线性调频信号激励过阻尼双稳系统的随机共振现象研究

线性调频信号是工程中常见的一种信号, 由于其为非周期信号, 无法以频域信噪比作为衡量其是否产生随机共振的测量手段, 故鲜有文献研究以线性调频信号为激励信号的随机共振现象. 本文利用线性调频信号在最优分数阶Fourier变换域上的能量聚集性, 首次提出以最优分数阶Fourier变换域上定义的信噪比作为测量手段, 研究了线性调频信号叠加高斯白噪声激励过阻尼双稳系统的随机共振现象, 且发现了以线性调频信号为激励信号时产生的新现象, 即随着信号频率的增大, 随机共振将逐渐减弱, 并给出了合理的解释.仿真的结果与理论分析一致, 验证了本文所提出方法的有效性. 关键词： 线性调频信号 分数阶Fourier变换 随机共振     

第一性原理研究硫化镉高压相变及其电子结构与弹性性质

采用第一性原理研究了CdS的六方纤锌矿(WZ), 立方闪锌矿(ZB) 和岩盐矿(RS)相在高压条件下的相稳定性、 相变点、电子结构以及弹性性能.WZ相与RS 相可以在相应的压强范围内稳定存在, 而ZB相不能稳定存在.压强大于2.18 GPa时, WZ相向RS相发生金属化相变.WZ相中S原子电负性大于Cd, 且电负性差值小于1.7, CdS的WZ相为共价晶体.高压作用下, S原子半径被强烈压缩, 有效核电荷增加, 对层外电子吸引能力提高, 电负性急剧增大, 导致S与Cd的电负性差值大于1.7, CdS的RS相以离子晶体存在. WZ相的C₄₄随压强增加呈下降趋势, 导致WZ相力学不稳定, 并向RS相转变.当压强大于2.18 GPa时, RS相C₁₁, C₁₂随压强增加而增大, 并且C₄₄保持稳定, 说明RS相具有良好的高压稳定性与力学性能. 关键词： 第一性原理 相变 电子结构 弹性性质     

THz振荡器双光栅开放腔的高频特性

针对双开槽的双光栅开放式谐振腔,提出了一种基于矩形波导谐振腔理论的计算方法,给出TE04模式场分布的结构设计修正公式,用于进行准光学谐振腔结构设计优化,通过增强电子注区域的电场强度,可有效提高电子效率。同时对双光栅慢波结构"冷"色散特性进行了计算,分析了不同参数对其高频特性的影响。结果表明,双光栅槽宽和电子注通道宽度的变化会对其色散特性的频带上限产生较大影响。     

Raman spectroscopy of very small Cd1−xCoxS quantum dots grown by a novel protocol: direct observation of acoustic‐optical phonon coupling

Glass‐embedded Cd_1−xCo_xS quantum dots (QDs) with mean radius of R ≈ 1.70 nm were successfully synthesized by a novel protocol on the basis of the melting‐nucleation synthesis route and herein investigated by several experimental techniques. Incorporation of Co²⁺ ions into the QD lattice was evidenced by X‐ray diffraction and magnetic force microscopy results. Optical absorption features with irregular spacing in the ligand field region confirmed that the majority of the incorporated Co²⁺ ions are under influence of a low‐symmetry crystal field located near to the Cd_1−xCo_xS QD surface. Electron paramagnetic resonance data confirmed the presence of Co²⁺ ions in a highly inhomogeneous crystal field environment identified at the interface between the hosting glass matrix (amorphous) and the crystalline QD. The acoustic‐optical phonon coupling in the Cd_1−xCo_xS QDs (x ≠ 0.000) was directly observed by Raman measurements, which have shown a high‐frequency shoulder of the longitudinal optical phonon peak. This effect is tuned by the size‐dependent sp‐d exchange interaction due to the magnetic doping, causing variations in the coupling between electrons and longitudinal optical phonon. Copyright © 2013 John Wiley & Sons, Ltd.     

胞嘧啶核苷在纳米银膜上的NIR-SERS光谱检测

采用一种高活性的纳米银膜作为表面增强拉曼散射(SERS)基底,以近红外激光(785 nm)作为激发光源,对胞嘧啶核苷(胞苷)水溶液(10-2～10-8 mol·L-1)进行了近红外表面增强拉曼散射(NIR-SERS)光谱检测。实验结果表明,当胞苷水溶液浓度等于或低于10-7 mol · L-1时,可在300～2 000 cm-1范围内获得信噪比较好的NIR-SERS光谱。将胞苷水溶液(10-2～10-5 mol · L-1)分别滴在10片不同的纳米银薄膜上进行检测,结果表明该纳米银膜体现出了较好的光谱重现性。通过对纳米银膜表面形貌进行表征发现聚乙烯醇(PVA)包覆的纳米银颗粒在铝片表面形成“草状”结构。并通过对吸附了胞苷分子的纳米银膜进行紫外-可见光反射光谱检测,发现在800 nm处出现等离子共振峰。因此采用785 nm的近红外激光作为激发光时,该体系能够体现出强烈的表面等离子共振(surface plasmon resonance, SPR)特性。同时采用DFT-B3LYP/6-311G对胞苷分子进行了拉曼光谱计算,计算所采用入射光波长为785 nm,通过计算结果与实验测得的胞苷固体的拉曼光谱对比发现在300～2 000 cm-1范围内两者匹配得较好,进而对其振动进行了归属。最后通过比较胞苷的拉曼光谱和NIR-SERS光谱对胞苷分子在纳米银膜上的可能吸附方式进行了分析。分析结果表明胞苷分子主要为其核糖部分吸附纳米银颗粒上,同时该分子的17NH₂基团可能靠近局域电磁场增强区域。     

三值噪声调制下电路中的随机共振现象

研究了在三值噪声和热噪声扰动下的RLC电路的平均输出幅度增益. 利用随机平均法和Shapiro-Loginov公式, 得到了平均输出幅度增益的精确表达式. 通过计算机模拟, 画出了平均输出幅度增益与平坦系数、有噪声和无噪声时, 输出信号振幅与输入信号频率之间的关系曲线. 从结果来看, 系统产生了双值噪声驱动的RLC电路中(文献[11])没有的随机共振现象, 甚至还出现了双共振峰.     

多频信号的自适应随机共振检测方法

针对系统输入端待测信号为低频与中高频情形,分别建立了多频信号的自适应随机共振检测方法,即通过分析待测输入信号的频率特性和信号并行输入个数,对于低频信号,固定步长,自动调节系统参数;对于中高频信号,利用外加周期信号的频率,选取合适的测量指标来衡量随机共振现象的发生,并且寻求最优系统参数,在随机共振输出信号频域中提取待测信号的频率.与传统的自适应随机共振信号检测方法相比,采用色噪声,更符合实际背景,更精确地提取待测信号的频率.仿真结果与分析的一致,表明方法有效可行,有良好的应用前景.     

Vocal Tract Resonance Analysis of Aging Voice Using Long-Term Average Spectra

This study is the first to use long-term average spectra (LTAS) to investigate resonance characteristics of dynamic speech in young adulthood and old age. A total of 80 speakers participated, divided equally by age group and gender. All elderly speakers were healthy, active members of the community. Measurement of the first three spectral peaks in LTAS from the first paragraph of the Rainbow Passage revealed significant lowering of peak 1 from young adulthood to old age in both men and women. Peaks 2 and 3 also lowered significantly across the adult lifespan in women and showed a tendency to lower in men. These acoustic findings are consistent with anatomic data suggesting that aging results in lengthening of the supraglottic vocal tract. Findings that women demonstrate more substantial lowering of spectral peaks with aging than men suggest that women may undergo more pronounced age-related lengthening of the supraglottic vocal tract. Alternatively, it is possible that elderly men systematically alter tongue position during vowel articulation while elderly women are less inclined to do so. Taken in conjunction with previous research, these findings suggest a "mixed model" of vocal tract resonance changes with aging in which an interaction exists between gender, the resonance effects of laryngeal lowering, and vowel articulatory patterns.     

Adiabatic Hyperspherical Approach to the Problems of Muon Catalyzed Fusion

The adiabatic hyperspherical approach (AHSA) is applied for the numerical investigation of the scattering processes and resonances in Coulomb three-body mesic atomic systems. The results of the calculations of elastic and inelastic cross sections in low-energy collisions aμ + b (a, b = p, d, t), energies, lifetimes and local characteristics of resonant states of mesic molecular ions ⁿ Heaμ⁺ (n = 3, 4) are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Transport and Coulomb drag for two interacting carbon nanotubes

We study nonlinear transport for two coupled one-dimensional quantum wires or carbon nanotubes described by Luttinger liquid theory. Transport properties are shown to crucially depend on the contact length L _c. For a special interaction strength, the problem can be solved analytically for arbitrary L _c. For point-like contacts and strong interactions, a qualitatively different picture compared to a Fermi liquid emerges, characterized by zero-bias anomalies and strong dependence on the applied cross voltage. In addition, pronounced Coulomb drag phenomena are important for extended contacts. Received 28 July 2000