is shown that this system possesses a three-variable offset boosting system. In conformity with the performed simulations, it also turns out that the resultant hidden attractors can be distributively ordered in a grid of three dimensions, a lattice of two dimensions, a line of one dimension, and even arbitrariness in the phase space. Through considering the Caputo fractional-order operator in all performed simulations, phase portraits in two- and three-dimensional projections, Lyapunov exponents, and the bifurcation diagrams are numerically reported in this work as beneficial exit results. 相似文献
9.
Rita Simões Christoph Mönninghoff Martha Dlugaj Christian Weimar Isabel Wanke Anne-Marie van Cappellen van Walsum Cornelis Slump 《Magnetic resonance imaging》2013
Magnetic Resonance (MR) white matter hyperintensities have been shown to predict an increased risk of developing cognitive decline. However, their actual role in the conversion to dementia is still not fully understood. Automatic segmentation methods can help in the screening and monitoring of Mild Cognitive Impairment patients who take part in large population-based studies. Most existing segmentation approaches use multimodal MR images. However, multiple acquisitions represent a limitation in terms of both patient comfort and computational complexity of the algorithms. In this work, we propose an automatic lesion segmentation method that uses only three-dimensional fluid-attenuation inversion recovery (FLAIR) images. We use a modified context-sensitive Gaussian mixture model to determine voxel class probabilities, followed by correction of FLAIR artifacts. We evaluate the method against the manual segmentation performed by an experienced neuroradiologist and compare the results with other unimodal segmentation approaches. Finally, we apply our method to the segmentation of multiple sclerosis lesions by using a publicly available benchmark dataset. Results show a similar performance to other state-of-the-art multimodal methods, as well as to the human rater. 相似文献
10.
李子臣和杨义先基于离散对数和素因子分解两个困难问题提出了具有消息恢复的数字签名方案-LY方案,武丹和李善庆指出了LY方案的安全性仅仅依赖于因子分解问题,为弥补这个缺陷他们同时给出了一个改进方案——WL方案.但是,该改进方案的安全性并不象作者所认为的那样依赖于两个难题.一旦因子分解问题可解,攻击者就可以伪造签名. 相似文献