Enhancing the magnetoelectric coupling of Co4Nb2O9[100] by substituting Mg for Co

We report experimental studies on enhancing the magnetoelectric (ME) coupling of Co₄Nb₂O₉ by substituting the non-magnetic metal Mg for Co. A series of single crystal Co_4−xMg_xNb₂O₉ (x = 0, 1, 2, 3) with a single-phase corundum-type structure are synthesized using the optical floating zone method, and the good quality and crystallographic orientations of the synthesized samples are confirmed by the Laue spots and sharp XRD peaks. Although the Néel temperatures (T_N) of the Mg substituted crystals decrease slightly from 27 K for pure Co₄Nb₂O₉ to 19 K and 11 K for Co₃MgNb₂O₉ and Co₂Mg₂Nb₂O₉, respectively, the ME coupling is doubly enhanced by Mg substitution when x = 1. The ME coefficient α_ME of Co₃MgNb₂O₉ required for the magnetic field (electric field) control of electric polarization (magnetization) is measured to be 12.8 ps/m (13.7 ps/m). These results indicate that the Mg substituted Co_4−xMg_xNb₂O₉ (x = 1) could serve as a potential candidate material for applications in future logic spintronics and logic devices.     

离子负载的双(三氟乙酰氧基)碘苯对乙酰苯胺类化合物的乙酰氧基化反应

制备了1种新的含有酰胺键的离子负载的双(三氟乙酰氧基)碘苯固体试剂--1-甲基-3-[4鄄(双三氟乙酰氧基碘)苯甲酰胺基乙基]咪唑四氟硼酸盐. 该试剂无吸湿性, 在空气中长期放置不变质. 研究了1种离子负载的二(乙酰氧基)碘苯试剂和3种离子负载的双(三氟乙酰氧基)碘苯试剂对乙酰苯胺的对位乙酰氧基化反应. 结果表明, 离子负载的二(乙酰氧基)碘苯试剂氧化能力较弱, 乙酰氧基化产物产率较低; 含有酰胺键的2种离子负载的双(三氟乙酰氧基)碘苯试剂不发生乙酰氧基化反应; 而1-甲基-3-[4-双(三氟乙酰氧基)碘苯甲基]咪唑四氟硼酸盐是理想的氧化剂. 以1-甲基-3-[4-双(三氟乙酰氧基)碘苯甲基]咪唑四氟硼酸盐为氧化剂, 室温下乙酰苯胺及其衍生物与乙酸可区域选择性地发生乙酰氧基化反应, 产率较高. 回收后的离子负载的碘苯容易再生成试剂, 而再生试剂的乙酰氧基化反应活性几乎保持不变.     

基于单向耦合映象格子生成伪随机位序列的两种新方法

基于时空混沌单向耦合映象格子模型,提出生成伪随机位序列的两种新方法：利用耦合映象格子状态的不变分布特性,选取合适基准对系统中格点状态序列进行判决,生成伪随机位序列;以及基于方向相的思想,通过比较一个格点相邻时间的两个状态值来生成伪随机位序列.对生成序列的性能进行了详细分析,数值实验结果表明,它们具有理想的平衡性、相关性和游程特性,可以应用于信息安全、密码学和数字通信等领域. 关键词： 伪随机位序列 单向耦合映象格子 不变分布 方向相     

强流离子束发射度测量技术

强流离子束的发射度测量与弱流相比有很多需要特殊考虑之处.介绍了北京大学先后研制的三台强流离子束发射度测量仪所采用的测量和数据处理方法,包括本底扣除和阈值设置、强流脉冲束的测量方法、误差的主要来源及其控制与校正等.     

Photoinduced triplet-triplet energy transfer via the 2-ureido-4[1H]-pyrimidinone self-complementary quadruple hydrogen-bonded module

Phosphorescence quenching and flash photolysis experiments demonstrate that photoinduced intra-assembly triplet-triplet energy transfer can take place via a 2-ureido-4[1H]-pyrimidinone-bridged benzophenone-naphthalene assembly I with a rate constant of 3.0 x 106 s-1 and an efficiency of 95% in CH2Cl2. This new finding suggests that with high binding strength and directionality, the 2-ureido-4[1H]-pyrimidinone hydrogen-bonded module may serve as a new model to illustrate the fundamental principles governing the triplet-triplet energy-transfer process through hydrogen bonds.     

Enantioselective Synthesis of Chiral Amides by a Phosphoric Acid Catalyzed Asymmetric Wolff Rearrangement**

The enantioselective addition of potent nucleophiles to ketenes poses challenges due to competing background reactions and poor stereocontrol. Herein, we present a method for enantioselective phosphoric acid catalyzed amination of ketenes generated from α-aryl-α-diazoketones. Upon exposure to visible light, the diazoketones undergo Wolff rearrangement to generate ketenes. The phosphoric acid not only accelerates ketene capture by amines to form a single configuration of aminoenol intermediates but also promotes an enantioselective proton-transfer reaction of the intermediates to yield the products. Mechanistic studies elucidated the reaction pathway and explained how the catalyst expedited the transformation and controlled the enantioselectivity.