D. G. Pettifor

Half-Metallic Ferromagnetism and Structural Stability of Zincblende Phases of the Transition-Metal Chalcogenides

An accurate density-functional method is used to study systematically half-metallic ferromagnetism and stability of zincblende phases of 3d-transition-metal chalcogenides. The zincblende CrTe, CrSe, and VTe phases are found to be excellent half-metallic ferromagnets with large half-metallic gaps (up to 0.88 eV). They are mechanically stable and approximately 0.31-0.53 eV per formula unit higher in total energy than the corresponding nickel-arsenide ground-state phases, and therefore would be grown epitaxially in the form of films and layers thick enough for spintronic applications.

The tight-binding bond model

The authors present a tight-binding model of cohesion and interatomic forces which exploits the variational principle of density functional theory. The binding energy of a solid is expressed as a sum of four terms, each of which has a clear physical meaning. The first two terms are the covalent bond and promotion energies, which are found by solving the electronic Hamiltonian to obtain the density matrix. The remaining two terms describe changes in the total electrostatic and exchange-correlation energies on forming the solid from isolated atoms. The variational principle allows these two terms to be expressed as functionals of a superposition of frozen atomic charge densities. The authors show that they may then be approximated by a sum of pair potentials. The importance of self-consistency in tight-binding models is discussed with particular attention to the evaluation of the bulk modulus and certain interatomic force constants by frozen-phonon calculations. It is shown that serious errors may arise in non-self-consistent models due to the violation of charge conservation and the neglect of variations in the potential caused by charge flow. The authors advocate local charge neutrality as the simplest approximation to self-consistency which overcomes these problems. This assumption leads to a remarkably simple expression for the force on an atom due to its neighbours, which is both physically transparent and computationally efficient. These concepts are illustrated for three-dimensional solids by calculations of covalent bond energies in BCC, FCC and HCP transition metals using a canonical d-band model. Model one-dimensional calculations are also presented which illustrate the computation of covalent bond energies and interatomic forces at surfaces and interfaces, and the importance of local charge neutrality in the model.

Modelling of spin-polarized electron tunnelling from 3d ferromagnets

Spin-polarized electron tunnelling from ferromagnetic Fe and Co films is modelled within a quantum-mechanical treatment of the electronic transport and a tight-binding approximation accounting for an accurate band structure of the 3d metals. Calculations have been performed assuming that the band gap of the insulator is much larger than the hopping integrals between the metal and the insulator, the electronic structure of the latter being approximated by two non-coupled s-type tight-binding bands separated by a gap. It is found that within the ballistic regime of conductance the spin polarization of the tunnelling current depends strongly on the type of covalent bonding between the ferromagnet and the insulator. In the case of ss bonding the tunnelling current is carried only by the s electrons of the ferromagnet and the spin polarization is positive. This is due to the strong s - d hybridization within the ferromagnet which reverses the sign of the spin polarization of the s-electron partial density of states at the Fermi level with respect to the total surface density of states. The absolute values of the spin polarization of the tunnelling current in this case of ss bonding across the metal - insulator interface are in very good agreement with experimental data on tunnelling between 3d ferromagnets and aluminium through an alumina spacer. Including the sd bonding at the interface, however, results in a large contribution of the d-electron tunnelling current, which reduces the spin polarization and leads to a change in its sign for the case of Co.

Electronic structure and energetics of LaNi5, α-La2Ni10H and β-La2Ni10H14

Abstract The electronic structure and energetics of LaNi 5 , its hydrogen solution (α-La 2 Ni 10 H) and its hydride (β-La 2 Ni 10 H 14 ) were investigated by means of the tight-binding linear muffin-tin orbital method within the atomic sphere approximation (TB-LMTO-ASA). Preferred site occupation of the absorbed hydrogen atoms was investigated in terms of the charge density of the interstitial sites and the total energy, both of which indicate that the 6m site in the P 6/ mmm symmetry is the most preferred. A negative heat of formation of La 2 Ni 10 H 14 was obtained after optimising the kinetic energy of the electrons outside the atomic spheres and the interstitial sphere radii.

Half-metallic ferromagnetism of MnBi in zincblende phase

The full-potential augmented plane wave plus local orbitals method within density-functional theory is used to predict that MnBi in the zincblende phase is a true half-metallic ferromagnet with a magnetic moment of 4.000mu(B) per formula. This phase of MnBi is found to be robust against volume changes from -12% to +30% and remains qualitatively the same under various exchange-correlation approximations

Atomistic simulation of titanium. I. A bond-order potential

The bond-order potential for hcp Ti has been constructed in the framework of a tight-binding description of the binding energy. In this scheme the energy consists of two parts: the bond part that comprises the d-electron contribution to bonding, and a pairwise part. Both parts contain fitting parameters but are treated independently. The bond part reproduces the most important characteristics of the d-band and Cauchy pressures, and the pairwise part complements the bond part so as to reproduce exactly the equilibrium lattice parameters and to a good approximation the elastic moduli. The potential is tested by examining the mechanical stability of the hcp lattice with respect to a variety of large deformations. It is applied to a study of dislocation behaviour in Ti in the accompanying paper, part II.

Oxygen-induced positive spin polarization from Fe into the vacuum barrier

Bonding at the ferromagnet–insulator interface is an important factor which influences spin polarization of the tunneling current in ferromagnetic tunnel junctions. In this article we investigate the spin-polarized electronic structure of the (001) surface of body-centered-cubic iron covered by an oxygen overlayer, as this could reflect the mechanism of bonding and spin polarization in iron/oxide tunnel junctions. The Fe/O atomic structure is optimized using the plane-wave code CASTEP within the spin-polarized generalized-gradient approximation. The electronic structure and local densities of states are calculated using the linear muffin-tin orbital method. The results show hybridization of the iron 3d orbitals with the oxygen 2p orbitals, the strong exchange splitting of the former resulting in exchange-split bonding and antibonding oxygen states. These antibonding states are partially occupied for the majority spins but are almost unoccupied for the minority spins, which leads to a positive spin polariz...

Functionalized Fullerenes in Self-Assembled Monolayers

Anisotropy of intermolecular and molecule-substrate interactions holds the key to controlling the arrangement of fullerenes into 2D self-assembled monolayers (SAMs). The chemical reactivity of fullerenes allows functionalization of the carbon cages with sulfur-containing groups, thiols and thioethers, which facilitates the reliable adsorption of these molecules on gold substrates. A series of structurally related molecules, eight of which are new fullerene compounds, allows systematic investigation of the structural and functional parameters defining the geometry of fullerene SAMs. Scanning tunnelling microscopy (STM) measurements reveal that the chemical nature of the anchoring group appears to be crucial for the long-range order in fullerenes: the assembly of thiol-functionalized fullerenes is governed by strong molecule-surface interactions, which prohibit formation of ordered molecular arrays, while thioether-functionalized fullerenes, which have a weaker interaction with the surface than the thiols, form a variety of ordered 2D molecular arrays owing to noncovalent intermolecular interactions. A linear row of fullerene molecules is a recurring structural feature of the ordered SAMs, but the relative alignment and the spacing between the fullerene rows is strongly dependent on the size and shape of the spacer group linking the fullerene cage and the anchoring group. Careful control of the chemical functionality on the carbon cages enables positioning of fullerenes into at least four different packing arrangements, none of which have been observed before. Our new strategy for the controlled arrangement of fullerenes on surfaces at the molecular level will advance the development of practical applications for these nanomaterials.

Nanoscale solid-state quantum computing

Most experts agree that it is too early to say how quantum computers will eventually be built, and several nanoscale solid–state schemes are being implemented in a range of materials. Nanofabricated quantum dots can be made in designer configurations, with established technology for controlling interactions and for reading out results. Epitaxial quantum dots can be grown in vertical arrays in semiconductors, and ultrafast optical techniques are available for controlling and measuring their excitations. Single–walled carbon nanotubes can be used for molecular self–assembly of endohedral fullerenes, which can embody quantum information in the electron spin. The challenges of individual addressing in such tiny structures could rapidly become intractable with increasing numbers of qubits, but these schemes are amenable to global addressing methods for computation.

The influence of impurities within the barrier on tunneling magnetoresistance

Tunneling magnetoresistance (TMR) of a ferromagnetic tunnel junction is calculated in the presence of nonmagnetic metal impurities and disorder in the barrier layer within a single-band tight-binding model. We found that the introduction of impurities which have onsite atomic energies close to the Fermi level leads to a rapid drop in TMR as a function of the impurity concentration. This results from the increasing number of localized electronic levels close to the Fermi energy which promote the formation of highly conducting channels within the insulator. With increasing atomic energy of the impurities, tunneling via the impurity band results in higher values of TMR due to a higher effective potential barrier associated with this mechanism of tunneling. In this case introduction of impurities within the barrier can provide enhanced values of conductance without a significant reduction in TMR, which is required for MRAM applications of tunnel junctions.