Tight-binding Recursion Research Articles

Conductance of Ni nanocontacts within first-principle approach

Abstract Electronic structure and conductance of atomic-size Ni contacts are calculated using a real-space tight-binding LMTO method and recursion technique within the frame work of density functional theory. The Landauer–Buttiker approach is used to calculate the conductance. The spin-dependent conductance as a function of energy shows ballistic bulk-like behavior. It decreases appreciably on considering the structural relaxation in the nanocontact region which indicates the significant effect of the interatomic distance on the conductance.

Study of surface segregation in Mo 0.75Re 0.25[formula omitted] random alloy

We compute from the electronic structure surface compositions in binary alloys. In the context of effective cluster interactions, we show that the surface segregation is driven by the point interaction. Self-consistent calculations of point and pair interactions are presented within the tight-binding recursion method and the Bragg–Williams approximation. This framework is used to calculate the equilibrium segregation profile of Mo 0.75Re 0.25(0 0 1) alloy above the bulk order–disorder temperature. The results are found to be in good agreement with experimental data.

Magnetism of ultrathin Fe films on Mo substrates with low and high Miller indices

The magnetism of iron (Fe) monolayers (ML) adsorbed on low- and high-index orientations of molybdenum (Mo) surfaces is investigated using the real-space self-consistent tight-binding recursion method in the Hartree–Fock approximation with Hubbard Hamiltonian. The (0 1 1) orientation is found to be the most favored to exhibit magnetism due to Fe neighboring atoms. The least magnetic moment is found in (0 0 1) orientation due to the larger number of Mo neighboring atoms. In the vicinal systems, the net magnetization decreases as the step length increases.

Spin polarization of Cr/V vicinal structures

The magnetism is investigated for chromium (Cr) overlayers adsorbed on vanadium (V) vicinal substrates. The calculations of the magnetic moments are carried out by the real-space self-consistent tight-binding (TB) recursion method in the Hartree–Fock approximation. These structures exhibit the largest magnetic moment at the edge atom and the least at the kink. Appreciable magnetic moments are induced on V sublayers antiferromagnetically coupled with the surface moments where this coupling is frustrated at kink atoms. The net magnetization increases as a function of the step length and persists for few layers below the surface.

Magnetism of (Fe, V, Mo)/Pd(0 0 1) systems

The magnetism is investigated for vanadium (V), iron (Fe) and molybdenum (Mo) transition metals epitaxially grown on semi-infinite palladium Pd(0 0 1) surfaces. V (Mo) overlayers on Pd(0 0 1) substrate exhibit a metastable antiferromagnetic c(2×2) configuration. Accordingly, no induced magnetism appears in the interfacial Pd(I) layer. In contrast, the alloyed overlayer systems (Fe, V, Mo) on Pd(0 0 1) exhibit net surface magnetization that causes induced magnetic moments in the Pd(I) layer. The magnetic moments in the lower Pd(I−1) layer are antiferromagnetically coupled with the surface moments. The calculations of the magnetic moment are carried out by the real-space self-consistent tight-binding recursion method in the Hartree–Fock approximation.

Magnetism in vanadium-molybdenum systems

The magnetism of relaxed and nonrelaxed vanadium-molybdenum(Vn/Mo) systems is investigated for n = 1-3 layers in the (001) and (111)orientations. This study is carried out using the the real-spaceself-consistent tight-binding recursion method in the Hartree-Fockapproximation with Hubbard Hamiltonian. The magnetic moment of a V monolayerepitaxially grown on a semi-infinite Mo substrate is found to be larger in the(001) orientation than that in the (111). The magnetisms in bilayer and threelayer V systems become comparable in the (001) and the (111) orientations inthe nonrelaxed case, but are larger in the (001) for the relaxed systems. Themagnetic moments are found to decrease in the relaxed systems for both cases.

Effects of doping on crystal and grain boundary in human enamel

In this paper, by use of the tight binding recursion method, the effects of doping on the electronic structure and the energy of grain boundary in hydroxyapatite (OHAP) are investigated. Some important energetic information, such as structural energy of atom, interatomic energy and grain boundary energy are calculated. The results show that the crystal structure of OHAP will be stabilized by the substitution of F for OH, and the substitution of CO 3 for PO 4 may result in deficiency of Ca. Doping Na, Mg, Sr, Ba, Ca or Pb into the grain boundary will lower the grain boundary energy. The grain boundary energy will be influenced from doping sensitively, such as Na, Mg, Sr, Ba, Ca or Pb. Especially, the grain boundary energy will be lowered dramatically by doping with Pb.

On the magnetism of iron–vanadium systems

The magnetic structure of expanded semi-infinite surface V( 0 0 1 ) (vanadium, V) and bulk V is studied for different values of the intra-atomic exchange parameter J V. An anti ferromagnetic (AF) inter-layer coupling is found for the semi-infinite V(0 0 1) surface. The ferromagnetic (F) and AF phases are investigated for bulk V at expanded volumes. The F phase occurs for values of J>1 eV, whereas the AF phase occurs for values of J>0.5 eV. The intra-atomic exchange parameter J V decreases as a function of the lattice constant. We present also a study of the magnetic behavior of Fe monolayer deposited on a semi-infinite V(0 0 1) substrate and V monolayer on a semi-infinite Fe substrate. The investigations of such structures show a decrease in the magnetic moment of Fe as compared to its bulk value. The calculations of the magnetic moment are carried out by the real-space self-consistent tight-binding recursion method in the Hartree–Fock approximation.

Correlation between bond distortion and the band-tail electronic density of states in amorphous silicon: a tight-binding recursion study

We study the spatial structure of the electronic states in amorphous silicon using a tight-binding Hamiltonian and the recursion method. A realistic cluster consisting of 4096 atoms in a periodic cubic supercell, obtained using reverse Monte–Carlo methods, is used as a model of amorphous silicon [B.R. Djordjevic, M.F. Thorpe, F. Wooten, Phys. Rev. B 52 (1995) 5685]. The local density of states for each of the 16 384 orbitals is computed. Our aim is to provide some insight into the nature of the band tail states and we particularly address the following issues. We examine the relative contributions of the p-orbitals and the s-orbitals to the density of states in the region of the band tail and gap. We investigate the geometric nature of these states. Finally, we study the correlation between the structural distortions in the amorphous silicon cluster and the spatial distribution of the band tail and gap states. Our results show that the p-orbitals contribute a large fraction of the density of states in the valence band tail. This fraction decreases as the energy increases across the gap into the conduction band. We observe the expected change from extended states deep in the valence and conduction band to localized states in the band gap. An attempt was made to characterize this change in terms of the fraction of the total density of states required for a percolating nearest-neighbor path in the cluster as a function of the energy. No abrupt change was observed in this quantity for energies in the region of the band tails. Although there are 4096 atoms in the cluster, 90% of the density of states in the band tail and gap is localized on only 416 atoms, that is, we find a backbone of atoms in the cluster responsible for a large fraction of the states in this energy range. We also find a strong correlation between atoms contributing a high density of states in the band tail and gap region and atoms with a large bond angle distortion.

Electronic Structure of ∑5[100] Grain Boundary and Doping Effect in Iron

Based on CSL model and MD relaxation, the atomic structure of Σ5 [100] grain boundary in α-Fe are obtained. By using the tight-binding recursion method, the electronic structure and boron doping effect on Σ5 [100] grain boundary in α-Fe are investigated. The calculated results indicate that the boron enhances the atomic interaction between the host atoms on grain boundary, as well as between the boron and host atoms. The calculation of the energy of impurity segregation to grain boundary shows that the boron has the tendency to segregate onto the grain boundary, and can improve the cohesion of grain boundary.

Electron structure of a grain boundary and a crystal of calcium phosphate bioceramic (hydroxyapatite)

Abstract The electron structure of the typical calcium phosphate ceramic, hydroxyapatite (OHAP), is investigated by the use of the tight-binding recursion method. The local density of states, partial density of states, structural energy and change transfer are calculated, as well as the interatomic energy between atoms and bond order integral. Some physical and chemical properties of OHAP are discussed. The calculated results show that both the OH group and the PO4 group can be regarded as individual structure units in OHAP owing to strong interaction between atoms. Meanwhile, the atomic structure of a grain boundary is determined on the basis of the near-coincidence site lattice model with consideration of the bond length in the crystal and the lattice distortion. The electron structure and energy of a Σ = 7 grain boundary are calculated. The results show that a grain boundary with a slight lattice misfit has a lower energy.

Properties of compressed overlayers: Tight-binding study of Pd/Co(0001) and CO chemisorption

By using a semiempirical self-consistent tight-binding recursion scheme method we consider the Pd/Co(0001) system. Ideally pseudomorphic, $11\ifmmode\times\else\texttimes\fi{}1$, and ``noncompressed'' Pd overlayers are considered, the latter two being treated by a mean-field method. The origin of the ``inverted exchange splitting'' of interface states [W. Weber et al., Phys. Rev. B 46, 6199 (1992)] is clarified. To assess the overlayer reactivity the chemisorption of CO is considered. It is shown that the (effective) Pd-Pd distance is a controlling parameter together with the chemical nature of the substrate. The relations to similar systems Pt/Co(0001) and Pt/Ni(111) and to the (111) face of corresponding alloys are also mentioned.

Lattice constant dependence of the electronic states of the simple-cubic Na 2MC 60 (M=Cs, Rb)

Lattice constant dependence of the electronic states of the simple cubic Na 2MC 60 (M= Cs, Rb) are studied taking into account the effects of molecular orientational disorder, using the tight-binding recursion method. We find that there exists a significantly steeper interfullerene spacing a dependence of the density of states (DOS) at the Fermi level ( N( E F )) for Na 2MC 60 than for the merohedrally disordered facecentered-cubic A 3C 60 (A=K,Rb) for some appropriate interaction radius. This result suggests the possible dominance of the BCS model over the T c - a correlation of all alkali-metal-doped fullerenes.

Tight-binding calculations of cohesive properties and phase diagram of Ni-Al-X alloy systems

A tight-binding recursion scheme, coupled to the linear muffin-tin orbital method, is used to study the chemcial bonding and thermodynamical properties of Ni-Al-X bcc alloys, where X denotes solute elements such as Mn, Fe, and Co. The electronic structures are calculated by tridiagonalizing a tight-binding Hamiltonian using the continued fraction technique. The authors estimate the effective pair (cluster) interaction energies VNi-Al VAl-X and VAl-x using the direct configurational averaging method and discuss the effects of the ternary solute atoms on the thermodynamical properties of the Ni-Al-X alloys, using the cluster variation method. The effective pair interaction energies are calculated by averaging over a small number of randomly generated configurations.

Tight-binding recursion calculations of step energetics on the GaAs(110) surface

On the GaAs(110) surface steps perpendicular to the [001] direction may be either arsenic-terminated or gallium-terminated. We compute the energy difference between these steps using a tight-binding recursion method. We find that the arsenic-terminated step is more stable by approximately 0.5 eV. Our results suggest that some recent experimental observations on the shape of islands formed during homoepitaxy on GaAs(110) may be the consequence of an energy-driven rather than a kinetics-driven epitaxial growth.

Application of the Recursion Method to the Electronic Structures of Simple-Cubic Na2CsC60 and Body-Centered-Cubic K6C60

Electronic structures of simple cubic (sc) Na2CsC60 and body-centered-cubic (bcc) K6C60 are studied using the tight-binding recursion method. We find that the orientational disorder modifies the electronic states of sc Na2CsC60 significantly. The ground state of K6C60 is found to be insulating with a gap of 0.20 eV. The valence electrons of the alkali-metal atoms in both Na2CsC60 and K6C60 are almost completely transferred to the lowest unoccupied bands of the C 60 molecular solid. The strength of the interaction between the alkali-metal atom and the C 60 molecule is estimated.

The large- U Hubbard model for a semi-infinite crystal: a moment approach and an energy-dependent recursion method

Electron-correlation effects at metal surfaces can be studied qualitatively within the framework of the Hubbard model for a semi-infinite lattice. We propose a moment approach for an approximate determination of the local electronic self-energy that is applicable for systems with reduced translational symmetry. For the strong-correlation regime a one-pole ansatz for the self-energy can be motivated. All a priori unknown parameters in the ansatz for the self-energy are calculated self-consistently by exploiting the equality between two alternative but exact representations for the first four moments of the spectral density. With the resulting expression for the self-energy at hand, the many-body problem reduces to the problem of finding the local density of states (LDOS) for an energy-dependent effective one-particle Hamiltonian. We determine the LDOS for the semi-infinite system using a straightforward generalization of the standard tight-binding recursion method that is suitable for treating an energy-dependent Hamiltonian. The resulting energy dependence of the recursion coefficients is studied in detail for the (100) surface of a bcc crystal and an interpolation procedure for a numerically feasible evaluation of the theory is suggested.

Energy gap of nanoscale Si rods

The electronic structure of silicon rods has been studied by means of the tight-binding recursion method to investigate the dependence of the energy gap (Eg) on a rod length and the direction of the rod axis. An empirical expression for Eg is derived from numerical results for the rods in 〈100〉, 〈110〉, and 〈111〉 directions. This expression is applicable to the energy gaps of wires and crystallites, which can be regarded as limiting cases of rods.

Magnetism of relaxed V(001) slabs

The magnetism of vanadium (001) layers has been studied by calculating the local magnetic moment of each atomic site in addition to other parameters related to the local electronic structure. These calculations were carried out using the tight binding recursion method and the Hubbard model Hamiltonian of magnetism. The dependence of the magnetism of vanadium layers on the interatomic distance is the central part of the study. The ferromagnetic (F) and antiferromagnetic (AF) solutions of the coupling between magnetic vanadium layers are studied in terms of the interatomic distance. Also the stability of each solution is tested using the calculated values of the total energy. Using a good range of the interatomic distances, the accepted (AF or F) solution is determined for the coupling between vanadium (001) layers.

Magnetic anisotropy of transition-metal thin films: Convergence properties.

On the basis of extended recursion-scheme calculations and general analysis we show that the instabilities and oscillations of the magnetic anisotropy energy (MAE) curve result from numerical inaccuracies. We explain how to obtain quickly convergent results within the tight-binding recursion method. For an Fe(001) monolayer we find a negligible MAE, for a Co(111) monolayer the in-plane anisotropy is preferred whereas for a Co(111) bilayer the result is sensitive to the choice of parameters. Crystal-field effects are also discussed.