软 X射线共振非弹性光散射及其应用

软 X射线共振非弹性光散射是近年来随着高亮度第三代同步辐射出现而发展起来的光散射光谱技术 ,可以用于对多原子构成的分子、凝聚态物质进行位置选择的价电子态结构的研究。与 X射线光电子能谱 ( XPS)或紫外光电子能谱 ( UPS)相比 ,这一光谱方法由于测量样品激发后产生的散射 X射线 ,因而不仅可以获得表面的原子分子信息 ,而且可以用于研究样品体内或掩埋薄层的原子分子。同时这一方法的共振特性使得其可以进行灵敏的元素选择测量。本文将介绍这一光散射光谱技术及其在原子分子物理、表面物理及凝聚态物理中的应用     

Theoretical calculations of hardness and metallicity for multibond hexagonal 5d transition metal diborides with ReB_2 structure

The hardness, electronic, and elastic properties of 5d transition metal diborides with ReB_2 structure are studied theoretically by using the first principles calculations. The calculated results are in good agreement with the previous experimental and theoretical results. Empirical formulas for estimating the hardness and partial number of effective free electrons for each bond in multibond compounds with metallicity are presented. Based on the formulas, IrB_2 has the largest hardness of 21.8 GPa, followed by OsB_2(21.0 GPa) and ReB_2(19.7 GPa), indicating that they are good candidates as hard materials.     

表面桥氧空位对甲醛在二氧化钛表面吸附的影响

Oxygen vacancy (O_v) has significant influence on physical and chemical properties of TiO₂ systems,especially on surface catalytic processes.In this work,we investigate the effects of O v on the adsorption of formaldehyde (HCHO) on TiO₂(110) surfaces through firstprinciples calculations.With the existence of O_v,we find the spatial distribution of surface excess charge can change the relative stability of various adsorption configurations.In this case,the bidentate adsorption at five-coordinated Ti (Ti_5c) can be less stable than the monodentate adsorption.And HCHO adsorbed in O_v becomes the most stable structure.These results are in good agreement with experimental observations,which reconcile the long-standing deviation between the theoretical prediction and experimental results.This work brings insights into how the excess charge affects the molecule adsorption on metal oxide surface.     

Local density approximation in site-occupation embedding theory

ABSTRACT

Site-occupation embedding theory (SOET) is a density functional theory (DFT)-based method which aims at modelling strongly correlated electrons. It is in principle exact and applicable to model and quantum chemical Hamiltonians. The theory is presented here for the Hubbard Hamiltonian. In contrast to conventional DFT approaches, the site (or orbital) occupations are deduced in SOET from a partially interacting system consisting of one (or more) impurity site(s) and non-interacting bath sites. The correlation energy of the bath is then treated implicitly by means of a site-occupation functional. In this work, we propose a simple impurity-occupation functional approximation based on the two-level (2L) Hubbard model which is referred to as two-level impurity local density approximation (2L-ILDA). Results obtained on a prototypical uniform eight-site Hubbard ring are promising. The extension of the method to larger systems and more sophisticated model Hamiltonians is currently in progress.     

Synthesis and Electrolytic Properties of Phases Based on Calcium Thiogadolinate as a Function of Method of the Precursor Synthesis

Differently treated phases based on CaGd₂S₄ are studied by electrochemical methods. The range of solid solutions, electrolytic temperature interval, and averaged transport numbers for ions and electrons are determined. Compositions with best electrolytic properties are established. The origin of conduction in a CaGd₂S₄-based solid electrolyte is investigated. The obtained data show the phases to be sulfide-conducting solid electrolytes with a small share of cationic conduction.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 633–639.Original Russian Text Copyright © 2005 by Medvedeva, Kalinina, Metlin, Ushakova.     

Vertical transport and relaxation mechanisms in δ-doping superlattices

We report on electron transport in growth direction and relaxation mechanisms in δ-doped GaAs-superlattices. In order to investigate pure electron transport, n-type δ–n–i–p–i structures sandwiched between two n⁺-cladding layers have been investigated, with doping induced barrier heights ΔV smaller than the band gap energy E_g of the host material. An exponential increase of the current is expected with increasing bias due to tunneling through a decreasing barrier. At elevated temperatures, thermally activation over the barriers becomes possible. A simple WKB-model describes the experimental data reasonably well. Moreover, a current step appears in the I–V characteristics at a critical field which is clearly below the breakthrough value. Opto-electrical measurements confirm the existence of holes in the structure at fields larger than the critical field. A model is presented which explains the photocurrent and electroluminescence measurements consistently. The key mechanism is based on a few ballistically traveling electrons that can gain enough energy for interband avalanche multiplication.     

Effective spin susceptibility, electron mass and Landé factor in two-dimensional (2D) Si inversion layers

We have studied the silicon (Si) band-structure, electron–electron and electron-ionized donor interaction effects on our accurate and approximate results (AcR and ApR) for renormalized effective spin susceptibitity (RESS), electron mass (EEM), Landé factor and spin polarization in the impure 2D Si (electron system), showing that:(i) our ApR, being strongly deviated from our AcR, reproduces approximately all the data obtained recently by Pudalov et al. (Phys. Rev. Lett. 88 (2002) 196404) [in particular, RESS =4.7 at the critical value of Wigner–Seitz radius r_s: r_s=r_c≈8.5 at which occur the “apparent” metal–insulator transition (MIT)] and can also be compared with other ApRs found in the recent literature,(ii) both the RESS and EEM produce physical singularities at the same critical value: r_s=r_c11.05661 (weakly disordered samples) at which occurs the “true” MIT; the existence of such two “apparent and true” critical values in this impure system agrees with a recent discussion by Abrahams et al. (Rev. Mod. Phys. 73 (2001) 251), and(iii) at r_s=r_c=8.5, at which occurs the “apparent” MIT, our AcR for effective spin polarization and the corresponding result, obtained using a disordered Hubbard model and a determinant quantum Monte Carlo method by Denteneer and Scalettar (Phys. Rev. Lett. 90 (2003) 246401), both give the same result: ξ_eff.=ξ_c0.31 at B0.4 T, which is found to be lower than the critical parallel magnetic field for full spin polarization, B_c=1.29 T, supporting thus the existence of such an “apparent” MIT.     

Electron Transitions of Cadmium Oxide

The bands, densities of states, and spectrum of the permittivity ₂ of a CdO crystal have been calculated by the FP–LMTO method. The ₂, ₁, –Im ^–1, and Re ^–1 spectra for the CdO crystal in the range 1–30 eV have been obtained on the basis of the experimental spectrum of reflection. The spectra were decomposed into elementary components and their main parameters were calculated. The obtained spectrum of the transverse components was compared with the theoretical spectrum of ₂.     

新型具有两极性质的电致发光聚合物

合成了一种新型含有三苯胺和恶二唑基元的共聚物。聚合物在溶液中的紫外吸收光谱表明吸收是由电子的π－π^*跃迁引进的，在溶液和薄膜中发射蓝色的荧光，聚合物在溶液中的氧化还原性质证明其既具有电子传输性质又具有空穴传输性质，同时电化学方法得到的聚合物的禁带宽度与紫外吸收光谱的结果是一致的。     

Electron scattering from nonlinear molecules using multicentre wavefunction expansions

Summary The scattering of slow electrons by polyatomic molecules is studied within a full quantum formulation of the problem, and the wavefunctions of the target molecular bound electrons are obtained from a multicentre expansion (MCE) of Gaussian-type orbitals (GTOs) within the single-determinant Hartree-Fock (HF)-SCF scheme. It is shown that the scattering calculations require the solution of radial coupled equations where the interaction is obtained as a further multipolar expansion at the target centre of mass (c.o.m.), while the relevant numerical procedures are discussed for systems like SiH₄, GeH₄ and CF₄. The ensuing interaction coefficients are used to carry out convergence studies of the scattering cross-sections, over a broad range of energies, in the case of silane targets. The results are found to be rapidly converging for such a system, while the different behaviour expected for other targets is also analysed and explained.