The nonsymmetric pressure tensor in polyatomic fluids

Nonequilibrium molecular dynamics calculations are used to show that polyatomic fluids can support antisymmetric stress. In a homogeneous system where the time dependence of vorticity is a step function, it is shown that the rate at which intrinsic angular velocity approaches its steady-state value ( = 1/2 × u) is determined by the magnitude of the antisymmetric part of the pressure tensor.     

透镜球差对拉盖尔-高斯涡旋光束聚焦特性的影响及改善方法

涡旋光场调控对涡旋光在激光通信、生物操控等领域的应用研究具有重要意义。为了揭示透镜球差对拉盖尔-高斯(Laguerre-Gaussian,LG)涡旋光束传输特性的影响,本文基于菲涅尔衍射理论,模拟研究了不同透镜球差系数下的LG光束聚焦场的强度、相位以及螺旋谱分布。结果表明:光学系统初级球差系数的增大恶化了LG光束聚焦场的强度分布和螺旋相位,弥散了轨道角动量(orbital angular momentum,OAM) 。由此,本文提出了一种光阑补偿方案,通过在透镜前添加大小合适的圆孔光阑,以消除透镜像差。光阑补偿后的LG光束聚焦场由星芒状空心分布恢复为空心圆环分布,OAM态保持单一稳定。本研究工作为改善透镜像差对涡旋光的传输影响提供了解决方案。     

轨道角动量技术在无线通信中的应用

智能终端的普及以及移动互联网应用的蓬勃发展,使容量需求与频谱资源短缺的矛盾日益突出。轨道角动量可以在不增加系统带宽的情况下,极大地提高系统容量,引起业内广泛关注。本文分析了轨道角动量的工作原理,介绍了轨道角动量技术在无线通信中的研究进展,探讨了轨道角动量在无线通信中应用所面临的问题及挑战。     

非稳态油膜力作用下偏角不对中碰摩转子系统振动特性分析

建立了非稳态油膜力、碰摩力作用下,偏角不对中轴系的横向振动模型,将Jeffcott转子模型推广至约束条件下的轴系振动模型,通过引入转子间的位移约束,由Lagrange方程导出了不对中轴系系统的振动方程.仿真研究表明:由于不对中的存在,轴系横向振动是强非线性、非定常、同时含有自激励、参数激励和外激励的三自由度振动系统;由于从动轴与主动轴的不对中,导致了转轴间的运动约束,使轴系横向振动系统比Jeffcott转子动力学模型所描述的振动系统少了一个自由度.随着偏角不对中量、转轴质量比的变化,系统出现周期、概周期、混沌运动特征.不对中轴系处于概周期运动时,呈现出自激振动特征.其结果可为旋转机械故障诊断、轴系对中及设备的安全运行提供更为准确的理论参考.     

Discrimination of Excited States of Acetylacetone through Theoretical Molecular-Frame Photoelectron Angular Distributions

Photoelectron angular distribution (PAD) in the laboratory frame for randomly oriented molecules is typically described by a single anisotropy parameter, the so-called asymmetry parameter. However, especially from a theoretical perspective, it is more natural to consider molecular photoionization by using a molecular frame. The molecular frame PADs (MFPADs) may be used to extract information about the electronic structure of the system studied. In the last decade, significant experimental efforts have been directed to MFPAD measurements. MFPADs are highly characterizing signatures of the final ionic states. In particular, they are very sensitive to the nature of the final state, which is embodied in the corresponding Dyson orbital. In our previous work on acetylacetone, a prototype system for studying intra-molecular hydrogen bond interactions, we followed the dynamics of the excited states involved in the photoexcitation–deexcitation process of this molecule. It remains to be explored the possibility of discriminating between different excited states through the MFPAD profiles. The calculation of MFPADs to differentiate excited states can pave the way to the possibility of a clear discrimination for all the cases where the recognition of excited states is otherwise intricate.     

线偏振激光场中原子的多光子电离的光电子角分布研究

非微扰量子电动力学的发展使我们可以利用精确的波函数和非微扰的散射理论来研究多光子电离问题.文章作者及其合作者发展了光电子角分布的处理方法,利用复合相位Bessel函数来表征光电子的跃迁几率幅,将光电子的角分布与复合相位Bessel函数直接联系起来.研究发现,复合相位Bessel函数的性质决定了光电子角分布的主要特点及其随激光强度、频率以及光电子能量的演化.该理论不但证实了实验上已经观测到的各种光电子角分布,而且还预言了实验上尚未观测到的光电子角分布,确立了光电子角分布的标度定律.     

Lattice dynamical study of body-centred tetragonal indium

The phonon dispersion curves, phonon frequency distribution function as well as the lattice specific heat of body-centred tetragonal indium have been deduced using a lattice dynamical model which includes central, angular and volume forces. Six elastic constants, four zone boundary frequencies and an equilibrium condition were used in the evaluation of the force constants. It is shown that this model is elastically consistent and satisfies the symmetry requirements of the lattice, the phonon frequencies of indium deduced from it are in very good agreement with the experimental values of Reichardt and Smith and the theoretical values of Garrett and Swihart, and theθ _D values compare well with the experimental values over a wide temperature range. The apparent discrepancies in the phonon dispersion curves and theθ _D-T curves obtained from deficient models, importance of umklapp processes and the significance of angular forces in the lattice dynamical models are discussed.     

4fN-15d energy levels of lanthanides: aquasi-angular-momentum approach and its application to Cs2NaYF6:Er3+

<正>The spin-orbit interaction of the 5d electron needs to be taken into account to give the proper energy structure for the 4f^N-15d configuration of heavy lanthanide ions occupying a site with ligands forming an octahedron.This paper derives theoretical results for the energy structure by treating the t₂ orbitals as quasi p orbitals and then using angular-momentum coupling techniques.An analytic expression for the electric dipole absorption line strengths between 4f^N multiplets and 4f^N-15d states is given in terms of various angular-momentum quantum numbers and re-coupling coefficients.The result is then applied to interpret the excitation spectrum of Cs₂NaYF₆:Er³⁺.The high-spin and low-spin states of Cs₂NaYF₆:Er³⁺ are discussed in terms of the wavefunctions obtained by using the developed theoretical model.     

Nucleon internal structure: a new set of quark, gluon momentum, angular momentum operators and parton distribution functions

It is unavoidable to deal with the quark and gluon momentum and angular momentum contributions to the nucleon momentum and spin in the study of nucleon internal structure. However we never have the quark and gluon momentum, orbital angular momentum and gluon spin operators which satisfy both the gauge invariance and the canonical momentum and angular momentum commutation relation. The conflicts between the gauge invariance and canonical quantization requirement of these operators are discussed. A new set of quark and gluon momentum, orbital angular momentum and spin operators, which satisfy both the gauge invariance and canonical momentum and angular momentum commutation relation, are proposed. The key point to achieve such a proper decomposition is to separate the gauge field into the pure gauge and the gauge covariant parts. The same conflicts also exist in QED and quantum mechanics and have been solved in the same manner. The impacts of this new decomposition to the nucleon internal structure are discussed.     

Clebsch-Gordan系数的对称性

利用Schur引理确定了角动量耦合的Clebsch-Gordan系数按照其定义所允许的任意程度,然后利用正交归一关系和相位知识,导出了Clebsch-Gordan系数的对称性.