The orbital characters and kz dispersions of bands in iron-pnictide NaFeAs

The electronic structure and orbital characters of iron-pnictide NaFeAs have been studied by polarization dependent angle-resolved photoemission spectroscopy. Some of the bands are mixed with the orbitals of opposite symmetries, which could be interpreted by the hybridization among the bands. According to the photon energy dependent experiment, the k_z dispersions of the bands that cross the Fermi energy are weak in both paramagnetic and spin density wave states. However, a band well below the Fermi level shows a k_z dispersion of 41 meV, which mainly contains the d_z² orbital.     

Electronic structure of TiH2

The electronic structure of TiH₂ has been studied using the augmented-plane-wave method and the LCAO interpolation. The density of states and its orbital components show that the conduction band is Ti d-like and that the valence band is largely derived from the hydrogen orbitals with small Ti 3d hybridization. The electronic charges on the hydrogen atom are ～ 1.5 as compared to 1.6–1.7 of the rare-earth metal hydrides.     

Calculations of electronic structure and density of states in?the?wurtzite structure of Zn1? x Mg x O alloys using sp3 semi-empirical tight-binding model

Calculating the electronic structure and the density of states in the wurtzite structure of Zn_1−x Mg _x O (ZMO) alloys using sp³ semi-empirical tight-binding model, we observed increases of both band gap and electron effective mass that agree with the experimental results as increasing Mg composition up to x=0.3. From the calculated total density of states, the increasing electron effective mass is a result of less orbital overlap of cation sites due to extra density of modes coming from Mg3s and Mg3p orbitals as introducing more Mg composition. Additionally, reducing electronegative characteristic of oxygen was caused by that the O2p was less localized around the oxygen atom.     

Influence of one CO molecule on structural and electronic properties of monatomic Cu chain

By using first-principles calculations, we have systematically investigated the structural and electronic properties of an infinite linear monatomic Cu chain with an adsorbed CO molecule. We find that the bridge geometry is energeticabsally favored not only when the Cu–Cu bond below the molecule is unstretched, but also for a wide range of d_Cu–Cu up to about 4.20 Å, while the substitutional geometry is favored only in the hyperstretched situation d_Cu–Cu>4.80 Å. Charge density differences point out the electron transfer is from the Cu atoms to the adsorbed CO molecule. The binding mechanism of CO to Cu chain can be described by the Blyholder’s model, in terms of σ-donation of electron density from the nonbonding CO-5σ orbital into empty metal orbitals and π-backdonation from the occupied metal d orbitals to empty CO-2π^⁎ orbital. The donation/backdonation process leads to the formation of bonding/antibonding pairs, 5σ_b/5σ_a and 2π_b^⁎/2π_a^⁎, with the 5σ_a lying above E_f and the 2π_b^⁎ below E_f.     

Bonding of nitrogen atoms on Cu(001) surfaces: A cluster approach

A study of the chemisorption of nitrogen atoms on a copper surface has been performed, based on an analysis of the electronic structure of the Cu₅N cluster obtained from self-consistent-field Xα scattered-wave calculations. Our calculations show that the chemisorption of nitrogen on Cu(001) surfaces induces peaks below and above the Cu d-band region in the total density of states curve. The bonding orbitals formed between the N 2p and the Cu valence orbitals are generally found near the bottom of the Cu d-band region, while the antibonding orbitals formed between the N 2p and Cu orbitals are found to lie above the Cu d-band region. These hybridized orbitals involving the N 2p orbital gave a satisfactory interpretation of the adsorbate-induced structure reported in N/Cu(001) ultraviolet photoemission (UPS) studies. In addition, the separate contributions of the N 2p⊥ and 2p∥ states to the total density of states curve of the Cu₅N cluster are given. This information may be useful in interpreting angleresolved UPS data.     

Progressive changes in atomic structure and gap states on Si(0 0 1) by Bi adsorption

From ab initio studies employing the pseudopotential method and the density functional scheme, we report on progressive changes in geometry, electronic states, and atomic orbitals on Si(0 0 1) by adsorption of different amounts of Bi coverage. For the 1/4 ML coverage, uncovered Si dimers retain the characteristic asymmetric (tilted) geometry of the clean Si(0 0 1) surface and the Si dimers underneath the Bi dimer have become symmetric (untilted) and elongated. For this geometry, occupied as well as unoccupied surface states are found to lie in the silicon band gap, both sets originating mainly from the uncovered and tilted silicon dimers. For the 1/2 ML coverage, there are still both occupied and unoccupied surface states in the band gap. The highest occupied state originates from an elaborate mixture of the p_z orbital at the Si and Bi dimer atoms, and the lowest unoccupied state has a ppσ* antibonding character derived from the Bi dimer atoms. For 1 ML coverage, there are no surface states in the fundamental bulk band gap. The highest occupied and the lowest unoccupied states, lying close to band edges, show a linear combination of the p_z orbitals and ppσ* antibonding orbital characters, respectively, derived from the Bi dimer atoms.     

Theoretical studies on the electronic and optoelectronic properties of [A.2AP(w)/A*.2AP(WC)/C.2AP(w)/C*.2AP(WC)/C.A(w)/C*.A(WC)]–Au8 mismatch nucleobase complexes

The electronic and optoelectronic properties of [A.2AP(w)/A^*.2AP(WC)/C.2AP(w)/C^*.2AP(WC)/C.A(w)/ C^*.A(WC)]–Au₈ metal-mismatch nucleobase complexes are investigated by means of density functional theory and time-dependent methods. We selected these mispairs as 2-aminopurine (2AP) produces incorporation errors when binding with cytosine (C) into the wobble (w) C·2AP(w) mispair, and is tautomerised into Watson–Crick (WC)-like base mispair C^*·2AP(WC) and less effectively produces A.2AP(w)/A^*.2AP(WC) mispairs. The vertical ionisation potential, vertical electron affinity, hardness and electrophilicity index of these complexes have also been discussed. The modifications of energy levels and charge density distributions of the frontier orbitals are also analysed. The absorption spectra of these complexes lie in the visible region, which suggests their application in fluorescent-bio imaging. The mechanism of cooperativity effect is studied by molecular orbital potential (MEP), atoms-in-molecules (AIM) and natural bond orbital analyses. Most metalated pairs have smaller HOMO–LUMO band gaps than the isolated mismatch nucleobases which suggest interesting consequences for electron transfer through DNA duplexes.     

Rotational Analysis of the ÃA1-X?A1 Band System of Yttrium Dicarbide, YC2

The 000-000 and 3¹₀ bands of the 775-nm electronic transition of YC₂(òA₁←X?²A₁) have been studied at high resolution, using the laser-induced fluorescence from a supersonic jet expansion. Three types of experiment have been carried out. First, the complete rotational and hyperfine structures of the two bands were recorded. To measure the small asymmetry splittings in the K=2 levels of the X?²A₁ state, portions of the b-type 3¹₀ band were then recorded in the presence of a weak static electric field. Finally, a number of pure rotational transitions between the K=0 levels of the ground state were recorded by pump/probe microwave optical double resonance. A few small rotational perturbations occur in the upper electronic state but, omitting the perturbed lines, the combined data sets could be modeled using an effective Hamiltonian operator appropriate for the rotation, electron spin, and hyperfine structure of a rigid asymmetric top molecule. The molecule is confirmed as being “T-shaped,” where the Y atom is bonded to the side of a C₂ group; the rotational constants determined are for the òA₁, 3¹ level, A=1.76128, B=0.189949, C=0.170056 cm⁻¹, and for the X?²A₁, v=0 level, A=1.742731, B=0.201947, C=0.181285 cm⁻¹. Allowing for electron orbital corrections to the rotational constants, the geometrical structures are found to be òA₁ state, r (Y-C)=2.2795 Å, r (C-C)=1.2630 Å, ∠C-Y-C=32.17°; X?²A₁ state, r (Y-C)=2.1946 Å, r (C-C)=1.2697 Å, ∠C-Y-C=33.63°. A molecular orbital diagram is given for the states of YC₂ and the interpretation of the electron spin and hyperfine parameters is discussed.     

Electronic structures of ternary iron arsenides <Emphasis Type="Italic">A</Emphasis>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> (<Emphasis Type="Italic">A</Emphasis> = Ba,Ca, or Sr)

We have studied the electronic and magnetic structures of the ternary iron arsenides AFe₂As₂ (A = Ba, Ca, or Sr) using the first-principles density functional theory. The ground states of these compounds are in a collinear antiferromagnetic order, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions bridged by As 4p orbitals. The correction from the spin-orbit interaction to the electronic band structure is given. The pressure can reduce dramatically the magnetic moment and diminish the collinear antiferromagnetic order. Based on the calculations, we propose that the low energy dynamics of these materials can be described effectively by a t-J _H -J ₁-J ₂-type model [2008, arXiv: 0806.3526v2].     

Spectral properties of chlorines and electron transfer with their participation in the photosynthetic reaction center of photosystem II

Structural factors that provide localization of excited states and determine the properties of primary donor and acceptor of electron in the reaction center of photosystem II (PSII RC) are studied. The results of calculations using stationary and time-dependent density functional theory indicate an important role of protein environments of chlorophylls P_A, P_B, B_A, and B_B and pheophytins H_A and H_B in the area with a radius of no greater than ≤10 Å in the formation of excitonic states of PSII RC. When the neighboring elements are taken into account, the wavelength of long-wavelength Q _y transition of chlorophyll molecules is varied by about 10 nm. The effect is less developed for pheophytin molecules (Δλ ? 2 nm). The following elements strongly affect energy of the transition: His^A198 and His^D197 amino-acid residues that serve as ligands of magnesium atoms affect P_A and P_B, respectively; Met^A183 affects P_A; Met^A172 and Met^D198 affect B_A; water molecules that are located above the planes of the B_A and B_B macrocycles form H bonds with carbonyl groups; and phytol chains of P_A and P_B affect B_A, B_B, H_A, and H_B. The analysis of excitonic states, mutual positions of molecular orbitals of electron donors and acceptors, and matrix elements of electron transfer reaction shows that (i) charge separation between B_A and H_A and P_B and B_A is possible in the active A branch of cofactors of PSII RC and (ii) electron transfer is blocked at the B_B - H_B fragment in inactive B branch of PSII RC.     

Ab initio calculation of neutral and singly charged Mgn (n?11) clusters

Under generalized gradient approximation (GGA), geometrical structure, size dependence of stability and electronic properties of neutral Mg_n, singly charged cationic Mg_n⁺ and singly charged anionic Mg_n⁻ clusters consisting of up to 11 atoms have been studied systematically by ab initio method within the norm-conserving pseudopotentials. In addition to the electronic shell effects, the “closed” geometrical structure can also enhance the stability of the clusters. The enhanced stability for the cationic cluster resulted from the removal of an antibonding electron is larger than that for the anionic cluster by promoting an extra electron to occupy a bonding orbital. The density of states (DOS) shows the increase in interaction between valence and unoccupied states leads to an increase in s-p hybridization.     

Laser fluorescence spectroscopy of 2B1 (K > 0) vibronic states of NO2

The low-pressure fluorescence spectrum of NO₂ excited by the narrow (0.5 GHz) HeCd laser transition at 441.6 nm is found to be quite complex, indicating a rather high density of transitions to final states at this energy. Seven fluorescing states exhibiting perpendicular selection rules have been assigned to ²B₁ (K > 0) vibronic states of NO₂. From the presence of final states with v₃″ = 2, a considerable amount of ²A₂ electronic state character is attributed to at least some of these states.     

Double-exchange model for molecule-based magnets

We report a detailed study of a double-exchange model proposed for the molecule-based magnets. The model is applied to a two-dimensional periodic complex made of a transition metal and an organic molecule in which the electronic structure is described by effective d orbitals of the transition metal ion at infinite Hund's coupling limit and the lowest unoccupied molecular orbital of the organic molecule, π^∗. Depending on the average electron density of the organic molecules and various superexchange couplings between metal ions' core spins, magnetic states of the complex are investigated. Performing Monte Carlo calculations on a model Hamiltonian for various electron densities of the organic molecule, the average magnetization and critical magnetic ordering temperatures are determined.     

Additional crystalline electric field potentials for heavy rare earth metals

A derivation is given of additional crystalline potential energy terms due to the interaction of conduction electrons with the localised electrons of ions in pure heavy rare earth metals. Explicit expressions for theA ₄ ⁰ andA ₆ ⁰ coefficients are given to supplement those ofA ₂ ⁰ andA ₆ ⁶ given in a recent paper. Additional contributions which arise from the unfilledf shells of the neighbours of any particular ion under consideration, are derived but shown to be of such a magnitude that, for most purposes, they can be neglected. The use of Slater orbitals in calculating enhancement factors forA ₄ ⁰ andA ₆ ⁰ is discussed with particular reference to those contributions which involve resonance amplitudes and mainly involve thef orbitals.     

Projection technique for population analysis of atomic orbitals in crystals

The one-electron density matrix of a crystal in the basis set of localized orbitals is calculated using two variants of the projection technique, namely, the projection of crystal orbitals onto the space of atomic orbitals (technique A) and the projection of atomic functions onto the space of crystal orbitals (technique B). A comparative analysis of the one-electron density matrices thus obtained is carried out, and a simplified version of technique B is proposed to avoid cumbersome calculations with a large number of vacant crystal orbitals. Both techniques are used to calculate the local characteristics of the electronic structure (atomic charges, atomic covalences, bond orders) for a number of crystals (Si, SiC, GaAs, MgO, cubic BN, TiO₂ rutile) in the framework of the density-functional theory within the generalized gradient approximation in the plane wave basis set with the norm-conserving pseudopotentials. It is revealed that both variants of the projection technique lead to close local characteristics of the electronic structure. The local characteristics of the electronic structure of the TiO₂ crystal with a rutile structure are determined by the projection technique and by constructing the Wannier-type atomic functions (WTAF) in the minimal valence basis set in the framework of the variational method with the crystal orbitals calculated in the linear combination of atomic orbitals (LCAO) approximation. It is demonstrated that, although the basis sets used for calculating the crystal orbitals differ significantly (plane waves in the projection technique, LCAO in the WTAF method), the local characteristics of the electronic structure are in good agreement.     

Exploring the electronic structure of Pb2+ ions containing material Pb16(OH)16(NO3)16

A theoretical band structure calculation for lead nitrate hydroxide Pb₁₆(OH)₁₆(NO₃)₁₆ single crystal was performed based on the experimental crystallographic data obtained by Chang et al. Calculations exhibit that the conduction band minimum (CBM) is situated at Γ the center of the Brillouin zone (BZ) while the valence band maximum (VBM) is located between Γ and Y points of the BZ, resulting in an indirect energy band gap of about 3.70 eV in close agreement to the measured one (3.78 eV). The angular momentum resolved projected density of states reveals the existence of the strong hybridization between the orbitals and the VBM is originated from Pb-6s/6p and O-2p orbitals while the CBM from N-2p and Pb-6p orbitals. The calculated valence electronic charge density distribution explore the bond characters and the dominancy of the covalent bonding between Pb–O of PbO_n ployhedra and N–O of [NO₃]⁻ triangle. The calculated bond lengths and angles show good agreement with the experimental data.     

清洁Si(111)表面电子结构的理论研究

本文报道在原子集团模型下用CNDO-SCF方法对清洁Si(111)表面电子结构的系统研究结果:(1)计算了表面上的净电荷分布、电荷转移以及局域在各原子轨道上的电荷;发现T₃⁰,T₃⁺和T₃^-式的表面悬挂键结构较难存在;表面原子趋于形成带有分数电荷的悬挂键,而实际上这些悬挂键彼此结合成弯键;表面原子及其悬挂键上有净电荷积累,且有很强的定域性和取向性。(2)计算了原子集团模型的静 关键词：     

氧化钒晶体的半导体至金属相变的理论研究

本文利用第一性原理方法研究了金红石相和单斜相VO₂晶体的电子结构和热力学性质.在计算中采用局域密度近似结合Hubbard U模型(LDA+U)描述电子的局域强关联效应,同时也利用微扰密度泛函方法计算了两种相结构的声子谱.计算结果表明V原子3d电子轨道中x²-y²轨道能级分裂决定了VO₂晶体在不同相结构下的金属和绝缘体特性.零温状态方程计算揭示了在68 GPa时可以发生从单斜结构 关键词： 2')" href="#">VO₂ 相变 第一性原理     

Pressure effect on structural,electronic and thermodynamic investigations of europium filled skutterudite EuFe4Sb12: LDA and LSDA approximations

Numerical calculations based on the full potential muffin-tin orbitals method (FP-LMTO) within the local density approximation (LDA) and the local spin-density approximation (LSDA) to investigate the structural, electronic and thermodynamic properties of filled skutterudite EuFe₄Sb₁₂ are presented. The electronic band structure and density of states profiles prove that this material is a conductor. The present investigation is also extended to the elastic constants, such as the bulk modulus B, anisotropy factor A, shear modulus G, young's modulus E, Poisson's ratio ν, and the B/G ratio with pressure in the range of 0–40 GPa. The sound velocities and Debye temperatures are also predicted from the above constants. The variations of the primitive cell volume, expansion coefficient α, bulk modulus B, heat capacity (C_p and C_v), Debye temperature θ_D, Helmholtz free energy A, Gibbs free energy G, entropy S, and internal energy U with pressure and temperature in the range 0–3000 K are calculated successfully.