Structural distortion,charge modulation and local anisotropies in magnetite below the Verwey transition using resonant X‐ray scattering

The pattern of charge modulations and local anisotropies below the Verwey transition has been determined and quantified in high‐quality Fe₃O₄ single crystals and thin films grown on MgO by using resonant X‐ray scattering at the Fe K‐edge. The energy, polarization and azimuthal angle dependencies of an extensive set of reflections with potential sensitivity to charge or local anisotropy orderings have been analyzed to explore their origins. A charge disproportion on octahedral B sites of 0.20 ± 0.05 e^? with [0 0 1] and cubic periodicities has been confirmed, while no significant charge disproportion has been obtained with [0 0 1/2] cubic periodicity. Additional charge modulations in the monoclinic a–b plane are also present. In addition, the occurrence of new forbidden (1, 1, 0) and (0, 0, 2n + 1/2) cubic reflections that arise from the anisotropy of the local structure around different tetrahedral and octahedral Fe atoms is shown. This complex pattern of weak charge modulations and local anisotropies is fully compatible with the low‐temperature crystal structure refined in the non‐polar C2/c space group and disproves any bimodal charge disproportion of the octahedral Fe atoms.     

Resonant X-ray scattering from a rotating medium: The Nuclear Lighthouse Effect

A coherently excited nuclear state is carried with a rotating sample so that its radiative decay is redirected by the rotation angle that has developed during its lifetime. As a result, the time spectrum of the nuclear decay is mapped to an angular scale. This effect has been observed in nuclear resonant scattering of synchrotron radiation from a rotating ⁵⁷Fe metal foil. Applications with respect to elastic and inelastic nuclear resonant scattering are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cd_(0.96)Zn_(0.04)S/Cd_(0.97)Mn_(0.03)S/Cd_(0.96)Zn_(0.04)S多层纳米线中s-d交换作用的研究

以氧化铝纳米孔为模板,采用直流电化学沉积的方法制备了Cd_(0.96)Zn_(0.04)S/Cd_(0.97)Mn_(0.03)S/Cd_(0.96)Zn_(0.04)S量子阱纳米线阵列,并系统研究了该纳米线阵列在不同温度和不同磁场下的电学输运特性.随着外磁场的变化,样品表现出共振传输特性.通过量子阱理论对实验现象进行了分析,直接得到了稀磁层Cd_(0.97)Mn_(0.03)S中s-d交换作用常数N_0α的定量结果.研究发现该交换作用常数随温度具有e~(-1/T)的变化趋势.     

利用矩形金属周期阵列实现局域电场的增强和共振波长的可控

基于表面等离激元共振(SPR)的原理,设计了一种具有矩形凹槽周期阵列微结构的金属增强基底。利用有限元方法对基底表面附近电场的分布进行了理论模拟分析,结果表明在SPR共振情况下,其凹槽微结构坑口处可得到强局域场,局域电场强度E_max／E₀可达20。通过改变结构的周期、凹槽长度l、宽度w以及环境介质,SPR共振峰发生规律性的移动,波长覆盖范围为500～1 000 nm。入射光沿x方向偏振的情况下,随着结构x方向周期P_x的增加,SPR共振峰明显红移。当入射光波长与P_x相当时,观察到凹槽内局域电场突然减小的现象。这是由于满足了波矢匹配条件,传播型SPP被激发导致的。改变凹槽长度l,发现共振波长随l的增加红移,近似呈线性关系。环境介质折射率的增加也会引起共振峰的红移。而凹槽宽度w的增加将导致其蓝移。这种规律性的移动为实现共振波长的调控提供了途径。受Jain研究报道的启发,矩形凹槽结构可以等效为两对偶极耦合模型的组合,从而解释SPR共振峰随结构参数变化而发生的移动。     

Model reduction of state space systems via an implicitly restarted Lanczos method

The nonsymmetric Lanczos method has recently received significant attention as a model reduction technique for large-scale systems. Unfortunately, the Lanczos method may produce an unstable partial realization for a given, stable system. To remedy this situation, unexpensive implicit restarts are developed which can be employed to stabilize the Lanczos generated model.This work was supported in part by ARPA (US Army ORA4466.01), by ARPA (Grant 60NANB2D1272), by the Department of Energy (Contract DE-FG0f-91ER25103) and by the National Science Foundation (Grants CCR-9209349 and CCR-9120008).     

CCl自由基A 2Δ←X 2Ⅱ跃迁REMPI光谱研究

在271～279 nm波长范围内采用REMPI技术研究CCl自由基的A 2Δ光谱.实验观察到12C35Cl以及同位素分子12C37Cl 的A 2Δ←X 2Ⅱ1/2,0-0,1-0和2-1跃迁谱带,其中2-1的跃迁为新观察到谱带.获得了A 2Δ←X 2Ⅱ1/2电子跃迁0-0,1-0,2-1谱带的同位素位移.     

用紧束缚方法描述光子晶体缺陷耦合的共振频率移动

通常采用含两个耦合参数的紧束缚近似，就能很好地描述光子晶体缺陷因耦合而导致的共振频率分裂.然而，缺陷耦合造成的共振频率移动，即包含奇数个缺陷的耦合系统的中央共振频率位置与原来单缺陷时的共振频率位置存在差异，则只有采用含三个耦合参数的严格紧束缚方法才能正确描述.根据耦合参数与共振频率的关系，利用三缺陷耦合系统的模拟计算结果确定了三个耦合参数的具体值，从而在理论上能够预言由任意个缺陷构成的耦合系统的共振频率的移动和分裂.理论预言与基于有限时域差分法的数值计算完全相符.     

硫化锰电子结构的软X射线共振非弹性散射研究

利用软X射线共振非弹性散射谱(resonant inelastic soft X-ray scattering, RIXS)对3d过渡金属硫化物中的硫化锰(MnS)电子结构进行了研究.通过分析Mn²⁺的2p⁶3d⁵→2p⁵3d⁶→2p⁶3d⁵二次光子过程，得到了共振非弹性散射谱中的两类非弹性峰，d-d电子跃迁和电荷转移(charge-transfer)跃迁.这两部分跃迁分别共振增强于L边附近及伴随峰附近.基于Hartree-Fock方法的多重态计算分别模拟了原子近似下和立方体O_h对称群下共振非弹性散射谱及吸收谱.计算得MnS实际晶体场10Dq值介于0.80eV—0.85eV之间.对MnS和MnO CT跃迁差异的讨论表明MnS较强的CT跃迁来源于其较窄的能隙宽度. 关键词： 软X射线共振非弹性散射 软X射线吸收谱 d-d跃迁 电荷转移     

火焰原子吸收光谱法测定土壤中有效态Cu、Zn、Fe和Mn

用稀HCl溶液提取土壤中有效态Cu、Zn、Fe和Mn,采用原子吸收光谱法测定其含量.样品加标回收率为93.0%-102.4%,相对标准偏差小于5.0%.该方法简便、快速,灵敏度高,检出限低,干扰小,结果令人满意.     

双波长外腔共振和频中Boyd-Kleinman因子优化的理论研究

双波长外腔共振是提高和频转换效率的有效手段,然而基于Boyd-Kleinman(BK)理论,和频效率除了受基频光功率密度影响外,主要受BK因子限制。本文通过分析BK因子随腔结构参数的变化关系,给出基于外腔频率转换过程中BK因子优化的一般方案,从而为搭建相关实验提供必要的理论基础。