载流子复合对时间分辨法拉第旋转光谱的影响

利用二能级体系速率方程,推导了半导体中探测光探测到的法拉第旋转光谱的理论模型,发现电子-空穴对的复合对法拉第旋转信号随时间的衰减有重要影响,并利用该模型对GaAs量子阱中实验测得的法拉第旋转光谱进行拟合,得到GaAs量子阱材料中的电子自旋弛豫时间为73.5 ps,而直接利用单指数进行拟合得到的电子自旋弛豫时间仅为51.3 ps. 因此,直接利用单指数对法拉第旋转光谱进行拟合得到电子自旋弛豫时间的传统做法是不准确的. 关键词： 自旋弛豫时间 时间分辨法拉第旋转光谱 GaAs量子阱     

14N核四极共振自旋系统自旋-晶格弛豫时间的测量

¹⁴N核四极共振自旋系统的自旋-晶格弛豫是一种双指数弛豫。本文介绍了¹⁴N核四极共振自旋系统的自旋-晶格弛豫时间的3种测量方法,利用可变多面体方法对实验数据进行拟合,获得了¹⁴N核四极共振自旋系统的自旋-晶格弛豫时间T_1s和T₁₁,对有关文献中关于核四极共振弛豫时间的测量的3个观点提出了质疑。     

基于改进非线性拟合的核磁共振T2谱多指数反演

核磁共振T₂谱多指数反演算法是开展复杂体系样品核磁共振(NMR)弛豫研究最重要的数学工具. 常用的T₂谱多指数反演算法一般都是事先给出弛豫时间T₂分布的布点, 然后转化为线性拟合问题进行求解. 在求解的T₂谱较为分散的时候, 反演得到的T₂谱精确度不高, 分辨率较低. 非线性拟合是解决这个问题的有效办法. 本文针对分散T₂谱反演利用非线性拟合时遇到的初值依赖及运算复杂问题, 利用线性回归最小二乘方法, 改进了其中的带非负约束非线性优化模型, 将搜索的反演参数从T₂, f 减少为T₂, 加快了收敛速度, 减少了对初值的依赖, 提高了反演精度, 使算法更加稳健. 通过用改进的Levenberg-Marquardt算法和差分进化算法进行计算机模拟反演及实验数据反演, 验证了改进方法在核磁共振T₂ 谱反演中的有效性.     

基于差分进化算法的核磁共振T2谱多指数反演

提出了一种基于差分进化(DE)算法的核磁共振弛豫信号多指数反演新方法. 将核磁共振T₂谱反演问题转化为带非负约束的非线性优化问题,不需要预先给定横向弛豫时间T_{2分布,直接利用差分进化算法进行反演计算. 在测量信号低信噪比的情况下,计算机模拟和实验数据反演都表明了该方法在分析处理NMR弛豫信号中的有效性. 关键词： 核磁共振 多指数反演 差分进化 岩心分析}     

C60分子在有序-无序和玻璃态相变间的取向概率与弛豫行为

用极限动力学模型研究了Ｃ₆₀分子在有序-无序相变和玻璃态相变温度区间取向 角为98°和38°的取向概率与温度的关系.计算结果在玻璃态相变点附近的85K,90K和有序- 无序相变点的260K分别与实验值相吻合,取向概率对实验值更精确的拟合及其对温度的二阶 导数预言玻璃态相变点在84K.导出了弛豫规律,其结果表明：双能级的Ｃ₆₀分子从非平衡态到平衡态的弛豫行为与非指数因子β有关,其总的弛豫时间决定于其中一个较 短的弛豫时间,展宽指数形式保持不变.讨论了KWW方程的非 关键词： 60')" href="#">Ｃ₆₀ 取向概率 非平衡态弛豫     

1,2-聚丁二烯13C-NMR弛豫的立体化学依赖性

本工作用200MHz脉冲付利叶变换NMR波谱仪测定了一系列1,2-链节立体构型不同的1,2-聚丁二烯样品的¹³C自旋-晶格弛豫时间T₁和核Overhauser效应NOE值,并用Cole-Cole、Fuoss-KirK wood经验相关时间分布模型和构象跳跃、阻尼取向扩散分子模型对nT₁和NOE值进行了电子计算机拟合。利用所得数据讨论了1,2-聚了二烯¹³C-NMR弛豫的立体化学依赖性。     

铁电体中极化长程涨落的光子关联谱实验研究

从极化团簇的随机涨落出发, 利用维纳过程模型, 推导了铁电体中极化长程涨落的弛豫规律以及光强自相关函数所可能的表现形式. 阐述了微观极化团簇的弛豫过程与宏观测量弛豫规律之间的联系. 通过对原有氦氖激光光子关联谱实验装置进行改进, 观测了BaTiO₃和0.71Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃单晶中极化团簇长程涨落在居里点和立方到四方相变点附近的弛豫过程. 在BaTiO₃中发现极化团簇长程涨落在居里点之上4 K存在双弛豫现象, 此现象与其有序无序相变机理相联系. 在Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃中发现极化团簇长程涨落在相变点两侧都存在双弛豫现象. 利用推导的理论结果很好地拟合了实验结果并提取了极化团簇长程涨落的弛豫时间. 两种样品中极化团簇长程涨落的弛豫时间都在相变点出现突变, 并呈现临界慢化现象.     

反演与拟合相结合处理核磁共振弛豫数据的方法

讨论非负最小二乘（NNLS）法和非线性拟合在分析处理核磁共振（NMR）弛豫数据中的应用.同时将二者结合，提出用NNLS的反演结果来设定非线性拟合初值的方法，并用计算机模拟和实验证明了该方法在分析处理NMR弛豫数据中的有效性. 关键词： 非负最小二乘法 非线性拟合 核磁共振 弛豫时间     

利用电子结构数据拟合H2分子的电子壳模型势函数参数

电子壳模型势函数在离子晶体的原子级计算机模拟中有广泛应用,其势参数主要通过拟合晶体的实验数据或电子结构数据得到.提出了通过拟合双原子分子的量子化学从头计算电子结构数据来获得该势函数的方法,并由H₂分子的电子结构数据建立了H原子间的电子壳模型势函数.此外,还应用该势函数对H⁺₂分子离子进行了计算.该势函数拟合方案更适合于共价键型的分子. 关键词： 电子壳模型势 参数拟合 共价键 2分子')" href="#">H₂分子     

￣（14）N核四极共振自旋系统自旋－晶格弛豫时间的测量

￣１４Ｎ核四极共振自旋系统的自旋－晶格弛豫是一种双指数弛豫。本文介绍了￣１４Ｎ核四极共振自旋系统的自旋－晶格弛豫时间的３种测量方法，利用可变多面体方法对实验数据进行拟合，获得了￣１４Ｎ核四极共振自旋系统的自旋－晶格弛豫时间Ｔ＿１ｓ和Ｔ＿１１，对有关文献中关于核四极共振弛豫时间的测量的３个观点提出了质疑。     

Direct integration transmittance model

A transmittance model has been developed for interpretation of high spectral resolution measurements of laboratory absorption and of planetary thermal emission. The high spectral resolution requires transmittances to be computed monochromatically by summing the contribution of individual molecular absorption lines. A magnetic tape atlas of H₂O, O₃, and CO₂ molecular line parameters serves as input to the transmittance model with simple empirical representations used for continuum regions wherever suitable laboratory data exist. The theoretical formulation of the transmittance model and the computational procedures used for the evaluation of the transmittances are discussed, and application of the model to several homogenous path laboratory absorption examples is demonstrated.     

Temperature dependence of Szigeti effective charge of alkali halides

The second Szigeti relation was used to obtain the temperature dependence of the Szigeti effective charge, e_s. The results are discussed in the framework of the deformation dipole model. Recent experimental data are used to show that the volume derivatives of e_s of most ionic solids are positive, thus providing evidence that the deformation dipole model is qualitatively valid.     

混凝土HJC模型抗侵彻参数敏感性数值模拟研究

 混凝土是一种应用广泛的复合材料,其动态性能及抗侵彻毁伤效应的研究越来越依赖于数值模拟,其中混凝土HJC模型是应用最广泛的本构模型之一。在介绍HJC模型的基础上,应用LS-DYNA数值模拟方法进行HJC本构模型抗侵彻性能的参数敏感性研究,通过大量数值模拟得出HJC模型的抗侵彻敏感参数有：无侧限单轴抗压强度f_c、压实点压力p_lock、特征化粘性强度A、特征化压力硬化因子B和压力硬化指数N。在混凝土的抗侵彻数值模拟过程中,重点是确定敏感参数的取值,以提高数值模拟的精度和质量,做到合理地选择试验方法和安排试验量。     

Core level shake up structures of N2 adsorbed on nickel surfaces: Cluster models

Self-consistent Hartee-Fock-LCAO calculations on a linear NiN₂ cluster are used to explain the two-peak structure observed in core level photoemission from N₂ adsorbed on the Ni (100) surface. The two observed peaks are due to different N₂ core hole final states, a screened and an unscreened final state that can be clearly identified in the cluster model. The energy separation and intensity ratio (computed using the sudden approximation) of the two final states in NiN₂ are comparable with the experimental peak separation and intensity ratio for reasonable cluster geometries. A comparison with previous model studies on NiCO is used to explain the different shape of the core level spectra of N₂ and CO adsorbed on nickel surfaces.     

Tb0.3Dy0.7Fe2合金的本构参数辨识方法研究

建立了一种简便的、适用于磁畴模型应用的Tb_0.3Dy_0.7Fe₂ 合金本构参数辨识方法. 针对Tb_0.3Dy_0.7Fe₂合金磁畴模型中本构参数不明确且直接实验测试困难的问题, 提出了一种数值计算与实验测试相结合的参数辨识方法. 采用坐标变换与绘制自由能等势曲线相结合的方法, 简化了载荷作用下Tb_0.3Dy_0.7Fe₂ 合金内磁畴角度偏转的数值计算, 研究了合金磁畴角度偏转模型的参数依赖性. 在此基础上, 结合简单的实验测试, 建立了Tb_0.3Dy_0.7Fe₂合金各向异性常数K₁ 和K₂、能量分布因子ω、晶轴取向分布的辨识及修正方法. 该方法能够简单、快速地完成Tb_0.3Dy_0.7Fe₂ 合金磁畴模型中本构参数的辨识, 对完善磁致伸缩材料磁畴偏转的数值计算模型非常有意义. 理论分析可为类磁致伸缩材料磁机耦合模型的建立、完善, 以及材料本构参数的辨识、获取提供参考.     

Fractional electrical model for modified bismuth oxide

Modified bismuth oxide (Bi₂O₃) is of great importance to fuel cells and oxide varistors. The electrical parameters of Bi₂O₃ vary with its modifying components as well as with its aging temperature. This paper presents an analysis of impedance test results carried out to find an equivalent electric model and the relationship between the model's elements to various additives used to modify Bi₂O₃ and to changes in aging temperature. The proposed model was tested via computer simulation and the model parameters correlated to individual modifiers.     

Consideration of slice profiles in inversion recovery Look–Locker relaxation parameter mapping

Purpose

To include the flip angle distribution caused by the slice profile into the model used for describing the relaxation curves observed in inversion recovery Look–Locker FLASH T₁ mapping for a more accurate determination of the relaxation parameters.

Materials and methods

For each inversion time, the flip angle dependent signal of the mono-exponential relaxation model is integrated across the slice profile. The resulting Consideration of Slice Profiles (CSP) relaxation curves are compared to the mono-exponential signal model in numerical simulations as well as in phantom and in-vivo experiments.

Results

All measured relaxation curves showed systematic deviations from a mono-exponential curve increasing with flip angle and T₁ but decreasing with repetition time. Additionally, the accuracy of T₁ was found to be largely dependent on the temporal coverage of the relaxation curve. All these systematic errors were largely reduced by the CSP model.

Conclusion

The proposed CSP model represents a useful extension of the conventionally used mono-exponential relaxation model. Despite inherent model inaccuracies, the mono-exponential model was found to be sufficient for many T₁ mapping situations. However, if only a poor temporal coverage of the relaxation process is achievable or a very precise modeling of the relaxation course is needed as in model-based techniques, the mono-exponential model leads to systematic errors and the CSP model should be used instead.     

Investigation of the effective interaction by means of a modified shell model

The effective InteractionU for a shell model with hamiltonianH _m is investigated by means of a “mixed” model with hamiltonianH _m . For this model the Bornv. Neumann expansion of the effective interaction? reduces to its first two terms. Within the finite dimensional function space ? _n used in the shell model calculation the operatorsU and? are exactly equivalent. This is used to show that an energy independent approximation forU- if it is possible at all-is necessarily equal to the residual interaction, i.e. to the difference between nucleon-nucleon interaction and shell model interaction.     

A general pseudopotential model for molecules with many valence electrons

Criteria are discussed for the practical application of ‘valence-electron only’ calculations aimed at simulating ab initio all-electron calculations on molecules containing heavy atoms with many core electrons. A theoretical model is outlined based on the use of pseudopotentials derived from atomic SCF calculations. The model has been used in calculations for NaH, Na₂, H₂S, S₂, H₂S₂, HCl, Cl₂ and NaCl, and has been tested by comparison with reference all-electron calculations. Most of these tests employ double-zeta basis sets and SCF wavefunctions, although polarization functions and complete valence-electron CI wavefunctions have been used in some cases. The model is generally successful for ground-state equilibrium structures, charge distributions and orbital energies.     

A theory of magnetism for semi-metallic uranium compounds

A theoretical study is carried out to explain the magnetic properties of uranium compounds of NaCl, anti-Cu₂Sb and Th₃P₄ type. The electronic model used is a modified free electron model along the lines of the band structure for NaCl type uranium compounds previously reported. An exchange interaction via the conduction electrons is assumed for the mechanism of magnetic ordering. The molecular field approximation is used to calculate θ, T_N and the exchange interaction constants. The stability range of various magnetic ordering states and the variation of θ, T_N and exchange interaction constants are obtained as functions of lattice parameter. These results well explain the experimental data.