Electronic structure of BaTiO3 using resonant X-ray emission spectroscopy at the Ba-L3 and Ti-K absorption edges

We have studied the electronic properties of the ferroelectric barium titanate BaTiO₃ using two complementary bulk-sensitive spectroscopic probes, resonant X-ray emission spectroscopy (RXES) and X-ray absorption spectroscopy in the partial fluorescence mode (PFY-XAS) at the Ba-L₃ and Ti-K absorption edges. Contrary to a previous study, we found no fine structure in the pre-edge area of the PFY-XAS spectrum at the Ba-L₃ edge, and no temperature-induced spectral change was observed between room temperature and 150 °C. This result is not supportive of the possible presence of the displacement around Ba²⁺ at the Curie temperature. RXES spectra were measured at the Ti-K edge for BaTiO₃, along with SrTiO₃ and La-doped metallic SrTiO₃. The photon energy of the emission peak is found to be nearly constant throughout the absorption edge for all three compounds. We deduce the Ti 3d states to have a delocalized character, in contrast with the Ba 5d states, a property which is consistent with the proposed scenario of the formation of electric dipoles in BaTiO₃.     

小鼠卵母细胞染色体三维双光子荧光图像的轴向衰减

双光子荧光显微镜作为一种高分辨光学仪器,已经被广泛应用于生物样品的非侵入式三维光学成像中。相比共聚焦显微镜,双光子荧光显微镜拥有更深的探测深度。然而,即便如此,在对较厚的生物样品进行非侵入式光学三维成像时,样品的成像质量也往往会随着探测深度的增加而下降。在临床和生物学领域对研究母性遗传起重要作用的小鼠卵母细胞拥有较大的直径(80～100 μm),吸收和散射效应较为明显。本文研究小鼠卵母细胞染色体的三维双光子荧光图像随探测深度增加图像质量的衰减程度。通过对所得图像进行轴向衰减矫正,利用体积作为参数,将矫正前后小鼠卵母细胞内染色体三维双光子荧光图像进行对比。结果表明,由于吸收和散射效应,卵母细胞存在较严重的光学轴向衰减问题,因此,对用双光子荧光三维成像手段获得的小鼠卵母细胞图像进行衰减矫正是有必要的。这为进一步精确定量的研究卵母细胞内染色体的三维构像打下良好的基础。     

基于三维成像技术的四轮定位参数建模方法

鉴于现有四轮定位仪参数求取方法误差大、参数项目少、对测量环境要求高等不足,提出了一种基于三维成像技术的四轮定位参数建模方法;该方法通过双摄像机对目标板拍摄获得的图像信息,应用空间向量方法,求取车体运动状态下的轮轴信息,动态建立四轮定位参数测量模型,解决测量平面的动态建立、测量平台的补偿等问题;经过零标定平台测试,当车轮旋转在18~22°时,定位参数求取方法正确有效,通过实车对比测量验证,平均误差小于0.07°。     

提高生物大分子NMR分辨率和灵敏度的有效方法：TROSY和CRINEPT

随着NMR谱仪和实验技术的发展,以及²H、¹⁵N、¹³C标记技术的发展和完善,先后出现了多种异核多维和多共振脉冲实验方法,测定分子量不大于30 kDa的蛋白质的溶液三维结构不再是困难的事. 但是,对于分子量大于30 kDa的蛋白质,由于共振峰数目的增多和谱线增宽,其结构测定依然很困难. 虽然提高磁场强度能够提高分辨率和灵敏度,但是,800 MHz以上的NMR谱仪造价昂贵,难以普及,促使NMR工作者不断寻求能够提高分辨率和灵敏度的新的脉冲实验方法. 到目前为止,比较成功的方法是TROSY^[1]和CRINEPT^[2], 以及在此基础上发展的新方法. 文中对TROSY和CRINEPT的基本原理、最新发展,以及它们在蛋白质结构测定和分子间相互作用研究等方面的广泛应用做了综述.      

Time-asymptotic boson states from infinite mean field quantum systems coupled to the boson gas

By means of cocycle techniques in a recent paper, the global dynamics of mean field-boson couplings has been studied. Here, by restricting to the bosonic system the infinite time limit (t ) for very general initial states, one obtains time-asymptotic states on the bosonicC ^*-Weyl algebra, in which one partially rediscovers the collective ordering of the infinite mean field lattice.     

Parallel numerical simulations for quantized vortices in Bose--Einstein condensates

We employ the parallel computing technology to study numerically the three-dimensional structure of quantized vortices of Bose--Einstein condensates. For anisotropic cases, the bending process of vortices is described in detail by the decrease of Gross--Pitaevskii energy. A completely straight vortex and the steady and symmetrical multiple-vortex configurations are obtained. We analyse the effect of initial conditions and angular velocity on the number and shape of vortices.     

First-principles study of electronic structure and ferromagnetism in Ti-doped ZnO

We perform first-principles spin polarized calculations of the electronic structure of Ti-doped in ZnO. Ferromagnetism in Ti-doped ZnO is identified, which is in agreement with recent experimental and calculated results. A net magnetic moment of 0.715μ_B is found per Ti. At a Ti concentration of 12.5%, total energy calculations show that the ferromagnetic state is 68 meV lower than the antiferromagnetic state. The electronic states near Fermi energy are dominated by strong hybridization between O 2p and Ti 3d, which is just the origin of impurity band in Ti-doped ZnO and also implies that the Ti-O bond is quite covalent instead of purely ionic. Since there is no magnetic element in this compound, Ti-doped ZnO appears to be an unambiguous dilute magnetic semiconductor.     

Macroscale and microscale evolutions of mechanically alloyed FeZn systems

The present work aims to correlate, in time, macroscale and microscale phenomenological evolutions of the microstructure of Fe and FeZn alloys processed by mechanical milling (MM) and alloying (MA), respectively. Powders were characterized for particle size distribution (PSD), particle morphology (optical microscopy, OM, scanning electron microscopy, SEM), microhardness, crystallite size, differential scanning calometry (DSC) and transmission electron microscopy (TEM). Two macroscopic regimes of PSD behavior were distinguished: the first one dominated by the cold welding process; and, the other where both fracture and agglomeration play a significant role. Solid solubilization of Zn on bcc Fe was found to reduce the final microhardness as well as increase the lattice parameter and is very well predicted by Miedema's thermodynamical approach. Microhardness and solid solution formation kinetics were correlated in time and both could be precisely described by a logistic function. After 5 h of planetary milling, microhardness and the lattice parameter become stable as well as the PSD and particle morphology, indicating that the system has already reached steady state. Indeed, this condition can be monitored by both macroscopic and microscopic parameters. Prior to an homogeneous powder, DSC results suggest an endothermic solid-state amorphization reaction for samples processed for up to 1 h as a result of the formation of clean Fe/Zn interfaces during MA.     

EPR study of Cu(II) dopant ions in single crystals of bis(l-asparaginato)Zn(II)

We report an electron paramagnetic resonance (EPR) study at 33.9 GHz and room temperature of oriented single crystal samples of bis(l-asparaginato)Zn(II) doped with Cu(II). The variation of the spectra with magnetic field orientation was measured in three crystal planes (a*b, bc and a*c, with a*=b×c). These spectra display two groups of four peaks arising from the hyperfine interaction with the I_Cu=3/2 nuclear spins of copper. They were assigned to Cu(II) ions in two lattice sites related by a 180° rotation around the b-crystal axis. The g and hyperfine coupling (A) tensors of the Cu(II) ions were evaluated from the single crystal data. Some indeterminacy in the assignment of the signals was avoided measuring the EPR spectrum of a powder sample. Their principal values are g₁=2.060(1), g₂=2.068(2), g₃=2.283(2), and A₁≈0.1×10⁻⁴, A₂=13×10⁻⁴ and A₃=165×10⁻⁴ cm⁻¹. The eigenvectors corresponding to g₃ and A₃ are coincident within the experimental error; the other eigenvectors are rotated 5.6° in the perpendicular plane. Considering the crystal structure of bis(l-asparaginato)Zn(II), our EPR results indicate that the Cu(II) impurities replace Zn(II) ions in the host crystal. We propose a molecular model based on the EPR data and the structural information, and analyse the results comparing the measured values with those obtained in similar systems.