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61.
穿过大气湍流的高斯激光脉冲在时间域的展宽行为与高斯脉冲通过理想高斯滤波器后的时域展宽行为相似。因此,为了激光通信信道仿真的需要,提出用级联巴特沃斯滤波器来逼近理想高斯滤波器的方法对这种时间域上的展宽行为进行建模。逼近后的高斯滤波器的3dB频率点与大气湍流的物理参数(包括折射率结构常数和湍流外尺度)和传播几何路径长度联系在一起,从而将高斯脉冲的展宽与所通过的大气湍流环境参数相结合并得到了模拟高斯脉冲展宽的解析表达式。该模型和由模型参数表示的高斯脉冲展宽的解析表达式不仅在湍流弱起伏区有效,而且在从弱起伏到中等起伏再到强起伏的整个区域中都有效。 相似文献
62.
63.
Y.D. Su 《Applied Surface Science》2009,255(18):8164-8170
We deposit ternary WCxNy thin films on Si (1 0 0) substrates at 500 °C using direct current (DC) reactive magnetron sputtering in a mixture of CH4/N2/Ar discharge, and explore the effects of substrate bias (Vb) on the intrinsic stress, preferred orientation and phase transition for the obtained films by virtue of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and selective area electron diffraction (SAED). We find that with increasing the absolute value of Vb up to 200 V the carbon (x) and nitrogen (y) atom concentrations of WCxNy films keep almost constant with the values of 0.75 and 0.25, respectively. The XPS and SAED results, combined with the density-functional theory (DFT) calculations on the electronic structure of WC0.75N0.25, show our obtained WCxNy films are single-phase of carbonitrides. Furthermore, we find that the compressive stress sharply increases with increasing the absolute value of Vb, which leads to a pronounced change in the preferred orientation and phase structure for the film, in which a phase transition from cubic β-WCxNy to hexagonal α-WCxNy occurs as Vb is in the range of −40 to −120 V. In order to reveal the relationship between the stress and phase transition as well as preferred orientation, the DFT calculations are used to obtain the elastic constants for β-WCxNy and α-WCxNy. The calculated results show that the preferred orientation is dependent on the competition between strain energy and surface energy, and the phase transition can be attributed to a decrease in the strain energy. 相似文献
64.
Based on the spin generalized gradient approximation (σGGA) of the density functional theory (DFT), the structural, magnetic, and electronic properties of Mn-doped ZnO structure have thoroughly been investigated. It is found that the Mn atom prefers to substitute one of the Zn atoms, producing the energetically most stable configuration for the Mn-doped ZnO structure. Employing the Hubbard potential within the calculations suggests various changes and modifications to the structural, magnetic and electronic properties of the Mn-doped ZnO. Our calculations reveal that the local magnetic moment at the Mn site using the ordinary σGGA functional is 4.84 μB/Mn, which is smaller than that evaluated by including the Hubbard potential of 5.04 μB/Mn. Overall, the electronic band structure of the system, within the σGGA+U, is half-metallic, with metallic nature for the majority state and semiconducting nature for the minority state. Simulated scanning tunneling microscopy (STM) images for both unoccupied and occupied states indicate siginficant brightness on both Zn and Mn atoms and much brighter protrusions around the O atoms, respectively. 相似文献
65.
Luis Miguel Azofra Ibon Alkorta José Elguero 《Journal of Physical Organic Chemistry》2012,25(12):1286-1292
A theoretical study of the hemiacetal formation reaction between methanol and CX3CHO (X = H, F, Cl, Br, and I) has been carried out using density functional theory and Becke, three‐parameter, Lee–Yang–Parr/6‐311++G(d,p) computational methods. The stationary points of the reaction between the isolated molecules and the reaction catalyzed by an additional methanol molecule have been characterized. Because the final products present a stereogenic center, the potential autocatalysis of the reaction has been examined and also the possibility of spontaneous generation of chirality when the hemiacetal molecules are involved in the transition state structure. High barriers are found in the reaction between the isolated molecules that are reduced by the assistance of an additional molecule (methanol or hemiacetal product). The reactions catalyzed by the hemiacetal products show higher barriers than the one catalyzed by methanol. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
66.
67.
直喷式柴油机燃烧系统均匀优化设计的多维仿真研究 总被引:1,自引:0,他引:1
以直喷式柴油机的动力性和经济性为优化目标,对柴油机燃烧系统进行了基于均匀设计的多维仿真研究。通过回归分析的方法,综合考虑了柴油机的进气系统、喷油系统和燃烧室形状等参数,得到了燃烧系统参数的最优组合。结果表明:均匀设计法能以较少的仿真计算次数得到优化结果;通过回归分析和回归方程的分析得出,涡流比、油束夹角,以及涡流比和油束夹角的交互作用对柴油机的动力性和经济性影响最大;均匀设计的优化结果准确可靠,因此均匀设计法可以用于柴油机的多维仿真研究;涡流比和油束夹角的交互作用对油气混合有较大的影响。 相似文献
68.
Quantum dynamics and statistics of an atom laser with nonlinear binary interactions are investigated in the
framework of mean-field approximation. The linearized effective
Hamiltonian of the system is accurately solvable. It is shown
that, although the input radio frequency field is in an ordinary
Glauber coherent state, the output matter wave will periodically
exhibit quadrature squeezing effects purely originated from the
nonlinear atom-atom collisions. 相似文献
69.
Microwave Doppler spectra of sea return at small incidence angles: specular point scattering contribution 下载免费PDF全文
It is well known that the sea return echo contains contributions from at least two scattering mechanisms. In addition to the resonant Bragg scattering, the specular point scattering plays an important role as the incidence angle becomes smaller (≤20o). Here, in combination with the Kirchhoff integral equation of scattering field and the stationary phase approximation, analytical expressions for Doppler shift and spectral bandwidth of specular point scattering, which are insensitive to the polarization state, are derived theoretically. For comparison, the simulated results related to the two-scale method (TSM) and the method of moment (MOM) are also presented. It is found that the Doppler shift and the spectral bandwidth given by TSM are insufficient at small incidence angles. However, a comparison between the analytical results and the numerical simulations by MOM in the backscatter configuration shows that our proposed formulas are valid for the specular point scattering case. In this work, the dependences of the predicted results on incidence angle, radar frequency, and wind speed are also discussed. The obtained conclusions seem promising for a better understanding of the Doppler spectra of the specular point scattering fields from time-varying sea surfaces. 相似文献
70.
K. Hyomi 《Journal of luminescence》2009,129(12):1715-1717
We present a micro-photoluminescence (m-PL) study of electron-spin injection under magnetic fields in self-assembled semiconductor quantum dots (QDs) of CdSe. A characteristic band line-up of the CdSe QDs coupled with a diluted magnetic semiconductor quantum well (DMS-QW) of ZnCdMnSe through a ZnSe barrier layer enabled us to inject the electron spins from the DMS-QW into QDs. An experimental evidence of the electron-spin injection was provided by observations of circularly polarized m-PL spectra from individual QDs in this coupled QD structure. We find anti-correlation of PL intensity in between the DMS-QW (spin injector) and the individual QDs (spin receiver). 相似文献