空位的第一性原理及经验势函数的对比研究

利用第一性原理及Stillinger-Weber（SW）,EDIP和Tersoff经验势函数对比研究了硅中单空位(V₁)、双空位(V₂)和六边形空位环(V₆)的结构特性及形成能.讨论了经验势函数描述空位时的优点和缺点.结果发现,第一性原理方法可以精确描述空位的原子结构及能量特性,而短程有效的经验势函数无法描述空位所固有的量子效应,如Jahn-Teller变形等.另外,由于经验势函数自身的缺陷,EDIP和T3无法应用于空位结构特性的计算.虽然 关键词： 空位 第一性原理 经验势函数     

First principles study of half-metallic ferromagnetism in zincblende Cd1−xVxSe

The structural, electronic and magnetic properties of zincblende (ZB) Cd_1?xV_xSe for different values of x were investigated using the full-potential linearized augmented plane wave plus local orbital (FPLAPW+lo) method based on spin-polarized density functional theory (DFT). It was confirmed that for all values of x, the ferromagnetic (FM) state is more stable than antiferromagnetic (AFM) state. The results show that Cd_0.25V_0.75Se and Cd_0.5V_0.5Se compounds exhibit a half-metallic (HM) characteristic while Cd_0.75V_0.25Se has nearly a HM nature. The obtained HM band gaps with EV-GGA have been considerably improved with respect to PBE-GGA scheme. The analysis of density of states (DOSs) curves confirms the hybridization between V d and Se p states in all compounds. The total magnetic moments of Cd_0.25V_0.75Se and Cd_0.5V_0.5Se compounds are 3μ_B (integer values), confirming their HM characteristic. Finally, the robustness of half-metallicity with changing the lattice parameters of Cd_1?xV_xSe alloys was discussed.     

Reconfigurations and diffusion of trivacancy in silicon

Disappearance of the divacancy (V₂) and trivacancy (V₃) complexes upon isochronal and isothermal annealing of electron irradiated Si:O crystals has been studied by means of deep level transient spectroscopy. The annealing studies have shown that the V₂ and V₃ defects are mobile in Si at T>200 °C and in oxygen-rich material are trapped by interstitial oxygen atoms so resulting in the appearance of V₂O and V₃O defects. The activation energies for diffusion of the V₂ and V₃ centers have been determined. Density functional modeling calculations have been carried out to investigate the migration and reorientation mechanisms of V₃ in large silicon supercells. It is proposed that these comprise a sequence of transformations between V₃(D₃) and V₃(C_2v) configurations.     

Quasiteilchen und Eigenwerte des Lippmann-Schwinger-Kerns

In the following we investigate several properties of the eigenvalues of the Lippmann-Schwinger kernel for potentials of Yukawa-type. In particular we derive the analogue of the Dashen-Frautschi formula, which was recently used for the calculation of the neutron-proton mass difference, and extend the result to resonances. We then use the eigenvalues of the Lippmann-Schwinger kernel to obtain a necessary and sufficient condition for the quotient of two Jost functions (corresponding to distinct potentialsV ₁,V ₂) to be the Jost function corresponding to the difference of their potentials (V ₁-V ₂). The result leads to separable potentials. Finally we examine the classical analogues of the vertex and wave-function renormalization constants and discuss the relationship betweenWeinberg's theory of quasiparticles andBohr's compound model of the atomic nucleous.     

正电子湮没谱和光致发光谱研究掺锌GaSb质子辐照缺陷

用正电子湮没谱和光致发光谱研究了质子辐照后掺锌GaSb中的缺陷.通过分析正电子的缺陷寿命τ₂及强度I₂的变化发现,在高能质子的辐照下产生了双空位缺陷V_GaV_Sb,可能同时产生了小的空位团.正电子平均寿命τ_av和S参数随着质子辐照剂量的变化也证明了这一结论.通过分析不同质子辐照剂量下掺锌GaS     

Geometries and electronic properties of NbnV(0, ±1) (n = 1−6) clusters studied by density-functional theory

Using density functional theory (DFT) with valence basis set LANL2TZ to study the relative stabilities and electronic properties of the most stable structures of Nb _n V^(0,?±1) (n = 1?6) clusters. The ground state structures of Nb _n V ^(0,?±1) keep the similar geometric structure as the host Nb _n clusters. The doping of vanadium atom enhances the chemical activities of Nb _n clusters. The Nb₃V and Nb₆V are more stable than other clusters. The average binding energy of charged systems (Nb _n V⁺ and Nb _n V^? clusters) are generally larger than neutral Nb _n V clusters natural population analysis shows that there are charge transfers from niobium to vanadium atoms in the small Nb_1?4V, however, for larger clusters (Nb₅V and Nb₆V), the charge transfers are from vanadium to niobium atoms. The vertical and adiabatic ionization potentials (VIP and AIP) are estimated and the vertical one is more close to experimental results.     

EPR identification of the triplet ground state and photoinduced population inversion for a Si-C divacancy in silicon carbide

It is shown that intrinsic defects responsible for the semi-insulating properties of SiC represent Si-C divacancies in a neutral state (V_Si-V_C)⁰, which have the triplet ground state. The energy level scheme and the mechanism of creating the photoinduced population inversion of the triplet sublevels of the divacancy ground state are determined. It is concluded that there is a singlet excited state through which spin polarization is accomplished, and this fact opens the possibility of detecting magnetic resonance on single divacancies.     

Magnetic behavior and structural feature of quasi-one-dimensional BaCu2V2O8 crystal

Magnetic properties of BaCu₂V₂O₈ single crystals obtained by the spontaneous nucleation method using V₂O₅ as a self-flux are investigated. Singlet ground states with spin gaps of 215 and 223 K are observed along the directions parallel and perpendicular to the c-axis of the crystal, respectively, being in good agreement with powder sample measurements. Reinvestigation of the crystal structure reveals that the whole structural framework of BaCu₂V₂O₈ crystal can be described as an assembly of alternating chains, which are built by edge-sharing pairs of CuO₄ square-plaquettes and VO₄ tetrahedra along the c-axis. The magnetic nature of BaCu₂V₂O₈ is well understood based on this structural interpretation.     

Theoretical investigation of the structures of unsupported 38-atom CuPt clusters

A genetic algorithm has been used to perform a global sampling of the potential energy surface in the search for the lowest-energy structures of unsupported 38-atom Cu–Pt clusters. Structural details of bimetallic Cu–Pt nanoparticles are analyzed as a function of their chemical composition and the parameters of the Gupta potential, which is used to mimic the interatomic interactions. The symmetrical weighting of all parameters used in this work strongly influences the chemical ordering patterns and, consequently, cluster morphologies. The most stable structures are those corresponding to potentials weighted toward Pt characteristics, leading to Cu–Pt mixing for a weighting factor of 0.7. This reproduces density functional theory (DFT) results for Cu–Pt clusters of this size. For several weighting factor values, the Cu₃₀Pt₈ cluster exhibits slightly higher relative stability. The copper-rich Cu₃₂Pt₆ cluster was reoptimized at the DFT level to validate the reliability of the empirical approach, which predicts a Pt@Cu core-shell segregated cluster. A general increase of interatomic distances is observed in the DFT calculations, which is greater in the Pt core. After cluster relaxation, structural changes are identified through the pair distribution function. For the majority of weighting factors and compositions, the truncated octahedron geometry is energetically preferred at the Gupta potential level of theory.     

New insights from variable-temperature and variable-pressure studies into coupling and decoupling processes for ion transport in polymer electrolytes and glasses

A fresh analysis of literature data shows how the influences of temperature and pressure on ion transport and structural relaxation in glass-forming systems may be combined within the framework of ‘master plots’ based on the equation E_A = M · V_A, to reveal new insights into coupling and decoupling effects in a wide range of systems. E_A,σ and V_A,σ are, respectively, instantaneous activation energies and volumes for ionic conductivity and the parameter, M_σ, is a corresponding ‘process modulus’. For structural relaxations occurring at the glass transition, the appropriate modulus is given by M_s = T_g · dP/dT_g. We can now identify typical behaviour patterns for fragile liquids on the one hand, and typical inorganic glasses on the other. Thus, the parameters, M_σ and M_s, for fragile systems such as molten Ca(NO₃)₂:KNO₃ (CKN) or a typical polymer electrolyte such as a complex of LiCF₃SO₃ in PPG, are found to remain constant over a wide range of temperatures down to T_g, despite changes in the temperature (and pressure) dependences of the ionic conductivities, as indicated for example by a return to Arrhenius behaviour in the case of CKN, or by so-called Stickel plots and changes in the VTF parameters for the polymer electrolytes. If E* and V* are activation energies and volumes assigned to elementary steps, when again E* = M · V*, we can go further and identify the microscopic processes driving forward structural relaxation. In the case of inorganic glasses, where usually we find the decoupling index R_τ ≈ 10¹², we identify two distinct decoupling paradigms represented by strong and fragile systems respectively, where in both cases the activation volumes for ion transport are very similar to the corresponding ionic volumes. In the former case (typified by the strongly cross-linked silicate and aluminosilicate systems), the negative activation volumes for structural relaxation (negative values of dT_g/dP) are clearly indicative of a ‘water-like’ behaviour attributable to the collapse of the network under pressure. On the other hand, for the more fragile fast-ion conducting silver iodomolybdate glass, the experimental results show that M_s (at T_g) ≈ M_σ (in glass), implying some recoupling of structural relaxation to ion transport. Arguments based on the dynamic structure model lead us to predict that a similar close link should exist between M_s (at T_g) and M_σ in the relatively fragile lithium and sodium borate glasses, thus highlighting the need for more information concerning the effects of pressure on the glass transition temperatures of common inorganic glasses.     

Ab initio calculations on the structural and lattice dynamical properties of TmX (X=As,P) compounds

The structural and lattice dynamical properties of TmX (X=As, P) compounds were investigated using normconserving pseudopotentials within the generalized gradient approximation correction (GGA) of Perdew–Burke–Ernzerhof (PBE) in the framework of density functional theory (DFT). The structural parameters (a₀, B, B′, E_coh) were determined through total energy and interatomic force minimization and the overall agreement was found to be good. The pressure dependence of the ratios of normalized lattice parameters a/a₀, normalized volume V/V₀, bulk modulus, elastic constants, Zener anisotropy factor, Poisson's ratio, Young's modulus, shear modulus, and the brittleness were presented and discussed. The thermodynamical properties such as thermal expansion, heat capacity, Debye temperature, and Grüneisen parameter were calculated employing the quasi-harmonic Debye model at different temperatures (0–1000 K) and pressures (0–30 GPa). The phonon dispersion curves and corresponding density of states (DOS) of TmX (X=As, P) were also obtained, and the salient results were interpreted.     

立方半导体中双空位态的电子结构(Ⅲ)——硅中双空位态的波函数

利用文献[1]中的基本方程和Vogl等人的紧束缚哈密顿量,计算了硅中理想双空位的E_g和E_u态的波函数,讨论了计算结果和ESR与ENDOR谱的比较。 关键词：     

Influence of doping of mercury atom(s) on optoelectronic properties of binary cadmium chalcogenides - A density functional theory based investigation with different exchange-correlation functionals and including spin-orbit coupling

Influence of doping of mercury atom(s) on optoelectronic properties of binary cadmium chalcogenides have been investigated theoretically by designing the mercury doped cadmium chalcogenide ternary alloys in B3 phase at some specific Hg-concentrations and studying their optoelectronic properties using DFT based FP-LAPW methodology. The structural properties are computed using WC-GGA, while spin-orbit coupling included electronic and optical properties are computed using TB-mBJ, EV-GGA, B3LYP and WC-GGA exchange-correlation functionals. In addition, electronic properties of mercury chalcogenides are calculated precisely using the GGA+U functionals. The concentration dependence of lattice parameter and bulk modulus of each of the Hg_xCd_1−xS, Hg_xCd_1−xSe, Hg_xCd_1−xTe alloy systems show almost linearity. For each of the alloy systems, band gap decreases almost linearly with increase in Hg-concentration in the unit cell and contribution from charge exchange to the band gap bowing is larger than that from for each of the volume deformation and structural relaxation. Also, covalent bonding exists between different constituent atoms in each compound. Optical properties of each specimen are computed from their spectra of dielectric function, refractive index, extinction coefficient, normal incidence reflectivity, optical conductivity, optical absorption coefficient and energy loss function. Several calculated results have been compared with available experimental and other theoretical data.     

Investigations of structural,electronic, mechanical and lattice dynamic properties of TiRu3 and TiOs3 compounds

Ab initio calculations of structural, electronic, elastic, and phonon properties of TiRu₃ and TiOs₃ compounds have been studied using the density functional theory (DFT) within the generalized gradient approximation (GGA). The basic structural properties such as lattice constants, bulk modulus and pressure derivative of bulk modulus of these compounds were studied and compared with the previous theoretical data. Electronic band structures and partial densities of states for TiRu₃ and TiOs₃ compounds were computed and analyzed. The electronic band calculations showed that the TiRu₃ and TiOs₃ compounds have metallic nature. Phonon spectra, their total and projected densities of states for these compounds were computed by using a linear-response method in the framework of the density functional perturbation theory. The specific heat capacities at a constant volume C_V and Debye temperature of TiCr₃ and TiOs₃ compounds were also calculated and discussed.     

Stress induced preferred orientation and phase transition for ternary WCxNy thin films

We deposit ternary WC_xN_y thin films on Si (1 0 0) substrates at 500 °C using direct current (DC) reactive magnetron sputtering in a mixture of CH₄/N₂/Ar discharge, and explore the effects of substrate bias (V_b) on the intrinsic stress, preferred orientation and phase transition for the obtained films by virtue of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and selective area electron diffraction (SAED). We find that with increasing the absolute value of V_b up to 200 V the carbon (x) and nitrogen (y) atom concentrations of WC_xN_y films keep almost constant with the values of 0.75 and 0.25, respectively. The XPS and SAED results, combined with the density-functional theory (DFT) calculations on the electronic structure of WC_0.75N_0.25, show our obtained WC_xN_y films are single-phase of carbonitrides. Furthermore, we find that the compressive stress sharply increases with increasing the absolute value of V_b, which leads to a pronounced change in the preferred orientation and phase structure for the film, in which a phase transition from cubic β-WC_xN_y to hexagonal α-WC_xN_y occurs as V_b is in the range of −40 to −120 V. In order to reveal the relationship between the stress and phase transition as well as preferred orientation, the DFT calculations are used to obtain the elastic constants for β-WC_xN_y and α-WC_xN_y. The calculated results show that the preferred orientation is dependent on the competition between strain energy and surface energy, and the phase transition can be attributed to a decrease in the strain energy.     

Effect of vanadium pentoxide on the electrical, dielectric, and optical properties of poly(vinyl alcohol)/vanadium pentoxide nanocomposites

In this study, the effect of vanadium pentoxide (V₂O₅) on the electrical, dielectric, and optical properties of poly(vinyl alcohol) (PVA)/V₂O₅ nanocomposites was examined. The PVA/V₂O₅ nanocomposites were prepared by solution mixing, followed by film casting. The as-prepared films were characterized by X-ray diffraction, thermogravimetric analysis, as well as impedance spectroscopy. The obtained results indicated that, with increasing V₂O₅ content, the PVA composite exhibits considerable vestige of crystallization and the PVA peaks become broader. The electrical conductivity, σ _dc, increases as the temperature and the dopant level of V₂O₅ increase. The frequency dependence of alternating current conductivity was governed by a critical frequency, ω _p. The values of ω _p are found to be thermally activated. The loss tangent (tan δ) shows a peak with the frequency, which indicates a dielectric relaxation in these composites with relaxation time decreases with both temperature and V₂O₅ content increase. Optical absorption edge and direct and indirect energies for all mentioned samples were determined and discussed.     

A microscopic description of nuclear molecules: Application to the 12C + 12C system

The interaction energy between two oblate ¹²C ions is calculated by means of the constrained Hartree-Fock method. The influence of the mutual orientation of the ions is investigated by considering two extreme configurations: an axial symmetric one where the two ions approach each with their symmetry axes aligned with the collision axis and a triaxial one where the axes of the fragments are perpendicular to the collision line. The corresponding potentials V₁ and V₂ display very distinct features. In particular the minima of the potentials occur for quite different interdistances. A method is devised for constructing from V₁ and V₂ the potentials and coupling factors between two ions rotating with definite angular momentum. Using these quantities in a coupled channel calculation, we explain the gross features of the elastic, single 2⁺ inelastic and double inelastic cross sections. The same calculation yields good agreement with the fusion data.     

Electronic theory for impurity segregation at lattice defects in metals

A tight-binding type electronic theory is used to study the impurity segregation at lattice defects in metals. It is shown that the heat of segregation E_segr results from a simple physical origin: It is roughly proportional to the difference in the diagonal matrix elements (V_i-V₀) of impurity potentials, where V_i (V₀) is determined for atomic sites in the vicinity of lattice defects (for a perfect lattice site). Applications to impurity segregation at cleaved surfaces, screw dislocations and tilt grain boundaries are discussed.     

硅中Jahn-Teller畸变的双空位超精细相互作用的理论计算

采用扩展的Koster-Slate模型,分别对硅中双空位的两种重要的Jahn-Teller畸变模,计算了缺陷电荷态V₂⁺,V₂^-中未配对电子的超精细相互作用常数a和b,给出与实验符合得很好的结果.由此还定出与实验相符合的能级和对称态:V₂⁺的未配对电子处于能级约为0.3eV的Ag对称态;而V₂^-的电子处于能级约为0.63eV的Bu态;EG1模可能是V₂⁺和V₂^-的最优选的畸变. 关键词：