铯氧比对砷化镓光电阴极激活结果的影响

实验和理论分析结果表明,激活成功的砷化镓光电阴极的铯氧比存在一个最佳值.砷化镓光电阴极铯氧比的控制可通过调节激活过程中铯源和氧源的加热电流大小来实现.激活实验结果表明,铯氧电流比适中的样品,首次进氧时,光电流上升速度最快,激活后的阴极量子效率最高,稳定性好.当偏离这个比例,过大或过小时,光电流上升速度都会减慢,激活结果也比前者差.随着铯氧电流比的增大,铯氧交替的总次数随之减少.最佳铯氧电流比的调节应以首次进氧时光电流的上升速度最快为准,一旦确定后在整个铯氧交替过程中保持不变.     

Niobium Monoxide Superstructures

JETP Letters - The cubic and orthorhombic models of Nb3O3 and Nb6O6 superstructures of nonstoichiometric niobium monoxide Nb0.75O0.75 with double defects have been proposed for the first time on...     

LEED crystallographic analysis for the structure formed by 2 ML of O at the Zr(0001) surface

A tensor LEED analysis is reported for the Zr(0001)-(1 × 1)-O surface which involves oxygen at a total coverage of 2 monolayers. The structure is indicated to have two layers of O: one forms an overlayer in which the O atoms bond to hollow sites of three-fold coordination on the regular metal surface, while the other layer has the O atoms in tetrahedral hole sites between the first and second metal layers. The stacking sequence, designated as (C)B(A)AB... corresponds to the first three layers of anion-terminated cubic ZrO₂, although some lateral compression is needed for superposition on the regular hcp Zr structure. The absorption of O in the tetrahedral holes results in a significant expansion in the first-to-second Zr---Zr interlayer spacing to about 3.44 Å from the bulk vaue of 2.57 Å. The O---Zr bond lengths are estimated to equal 2.07 Å for the overlayer O atoms, and 2.21 Å for the O atoms in tetrahedral hole sites. Comparisons are made with the structures of the corresponding 0.5 and 1 ML surfaces formed by the O/Zr(0001) system.     

铝掺杂锰酸锂正极材料制备及第一性原理研究

采用固相法制备出高纯度纳米LiAl0.25Mn1.75O4并用此制备成了半电池,对其进行充放电循环测试和阻抗测试,并与原始LiMn2O4的测试结果相比较。另采用基于密度泛函理论的第一原理方法,研究了掺铝锰酸锂LiAl0.25Mn1.75O4的能带结构、态密度和原子布居,实验与计算分析结果表明LiAl0.25Mn1.75O4在室温下0.01C放电时首次放电容量为124.8mAh/h,室温0.2C下50个循环周期后放电比容量保持率可达到83.6%;LiAl0.25Mn1.75O4的能带带隙为0.21eV,分态密度中Al-s轨道与O-s轨道在-20eV左右的明显杂化,均表明LiAl0.25Mn1.75O4材料具有高导电率、高结构稳定性、高比容量保持率的性能,这为推动锂离子电池锰酸锂正极材料的发展提供理论依据。     

Verwey相变处Fe3O4的结构、磁性和电输运特性

作为典型的金属–绝缘体转变,Fe3O4的Verwey相变蕴涵的丰富物理现象与微观机制,因而受到了人们的广泛关注.在Verwey相变处,Fe3O4的晶体结构、电子结构以及磁各向异性等均发生转变,但其磁基态并未发生改变.与其他强关联体系相比,Fe3O4的Verwey相变不需要考虑磁交换耦合作用的变化,有利于揭示强关联体系中金属–绝缘体转变的物理本质.本文从晶体结构、电荷有序、电输运特性、磁性和铁电特性等方面简要地介绍了Fe3O4的Verwey相变的研究历史和现状.     

在硅酸盐矿物,玻璃和熔体中Si和Al的配位与局部结构：K—边 …

本文采用步辐射的ＳｉＫ－边Ｘ－射线吸收近边结构（ＸＡＮＥＳ）谱研究了Ｓｉ在ＳｉＯ２－Ｐ２Ｏ５和Ｎａ２Ｏ－ＳｉＯ２－Ｐ２Ｏ５的低压磷硅酸盐玻璃中结构与配位,以及Ｓｉ的配位几何随玻璃中Ｐ２Ｏ５含量而变化：同步辐射的Ａｌ Ｋ－边ＸＡＮＥＳ谱研究了Ａｌ在铝硅酸盐成分为ＮａＡｌＳｉ２Ｏ６－ＮａＡｌＳｉ３Ｏ８的玻璃和熔体中的配位和局部结构,并提供了直接的实验证据该成分的玻璃体系中由于压力的变化所诱导Ａｌ配位的     

High-energy X-ray diffraction of a hydrous silicate liquid under conditions of high pressure and temperature in a modified hydrothermal diamond anvil cell

In situ high-energy X-ray diffraction measurements were made for the first time on a water-saturated silicate melt at high pressure and temperature. A modified hydrothermal diamond anvil cell (HDAC), designed to minimize the path length of the X-ray beam within a diamond anvil and to increase the solid angle of the diffracted beam, was used to reduce high background contributions and extend X-ray diffraction data collection in Q space. Quantitative differential pair distribution function (PDF) analysis of X-ray diffraction data show that the first measurable (Si–O) peak is 0.095 Å greater in length in the hydrous melt than in the starting glass. Contributions from the H₂O O–O correlations, as well as from the second nearest neighbor O–O correlations within the silicate melt, are evident within the second peak of the differential PDF. The procedure described opens new opportunities to directly investigate volatile-rich melts at high pressure and temperature.     

放射性核素铀在针铁矿中的占位研究

用第一性原理研究放射性同位素铀在针铁矿(α-FeOOH)中的占位情况, 分别考虑铀原子替代针铁矿中的铁的替位缺陷和铀的多种八面体和多种四面体间隙缺陷. 计算发现了三个最稳定的缺陷构型, 它们分别对应于一个铀替位缺陷(S) 及其中的一个铀的八面体(O)和四面体(T)间隙缺陷, 其形成能分别为-13.49, -3.86, -1.60 eV. 也研究了两个相邻的铀原子在针铁矿中的占位情况, 发现双铀原子很容易掺入到相邻的SS或OS位, 它们的形成能分别为-27.392和-16.214 eV, 结合能分别为-0.417和1.131 eV. 表明双原子铀在针铁矿中会以SS形式发生偏聚而较难以OS形式偏聚. 关键词： 铀 针铁矿 占位 第一性原理     

计算氢同位素水分子汽化焓的比较法

本文首次提出计算氢同位素水分子汽化焓的比较法,得到0～374℃时H2O,D2O,DTO和T2O的汽化焓,并提出用D2从含氚重水中分离出DT气的适宜温度.     

Nearly free electrons in the layered oxide superconductor Ag5Pb2O6

We present first measurements of quantum oscillations in the layered oxide superconductor Ag5Pb2O6. From a detailed angular and temperature dependent study of the de Haas-van Alphen effect we determine the electronic structure and demonstrate that the electron masses are very light, m* approximately 1.2me. The Fermi surface we observe is essentially that expected of nearly free electrons--establishing Ag5Pb2O6 as the first known example of a monovalent, nearly free electron superconductor at ambient pressure.     

An isotopic labeling study of the diffusion mechanism during oxidation of Si(100) in water vapor by successive oxidation

The diffusion mechanism during the wet oxidation of Si(100) at 1373 K was investigated by successive oxidations finally containing isotopic water. SiO₂ was first thermally grown on Si in non-labeled oxidizing ambient (dry O₂ or H₂O) followed by isotopic water (H₂¹⁸O) to trace ¹⁸O species in SiO₂. The distributions of ¹⁶O and ¹⁸O in the oxide film were analyzed by means of secondary ion mass spectroscopy (SIMS). SIMS depth profiles show that there was a wide overlap of both isotopes (¹⁸O and ¹⁶O) throughout the SiO₂ layer, no matter whether the first oxidation step was carried out in dry O₂ or H₂O, and the concentration gradient of ¹⁸O decreased with increasing oxidation time at the second oxidation step by H₂¹⁸O. The results suggest that the diffusion mechanism in SiO₂ during water vapor oxidation is exchange diffusion; H₂O related oxidizing species diffuse through the network with significant exchange with the pre-existing oxygen in it.     

化学反应中的量子干涉

波长为157nm的 O2 在受激准分子激光器作用下可离解为 O3P 和 O1D .为了在理论上研究化学反应 的量子干涉效应,利用一阶含时波恩近似及各向异性相互作用势建立了量子干涉效应模型,讨论了完全干涉和完全非干涉两种情况。     

First-principles study of ferromagnetism in Zn- and Cd-doped SnO2

The electronic structures and magnetic properties of Zn- and Cd-doped SnO₂ are investigated using first-principles calculations within the generalized gradient approximation (GGA) and GGA+U scheme. The substitutional Zn and Cd atoms introduce holes in the 2p orbitals of the O atoms and the introduced holes are mostly confined to the minority-spin states. The magnetic moment induced by doping mainly comes from the 2p orbitals of the O atoms, among which the moment of the first neighboring O atoms around the dopant are the biggest. The U correction for the anion-2p states obviously increases the moment of the first neighboring O atoms and transforms the ground states of the doped SnO₂ from half-metallic to insulating. The magnetic coupling between the moments induced by two dopants is ferromagnetic and the origin of ferromagnetic coupling can be attributed to the p–d hybridization interaction involving holes.     

Selective population and neutron decay of an excited state of 23O

We have observed a resonance in neutron-fragment coincidence measurements that is presumably the first excited state of 23O at 2.8(1) MeV excitation energy which decays into the ground state of 22O. This interpretation is consistent with theory. The reaction mechanism supports the assignment of the observed state as the 5/2+ hole state. This assignment and the recently observed 3/2+ particle state advance the understanding of 23O.     

Isotope Effects on the 17O, 1H Coupling Constant and the 17O-{1H} Nuclear Overhauser Effect in Water

The NMR spectra of solutions of 30% 17O-enriched H2O and D2O in nitromethane display the resonances of the three isotopomers H2O, HDO, and D2O. All 17O, 1H and 17O, 2H coupling constants and the primary and secondary isotope effects on J(17O, 1H) have been determined. The primary effect is -1.0 +/- 0.2 Hz and the secondary effect is -0.07 +/- 0.04 Hz. Using integrated intensities in the 17O NMR spectra, the equilibrium constant for the reaction H2O + D2O right harpoon over left harpoon 2HDO is found to be 3.68 +/- 0.2 at 343 K. From the relative integrated intensities of proton-coupled and -decoupled spectra the 17O-{1H} NOE is estimated for the first time, resulting in values of 0.908 and 0.945 for H2O and HDO, respectively. This means that dipole-dipole interactions contribute about 2.5% to the overall 17O relaxation rate in H2O dissolved in nitromethane. Copyright 1999 Academic Press.     

Experimental evidence of core modification in the near drip-line nucleus 23O

The longitudinal momentum (P(axially)) distributions of one- and two-neutron removal fragments ((21,22)O) of 23O from the reaction with a C target at 72 A MeV have been measured for the first time using a new direct time-of-flight method with nearly full acceptance for the breakup fragments. The unexpectedly narrow width of 21O ( 115 +/- 34 MeV/c in FWHM) is consistent with two neutrons occupying the 2s(1/2) orbital in 23O. This indicates modification of core (22O) structure for neutron halo-like sd shell nuclei near the drip line. This also suggests the lowering of the s orbital providing a justification for the N = 16 magic number.     

高压合成的硼酸锶掺Eu2+离子发光特性的研究

 本文首次采用高压方法合成了Sr₃B₂O₆:Eu²⁺、Sr₂B₂O₅:Eu²⁺、SrB₄O₇:Eu²⁺一系列硼酸盐，研究了它们和SrB₂O₄:Eu²⁺的常压与高压合成产物的发光光谱、强度及效率的变化，以及发光与其结构的关系。由于硼酸盐在高压处理后，结构发生变化，因而光谱及发光强度均有所改变。尤其对SrB₂O₄:Eu²⁺，在适当的合成压力条件及一定的激光条件下，其量子发光效率比常压提高80倍以上。     

"Hot-atom" O2 dissociation and oxide nucleation on Al(111)

We propose an atomistic model for the nucleation of aluminum oxide on the Al(111) surface derived from first principles molecular dynamics simulations. The process begins with the dissociative adsorption of O2 molecules on the metal surface, which occurs via a "hot-atom" mechanism driven by the partial filling of the sigma* antibonding molecular orbital of O2. During the subsequent hyperthermal motion, O atoms can be spontaneously incorporated underneath the topmost Al surface layer, initiating the nucleation of the oxide far below the saturation coverage of one (1 x 1) O adlayer.     

Ab initio calculation of correlation effects for the O 1s core electron binding energy in MgO

Wavefunction based ab initio embedded cluster calculations are employed to calculate the O 1s core electron binding energies (CEBEs) of bulk MgO and the MgO(001) surface. A quantum cluster consisting of 61 atoms in five layers and embedded in a large point charge field is used for bulk MgO, the cluster for the MgO(001) surface is chosen accordingly. The O 1s CEBEs are calculated at the Koopmans' theorem (KT) and ΔSCF levels and with inclusion of correlation effects by means of the MC-CEPA method (multi-configuration coupled electron pair approximation), which is an approximate multi-reference coupled cluster approach. The correlation contributions to the O 1s CEBE of the central O atom due to the Mg atoms in the first and the O atoms in the second coordination shell turned out to be additive to a large extent. Therefore, they could be evaluated in an incremental fashion by a series of smaller calculations, where only a few atoms are included in the correlation treatment rather than all atoms of the first coordination shells or of the full quantum cluster. This makes the calculations feasible, even if large basis sets are used. The final results for the O 1s CEBEs are 533.47 and 533.50 eV for bulk MgO and the MgO(001) surface, to which electron correlation contributes 0.77 and 0.70 eV, respectively.     

Electronic and optical properties of Au-doped Cu_2O:A first principles investigation

The Cu2O and Au-doped Cu2O films are prepared on MgO(001) substrates by pulsed laser deposition. The X-ray photoelectron spectroscopy proves that the films are of Au-doped Cu2O. The optical absorption edge decreases by 1.6%after Au doping. The electronic and optical properties of pure and Au-doped cuprite Cu2O films are investigated by the first principles. The calculated results indicate that Cu2O is a direct band-gap semiconductor. The scissors operation of 1.64 eV has been carried out. After correcting, the band gaps for pure and Au doped Cu2O are about 2.17 eV and2.02 eV, respectively, decreasing by 6.9%. All of the optical spectra are closely related to the dielectric function. The optical spectrum red shift corresponding to the decreasing of the band gap, and the additional absorption, are observed in the visible region for Au doped Cu2O film. The experimental results are generally in agreement with the calculated results.These results indicate that Au doping could become one of the more important factors influencing the photovoltaic activity of Cu2O film.