Effects of oxygen vacancy location on the electronic structure and spin density of Co-doped rutile TiO2 dilute magnetic semiconductors

According to density functional theory （DFT） using the plane wave base and pseudo-potential, we investigate the effects of the specific location of oxygen vacancy （Vo） in a （Ti,Co）06 distorted octahedron on the spin density and magnetic properties of Co-doped rutile Ti02 dilute magnetic semiconductors. Our calculations suggest that the Vo location has a significant influence on the magnetic moment of individual Co cations. In the case where two Co atoms are separated far away from each other, when the Vo is located at the equatorial site of a Co-contained octahedron, the ground state of the two Co cations is d6（t3g↑, t23g ↓） without any magnetic moment. However, if the Vo is located at the apical site, these two Co sites have different ground states and magnetic moments. The spin densities are also observed to be modified by the exchange coupling between the Co cations and the location of Vo. Some positive spin polarization is induced around the adjacent O ions.     

Effects of oxygen vacancy location on the electronic structure and spin density of Co-doped rutile TiO₂ dilute magnetic semiconductors

According to density functional theory (DFT) using the plane wave base and pseudo-potential, we investigate the effects of the specific location of oxygen vacancy (VO) in a (Ti,Co)O 6 distorted octahedron on the spin density and magnetic properties of Co-doped rutile TiO2 dilute magnetic semiconductors. Our calculations suggest that the V O location has a significant influence on the magnetic moment of individual Co cations. In the case where two Co atoms are separated far away from each other, when the V O is located at the equatorial site of a Co-contained octahedron, the ground state of the two Co cations is d6 (t3 2g ↑, t 3 2g ↓) without any magnetic moment. However, if the V O is located at the apical site, these two Co sites have different ground states and magnetic moments. The spin densities are also observed to be modified by the exchange coupling between the Co cations and the location of V O . Some positive spin polarization is induced around the adjacent O ions.     

氧空位对Co掺杂TiO2稀磁半导体中杂质 分布和磁交换的影响

本文使用基于密度泛函理论的第一性原理方法研究了Co掺杂TiO₂稀磁半导体中氧空位对体系能量和磁性的影响. 通过对总能量的计算发现当引入氧空位后近邻杂质体系能量高于均匀掺杂体系, 同时氧空位易在Co近邻位置富集. 进而发现氧空位的存在及其占位可以影响Co离子间的磁交换, 近邻Co离子体系下氧空位的引入使Co离子间的铁磁耦合减弱; 非近邻Co离子体系下, 底面氧空位使Co离子间呈反铁磁耦合而顶点氧空位使Co离子间呈铁磁耦合. 总之, 氧空位的存在对Co掺杂TiO₂材料的能量及磁性都有较大影响.     

Direct N-alkylation of amines with alcohols using AlCl3 as a Lewis acid

A substitution reaction of amines with alcohols for N-alkylated amines has been developed using inexpensive AlCl_3 without any ligand or additive.Either aromatic or aliphatic amines and primary or secondary alcohols perform the AlCl_3-mediated reaction smoothly to afford various N-alkylated amines in satisfactory yields.     

VO2金属-绝缘体相变机理的研究进展

VO₂是一种热致相变金属氧化物. 在341 K附近, VO₂发生由低温绝缘体相到高温金属相的可逆转变, 同时伴随着光学、电学和磁学等性质的可逆突变, 这种独特的性质使得VO₂在光电开关材料、智能玻璃、存储介质材料等领域有着广阔的应用前景. 因此, VO₂金属-绝缘体可逆相变一直是人们的研究热点, 但其相变机理至今未有定论. 首先, 简要概述了VO₂相变时晶体结构和能带结构的变化情况: 从晶体结构来讲, 相变前后VO₂从低温时的单斜相VO₂(M)转变为高温稳定的金红石相VO₂(R), 在一定条件下此过程也可能伴随着亚稳态单斜相VO₂(B)与四方相VO₂(A)的产生; 从能带结构来看, VO₂处于低温单斜相时, 其d_//能带和π^*能带之间存在一个禁带, 带宽约为0.7 eV, 费米能级恰好落在禁带之间, 表现出绝缘性, 而在高温金红石相时, 其费米能级落在π^*能带与d_//能带之间的重叠部分, 因此表现出金属导电性. 其次, 着重总结了VO₂相变物理机理的研究现状. 主要包括: 电子关联驱动相变、结构驱动相变以及电子关联和结构共同驱动相变的3种理论体系以及支撑这些理论体系的实验结果. 文献报道争论的焦点在于, VO₂是否是Mott绝缘体以及结构相变与MIT相变是否精确同时发生. 最后, 展望了VO₂材料研究的发展方向.     

An electron spin resonance study of Eu doping effect in La4/3Srs/3Mn2O7 single crystal

The effect of Eu3＋ ion doping in the La sites of single-crystal La4/3Srs/3Mn2O7 was investigated. Electron spin resonance （ESR） was applied to La4/3Sr5/3Mn2O7 and （Lao.8Euo.2）4/3Sr5/3Mn2O7 single crystals. A phase separation and phase transitions were observed from the ESR spectra data. Between 350 K and 300 K, both paramagnetic resonance （PMR） and anisotropic ferromagnetic resonance （FMR） lines were observed in the ab plane and the c axis direction, suggesting a coexistence of the paramagnetic （PM） phase and the ferromagnetic （FM） phase. The magnetization measurement reveals a spin-glass-like behavior in single-crystal （Lao.8Euo.2）4/3 Sr5/3Mn2O7 below the temperature of spin freezing Tf （- 29.5 K）.     

Robust univariate spline models for interpolating interval data

We consider spline interpolation problems where information about the approximated function is given by means of interval estimates for the function values over ranges of x-values instead of specific knots. We propose two robust univariate spline models formulated as convex semi-infinite optimization problems. We present simplified equivalent formulations of both models as finite explicit convex optimization problems for splines of degrees up to 3. This makes it possible to use existing convex optimization algorithms and software.