颗粒介质弹性的弛豫

颗粒介质是复杂的多体相互作用体系, 其弹性源自内部的力链结构, 弹性能量处在亚稳态, 具有复杂的弛豫行为. 在常规作用下, 颗粒介质往往呈现明显的弹性弛豫. 应力松弛是应变恒定时应力的衰减现象, 弹性弛豫是应力松弛的主要原因. 在前期工作基础上, 从弹性势能面和双颗粒温度热力学角度分析了弹性弛豫的机理, 量化了弹性应力演化不可逆过程; 基于双颗粒温度热力学计算得到了弹性能、颗粒温度和应力的演化, 其中应力松弛的计算结果与实验结果基本一致, 讨论了颗粒温度初值和输运系数的影响. 指出, 开展力链结构及其动力学研究是揭示宏观弹性弛豫机理的关键.     

光磁相互作用及自旋极化的光学调控

近年来,超短激光脉冲的发展为人们在极端时间尺度上研究光与物质的相互作用提供了有 效工具。皮秒时间尺度上研究磁有序材料中的自旋动力学过程已经成为凝聚态物理研究的热点, 促进了自旋电子学的发展。本论文基于飞秒激光抽运–探测技术,介绍了半导体及其纳米结构中 光注入自旋极化及其弛豫过程;综述了铁磁性薄膜中的超快退磁,激光诱导磁阻尼进动和逆法拉 第效应; 利用太赫兹脉冲的磁场分量,研究了反铁磁晶体中自旋共振模式的激发,相干控制以 及自旋重取向的探测; 最后介绍了超快光谱用于研究多铁性材料中电子、晶格和自旋间的耦合。 了解不同材料在超快时间尺度上的磁光、光磁效应的最新进展,有助于在磁有序的超快光控制研 究领域做出原创性的工作。     

超离子玻璃中的超声吸收理论

本文依据红外发散理论讨论超离子玻璃中无序引起的多体相互作用所导致的红外发散响应。在研究新的弛豫机制的基础上,修正了滞弹性应变的速率方程,并利用玻耳兹曼叠加原理,导得具有红外发散的超声吸收公式。由此导出超声吸收与频率的ωⁿ关系和表观激活能。这些都与超离子玻璃(AgI)_x(Ag₂O·nB₂O₃)_1-x的实验数据符合。 关键词：     

FOI-PERFECT程序对电磁驱动高能量密度系统的三维弛豫磁流体力学模拟

提出一个完整的弛豫磁流体力学模型用于电磁驱动高能量密度系统的数值模拟, 它由弛豫电磁波动、弛豫热输运、P1/3近似辐射输运以及流体力学构成。电磁部分在真空区退化为电磁传播, 在等离子体物质区退化为磁扩散近似, 并且相速和群速是有上界的。这意味着弛豫磁流体力学能退化到传统的电阻性磁流体力学, 并且能用显式方法数值求解, 便于大规模高效并行化。基于此弛豫磁流体力学模型开发了三维辐射磁流体力学程序FOI-PERFECT, 指出了所采用的关键数值技术, 并给出了一些应用例子。     

场诱导下自旋冰系统[111]方向磁相变过程的蒙特卡洛模拟

自旋冰系统因为具有与固体水(冰Ih)中氢原子构型相同的自旋排列局域规则而具有宏观量子简并的基态,即使十分接近零温也不会达到长程磁有序态.外场的诱导将破坏基态的简并而使系统达到长程磁有序.我们在最近邻铁磁相互作用的简化模型下用蒙特卡洛方法模拟了自旋冰系统在[111]方向外场下的磁相变行为.分别给出了不同温度和不同相互作用强度下的磁化曲线,分析了系统的磁化过程中自旋的取向得有序程度以及相变,探讨了相互作用强度对系统磁相变行为的影响.     

应变晶体的弹性偶极子模型

将物体视为一弹性连续介质,其中任一处在应变状态的体积元以一弹性偶极子模拟之,本文给出了它的等效偶极矩的表示式,并将其划分为与此体积元及其周围基体的固有性质有关的永久偶极矩和决定于介质所处的应变状态的感生偶极矩两部分,它们分别使弹性介质具有顺弹性和介弹性。本文给出了在各向同性的弹性连续介质中弹性偶极子所产生的位移场的表示式,以及在考虑到其他应力源的强度和偶极子矩间存在着相互弛豫作用的情况下,弹性偶极子与其他应力场间相互作用的式子,它是Kroner公式加上高级修正项,将此模型用于讨论熟知的Cottrell气团和两对称中心间的相互作用时,得到与前人一致且更为细致的结果。 关键词：     

石英中Al~（3＋）－空穴取向变化弛豫过程中的红外发散响应

基于ａ石英的晶体结构，将红外发散响应模型和双势阱模型应用到含Ａｌ杂质的ａ石英中Ａｌ￣（３＋）－空穴的取向变化弛豫过程，研究其低温介电损耗特性，结果表明Ｔ＜６．５Ｋ时，介电损耗的主要贡献来自于单声子助隧道弛豫过程；Ｔ＞１０Ｋ时，主要贡献来自于热跃迁弛豫过程；而在中间温区，介电损耗是两种过程的迭加，同一弛豫体不同的弛豫过程对应于不同的红外发散响应，还讨论了同一弛豫体引起的超声弛豫损耗。     

高浓度配体低浓度顺磁离子体系NMR的顺磁弛豫的研究（Ⅰ）：顺磁…

本文导出了高浓度配体低浓度顺磁金属离子体系ＮＭＲ的顺磁弛豫谱峰增宽定量处理的公式，该公式把谱峰增宽的变化与顺磁离子浓度，给合能力强等进行关联，并得到实验上的证实。     

光子-磁振子相互作用过程：铁磁共振线宽的一种机制

通过分析磁振子的激励与弛豫过程，指出光子-磁振子的相互作用是铁磁共振线宽的机制之一， 利用幺正变换得到了3玻色子相互作用项的对角化哈密顿量，为精确计算共振线宽提供了依据。     

异核偶极相互作用非久期项对交叉极化弛豫速率的影响

本文通过计入异核偶极相互作用的非久期项对交叉极化弛豫速率的影响而推广交叉极化理论。自旋系统内部的快运动可以使得异核偶极相互作用的非久期项对交叉弛豫的贡献变得可观。本文给出了交叉弛豫速率的一般公式,并讨论了本理论在交叉极化实验中的应用。     

Berry phase of many-body system: time-dependent representation method

In this paper, we investigate the Berry phase of many-body system in the time-dependent representation. The formula of Berry phase of many-body system calculating in the time-dependent representation is derived. In this scenario, the Berry phase can be decomposed into two terms, which have different physical meanings respectively. Using these formula, we calculated the Berry phase for the many-spin system coupled by the XXZ exchange interaction. The results show that the Berry phase consists of two contributions which can be interpreted as the flux of a nonquantized monopole and the flux of a Dirac ring respectively.     

Magnetic susceptibility and short-range order in iron pnictides: Anisotropic J1-J2 Heisenberg model

The electron spin relaxation times by piezoelectric and polar optical phonon scattering in GaAs are calculated using the formula derived from the projection-reduction method. The temperature, magnetic field, and electron density dependences of the relaxation time are investigated. The electrons are found to be scattered mostly by piezoelectric phonons at low temperatures and polar optical phonons at high temperatures. The electron density affects the magnetic field dependence of the relaxation time at low temperatures but have only slight affects at high temperatures.     

Matrix coupled mode theory model of strongly-coupled multiwaveguide optical nonlinear directional couplers

This paper discusses an improved coupled mode theory model for the nonlinear directional coupler in the strong-coupling range. A new single-line matrix equation is derived for the multiwaveguide case. For the two-guide case, a new critical power formula and two exact identities are derived. Interrelation between the overlap integral and nonlinear cross-coefficients is demonstrated. It is shown that inconsistent inclusion of these terms in previous approaches leads to inaccuracies. Results obtained by consistent inclusion of these terms and our critical power formula show a great improvement in accuracy over previous analyses in strong-coupling.     

A quantitative analysis of recombination data in high magnetic fields

Magnetic field effects on the hydrogen abstraction reaction of 4-methoxybenzophenone with thiophenol in several solvents of different viscosity have been reported, and the observed magnetic field dependence was explained as caused by the Δg and a polarized initial triplet radical pair state. The present work reports a quantitative analysis of the data based on a recently derived general analytical formula. It is found that the observed magnetic field dependence can be explained as originating from an unpolarized triplet state, if both the coherent mixing caused by different g values of the two radicals and the incoherent mixing due to spin relaxation are included. Several different expressions for the magnetic field dependence of the longitudinal and transverse relaxation rates were applied. Rather surprisingly, the different models gave almost identical fits. However, the values obtained of the microscopic parameters depended significantly on the model. Physically sensible parameter values were obtained only when the complete magnetic field dependence of the two relaxation times were used. For this model it was found that both the anisotropy factor of the g tensors and the diffusion coefficient agreed with expectations.     

Ultrafast magnon generation in an Fe film on Cu(100)

We report on a combined experimental and theoretical study of the spin-dependent relaxation processes in the electron system of an iron film on Cu(100). Spin-, time-, energy- and angle-resolved two-photon photoemission shows a strong characteristic dependence of the lifetime of photoexcited electrons on their spin and energy. Ab?initio calculations as well as a many-body treatment corroborate that the observed properties are determined by relaxation processes involving magnon emission. Thereby we demonstrate that magnon emission by hot electrons occurs on the femtosecond time scale and thus provides a significant source of ultrafast spin-flip processes. Furthermore, engineering of the magnon spectrum paves the way for tuning the dynamic properties of magnetic materials.     

核多体系统输运现象的微观理论

首先回顾了描写核多体系统输运现象的一些主要模型和方法,然后介绍了输运现象微观动力学基础研究上一些新的结果,强调了单粒子运动动力学特征在建立集体输运方程和理解超重核合成机制上的重要作用。能量耗散和熵产生的数值计算结果表明,集体运动耗散过程可分为退相干、弛豫和定态等3 个阶段,弛豫过程通常表现为非常复杂的反常扩散过程。在这些理论工作的基础上,提出了一种自洽地分离核多体系统集体和单粒子变量的可能途径。In this article, I provide a simple review on conventional methods and models on the transport phenomenon of nuclear many-body systems. By exploiting the basic idea of time-dependent projection operator, I recommend a novel method to derive the transport equation for collective motion which is embedded on the microscopic dynamics of timedependent single-particle motion. It is emphasized that the microscopic dynamics of single-particle motion should play an important role for understanding the dynamics of nuclear reaction and the synthesis mechanisms of new superheavy elements. The numerical results of energy dissipation and entropy production indicate that the collective motion passes through three stages, such as dephasing/decoherence, statistical relaxation and stationary state. The statistical relaxation is a complex anomalous diffusion process in general. With the aid of above analysis and results, a possible way to define the collective and single-particle variables for the realistic nuclear many-body systems is proposed.     

Molecule production via Feshbach resonance in bosonic systems

In this article, we review our recent theoretical works on producing ultracold molecules from ultracold bosonic atoms via magnetically tunable Feshbach resonances. Our analysis relies on a two-channel quantum microscopic model that accounts for many-body effects in the association process. We show that the picture of two-body molecular production depicted by the Landau-Zener model is significantly altered due to many-body effects. We derive an analytic expression for molecular conversion efficiency for the nonadiabatic linearly swept Feshbach resonance, that explains the discrepancy between the prediction of the Landau-Zener formula and the experimental data. With including the thermal dephasing effects in the oscillating magnetic field modulation Feshbach resoance, we reproduce the Lorentzian resonance lineshape and explain the maximum conversion efficiency observed in experiment.     

Giant charge relaxation resistance in the Anderson model

We investigate the dynamical charge response of the Anderson model viewed as a quantum RC circuit. Applying a low-energy effective Fermi liquid theory, a generalized Korringa-Shiba formula is derived at zero temperature, and the charge relaxation resistance is expressed solely in terms of static susceptibilities which are accessible by Bethe ansatz. We identify a giant charge relaxation resistance at intermediate magnetic fields related to the destruction of the Kondo singlet. The scaling properties of this peak are computed analytically in the Kondo regime. We also show that the resistance peak fades away at the particle-hole symmetric point.     

Correlations in charged bosons

A new chain diagram formula is derived for many-body systems in such a way that an ideal gas term does not explicitly appear. This form is simpler and more convenient than the previously known expression. The result is applied to a charged boson gas under different conditions. It is found that exponential type decrease of the correlations for large distances changes into 1/r⁸ type decrease at the transition point.