Theoretical study on the positron annihilation in Rocksalt structured magnesium oxide

Based on the atomic superposition approximation(ATSUP) and first-principles pseudopotential plane-wave methods,the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen-Pajamme and Boron’ski-Nieminen positron-annihilation-rate interpolation formula respectively.The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results.The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide.     

The properties and structures of the mono- and the di- vacancy in Cu crystal

The structures and energies of formation and migration of the mono- and di-vacancy in Cu crystal have been described and calculated with modified analytical embedded atom method (MAEAM). The lattice relaxation is considered with molecular dynamics (MD) method at T=0 K. The results show the FN di-vacancy is the most stable and likely occurs in practice from the energy minimization. Compared with the mono-vacancy, the formation energy of the FN di-vacancy is higher than that of a mono-vacancy, but lower than that of two isolated mono-vacancy. The preferred migration mechanism of the FN di-vacancy is multi-jump of either vacancy (rotating the di-vacancy). The calculated migration energy of the FN di-vacancy is lower than that of a mono-vacancy, so the FN di-vacancy is easier to migrate. All of the calculated results are in good agreement with the experimental values.     

Structural properties and diffusion processes of the Cu3Au (0 0 1) surface

The surface relaxation and surface energy of both the mixed AuCu and pure Cu terminated Cu₃Au (0 0 1) surfaces are simulated and calculated by using the modified analytical embedded-atom method. We find that the mixed AuCu termination is energetically preferred over the pure Cu termination thereby the mono-vacancy diffusion is also investigated in the topmost few layers of the mixed AuCu terminated Cu₃Au (0 0 1) surface. In the mixed AuCu terminated surface the relaxed Au atoms are raised above Cu atoms for 0.13 Å in the topmost layer. All the surface atoms displace outwards, this effect occurs in the first three layers and changes the first two inter-layer spacing. For mono-vacancy migration in the first layer, the migration energies of Au and Cu mono-vacancy via two-type in-plane displace: the nearest neighbor jump (NNJ) and the second nearest neighbor jump (2NNJ), are calculated and the results show that the NNJ requires a much lower energy than 2NNJ. For the evolution of the energy requirements for successive nearest neighbor jumps (SNNJ) along three different paths: circularity, zigzag and beeline, we find that the circularity path is preferred over the other two paths due to its minimum energy barriers and final energies. In the second layer, the NN jumps in intra- and inter-layer of the Cu mono-vacancy are investigated. The calculated energy barriers and final energies show that the vacancy prefer jump up to a proximate Cu site. This replacement between the Cu vacancy in the second layer and Cu atom in the first layer is remunerative for the Au atoms enrichment in the topmost layer.     

铝中氢-空位复合体的正电子湮没特性

本文从第一性原理出发,应用离散变分法、嵌入集团模型和三维数值弛豫技术计算了铝中一氢-空位复合体和多氢-空位复合体的正电子湮没特性,讨论了氢在铝单空位中可能的占据位置,表明正电子湮没技术是研究固体中缺陷的微观结构的有力工具。 关键词：     

POSITRON ANNIHILATION WITH VACANCIES IN THIN SURFACE LAYER OF As HEAVILY DOPED Si

The lifetime spectroscopy of slow positron accelerated with linear accelerator and pulse punch system was first used to analyze the vacancies in the thin surface layer of silicon heavily doped with arsenic. The results demonstrated that no mono-vacancy was detected to support the arsenic-vacancy complex models for explaining the electrical deactivation mechanism of arsenic-heavily-doped silicon.     

Mn掺杂ZnS(110)表面的电子结构和磁性

采用基于密度泛函理论的第-性原理方法,研究Mn掺杂ZnS(110)表面的电子结构和磁性.计算分析不同掺杂组态的几何参数、形成能、磁矩、电子态密度以及电荷密度.结果表明:单个Mn原子掺杂,替位于表面第二层的Zn原子时体系形成能最低,说明该层是最稳定的掺杂位置.对于两个Mn原子的掺杂,当Mn与Mn之间呈反铁磁耦合时体系最稳定.体系的总磁矩和自由Mn原子的磁矩差别很小,但是Mn原子的局域磁矩却依赖于Mn原子的3d态和近邻S原子的3p态的杂化作用,即受周围S原子环境的变化影响较大.此外,分析电荷密度图得出Mn原子替换Zn原子后与S原子形成了更强的共价键.     

First principles calculations of relationship between the Cu surface states and relaxations

In this paper the relationship between the surface relaxations and the electron density distributions of surface states of Cu(100), Cu(110), and Cu(111) surfaces is obtained by first-principles calculations. The calculations indicate that relaxations mainly occur in the layers at which the surface states electrons are localized, and the magnitudes of the multilayer relaxations correspond to the difference of electron density of surface states between adjacent layers. The larger the interlayer relaxation is, the larger the difference of electron density of surface states between two layers is.     

Lattice structures and electronic properties of WZ-CuInS_2/WZ-CdS interface from first-principles calculations

Using the first-principles plane-wave calculations within density functional theory, the perfect bi-layer and monolayer terminated WZ-CIS(100)/WZ-Cd S(100) interfaces are investigated. After relaxation the atomic positions and the bond lengths change slightly on the two interfaces. The WZ-CIS/WZ-Cd S interfaces can exist stably, when the interface bonding energies are-0.481 J/m~2(bi-layer terminated interface) and-0.677 J/m~2(monolayer terminated interface). Via analysis of the density of states, difference charge density and Bader charges, no interface state is found near the Fermi level.The stronger adhesion of the monolayer terminated interface is attributed to more electron transformations and orbital hybridizations, promoting stable interfacial bonds between atoms than those on a bi-layer terminated interface.     

Dielectric spectroscopy studies of ZnO single crystal

Dielectric relaxation and charge transport induced by electron hopping in ZnO single crystal are measured by using a novocontrol broadband dielectric spectrometer. Typical Debye-like dielectric relaxation originating from electronic hopping between electronic traps and conductive band in surface Schottky barrier region is observed for ZnO single crystal-Au electrode system. However, after insulation of ZnO single crystal by heat treatment in rich oxygen atmosphere, dielectric relaxation and alternating current conductance are observed simultaneously in the dielectric spectra, implying that dielectric relaxation and charge transport can be induced simultaneously by electronic hopping at high temperature in an ordered system. The intrinsic correlation between local dielectric relaxation and long range charge transport offers us a new method to explore complicated dielectrics.     

CeO2的电子结构，光学和晶格动力学性质：第一性原理研究

基于考虑了Ce-4f电子间的库仑作用U和交换作用J的LDA+U方案,应用第一性原理计算系统研究了CeO₂的电子结构,光学和晶格动力学性质.电荷密度和电子局域函数的分布特征表明,CeO₂是属于共价键的绝缘体.介电常数、玻恩有效电荷张量和声子色散曲线的计算值和相应的实验结果符合得比较好. 关键词： 电子结构 光学性质 晶格动力学 第一性原理计算     

Triple differential cross sections for the ionization of water by electron and positron impact

Triple differential cross section calculations for the ionization of several valence orbitals of the water molecule by electron and positron impact are reported. We found a very good agreement with the experimental data in case of the almost spherical 2A₁ orbital. By changing the projectile’s charge, significant differences were observed between the electron and positron TDCSs.     

Rule for structures of open metal surfaces

We present a clear and simple rule for determining the relaxation sequences on open (stepped, vicinal, or high-Miller-index) metal surfaces. At the bulk-truncated configuration of a surface, a surface slab is defined where the coordination of atoms is reduced from the bulk. The rule predicts that the interlayer spacings within this slab contract, while the interlayer spacing between this slab and the substrate expands. By first-principles calculations, we show that this rule is obeyed on all open Cu surfaces with interlayer spacings down to about 0.5 A. We also illustrate a direct relation of the relaxation sequences to the charge redistribution on these surfaces, which is demonstrated to be driving the multilayer relaxations. The applicability of the rule can be extended to other fcc and bcc metals, including unreconstructed and missing-row surfaces.     

3d过渡金属Co掺杂核壳结构硅纳米线的第一性原理研究

基于密度泛函理论的第一性原理计算,研究了横截面为五边形和六边形的核壳结构硅纳米线的过渡金属Co原子替代掺杂.通过比较形成能发现,核心位置掺杂、壳层单链掺杂以及外壳层全替代掺杂的硅纳米线都具有稳定性,其中核心位置掺杂结构的稳定性最高.掺杂体系均呈现金属性,随着掺杂浓度的增加,电导通道数增加.Co原子掺杂的硅纳米线呈现铁磁性,具有磁矩.Bader电荷分析表明,电荷从Si原子转移至过渡金属Co原子.与自由态时过渡金属Co原子的磁矩相比,体系中Co原子的磁矩有所降低,这主要是由Co原子4s轨道向3d/4p轨道的电荷转移以及4s,3d,4p的上自旋电子转移至下自旋导致的.     

Properties of positron annihilation in metals

Positron annihilation in bulk metals has been studied by examining the angular distribution of the annihilation photons in polycrystalline samples of magnesium, aluminum, copper, zinc, indium, tin, lead, and bismuth. It has been shown that conduction electrons as well as core electrons take part in this process. The conduction electron densities and Fermi energies have been determined. It is found that the electron density in the vicinity of a positron is significantly higher than the density of the free electron gas. We believe that this is due to the formation of Wheeler complexes and we estimate its charge. We have analyzed various means of measuring the conduction electron density and conclude that the positron method gives the most reliable information. Fiz. Tverd. Tela (St. Petersburg) 41, 929–934 (June 1999)     

First-Principles Study of Li Doping in a Double-Wall Carbon Nanotube

By performing first-principles calculations, we study Li doping in a double-wall carbon nanotube where a （5,0） tube is confined inside a （14,0） tube. There are three possible sites for Li doping and two of them are energetically favorable. The change of energy band structure is closely related to the doping sites and the charge transfer is investigated. Bader charge analysis indicates that Li prefers to donate its electron to the inner （5,0） tube. Moreover, the Li capacity of the system can reach LIC4.75 which makes it a promising candidate for Li-ion battery materials.     

E' centers in alpha quartz in the absence of oxygen vacancies: a first-principles molecular-dynamics study

The displacement of an oxygen atom in pure alpha quartz is studied via first-principles molecular dynamics. The simulations show that when an O atom in a Si-O-Si bridge is moved away from its original equilibrium position, a new stable energy minimum can be reached. Depending on the spin state and charge Q of the system, this minimum can give rise to either a threefold oxygen (singlet ground state and Q=+1) or to an unsaturated Si atom carrying a dangling bond (triplet state). In the latter case, the hyperfine parameters associated with the 29Si dangling bond are in rather good agreement with electron paramagnetic resonance/electron nuclear double resonance experiments.     

Fe-V-N稀合金中空位复合体特性的研究(Ⅱ)

本文利用离散变分法,集团模型和三维数值弛豫技术,研究了Fe-V-N稀合金中V-N-空位复合体的几何构型,正电子湮没特性和电子结构。根据正电子湮没寿命实验的结果和总能量极小原理,确定了V和N在V-N-空位复合体中的最佳占据位置。分析了V-N-空位复合体的成键特性和电荷转移情况。 关键词：     

Explanation of atomic displacement around lattice vacancies in diamond based on electron delocalization

The relationship between unpaired electron delocalization and nearest-neighbor atomic relaxations in the vacancies of diamond has been determined in order to understand the microscopic reason behind the neighboring atomic relaxation. The Density Functional Theory (DFT) cluster method is applied to calculate the single-electron wavefunction of the vacancy in different charge states. Depending on the charge and spin state of the vacancies, at outward relaxations, 84-90% of the unpaired electron densities are localized on the first neighboring atoms. The calculated spin localizations on the first neighboring atoms in the ground state of the negatively charged vacancy and in the spin quintet excited state of the neutral vacancy are in good agreement with Electron Paramagnetic Resonance (EPR) measurements. The calculated spin localization of the positively charged vacancy contrasts with the tentative assignment of the NIRIM-3 EPR signal to this center in (p-type) semiconductor diamond. The sign of the lattice relaxation in the diamond vacancy is explained based on the effect of electron delocalization on nearest-neighbor ion-ion screening, and also its effect on the bond length of neighboring atoms.     

Methane adsorption on graphite（0001） films： A first-principles study

Using first-principles methods, we have systematically investigated the electronic density of states, work function, and adsorption energy of the methane molecule adsorbed on graphite(0001) films. The surface energy and the interlayer relaxation of the clean graphite(0001) as a function of the thickness of the film were also studied. The results show that the interlayer relaxation is small due to the weak interaction between the neighboring layers. The one-fold top site is found most favourable on substrate for methane with the adsorption energy of 133 meV. For the adsorption with different adsorption heights above the graphite film with four layers, the methane is found to prefer to appear at about 3.21 A above the graphite. We also noted that the adsorption energy does not dependent much on the thickness of the graphite films. The work function is enhanced slightly by adsorption of methane due to the slight charge transfer from the graphite surface to the methane molecule.     

Relaxation and electronic states of Au(100), (110) and (111) surfaces

Using a first-principles method based on density functional theory, we investigate the surface relaxation and electronic states of Au(100), (110) and (111) surfaces. The calculated results show that the relaxations of the (100) and (110) surfaces of the metal are inward relaxations. However, the Au(111) surface shows an ‘anomalous’ outward relaxation, although several previous theoretical studies have predicted inward relaxations that are contrary to the experimental measurements. Electronic densities of states and the respective charge density distribution along the Z-axis of the relaxed surfaces are analyzed, and the origin of inward and outward relaxation is discussed in detail.