Local Density Formalism Research Articles

α-decay chains and cluster radioactivity of 295−299119 and 298−302120 isotopes using zero- and finite-range NN interactions

The α-decay half-lives of the superheavy nuclei (SHN) with Z = 119 and 120 are investigated by employing the density dependent cluster model. Within the double-folding model, the α-nucleus interaction potential is calculated microscopically using a realistic nucleon–nucleon (NN) interaction. The exchange part of the interaction potential is evaluated within a local density formalism, using the finite-range as well as the ordinary zero-range exchange components of the NN interaction. The influence of nuclear deformation is presented. The calculated values of α-decay half-lives for five isotopes of each of the SHN with Z = 119 and 120 have been compared with those values evaluated using other theoretical models. It was found that our theoretical values are in good agreement with their counterparts. The competition between α-decay and spontaneous fission is investigated and predictions for possible decay modes for the unknown nuclei 295−299119 and 298−302120 are presented. The probable cluster decay half-lives of 295119 and 298120 have been studied within the previously mentioned double-folding model as well as the unified formula (UF) of half-lives for α decay and cluster radioactivity, the scaling law by Horoi (Horoi) and the universal decay law (UDL). The cluster decay half-lives derived from the former three models are too large compared to α-decay half-lives, but the UDL formula predicts the possibility of heavy cluster emission. The mass number variations of the microscopic shell correction energy for different α-decay chains of Z = 119 and 120 isotopes are presented. The possible correlations between the shell effect and each of α-decay and cluster decay half-lives are discussed. We hope that the proposed theoretical predictions for α-decay chains and cluster radioactivity provide a new perspective to experimentalists.

Experimental and theoretical study of magnetic ordering and local atomic polarization in Ru-substitutedLu2Fe17

The distribution of magnetic moments in the iron-rich intermetallic compound ${\mathrm{Lu}}_{2}{\mathrm{Fe}}_{17}$ with Fe partly substituted by Ru has been investigated on ${\mathrm{Lu}}_{2}{\mathrm{Fe}}_{16.5}{\mathrm{Ru}}_{0.5}$ by using several techniques. The crystal structure and temperature evolution of the magnetic structure was studied using high resolution and high flux neutron powder diffractometers. In contrast to the parent ${\mathrm{Lu}}_{2}{\mathrm{Fe}}_{17}$, which transforms on cooling from an antiferromagnetic to a ferromagnetic state, ${\mathrm{Lu}}_{2}{\mathrm{Fe}}_{16.5}{\mathrm{Ru}}_{0.5}$ is an antiferromagnet (N\'eel temperature ${T}_{\mathrm{N}}$ = 208 K) down to the lowest temperatures. The magnetic polarization at the lutetium ${L}_{2,3}$ absorption edges, iron $K$ and ruthenium ${L}_{2}$ edges was studied by x-ray magnetic circular dichroism (XMCD) on a single crystal of ${\mathrm{Lu}}_{2}{\mathrm{Fe}}_{16.5}{\mathrm{Ru}}_{0.5}$ in a magnetic field of 3 T, i.e., above the field-induced metamagnetic transition. The Ru XMCD signal proves the existence of induced magnetic polarization parallel to the dominant Fe sublattice magnetization. A corresponding theoretical analysis in the framework of the local spin density formalism is provided in order to discern the structural disorder effects and magnetism in the system.

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Oxide-based resistive random access memories (RRAM) are based on a conductive filament of oxygen vacancies across the film. It is pointed out that the density of vacancies must be high in order to form a percolating path due to the highly localised nature of the vacancy defect orbital, which is much more localised than that given by local density formalism.

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Oxide-based resistive random access memories (RRAM) are based on a conductive filament of oxygen vacancies across the film. It is pointed out that the density of vacancies must be high in order to form a percolating path due to the highly localised nature of the vacancy defect orbital, which is much more localised than that given by local density formalism.

Defect densities inside the conductive filament of RRAMs

Highly localized oxygen vacancy state as calculated by screened exchange. • Estimate of defect densities in the conductive filament of an HfO 2 RRAM. • Discussion of defect localisation and its effects on percolation threshold. • Defect states in HfO 2 . Oxide-based resistive random access memories (RRAM) are based on a conductive filament of oxygen vacancies across the film. It is pointed out that the density of vacancies must be high in order to form a percolating path due to the highly localised nature of the vacancy defect orbital, which is much more localised than that given by local density formalism.

Combined fourier transform and discrete variational method approach to the self-consistent solution of the electronic band structure problem within the local density formalism

This novel approach combines a discrete variational treatment of all potential terms arising from the superposition of the spherical overlapping atomic charge densities with a rapidly convergent Fourier series representation of all multicenter nonspherical potential terms. The basis set consists of the exact numerical atomic valence orbitals, augmented by charge transfer states, virtual atomic states, and single analytic Slater orbitals for increased variational flexibility. The initial potential is a non-muffin-tin overlapping atomic potential including nongradient local density exchange and correlation terms. Full self-consistency is obtained by a procedure that combines an iterative scheme within the superposition model with a self-consistent optimization of the Fourier components of the nonspherical charge density terms. Ground-state properties such as structure factors and cohesive energy are computed. The results for diamond show very good agreement with experiment. Comparison of the results with the Hartree–Fock calculation is discussed.

Local density formalism approach to cohesive properties of solids: Diamond, BN, and LiF

Predictions of the local density formalism approach to cohesive properties of covalently bonded solids (diamond and cubic BN) and prototype ionic system (LiF) are described using results of recently developed fully self-consistent numerical basis set LCAO-DVM approach. Comparisons with restricted Hartree-Fock results and experiments for cohesive energies and equilibrium lattice constants are presented. Some of the principal bonding mechanisms in these crystals are discussed in terms of the contributions of local exchange and correlation to the binding and the charge redistribution relative to the noninteracting atoms.

Ga1−xAlxNsystem, Madelung, and strain energies: A study on the quality of cluster expansions

First-principles calculations within the local-density formalism were used to study the accuracy of cluster expansion techniques to predict the energy band gaps and enthalpy of the pseudobinary system ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{N}$, a technologically important alloy. The chosen pseudobinary system has the advantage of having small lattice mismatches, which minimizes the enthalpies of formation, and of being a semiconducting system with a direct band gap for any concentration $x$. Many different cluster expansion techniques were tested, some presenting clear advantages. The many cluster expansions were also compared against models of Madelung and strain energy, both long-range interactions. Though cluster expansions fail completely for the long-range Madelung interaction model, they behave remarkably well in the not so long-range strain model. The qualitative results for the strain model are similar to the results for the enthalpy and gap of the alloy system, thus giving us an assurance of our conclusions. Using only short-range interactions, all cluster expansions are clearly inadequate for the long-period orderings.

Ab initio calculations of the electronic and optical properties of 1 T-HfX 2 compounds

The electronic and optical properties of the 1 T-HfX 2 (X=S, Se and Te) compounds have been studied using the full potential linear augmented plane wave (FPLAPW) method within the local density formalism. Our calculations show that 1 T-HfS 2 and 1 T-HfSe 2 are semiconductors with indirect Kohn–Sham gaps of 0.62 and 0.55 eV, respectively, while 1 T-HfTe 2 is metallic having density of states at Fermi energy around 0.9 states/eV unit cell. Our calculations suggest that the trends in the band structures of the 1 T-HfX 2 series can be attributed to both the atomic species and structural parameters. Replacing S by Se and Te causes to separate the Hf–f states from the chalcogen–s states and the Hf–d states. The frequency dependent dielectric function, reflectivity and absorption coefficient show considerable anisotropy at low energies. We present a detailed comparison with the available experimental data and find good agreement.

Relationship between surface dipole, work function and charge transfer: Some exceptions to an established rule

Local-density-functional calculations are used to study the change of work functions induced by a layer of adsorbates. We investigated and compared the work function of a monolayer of Mo, Ag, Au, Fe, Co, Ni, Nb, Li, N, and O on W(100), W(110), W(211), and W(111) surfaces. While many systems obey the commonly accepted rule that electronegative adsorbates increase the work function of the surface, we find some exceptions. For example, overlayers of Fe, Co, and Ni increase the work function of W(100), W(211), and W(111), but decrease the work function of the W(110) surface, although the charge transfer is the same in all orientations. We found that even a layer of oxygen can decrease the work function of W(100), although there are always electrons transferred from the W substrate to the oxygen adsorbates. In order to understand these results, we established the relationship between surface dipole density and work function within the framework of local-density formalism. It turns out that subtle details of the charge transfer can determine the sign and magnitude of surface dipole change, leading to a strong dependence on the orientation of the substrate, with the consequence that the work-function changes are not always governed by the sign and quantity of adsorbate induced charge transfer.

Binding and surface diffusion of SiH3 radicals and the roughness of hydrogenated amorphous silicon

Local density formalism pseudopotential calculations find that the growth radical SiH3 binds to the hydrogen-terminated (111)Si surface. The bound site is not the three-center Si–H–Si bridging site previously assumed. It has a direct Si–Si bond between the SiH3 and the surface Si, and the terminal hydrogen is displaced to a bond center of a lateral surface Si–Si bond. This site is more stable as the unpaired electron can delocalize over more Si–Si bonds. A bound site validates the standard model of the growth of hydrogenated amorphous silicon (a-Si:H) and microcrystalline Si, in which a mobile growth species allows surface diffusion and creates smooth surfaces.

Surface diffusion of SiH 3 radicals and growth mechanism of a-Si:H and microcrystalline Si

Existing growth mechanisms of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si) assume that the growth species SiH 3 can diffuse over the hydrogen-saturated Si surface. However, recent calculations suggest that this could not happen. We have carried out local density formalism pseudopotential calculations of the binding of SiH 3 to hydrogen-terminated (1 1 1)Si surfaces. The bound site is not the three-centre SiHSi bridging site previously assumed. It has a direct SiSi bond between the SiH 3 and the surface Si, and the surface hydrogen is displaced to a bond centre of a surface SiSi bond. A bound site validates conventional models of growth of a-Si:H and μc-Si, in which a mobile growth species creates smooth surfaces.

Exchange correlation energy as a function of the orbital occupancies: Implementation on first principles local orbital methods

AbstractWe introduce a local density formalism, where the total energy of the system is obtained as a function of the orbital occupancies {nμσ}. The exchange energy, EX is written in terms of the electrostatic interaction between a charge nμσ and its hole (1 − nμσ), while the correlation energy is analyzed by means of a Hubbard Hamiltonian using a dynamic field approximation. Once the exchange correlation energy is obtained as an explicit function of the {nμσ}, our orbital occupancy method is completely defined. Our approach is then applied to bulk diamond, Si, and zinc‐blende GaN, as well as the benzene molecule. The results obtained show that this approach offers an interesting alternative to the well‐established density functional theory methods based on the electron density ρ($\vec{r}$ ). © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Band offsets and Schottky barrier heights of high dielectric constant oxides

Oxides with higher dielectric constants are required to act as gate dielectrics for future generations of electronic devices. The electronic structure and band offsets of the oxides on Si have been calculated for many candidate gate oxides using the local density formalism pseudopotential method. The predicted conduction band offsets are similar to those found earlier using the tight-binding method, and where available, with experimental values found by photoemission and internal photoemission. The oxides which are acceptable as gate oxides in terms of their band offsets are ZrO2, HfO2, La2O3, Y2O3, Al2O3, silicates such as ZrSiO4 and HfSiO4 and aluminates such as LaAlO3.

Electronic structure of p-type conducting transparent oxides

The band structure of p-type transparent conducting oxides CuAlO 2, CuGaO 2, CuInO 2 and SrCu 2O 2 have been calculated by the pseudopotential method within the local density formalism. The band gaps have been found using the B3LYP functional. The minimum indirect gap of CuXO 2 decreases along the series X=Al to In, and the valence band maximum is always at the zone boundary. The band maxima of SrCu 2O 2 are both at Γ.

Surface Diffusion of SiH3 Radicals and Growth Mechanism of a-Si:H and μc-Si

AbstractExisting growth mechanisms of hydrogenated amorphous silicon (a-Si:H) and micro-crystalline silicon assume that the growth species SiH3 can diffuse over the hydrogen-saturated Si surface. However, recent calculations suggest that this could not happen. Local density formalism pseudopotential calculations have been carried out of binding of SiH3 to hydrogen terminated (111)Si surfaces. The bound site is not the three-centre Si-H-Si bridging site previously assumed. It has a direct Si-Si bond between the SiH3 and the surface Si, and the surface hydrogen is displaced to a bond centre of a surface Si-Si bond. A bound site confirms conventional models of growth of a-Si:H and microcrystalline Si, in which a mobile growth species creates smooth surfaces.

A density functional theory study of the atomic and electronic structures of 3C-SiC( [formula omitted])-( [formula omitted])- R30° surface reconstruction

The electronic structures of SiC(1 1 1)-( 2 3 ×2 3 )- R30° surface reconstruction are studied using the discrete variational method in the framework of local density formalism. The calculation results suggest that the absence of quarter amount of Si atoms on the topmost layer of the Si-terminated surface is energetically favorable. The optimal geometry determined by the energy minimization includes the vertical and lateral displacements of the surface Si atoms and the subsurface C atoms. The calculated electronic structures show some features in the SiC band gap, which do not exist for a perfect Si-terminated SiC surface. The partial densities of states for different Si and C atoms are derived, from which the variation of the patterns and periodicities observed in the scanning tunneling microscopic experiment under different bias voltages can be explained.

Local-density approach and quasiparticle levels for generalized Hubbard Hamiltonians

This paper presents a general method to describe and analyze electron correlation effects in local-orbital electronic structure calculations using a generalized Hubbard Hamiltonian. In our approach, we first introduce a local density formalism where the total energy of the system is obtained as a function of the orbital occupancies $ni% associated with each local orbital; in particular, exchange and correlation local potentials are presented for a multilevel case. In parallel, using the dynamical mean field approximation, a many-body solution is obtained by means of a local self-energy that appropriately interpolates between the low and high correlation limits. We also show that the local density and the many-body solutions are linked through charge consistency conditions. These two solutions are applied to a multilevel Anderson impurity and to a multiband Hubbard lattice, our results showing the high accuracy of the approach presented in this paper. Further on, we discuss how to apply our previous analysis to the case of crystals and molecules and analyze several examples: bulk Si, and HF and H2O molecules. The good results obtained for these cases show that our approach for the description of correlation effects offers an interesting alternative to the well-established density functional methods based on the calculation of the electron density r(r). The prediction of the electronic and geometric structure of a solid requires the calculation of the quantum-mechanical properties of a system of interacting electrons in the presence of a given configuration of nuclei. Different approaches have been developed over the years to handle this complicated many-body problem. Density functional theory ~DFT !~ Refs. 1‐3! provides an exact mapping of the problem of a strongly interacting electron system ~in the presence of the nuclei! onto that of a single particle moving in an effective potential due to all the other electrons. With this approach, properties such as the total energy of the system could be calculated exactly. However, the effective potential—in particular, the so called exchange-correlation potential—is not known exactly and further approximations are needed. The local density approximation 4 ~LDA! assumes that the exchangecorrelation functional is purely local and can be calculated as a function of the local charge density. This approach provides accurate total energy differences between related structures but total cohesive energies can be in error by more than 20%. 5 The generalized gradient approximation 6 ~GGA!, where both the charge density and its gradient are used to calculate the exchange-correlation functional, improves the LDA results but does not fix completely the problem with the cohesive energies. Although total energy calculations can be performed quite accurately with these approximations, neither of them provides the correct quasiparticle spectrum of the system. 5 A new approach based on an energydependent nonlocal functional is necessary for this purpose, and further approximations are needed to make the problem tractable. The GW approximation 7‐10 has been applied to metals and semiconductors and provides an energy spectrum in good agreement with the experiment. We also mention the LDA1U method 11 which offers a very simple way of correcting for the main deficiency of the local density approximation as far as band gaps are concerned. All these methods have been traditionally implemented using a plane wave basis for the expansion of the electronic wave functions. 12 Recently, different methods based on local orbital basis have been developed. 13‐22 Local orbital basis sets may be used to improve significantly the computational performance of electronic structure calculations. For example, efficient first-principles tight-binding molecular dynamics methods can be devised using appropriate atomiclike basis sets, and order-N algorithms can be easily implemented in a local orbital framework. 23

Electronic structure of the antiferromagnetic ground state of La 2CuO 4 beyond LDA+U method

With regard the Hubbard Hamiltonian, we use a Bogolyubov transformation and within a spin density wave mean field, where the antiferromagnetic correlations of wave vector Q=( π/a, π/a) are included. This gives a new energy spectrum that is calculated starting from the band structure of La 2CuO 4 determined in the local density formalism. We calculate the self-energy in this new ground state using the random phase approximation and a double Lorentzian as a non-interacting density of states. With this self-energy, Dyson's equation is solved by diagonalizing the Green's function in k-space. The density of states is obtained by considering the renormalization factor and the lifetimes of the quasiparticle states in each pole. This leads to the splitting of the input density of states that is calculated in the local density formalism.

Quasiparticle structure and Fermi surface of Sr 2RuO 4

A quasiparticle structure calculation for Sr 2RuO 4 is performed. Starting from the band structure determined in the local density formalism, the Dyson's equation with self-energies arising from the Hubbard Hamiltonian, is solved by diagonalizing the Green's function in k-space. The density of states is obtained by considering the renormalization factor and the lifetimes of the quasiparticle states. We also obtain the Fermi surface without and with the inclusion of the strong correlations effects.