共查询到20条相似文献,搜索用时 93 毫秒
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利用转移矩阵方法计算了近周期超晶格体系(包括有限周期数的超晶格、耦合超晶格、层厚起伏和层厚渐变的超晶格)中声学声子的喇曼散射谱.结果表明上述近周期超晶格的光散射特性与理想严格周期的体系和完全无序的体系都不相同,呈现出许多独特的性质.对有限周期数的超晶格,由于边界的存在,喇曼谱中除了理想超晶格中存在的折叠声学声子峰(主峰)外,还会出现一系列等间距分布的卫星峰.对耦合超晶格体系,理想超晶格中存在的主峰将出现分裂.对层厚起伏变化的超晶格,主峰呈现非对称展宽,展宽主要出现在高波数端.计算结果和实验测得的谱线作了比
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文章作者提出了一种实现多准位相匹配的-维光学超晶格的新的设计方法--自调整方法,详细介绍了用自调整方法设计非周期光学超晶格和无周期光学超晶格的流程,并对结果进行了分析.数值模拟结果表明,自调整方法是一种高效灵活的专用于设计-维光学超晶格的优化算法,与其他优化算法相比,自调整方法能在更短的时间内搜索出更理想的优化结果.此外,自调整方法的优化结果不依赖于参数的设置和初始化条件,运行方便. 相似文献
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低维热电材料往往可以通过降低声子热导率实现其热电性能的提升。由两种材料交替生长获得的超晶格薄膜较单一材料薄膜具有更低的热导率,通过改变材料的厚度排布,随机排列的非周期性超晶格甚至可以实现更低的热导率。本文基于非平衡分子动力学模拟计算了硅和锗薄膜热导率和非对称界面热阻,构建了随机硅–锗超晶格热导率的数值拟合等效介质模型。引入邻间因子和修正函数后,获得了可以更为准确预测随机排列硅–锗超晶格热导率的修正等效介质模型。将此模型与遗传算法相结合,可以对大量随机超晶格结构进行高通量筛选,实现了热导率的快速优化。结果表明,即使总厚度大的超晶格最低热导率仍能维持在1.4~1.8 W·m-1·K-1,平均周期厚度稳定在2.0~2.5 nm。 相似文献
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在受二进制非周期信号和周期方波信号激励的分数阶双稳系统中,研究了非周期振动共振问题,用于微弱非周期信号的检测和增强.当非周期信号脉宽较大时,系统为小参数,通过调节周期方波信号的幅值,能够实现非周期振动共振.当非周期信号脉宽较小时,分别通过变尺度法和二次采样法实现了非周期振动共振.使用变尺度法,得到的大参数等价系统能够匹配任意小的非周期信号脉宽,其中变尺度系数是该方法在使用过程中需要选择的关键参数.使用二次采样法,二次采样后得到的非周期信号具有较大的脉宽,能够匹配原先的小参数系统,其中二次采样频率比是该方法使用过程中的关键参数.这两种方法虽然实现非周期振动共振的物理过程不同,但能够达到相同的效果.系统阶数对振动共振产生影响,随着阶数的增大,发生最佳振动共振时所需要的辅助信号幅值变大,同时系统输出的最佳时间序列与输入非周期信号之间的相似性增强. 相似文献
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由一束球面波和一束平面波或者两束球面波干涉所形成的体全息光栅,由于其光栅矢量在全息图体积内是随位置的变化而变化的,所以被称为非周期型的体光栅。主要研究了用于体全息三维成像系统的非周期体光栅的深度选择特性。根据叠加的原理,将非周期体光栅看作多个固定周期的基元体光栅的叠加,结合耦合波理论分析非周期体光栅的衍射特性。采用这一方法,利用Matlab软件模拟,研究了两束记录光夹角对所记录的非周期体光栅深度选择性的影响和在两束记录光夹角相同时,球面参考光体全息成像系统及平面参考光体全息成像系统深度选择性的差别,最后在光折变晶体材料中进行非周期型和周期型体光栅的记录和再现,对模拟结果进行了实验验证。 相似文献
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《Superlattices and Microstructures》1998,23(5):1117-1124
Using theab initioband structure results of double-zeta quality of polythiophene (PTP), polyfuran (PFU) and polyacetylene (PA), the effects of incorporation of varying percentages of vinylene units on the electronic structures and conduction properties of PTP and PFU have been investigated using a negative-factor counting method in the tight-binding approximation. Both periodic and aperiodic distributions of vinylene units have been studied. The results show that the incorporation of vinylene units makes the heterocyclic polymers better conductors of electricity both intrinsically as well as extrinsically and that the effects are more pronounced for the aperiodic distribution than for the periodic distribution. 相似文献
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This paper discusses the use of aperiodic (binary or ternary) sequences in combining pseudorandom number generators (RNG).
We introduce a method for combining two or three RNGs using cut and project sequences. This combination method produces aperiodic
number sequences having no lattice structure. Theoretical results are announced.
This work was partially supported by the Bell Canada University Laboratory, NSERC of Canada and FCAR of Québec.
Presented by L.-S. Guimond at the DI-CRM Woprkshop held in Prague, 18–21 June 2000. 相似文献
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The transition to aperiodic behavior in turbulent systems 总被引:1,自引:0,他引:1
Mitchell J. Feigenbaum 《Communications in Mathematical Physics》1980,77(1):65-86
Some systems achieve aperiodic temporal behavior through the production of successive half subharmonics. A recursive method is presented here that allows the explicit computation of this aperiodic behavior from the initial subharmonics. The results have a character universal over specific systems, so that all such transitions are characterized by noise of a universal internal similarity. 相似文献
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Liqin Li 《Physica B: Condensed Matter》2008,403(17):2888-2896
Based on more realistic physics we study the phonon spectra of the Fibonacci chain by taking into account a nonlinear resistance. It is found that the nonlinear force should be very weak and consequently, the continuity, range and gaps of the phonon spectra would be still controlled dominantly by the relative strength of spring constants and chain length. It means that even if no additional nonlinear resistance was taken into account, the conventional results of phonon spectra are exactly correct. On the other hand, in the framework of a conventional model we investigated the relationship between the biggest gaps of phonon spectra and defects of Fibonacci-like aperiodic chains. By means of numerical calculations one can obtain quantitatively the maximum of the length of a one-dimensional aperiodic chain sensitive to boundaries. This method would be useful for the calculation of quasiperiodic and aperiodic lattices. 相似文献
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T. Munakata M. Kamiyabu 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,53(2):239-243
Stochastic resonance(SR) in a FitzHugh-Nagumo neuron model
is investigated based on a dynamic mutual information (DMI)
between the input and the corresponding
output signals. The DMI is expressed in terms of the (cross)power spectra
of the input and output time series.
Both stochastic-periodic and aperiodic SR
are treated based on the DMI and our results are in good accord with
the SR measured by the signal to noise ratio(SNR) for the case of the
stochastic-periodic input and the power norm for the case of the aperiodic
input. 相似文献
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Scalar theory is a simple and rapid method to analyse and conceive diffractive optical element and gratings, in some limited cases. When we want to deflect wave fronts with large angles (for telecommunication devices, for instance) or when the features of the grating are around the order of the wavelength, scalar optics fail to be accurate, then adapted rigorous electromagnetic tools must be used. To correct the increasing cost in memory saving and time consuming with the size of the grating, we present here an original `field stitching' method adapted for aperiodic gratings (FSA). This method, initially developed for periodic gratings is extended here to analyse locally and rigorously aperiodic gratings with large sizes and small features without having some of the above-mentioned inconvenients. 相似文献
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Cascaded phase shift and intensity modulation in aperiodic quasi-phase-matched gratings 总被引:1,自引:0,他引:1
Cha M 《Optics letters》1998,23(4):250-252
I propose a method for enhancing the cascaded phase shift of the fundamental wave associated with second-harmonic generation in aperiodic quasi-phase matched (QPM) gratings. The position and length of one of the domains were varied to maximize the cascaded phase shift. A maximum enhancement of 84% in the cascaded phase shift relative to a perfectly periodic QPM grating was obtained at a moderate fundamental input intensity without initial seeding of the second-harmonic wave. All-optical intensity modulation is also proposed with another optimized aperiodic QPM grating. 相似文献
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A small bandwidth of periodic multilayers at wavelengths 2.4–4.4 nm presents problems for the spectral matching of mirrors.
This leads to low throughput of a Schwarzschild microscope and its sensitivity to technological errors in layer thickness.
We consider two cures for these difficulties: aperiodic coatings and a lateral gradient of layer thickness. Our design of
aperiodic multilayers maximizes the throughput of a microscope made at a fixed level of technology. The method includes a
new merit function, fast procedure for its minimization, linkage between multilayers and an equation for a lateral gradient
of layer thickness. Computation is performed for Sc/Cr coatings at 398 eV. It shows that the aperiodicity makes optics stable
to technological errors, while the lateral gradient increases throughput, but enhances sensitivity to the errors. The best
results are obtained for aperiodic mirrors with a lateral gradient of thickness, which assure both high microscope throughput
and stability to the errors. 相似文献