### Browsing by Author "Nieminen, R. M."

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Item Ab initio study of Cu diffusion in α-cristobalite(2012-11) Zelený, M.; Hegedüs, J.; Foster, A. S.; Drabold, D. A.; Elliott, S. R.; Nieminen, R. M.; Department of Applied PhysicsWe have studied the geometries, formation energies, migration barriers and diffusion of a copper interstitial with different charge states with and without an external electric field in the α-cristobalite crystalline form of SiO2 using ab initio computer simulation. The most stable state almost throughout the band gap is charge q = + 1. The height of the migration barrier depends slightly on the charge state and varies between 0.11 and 0.18 eV. However, the charge has a strong influence on the shape of the barrier, as metastable states exist in the middle of the diffusion path for Cu with q = + 1. The heights and shapes of barriers also depend on the density of SiO 2, because volume expansion has a similar effect to increase the positive charge on Cu. Furthermore, diffusion coefficients have been deduced from our calculations according to transition-state theory and these calculations confirm the experimental result that oxidation of Cu is a necessary condition for diffusion. Our molecular dynamics simulations show a similar ion diffusion, and dependence on charge state. These simulations also confirm the fact that diffusion of ions can be directly simulated using ab initio molecular dynamics.Item Ab initio study of properties of Co- and Cu- Doped Ni-Mn-Ga alloys(2015) Zelený, M.; Sozinov, A.; Björkman, T.; Straka, L.; Nieminen, R. M.; Department of Applied Physics; Department of Engineering Design and Production; Electronic Properties of Materials; LUT University; Brno University of TechnologyThe influence of Co and Cu doping on Ni-Mn-Ga alloy is investigated using the first-principles exact muffin-tin orbital method in combination with the coherent-potential approximation. The energy difference between the austenite (A) and the nonmodulated (NM) martensite δE A-NM depends linearly on the Cu concentration and distribution, with a minimum for all Cu at Mn sites and a maximum for all Cu at Ga sites. For alloys simultaneously doped by Co in Ni sublattice and Cu in Mn or Ga sublattice, the effects of the individual dopants on δE A-NM and (c/a)NM and are almost independent. The alloy with composition Ni46Co4Mn24Ga22Cu4 exhibits decreased equilibrium (c/a)NM and increased δE A-NM in comparison with Ni50Mn25Ga25, which is in agreement with the previous experimental results.Item Adsorption and migration of carbon adatoms on carbon nanotubes(2004) Krasheninnikov, A. V.; Nordlund, K.; Lehtinen, P.O.; Foster, A. S.; Ayuela, A.; Nieminen, R. M.; University of Helsinki; Department of Applied PhysicsWe employ density-functional plane-wave ab initio and tight-binding methods to study the adsorption and migration of carbon adatoms on single-walled carbon nanotubes. We show that the adatom adsorption and migration energies strongly depend on the nanotube diameter and chirality, which makes the model of the carbon adatom on a flat graphene sheet inappropriate. Calculated migration energies for the adatoms agree well with the activation energies obtained from experiments on annealing of irradiation damage in single-walled nanotubes and attributed to single carbon interstitials.Item Atoms embedded in an electron gas(1981) Puska, M. J.; Nieminen, R. M.; Manninen, M.; Department of Applied Physics; University of JyväskyläEnergies of atoms, H through Ar, embedded in a homogeneous electron gas are calculated within the density-functional scheme as a function of the electron-gas density. The energy-versus-density curves and the induced densities of states are analyzed and discussed in terms of the interaction properties of an atom with its environment. The low-density limit of the immersion energy is related to the electron-atom scattering length. The results should prove useful in detailed investigations of the recently suggested "quasiatom" or "effective-medium" approaches to chemical binding. The lowest-order estimates of the binding energies of diatomic molecules and chemisorbed atoms are obtained.Item Atoms embedded in an electron gas(1991-05-15) Puska, M. J.; Nieminen, R. M.; Department of Applied PhysicsThe chemical-binding properties of atoms belonging to the first three rows of the Periodic Table are studied within the atom-in-jellium model. The electronic structures are solved self-consistently using the density-functional theory. The extraction of the binding properties is carried out in the framework of the effective-medium theory. The emphasis is put on the systematic investigation of the trends along the 2p, 3p, and 3d series and on the effects due to different types of approximations for electron exchange and correlation. More specifically, in addition to the popular local-density approximation, the self-interaction-correction scheme and the generalized-gradient approximation are employed. The results provide insight into why the local-density approximation for solids (molecules or chemisorption systems) overestimates the cohesive (binding) energies but gives the lattice constants (bond lengths) and bulk moduli (vibration frequencies) rather well. The results obtained are also important because they give the basic parameters for the effective-medium theory, which is a versatile approximative method for calculating total energies of systems with many interacting atoms.Item Atoms embedded in an electron gas(1983-05-15) Puska, M. J.; Nieminen, R. M.; Department of Applied Physics; University of JyväskyläThe Fermi-level scattering phase shifts and the transport cross sections are reported for atoms embedded in a homogeneous electron gas. The applications of the results are discussed, using the electronic stopping power for slow ions and impurity resistivity as examples.Item Comment on "Paired Gap states in a Semiconducting Carbon Nanotube: Deep and Shallow Levels"(2007) Krasheninnikov, A. V.; Foster, A. S.; Nieminen, R. M.; Department of Applied PhysicsA Comment on the Letter by Sungjun Lee et al., [Phys. Rev. Lett. 95, 166402 (2005)]. The authors of the Letter offer a Reply.Item Comment on the positron surface-state lifetime(1984-09-24) Nieminen, R. M.; Puska, M. J.; Manninen, Matti; University of Jyväskylä; Department of Applied PhysicsItem Crystals from metallic clusters(1993-07-15) Seitsonen, A. P.; Puska, M. J.; Alatalo, M.; Nieminen, R. M.; Milman, V.; Payne, M. C.; Department of Applied PhysicsThe interactions of the magic Al12Si clusters are studied by first-principles electron-structure calculations. It is shown that clusters arranged into the fcc lattice do not conserve their separated-cluster icosahedral structure but coalesce to form a close-packed metal.Item Density-functional theory of positronium and electron bubbles in helium fluids(1980-05-01) Nieminen, R. M.; Välimaa, I.; Manninen, M.; Hautojärvi, P.; Department of Applied PhysicsThe density-functional method is applied to excess electrons and positronium atoms in helium fluids. The self-trapping is investigated in a fully self-consistent fashion, and formulas are given for the particle energy and Ps pick-off annihilation rate in quasifree as well as localized states. The numerical results compare well with experimental data. However, the need for a more sophisticated treatment of threshold effects near the onset of bubble formation is indicated.Item Electron and positron energy levels in solids(1987-11-15) Boev, O. V.; Puska, M. J.; Nieminen, R. M.; Department of Applied Physics; Cornell UniversityThe self-consistent electron densities and the corresponding positron states are calculated for several metals and semiconductors in the local-density approximation of the density-functional formalism. The calculations are performed with the linear-muffin-tin-orbital band-structure method. The emphasis of this work is on the energy levels of the delocalized positron and the electron chemical potential which are now calculated with respect to the same potential reference. These energies determine quantities such as the positron and positronium work functions and the deformation potentials which are important parameters in slow-positron-beam experiments. The theoretical results are compared to values extracted from experiments.Item Electronic polarizability of small metal spheres(1985-03-15) Puska, M. J.; Nieminen, R. M.; Manninen, M.; Department of Applied Physics; University of JyväskyläWe present the results of calculations for the ground-state electron structure, static polarizability, and dynamic response of small metal (jellium) spheres in vacuum or embedded in a dielectric. Fully self-consistent time-dependent density-functional methods are used. In particular, the static and dynamic responses to an incident electric field (dipolar polarizability and photoabsorption) are obtained. The results show substantial deviations from either classical or approximate quantum-mechanical solutions, and provide reference data for simplified treatments.Item Electronically induced trapping of hydrogen by impurities in niobium(1984-07-15) Manninen, Matti; Puska, M. J.; Nieminen, R. M.; Jena, P.; Department of Applied Physics; University of JyväskyläThe binding energies of hydrogen and its isotopes to substitutional impurities Ti, Cr, and V in niobium have been calculated. The hydrogen-metal interaction is based on the effective-medium theory. The wave mechanics of the hydrogenic interstitials are explicity dealt with, and the lattice distortion created by the hydrogen is incorporated through the method of lattice statics. The difference in the electronic structure between impurity and host atoms is shown to be largely responsible for the binding of hydrogen to the impurities. The results are in agreement with recent inelastic neutron scattering experiments.Item Embedded-atom calculations of Auger and x-ray photoemission shifts for metallic elements(1982-01-01) Nieminen, R. M.; Puska, M. J.; University of Jyväskylä; Department of Applied PhysicsChange in self-consistent-field energy density-functional calculations are reported for Auger and core-level binding-energy shifts in sp-bonded metals. The basic model, atom in jellium vacancy, gives good agreement with experiment, especially in the Auger case. The chemical and relaxation contributions to the shifts are discussed, and the extra-atomic response is analyzed in detail, both in position and energy space. The adequacy of the "excited-atom" approach to the energy shifts is discussed.Item First-principles simulation of intrinsic collision cascades in KCl and NaCl to test interatomic potentials at energies between 5 and 350 eV(1991-12-23) Keinonen, J.; Kuronen, A.; Tikkanen, P.; Börner, H. G.; Jolie, J.; Ulbig, S.; Kessler, E. G.; Nieminen, R. M.; Puska, M. J.; Seitsonen, A. P.; Department of Applied PhysicsTheoretical interatomic potentials for KCl and NaCl are tested at energies 5350 eV against experimental data from intrinsic collision cascades. The collisional scattering of Cl with Cl, K, and Na atoms was observed from Doppler-shifted rays depopulating an excited state in recoiling Cl36 produced through the thermal neutron capture Cl35(n,)36Cl. The collisional scattering was simulated with molecular dynamics. Interatomic potentials from the present Iab initioP atomic cluster calculations are proposed for the Cl-Cl, Cl-K, and Cl-Na interactions in KCl and NaCl.Item First-principles study of He in Si(1992-11-15) Alatalo, M.; Puska, M. J.; Nieminen, R. M.; Department of Applied PhysicsWe have performed first-principles calculations for He atoms in a Si lattice. From dynamic total-energy minimization we obtain the relaxations of the Si atoms around the impurity and the corresponding total energies. The calculated heat of solution and the diffusion constant of He in Si are in good agreement with experiment. There is a net attraction between two tetrahedral He interstitials, leading to a binding energy of 0.08 eV for He atoms at neighboring interstices. On the other hand, Si vacancies are found not to trap He atoms. The consequences of these results to He-bubble nucleation and growth are discussed.Item Hydrogen and deuterium decoration of In-vacancy complexes in nickel(1987-04-15) Puska, M. J.; Nieminen, R. M.; Jena, P.; Department of Applied Physics; University of JyväskyläThe quantum-mechanical states of hydrogen and deuterium in pure and defected nickel have been calculated using the effective-medium theory. The defects considered include monovacancies, the substitutional In impurity, a complex of four vacancies, and a complex of an In impurity decorated with a tetrahedron of four vacancies. While the substitutional In impurity does not trap hydrogen, the vacancy and the vacancy complexes with and without In association do. The calculated binding energy to the four vacancy complex is nearly insensitive to the hydrogen isotopic mass and to the In decoration. These results, along with the dependence of the hydrogen binding energy on multiple hydrogen occupancy of the In vacancy complex, are compared with recent perturbed-angular-correlation experiments.Item Indium and phosphorus vacancies and antisites in InP(1994-02-15) Seitsonen, A. P.; Virkkunen, R.; Puska, M. J.; Nieminen, R. M.; Department of Applied PhysicsWe present an extensive study of the structure and energetics of monovacancies and antisites in InP. Using a first-principles approach, the different charge states of indium and phosphorus vacancies and antisites are examined. The lattice distortions around the defects are derived fully self-consistently with respect to both electronic and ionic degrees of freedom. Jahn-Teller relaxations, defect-induced one-electron energy levels, and ionization potentials in the band gap are discussed. From the formation energies we predict the favored vacancies and antisites under different stoichiometry conditions.Item Interfacial oxide growth in silicon/high-k oxide interfaces: First principles modeling of the Si-HfO2 interface(2006) Hakala, M. H.; Foster, A. S.; Gavartin, J.L.; Havu, P.; Puska, M. J.; Nieminen, R. M.; Department of Applied PhysicsWe have performed first principles calculations to investigate the structure and electronic properties of several different Si–HfOx interfaces. The atomic structure has been obtained by growing HfOx layer by layer on top of the Si(100) surface and repeatedly annealing the structure using ab initio molecular dynamics. The interfaces are characterized via their geometric and electronic properties, and also using electron transport calculations implementing a finite element based Green’s function method. We find that in all interfaces, oxygen diffuses towards the interface to form a silicon dioxide layer. This results in the formation of dangling Hf bonds in the oxide, which are saturated either by hafnium diffusion or Hf–Si bonds. The generally poor performance of these interfaces suggests that it is important to stabilize the system with respect to lattice oxygen diffusion.Item Muon states in uniaxially strained iron(1984-04-01) Jena, P.; Manninen, Matti; Nieminen, R. M.; Puska, M. J.; University of Jyväskylä; Department of Applied PhysicsEffects of lattice relaxation, quantum motion, and uniaxial strain on the internal field at a positive-muon site in iron have been calculated. The uniaxial strain gives rise to a statistical shift of the muon population at interstitial sites. The effect of the population shift is found to be primarily responsible for the observed changes in the muon-precession frequency. The theory also predicts a 1T temperature dependence of the frequency shifts.