Comparative study of the magnetism of SrTcO3 and Ca(Sr)MnO3

By the first-principles calculations, we studied the electronic structures and the magnetic properties of SrTcO₃ and Ca(Sr)MnO₃. We found the strikingly high Néel temperature of SrTcO₃ is mostly due to the strong Tc(4d)–O(2p) hybridizations, since the Tc-4d states are more extended than the Mn-3d states. Such Tc(4d)–O(2p) hybridizations increase the super-exchange constants, hence increased the Néel temperatures.     

Equation of state and elastic properties of ACrO3 (A=Pb,Ca, Sr and Ba) perovskites under high pressure

We employ the spin-polarized generalized gradient approximation within the density functional theory to investigate the equation of state, magnetism and elastic constant of cubic ACrO₃ (A=Pb, Ca, Sr, and Ba) perovskite. The antiferromagnetic phase is the most stable state at zero pressure. Under pressure, the ferromagnetic state will transform to the non-magnetic state. Considering the effect of magnetism, the equilibrium lattice constant, the bulk modulus and the high pressure equations of state agree well with the available experiments. By using the energy-strain method, the predicted elastic properties are satisfactory.     

Influence of surface affinity of planar walls on effective interaction between the wall and colloids

Using density functional theory, we investigate the effective interaction between a big colloid immersed in a sea of small colloids and a wall which has different affinity to the small colloids. Steele 10-4-3 potential is introduced to mimic both short-range repulsive and long-range attractive interactions between the wall and the small colloids. It is found that the surface affinity of the wall has a significant influence on the effective interaction. In the short-range repulsive case, the repulsion greatly enhances the big colloid-wall effective attraction, which sensitively depends on the concentration of small colloids, and is not sensitive to the repulsive strength. In the long-range attractive case, both the concentration of small colloids and the attractive strength have great effect on the effective interaction, and with an increase of the attractive strength, a strong repulsion may be induced when the big colloid is close to the wall. In low density limit of small colloids, the present results agree well with those of the Asakura and Oosawa(AO) approximation.     

高温超导混合配对态与磁通涡旋格子

建立在对基于Gor’kov方法而导出的微观GinzburgLandau方程的分析揭示了高温超导体YBa2Cu3O7配对态对称性和磁通涡旋格子结构.分析指出,存在一个格子转变温度T,当温度高于T时,超导基态显示dx2-y2波对称性特征;低于该温度s波沟道幅值成为可观的量级;超导基态为混合s-dx2-y2态.对应单分量波函数磁通涡旋格子为三角的结构;而稳定的斜格子反映出混合波特征.s与d沟道间耦合约束了磁场下dx2-x2波对称性自由度,而对高温超导反常输运行为如上临界磁场温度曲线上翘现象所负责 关键词： 高温超导 GinzburgLandau理论 磁通涡旋     

从Mott绝缘体到高温超导体:自旋磁性与超导电性的互偶关系

作者最近的一项理论工作对高温超导体作为一种掺杂Mott绝缘体进行了新的探索 ,得到一个长波低能极限的有效理论 ,强调了自旋反铁磁性和超导电性之间的拓扑互偶关系 .这个理论能够为一大类具有本质性和挑战性的高温超导现象提供简明的解释 ,给出掺杂Mott绝缘体的超导电性的“指纹”特征 ,并给出若干有趣的理论预言 .     

Symmetry properties of the electron density and following from it limits on the KS-DFT applications

ABSTRACT

At present, the Density Functional Theory (DFT) approach elaborated by Kohn with co-authors more than 50 years ago became the most widely used method for study molecules and solids. Using modern computation facilities, it can be applied to systems with million atoms. In the atmosphere of such great popularity, it is particularly important to know the limits of the applicability of DFT methods. In this report, I will discuss two cases when the conventional DFT approaches, using only electron density ρ and its gradients, cannot be applied (I will not consider the Ψ-versions of DFT). The first case is quite evident. In the degenerated states, the electron density may not be defined, since electronic and nuclear motions cannot be separated, the vibronic interaction mixed them. The second case is related to the spin of the state. As it was rigorously proved by group theoretical methods at the theorem level, the electron density does not depend on the total spin S of the arbitrary N-electron state. It means that the Kohn–Sham equations have the same form for states with different S. The critical survey of elaborated DFT procedures, taking into account spin, shows that they modified only exchange functionals, the correlation functionals do not correspond to the spin of the state. The point is that the conception of spin cannot be defined in the framework of the electron density formalism, which corresponds to the one-particle reduced density matrix. This is the main reason of the problems arising in the study by DFT of magnetic properties of the transition metals. The possible way of resolving these problems can be found in the two-particle reduced density matrix formulation of DFT.     

Control of optical absorption edge of TiO2 through co-doped acceptors: The chemical trend

Tuning the optical adsorption edge of TiO₂ is attracting increasing attention as a potential solution to the worldwide energy shortage. A possible strategy to achieve high efficiency photocatalysis with TiO₂ is through dopants to modulate chemical composition. Based on first-principles calculations, we propose a hole-strain-mediated coupling mechanism between co-doped acceptors in anatase TiO₂. When the dopant complex on neighboring oxygen sites contains a large radius atom, and the doped system has at least one net hole, the dopants will strongly couple to form a pair through the local lattice strain induced by the large dopant. The coupling results in bandgap narrowing due to the appearance of the fully occupied mid-gap states, leading to a much more effective band gap reduction than that induced by mono-doping or conventional donor–acceptor codoping. The calculated absorption spectra show that acceptor–acceptor codopings could shift the absorption edge to the visible light region.     

Ferromagnetism of Pd(001) substrate induced by antiferromagnetic CoO

Our first-principles study has revealed unexpected spin polarization of the Pd(001) substrate in contact with antiferromagnetic CoO overlayers. We give an evidence that the ferromagnetism of Pd is caused by the zigzag positions of Co atoms with respect to the Pd interface, resulted from the lattice-mismatch driven structural relaxation. Because of the itinerant nature of its 4d electrons, we see that the ferromagnetic properties of Pd are highly sensitive to the local environment and can be enhanced further by increasing the thickness of CoO overlayer film or/and by applying an additional uniaxial pressure along c-axis exerted externally on the bottom layers of the Pd substrate. Our finding provides new functionality for the interfacial moments of the CoO/Pd system, which can be accessed experimentally, e.g., by the magneto-optical Kerr effect (MOKE) or/and by element-resolved X-ray magnetic circular dichroism (XMCD) measurement.     

Microscopic theory of atomic and electronic stretched exponential relaxation in high temperature superconductors

Measured dimensionless room-temperature conductivity relaxation stretching fractions β in YBCO are in excellent agreement with theoretical predictions, which in 1995 identified two magic fractions, β=3/5 and β=3/7. Thus, relaxation studies provide an absolute measure of “ideality” in these complex materials, independent not only of composition x but even of crystal structure. The relaxation stretching fractions β associated with T_c itself, reported in 2000, are also explained by the magic fraction β=3/5 predicted by microscopic theory. One can infer that the interactions responsible for high-temperature superconductivity are short range, non-magnetic, and primarily associated with resonant trapping centers in semiconductive layers.     

高温高压下钙钛矿晶体结构变化

最近高压单晶X射线衍射实验明确显示正交晶系（Pbnm或Pnma）（钙钛矿）ABO3的高压压缩行为完全由晶体框架结构（framework）中的BO6八面体和AO12多面体的相对压缩性所确定,即对一些钙钛矿晶体,BO6较AO12多面体更易压缩时,压力导致晶体结构向高对称性结构演变;对另外一些钙钛矿,BO6较AO12多面体更不易压缩时,压力导致晶体结构向低对称性结构演变.基于价键匹配关系假设,建立多面体压缩率之间关系：βB/βA=MA/MB,由价键参数和常压晶体结构数据计算参量Mi,可以估计BO6和AO12多面体的相对压缩率,从而正确地预言高压压缩行为.文章最后引入高温高压钙钛矿倾斜相变的一般性规律,即MA/MB＞1时,相变温度和高压关系为dTc/dP＜0,当MA/MB＜1时,dTc/dP＞0。     

高温高压下钙钛矿晶体结构变化

最近高压单晶X射线衍射实验明确显示正交晶系(Pbnm或Pnma) （钙钛矿）ABO3的高压压缩行为完全由晶体框架结构(framework)中的BO6八面体和AO12多面体的相对压缩性所确定,即对一些钙钛矿晶体,BO6较AO12多面体更易压缩时,压力导致晶体结构向高对称性结构演变;对另外一些钙钛矿,BO6较AO12多面体更不易压缩时,压力导致晶体结构向低对称性结构演变.基于价键匹配关系假设,建立多面体压缩率之间关系,由价键参数和常压晶体结构数据计算参量, 可以估计BO6和AO12多面体的相对压缩率,从而正确地预言高压压缩行为.文章最后引入高温高压钙钛矿倾斜相变的一般性规律.     

并五苯分子电荷注入导致自旋极化的密度泛函理论研究

用密度泛函理论研究了不带自旋的空穴注入并五苯后体系的自旋相关特性. 电荷注入后并五苯分子中存在自发自旋极化行为. 当注入电荷量达一定程度,分子磁矩随注入电荷量的增加呈线性增长,最大磁矩可达1 μB. 注入电荷和并五苯分子的相互作用导致分子体系结构发生变化,同时电荷密度分布及自旋密度分布也发生了变化. 注入电荷先填充自旋劈裂的碳原子p_z轨道.     

Partition density functional theory and its extension to the spin-polarized case

Partition density functional theory (PDFT) [P. Elliott, K. Burke, M.H. Cohen, and A. Wasserman, Phys. Rev. A 82 (2), 024501 (2010)] is a formally exact method for obtaining molecular properties from Kohn–Sham calculations on isolated fragments. Here, we express the partition energy of PDFT as an implicit functional of the molecular spin-densities for a given choice of fragmentation, and use the principle of von Barth and Hedin to formulate the spin-decomposed version of PDFT. We introduce a partition energy functional of the spin-up and spin-down electronic densities and derive the associated polarized partition potentials, which are found to be global quantities that influence every fragment in the molecule. Along with the formal theory, we propose a simplified approach to computing the spin-partition potentials, and illustrate its utility and accuracy with two simple examples. Finally, we propose a viable approach to including external electric and magnetic fields in the framework of spin-PDFT.     

A glimpse at ten years of microwaves and IR spectroscopy on high temperature superconductors

The different technical approaches are considered mostly to compare the accuracy attainable in microwaves to Far IR where new resonant tchniques have been developped. It is possible to use the fringes of the observed spectra to get directly the London penetration length. However in the present state of the Art there is more accuracy to use a two-fluid model with at least one Mid-IR oscillator, and a strategy is possible to fit separately the different parameters. The different results are reviewed and compared to the microwaves data. There is a good accordance, except perhaps for the power law-giving the concentration of quasiparticles in the superconductive phase. For a thin film of YBCO devoid of grain-boundaries (no twins), a θ⁴ law is found instead of a θ^1.5 law for an ordinary film. It should be highly valuable that both microwaves and Far-IR data should be taken on the same sample. presented at the 22nd International Conference on Infrared and Millimeter Waves, July 20–25, 1997, Wintergreen, Virginia 22958 USA.     

Ab initio calculation of the lattice dynamics of the Boron group-V compounds under high pressure

High-pressure effects on the lattice dynamics and dielectric properties of the BN, BP, BAs, BSb and BBi alloys have been carried out using the density-functional perturbation theory within the local density approximation. We study the variation of the optical phonon frequencies (ω_TO and ω_LO), the high-frequency dielectric coefficient (ε_∞) and the dynamic effective charge (Z*) with pressure. The ω_TO and ω_LO have a quadratic form with pressure for all boron compounds. The obtained ε_∞ and Z* for BN, BP remain constant with pressure. However, for BAs, BSb and BBi, ε_∞ and Z* have a quadratic form with pressure. Our results are in good agreement with the available experimental data for BN and BP and they allow prediction for BAs, BSb and BBi.     

InGaN薄膜中电子自旋偏振弛豫的时间分辨吸收光谱研究

采用飞秒时间分辨圆偏振光抽运-探测光谱对In_01Ga_09N薄膜的电子自旋注入和弛豫进行了研究.获得初始自旋偏振度约为02,此结果支持在圆偏振光激发下,重、轻空穴带的跃迁强度比为3∶1,而不支持1∶1或1∶094的观点同时获得自旋偏振弛豫时间为490±70ps,定性分析了自旋弛豫机理,认为BAP机理是电子自旋弛豫的主要机理. 关键词： 电子自旋 InGaN 自旋极化 自旋弛豫     

Quantitative high temperature absorption spectroscopy of NCO at 305 and 440 nm

A spectral survey of NCO absorption near the P₂ + ^pQ₁₂ head of the [A²Σ + (00⁰0)←X²Π_i(00¹0)] band was obtained at 1450°K, 0.6 atm using a remotely located cw ring dye laser source and a shock tube. Mixtures of hydrogen cyanide, oxygen and nitrous oxide diluted in argon were shock heated to provide a reproducible steady-state concentration of NCO, and narrow-line absorption was measured in repeated experiments with the laser set at different wavelengths. The peak absorption was found at 440.479 nm (vac). The experimental spectrum was compared with a theoretical model to yield an average Voigt parameter a ? 0.1. Additional experiments, in mixtures of cyanogen, oxygen and nitrous oxide diluted in argon, provided a known plateau level of NCO, which was used to infer an absolute absorption coefficient β(1450°K, 0.60 atm) = 110(-50, +130) cm^-1 atm^-1 at 440.479 nm. This value of β corresponds to an oscillator strength of 0.0026 for the (00⁰0←00¹0) band.Similar experiments were conducted to monitor the absorption around the R₁ head of the [B²Π_i(10¹0)←X²Π_i(00¹0)] band of NCO, using a frequency doubled cw ring dye laser. The observed spectrum displayed strong broadening, indicating predissociation of the upper state. At the peak absorption wavelength (304.681 nm, vac), we inferred β(1470°K, 0.63 atm) = 40(-19, +48) cm^-1 atm^-1 and a ? 9. This value of β corresponds to an oscillator strength of 0.0031 for the (10¹0←00¹0) band.     

磺胺甲恶唑的振动光谱和密度泛函理论研究

磺胺甲恶唑(Sulfamethoxazole, SMX)是一种磺胺类广谱抗生素.基于密度泛函理论(DFT)的B3LYP方法,在B3LYP/6-311++G (d, p)基组水平上对SMX初始构型进行结构优化,得出该分子最稳定结构构型.同时,给出了SMX的键长、键角和二面角等空间结构参数值,确定该分子空间构型为非平面结构.相同基组水平下进行频率计算,通过频率校正因子0.9630修正并绘制了SMX分子的红外光谱和拉曼光谱图.发现光谱谱段主要分布在3600-2800 cm^-1和1700-300 cm^-1频率段,并且理论计算值和实验测定值有较好吻合,表明DFT计算方法是可取和可信的.借助GaussView 6.0软件和计算得到的势能分布结果对频率范围内每一个振动模式进行了指认归属.此外,采用Multiwfn和VMD程序计算并分析SMX分子的静电势和前线分子轨道,预测了该分子的反应活性位点.该结果为SMX分子的振动光谱检测提供基础数据,为其结构鉴定提供技术参考.     

First-principles study of the high pressure phase transition and lattice dynamics of cerium

We present a first-principles study of the phase transition and lattice dynamics of Ce within the framework of the density functional theory using the GGA+U method. Our calculated results denote that under pressure the transition path is α-Ce (fcc)→α″-Ce (monoclinic, with two atoms per unit cell)→bct-Ce (body centered tetragonal), and the transition pressures are located at 5.36 and 14.37 GPa, respectively. The equation of state in a wide range of pressure is consistent with the experimental data. During the γ-α phase transition, the magnetic moment disappears gradually, which is mainly due to the strong interaction between the 4f and 5d electrons. By calculating the free energies from phonon dispersions including electronic contribution, the obtained γ-α transition temperature at zero pressure is 148 K. From the Blackman diagram of dimensionless elastic constant ratios, we can find that both γ- and α-Ce show negative Cauchy pressure—C₄₄>C₁₂.     

高温超导体电子结构和超导机理的角分辨光电子能谱研究

超导是一种奇异的宏观量子现象.100多年来,已发现的超导体主要分为两类:以金属或者合金为代表的常规超导体以及以铜氧化物和铁基高温超导体为代表的非常规超导体.常规超导体的超导机理能被BCS超导理论完美解释,但高温超导体的超导机理至今仍未达成共识,已经成为凝聚态物理领域中长期争论且充满挑战的重大科学问题.从实验上揭示非常规超导材料的微观电子结构,是理解其奇异正常态和超导电性机理、建立新理论的前提和基础.角分辨光电子能谱技术,由于可以实现对材料中电子的能量、动量和自旋的直接测量,在高温超导研究中发挥了重要的作用.本文综述了我们利用角分辨光电子能谱技术在铜氧化物和铁基高温超导体电子结构和超导机理研究中取得的一些进展,主要包括母体的电子结构、正常态的非费米液体行为、超导态的能带和超导能隙结构以及多体相互作用等.这些结果为理解铜氧化物和铁基高温超导体的物性及超导机理提供了重要的信息.