Electronic structure of the diamondlike systems BN1−x Mex (Me = Ti, Cr, Mn, Fe, Ni, Cu)

The local coherent potential method is used within the framework of multiple scattering theory to calculate the electron energy structure of the diamondlike systems BN_1−x Me_x (Me is a 3d transition metal) with the ZnS structure. We used the cluster version of the muffin tin approximation to calculate the crystal potential. The framework of a single approximation is used to compare the electronic energy structure of ternary and binary boron-nitride systems. The width of the hybridized band of the ternary systems BN-Me (Me = Cr, Mn, Fe, Cu) is greater than in the binary system by more than 8 eV and is due to the resultant B-Me interaction. Fiz. Tverd. Tela (St. Petersburg) 39, 1338–1341 (August 1997)     

Theory of librons for hydrogen crystals with low concentrations ofJ=0 impurities using the averageT-matrix approximation

The averaget-matrix approximation (ATA) has been used to describe librons in mixtures of ortho- and para-hydrogen atT=0 and low p-H₂ concentrations. The librons are treated within a harmonic approximation with coupling parameters fitted to the anharmonick=0 frequencies of Coll and Harris. It is shown that the spurious librational degrees of freedom at the p-H₂ sites can be completely removed with the help of a hard core pseudopotential similar to that of Elliott and Pepper. The density of states and the density of states of thek=0 lines are calculated as a function of the p-H₂ concentrationc _{p-H 2} forc _{p-H 2}<0.20. The averagek=0 libron frequencies are compared with the experimental data of Hardy, Silvera and McTague. Finally, first numerical results for the density of states atc _{p-H 2}=0.20 in the coherent potential approximation and the ATA which uses the virtual crystal approximation as a reference system are reported.Extract from Thesis, D26     

W形六角铁氧体BaFe18O27电子结构与导电性的第一性原理研究

采用基于第一性原理的GGA+U方法研究了BaFe₁₈O₂₇的晶体结构和基态电子结构. 以实验数据为初始结构的离子弛豫显示,由于稳定结构中离子半径的差异和2d位Fe的存在,位于BaO层中6h位的O离子脱离了实验结构中原胞的"表面"位置,产生畸变. 计算得到晶体磁矩为28 μ_B/f.u.,与实验相符. 电子态密度及能带计算表明该材料具有微弱的半金属特性,而且与c轴平行方向和垂直方向的能带色散关系有着很大不同,6g位Fe在该材料的输运特性中起着关键作用,它们形成一种"导电层",导致垂直电导率和平行电导率出现非常大的差异. 关键词： 第一性原理 W形六角铁氧体 电子结构 导电性     

Semiempirical studies of the electronic structure of aluminophosphide and Haeckelite boronitride nanotubes

The paper presents the results of semiempirical quantum-chemical calculations of the electron energy characteristics of Haeckelite boronitride nanotubes (n, n)-BN₄₆₈ (n = 3, ..., 14), (n, n)-BN₄₈ (n = 3, ..., 11), and (n, n)-BN₅₇ (n = 4, 5, 6). The nanotubes were found to be dielectrics with forbidden band widths from 5.5 to 8 eV. The forbidden band energy of the nanotubes increased as their diameter grew. The influence of substitution defects on the physical properties of Haeckelite boronitride nanotubes was studied for the example of the carbon atom.     

Structural,elastic, thermodynamic and electronic properties of LuX (X = N,Bi and Sb) compounds: first principles calculations

ABSTRACT

The structural, electronic, elastic and thermodynamic properties of LuX (X = N, Bi and Sb) based on rare earth into phases, Rocksalt (B1) and CsCl (B2) have been investigated using full-potential linearized muffin-tin orbital method (FP-LMTO) within density functional theory. Local density approximation (LDA) for exchange-correlation potential and local spin density approximation (LSDA) are employed. The structural parameters as lattice parameters a₀, bulk modulus B, its pressure derivate B’ and cut-off energy (E_c) within LDA and LSDA are presented. The elastic constants were derived from the stress–strain relation at 0 K. The thermodynamic properties for LuX using the quasi-harmonic Debye model are studied. The temperature and pressure variation of volume, bulk modulus, thermal expansion coefficient, heat capacities, Debye temperature and Gibbs free energy at different pressures (0–50 GPa) and temperatures (0–1600 K) are predicted. The calculated results are in accordance with other data.     

Calculation of the electron-ion interaction-potential components linear and quadratic in the ion displacements for YBa2Cu3O7

A simple method is proposed to calculate the component of electron interaction potential with the ionic subsystem of a crystal quadratic in ionic displacement. The electron-ion interaction potential is calculated for the high-frequency A _g modes in YBa₂Cu₃O₇. The important role of the Madelung potential, which results in violation of the selection rules valid in the rigid muffin-tin approximation, is pointed out. The quadratic component of the potential can greatly affect the calculated matrix elements which correspond to electronic transitions involving absorption or emission of zero wave-vector phonons. Fiz. Tverd. Tela (St. Petersburg) 39, 1323–1327 (August 1997)     

Electronic structure and physical properties of bcc selenium under high pressure

Abstract

Here we report a theoretical calculation of the band structure and superconductivity of Se in the bcc phase. The energy band structure and the effect of pressure on the band structure is obtained by means of the Linear Muffin-Tin Orbital method within the atomic sphere approximation. The superconducting transition temperature (T_c) is calculated using McMillan's formula and we predict the value of T_c at 115.3 Gpa as 2.3 K. Further increase in presssure decreases the T_c values. The normal state electrical resistivity at 115.3 Gpa is 1.43 fl cm, with further increase in pressure the resistivity decreases, which is a typical behaviour of number of elemental metals under pressure.     

Electronic structure of novel multiple-band superconductor SrPt<Subscript>2</Subscript>As<Subscript>2</Subscript>

The electronic structure of the recently discovered superconductor SrPt₂As₂ with T _c = 5.2 K has been calculated in the local-density approximation. Despite its chemical composition and crystal structure are somehow similar to FeAs-based high-temperature superconductors, the electronic structure of SrPt₂As₂ is very much different. The crystal structure is orthorhombic (or tetragonal if idealized) and has layered nature with alternating PtAs₄ and AsPt₄ tetrahedra slabs sandwiched with Sr ions. The Fermi level is crossed by Pt-5d states with rather strong admixture of As-4p states. Fermi surface of SrPt₂As₂ is essentially three-dimensional, with complicated sheets corresponding to multiple bands. We compare SrPt₂As₂ with 1111 and 122 representatives of FeAs-class of superconductors, as well as with isovalent (Ba,Sr)Ni₂As₂ superconductors. Brief discussion of superconductivity in SrPt₂As₂ is also presented.     

Calculating the dipolon binding energy in alkaline earth metal oxides and the ground state energy of F c + color centers (dipolon+electron) in MgO

The binding energy of the simplest neutral crystal lattice defect, a dipolon consisting of neighboring unlike vacancies, is calculated for the alkaline earth metal oxides having an NaCl structure (MgO, CaO, SrO, BaO) using a method developed by the author. The calculated values are found to correlate with a number of physical properties of these crystals. The ground state energy of an MgO crystal with an F _c ⁺ color center (dipolon1electron) is calculated using two independent quantum-mechanical methods. The results of these calculations are essentially identical. Fiz. Tverd. Tela (St. Petersburg) 41, 1976–1978 (November 1999)     

Specific features of the electronic band structure and x-ray spectra of boron nitride in sphalerite and wurtzite modifications

The electronic band structure of boron nitride compounds with crystal lattices of the sphalerite (c-BN) and wurtzite (w-BN) types is calculated by the local coherent potential method in the cluster muffin-tin approximation within the framework of the multiple scattering theory. The local partial densities of 2p states for boron and nitrogen in c-BN and w-BN modifications are compared with the experimental boron and nitrogen K x-ray emission spectra and band-structure calculations. A comparison of the total densities of states in c-BN and w-BN with the x-ray photoelectron spectra and the band calculations has revealed both similarities and differences in the electronic structures of these modifications. The fine structure in the vicinity of the valence band top of boron nitride in different crystal modifications is theoretically calculated for the first time. The specific features of the electronic structure and the x-ray spectra of boron nitride in different modifications are discussed.     

Effect of pressure on transition temperature of Rb3C60 fullerides

Electronic structure parameters play a significant role in fullerides leading to a superconducting state. Relevant electronic parameter as renormalized Coulomb repulsive parameter μ* and the attractive electron-phonon coupling strength λ are obtained within the dielectric function formalism for random phase approximation. As a first step, the superconducting transition temperature is deduced within the framework of McMillan approximation and strong coupling results using the widely spread phonon spectrum. In view of the importance of Coulomb screening for doped fullerides, the influence of pressure and volume on T _c are estimated to be within the range of experimental values. The isotope and dopant effects are also discussed. It is noticed that the high-T _c , the huge pressure effect, negative pressure derivative of T _c and positive volume derivative of T _c in alkali intercalated fullerides are dictated by the properties of Coulomb and on-ball-C₆₀ high energy intramolecular modes.     

Electronic structures of doped BaFe<Subscript>2</Subscript>As<Subscript>2</Subscript> materials: virtual crystal approximation versus super-cell approach

Using virtual crystal approximation and super-cell methods for doping, a detailedcomparative study of electronic structures of various doped BaFe₂As₂ materials by first principlessimulations is presented. Electronic structures remain unaltered for both the methods incase of passive site doping but in case of active site doping, the electronic structurefor virtual crystal approximation method differ from that of the super-cell methodspecially in the higher doping concentrations. For example, both of these methods giverise to a similar density of states and band structures in case of hole doping (replacingK in place of Ba) and isovalent P doping on As site. But in case of electron doped (Co inplace of Fe) systems with higher doping concentration, electronic structures calculatedusing virtual crystal approximation approach deviates from that of the super-cell method.On the other hand, in case of low isovalent Ru doping at the Fe site implemented byvirtual crystal approximation, one acquires an extra shift in the chemical potential incomparison to that for the super-cell method. This shift may be utilized to predict thecorrect electronic structure as well as the calculated Fermi surfaces within virtualcrystal approximation. But for higher Ru (that has different electronic configuration thanFe) doping concentration, simple shifting of chemical potential fails, the calculatedelectronic structure via virtual crystal approximation approach is very different fromthat by the super-cell formalism.     

The zero-point energy in the molecular hydrogen crystal

We propose a modified Einstein approximation to describe zero-point energy vibrations in a quantum crystal. Our aim was to develop a computationally cheap tool suitable for lattice structure optimisation. As in the classical Einstein model the representative atom vibrates in an effective potential due to the surrounding atoms of the crystal; the atoms however are not strictly placed at the positions corresponding to the crystal potential energy minima but their positions are described by the quantum mechanical density distributions. The effective potential computed that way is suitable for the application in solid para-hydrogen in contrast to the normal (unmodified) Einstein approximation. We compute the cohesive energy of the para-hydrogen crystal and perform lattice structure optimisation. The hexagonal closed packed is more stable than the fcc closed packed lattice and the lattice constants obtained are in very good agreement with the experimental values.     

Magnetostriction and thermal expansion anomalies near the Curie point of the compound La0.7Sr0.3MnO3 with the perovskite structure

It is found that the bulk part ω of the magnetostriction near the Curie temperature T _c in a La_0.7Sr_0.3MnO₃ single crystal with the perovskite structure is negative and that the temperature dependence of |ω| has a maximum near T _c . The quantity |ω| at the maximum increases rapidly with increasing magnetic field. The thermal expansion coefficient near T c increases with temperature much faster than linearly. The paramagnetic Curie temperature determined from the Curie-Weiss law, which the paramagnetic susceptibility of this crystal satifies, was found to be lower than T _c . These anomalies and also the near-T _c metal-insulator transition which is characteristic for this material are explained by the existence of a magnetically two-phase state consisting of a conducting ferromagnetic matrix containing antiferromagnetic insulating microregions occupying not more than 5% of the sample volume. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 449–453 (25 March 1997)     

Electron energy loss studies in solids; The transition metal dichalcogenides

The theory of characteristic electron energy losses is discussed in terms of the electronic band structure of a solid. The relationship between the observed plasmon energies, the average interband energy gap and the background dielectric constant of the solid is developed.

The transmission energy loss spectra of a number of the layer-type transition metal dichalcogenides, MX₂, where M=Zr, Hf, Nb, Ta, Mo and W and X=S and Se, have been measured in the range of 0–50 eV. In the experiments, a beam of 50 keV electrons is incident along the c-axis of the crystals and electrons inelastically scattered through an angle of 1 m radian are selected for energy analysis. This ensures that the momentum transfer and hence the electric vector for the excitations lies in the basal plane of the crystal (E⊥c).

Kramers-Kronig analysis has been applied to the energy loss data to deduce the complex dielectric function of each material. From this function, all other ‘optical’ constants, such as the reflectivity, and the oscillator integral function and joint density of states function have been calculated.

The results give substantial support to the existing band model for the family of materials and, in addition, provide the basis for a quantitative understanding of the band structure of individual compounds.     

First-principles study of electronic structure of transition metal nitride: ReN under normal and high pressure

First-principles calculation was performed using tight-binding LMTO method with local density approximation (LDA) and atomic sphere approximation (ASA) to understand the electronic properties of rhenium nitride. The equilibrium geometries, the electronic band structure, the total and partial DOS are obtained under various pressures and are analyzed in comparison with the available experimental data. The most stable structure of ReN is NiAs like structure. Our results indicate that ReN can be used as a super-hard conductor. We estimated the average electron-phonon coupling constant to be 1.65 and superconducting transition temperature (T_c) is 5.1 K. The T_c value increases with the increase in pressure.     

Pressure effect on structural,electronic optical and thermodynamic properties of cubic AlxIn1-xP: a first-principles study

ABSTRACT

First-principles total energy calculations have been performed using the full potential linearised augmented plane wave (FP-LAPW) method as implemented in the WIEN2k code based on the density functional theory (DFT) to investigate the Al-doping effects on the structural, electronic and optical properties of Al_xIn_1-xP ternary alloys in the zinc-blende (ZB) phase. Different approximations of exchange-correlations energy were used such as the local density approximation (LDA), the generalised gradient approximation within parameterisation of Perdew–Burke–Ernzerhof (PBE-GGA), and the Wu-Cohen (WC-GGA). In addition, we have calculated the band structures with high accuracy using the Tran-Blaha modified Becke–Johnson (TB-mBJ) approach. The pressure dependence of the electronic and optical properties of binary AlP, InP compounds and their related ternary alloys Al_xIn_1-xP were also investigated under hydrostatic pressure for (P?=?0.0, 5.0,10.0, 15.0, 20.0, 25.0?GPa), where it is found that InP compound change from direct to indirect band gap for P?≥?9.16?GPa. Furthermore, we have calculated the thermodynamic properties of InP and AlP binary compounds as well as the Al_xIn_1-xP solid solutions, where the quasi-harmonic Debye model has been employed to predict the pressure and temperature dependent Gibbs free energy, heat capacity, Debye temperature and entropy.     

Ground state properties of titaniumdiboride

Abstract

The electronics structure, the charge distribution and the total energy of hexagonal titaniumdiboride are calculated using non-local pseudopotentials in both the local density approximation (LDA) and the generalized gradient expansion approximation (GGA). In the LDA we obtain a = 3.023 Å, c = 3.166 Å and B_o = 271. GPa. For these quantities the GGA values are slightly lower and both compare well with experiment. We also determined selected elastic constants by fitting the total energies to a quadratic surface in the lattice parameters. Using strains that do not break the hexagonal symmetry we obtain C₁₁ + C₁₂ = 777.GPa, C₁₃ = 83. GPa and C₃₃ = 568. GPa. Again slightly lower values are obtained using the GGA. These values agree well with a recent experiment.