光腔中两原子共振荧光的相干性质

研究了置于光腔中的两个二能级原子，在强相干场驱动下的共振荧光光场的辐射场相干性质，发现在坏腔条件下，可以通过调节光腔与相干场的频率来提高一阶可见度和减小二阶关联函数.这是因为改变光腔与相干场的频率会引起原子修饰态布居数的改变，导致一阶可见度和二阶关联函数发生显著变化. 关键词： 光腔 两个二能级原子 一阶可见度 二阶关联函数     

从银电极上氢析出的温度效应得到的反应动力学启示

利用循环伏安法研究了多晶银电极在0.1 mol/L HClO₄溶液中氢析出反应的温度效应. 发现当从析氢起始电位负向扫描至零电荷电位(-0.4 V)时,氢析出反应的表观活化能(E_a,app)和指前因子(A)均随着电势的负移而增大(对应的E_a,app从24 kJ/mol增大至32 kJ/mol).继续负向扫描至零电荷电位以后,E_a,app随电势的负移而减小但A不随电势变化. 推测E_a,app和A在零     

Enhanced spontaneous emission factor for microcavity lasers

The microcavity and the influence of nonradiative recombination can control spontaneous emission. An analytic resolution of rate equation is studied for microcavity lasers. The relationship between output prop- erties and structural parameters of multi-quantum wells (MQWs) is obtained. One of the most important consequences of the incrcased spontaneous emission factor is the reduction of laser threshold. It is found that the characteristic curve of a "thresholdless" laser is strongly nonradiative depopulation-dependent. The light output is increased by the enhanced well number and the reduced width. In particular, there is an optimal well number corresponding to the lowest threshold current density for MQW structure in the microcavity lasers.     

Thermal lens determination of LD end-pumped solid-state laser with stable resonator by slit scanning method

A simple method to determine the thermal focal length of LD end-pumped solid-state laser with stable resonator is presented. The M² factor describing the quality of the beam can be obtained by scanning a slit through the multi-mode Gaussian beam field. The waist width of the beam and the corresponding TEM₀₀ under the same parameters of laser are then deduced through the law of multi-mode Gaussian beam propagation. Based on the standard matrix theory of stable resonator, the thermal focal length of the gain medium can be easily achieved. To show the application of this approach, the thermal focal length of an LD pumped Nd:YVO₄ laser is measured and the experimental results are in agreement with the theoretical prediction.     

Crack problem for superconducting strip with finite thickness

The inclined crack problems are considered for a thin strip and a strip with finite thickness in a perpendicular magnetic field. The critical current density is assumed to be a constant. The crack orientation is varied and the effect of crack on the magnetic field distribution is neglected. Based on the analytical results and variational inequality, the field and current distributions are computed for both thin strip and strip with finite thickness cases, respectively. Then, the stress intensity factors at the crack tip are determined using the finite element method for magnetic field loads. The numerical results are presented for different inclined crack angles, magnetization processes and geometry parameters of the strip. The results show that the fracture behavior of the strip with finite thickness is more complicated than that of the thin strip. With the numerical results, we can predict the largest possibility of cracking as the strip is in an external field.     

Convergence of local atomistic stress based on periodic lattice

Local stress in an atomic system, which provides an average stress measurement within a spatial volume containing a collection of atoms, is essential for determining the mechanical properties of a nanoscale structure as well as developing a proper multiscale modeling technique. Theoretically, the smaller averaging volume where a local stress can converge, the closer this atomistic stress definition can approach the ideal continuum stress. As a result, the more accurate stress concentration can be evaluated for the inhomogeneous case. With reference to the previous studies focusing on the spherical averaging volume, dependent on the type of crystals, the convergent radius of the virial stress or Hardy stress usually spans the size of several lattice constants. In this paper, we find that, once the averaging volume is periodic, the convergence of the virial stress and Hardy stress can be accomplished within one single lattice, which is much smaller than what is required by other non-periodic volumes such as a sphere. In the final section, a cracked sodium chloride crystal is considered to demonstrate that the crack opening stress described by the periodic lattices captures the stress concentration near the crack tip.     

A domain-independent interaction integral for magneto-electro-elastic materials

Magneto-electro-elastic (MEE) materials usually consist of piezoelectric (PE) and piezomagnetic (PM) phases. Between different constituent phases, there exist lots of interfaces with discontinuous MEE properties. Complex interface distribution brings a great difficulty to the fracture analysis of MEE materials since the present fracture mechanics methods can hardly solve the fracture parameters efficiently of a crack surrounded by complex interfaces. This paper develops a new domain formulation of the interaction integral for the computation of the fracture parameters including stress intensity factors (SIFs), electric displacement intensity factor (EDIF) and magnetic induction intensity factor (MIIF) for linear MEE materials. The formulation derived here does not involve any derivatives of material properties and moreover, it can be proved that an arbitrary interface in the integral domain does not affect the validity and the value of the interaction integral. Namely, the interaction integral is domain-independent for material interfaces and thus, its application does not require material parameters to be continuous. Due to this advantage, the interaction integral becomes an effective approach for extracting the fracture parameters of MEE materials with complex interfaces. Combined with the extended finite element method (XFEM), the interaction integral is employed to solve several representative problems to verify its accuracy and domain-independence. Good results show the effectiveness of the present method in the fracture analysis of MEE materials with continuous and discontinuous properties. Finally, the particulate MEE composites composed of PE and PM phases are considered and four schemes of different property-homogenization level are proposed for comparing their effectiveness.     

改进的强跟踪UKF算法及其在INS/GPS组合导航中的应用

针对标准UKF缺乏对系统状态异常的自适应调整能力,导致滤波精度降低的问题,提出一种改进的强跟踪UKF算法。该算法采用假设检验的方法对异常状态进行检测,当系统状态发生异常时,对预测协方差阵引入次优渐消因子自适应的调整滤波增益,实现对系统真实状态的强跟踪。该算法中次优渐消因子的确定无需计算系统模型的雅克比矩阵,提高了传统强跟踪UKF的实用性。将提出的算法应用于INS/GPS组合导航系统进行仿真验证,并与标准UKF进行比较,结果表明,在系统状态存在异常时,提出的带单重次优渐消因子的强跟踪UKF得到的东向、北向位置误差在[-13.7 m,14.9 m]以内,带多重次优渐消因子的强跟踪UKF得到的东向、北向位置误差在[-10.0 m,12.1 m]以内,滤波性能明显优于标准UKF,提高了组合导航系统的解算精度。     

单摆周期的系统误差分析

文章逐个分析计算了单摆的摆角、摆球的自转、摆线质量、空气浮力以及空气阻力对于单摆周期的影响,以1 m长5°小幅单摆为例,得到这5种因素带来的系统相对误差分别为+0.45‰、+0.02‰、-0.45‰、+0.07‰、-0.14‰,合计仅为-0.05‰,即这5种误差因素几乎相互抵消,小幅单摆实验在理论上的系统误差极小.文章给出了较为详细的推理、计算和分析过程,尤其空气阻力矩的推理过程以及图、表等,意在提供较为完整和准确的认识.文末,针对人们在空气阻力认识上可能存在的某些不足,文章给出了几点必要的说明.     

Microwave absorption properties regulation and bandwidth formula of oriented Y2Fe17N3-δ@SiO2/PU composite synthesized by reduction-diffusion method

As concepts closely related to microwave absorption properties, impedance matching and phase matching were rarely combined with material parameters to regulate properties and explore related mechanisms. In this work, reduction-diffusion method was innovatively applied to synthesize rare earth alloy Y$_{2}$Fe$_{17}$. In order to regulate the electromagnetic parameters of absorbers, the Y$_{2}$Fe$_{17}$N$_{3-\delta }$ particles were coated with silica (Y$_{2}$Fe$_{17}$N$_{3-\delta }$@SiO$_{2}$) and absorbers with different volume fractions were prepared. The relationship between impedance matching, matching thickness, and the strongest reflection loss peak (${\rm RL}_{\rm min}$) was presented obviously. Compared to the microwave absorption properties of Y$_{2}$Fe$_{17}$N$_{3-\delta }$/PU absorber, Y$_{2}$Fe$_{17}$N$_{3-\delta }$@SiO$_{2}$/PU absorbers are more conducive to the realization of microwave absorption material standards which are thin thickness, light weight, strong absorbing intensity, and broad bandwidth. Based on microwave frequency bands, the microwave absorption properties of the absorbers were analyzed and the related parameters were listed. As an important parameter related to perfect matching, reflection factor ($\sqrt {ărepsilon_{\rm r}/\mu_{\rm r}}$) was discussed combined with microwave amplitude attenuation. According to the origin and mathematical model of bandwidth, the formula of EAB (${\rm RL}<-10$ dB) was derived and simplified. The calculated bandwidths agreed well with experimental results.