Browsing by Author "Puska, Martti J."

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  • Ab initio study of fully relaxed divacancies in GaAs
    (1996) Pöykkö, S.; Puska, Martti J.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report calculations of the electronic and atomic structures of neutral and charged divacancies in GaAs using the first-principles Car-Parrinello method. It is found that the divacancy relaxes inwards in all charge states (2-,1-,0,1+) studied. The defect-induced electron levels lie in the lower half of the fundamental band gap. The doubly negative divacancy is the most stable one for nearly all values of the electron chemical potential within the band gap. The deep-level electron density is localized at the Ga-vacancy end of the divacancy and the ionic relaxation is stronger there than at the As-vacancy end. We have also calculated the thermodynamic concentrations for several different native defects in GaAs, and the implications for self-diffusion are discussed.
  • Ab initio transport fingerprints for resonant scattering in graphene
    (2012) Saloriutta, Karri; Uppstu, Andreas; Harju, Ari; Puska, Martti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have recently shown that by using a scaling approach for randomly distributed topological defects in graphene, reliable estimates for transmission properties of macroscopic samples can be calculated based even on single-defect calculations [A. Uppstu et al., Phys. Rev. B 85, 041401 (2012)]. We now extend this approach of energy-dependent scattering cross sections to the case of adsorbates on graphene by studying hydrogen and carbon adatoms as well as epoxide and hydroxyl groups. We show that a qualitative understanding of resonant scattering can be gained through density functional theory results for a single-defect system, providing a transmission “fingerprint” characterizing each adsorbate type. This information can be used to reliably predict the elastic mean free path for moderate defect densities directly using ab initio methods. We present tight-binding parameters for carbon and epoxide adsorbates, obtained to match the density-functional theory based scattering cross sections.
  • Adsorption structures of phenol on the Si (001)-(2 × 1) surface calculated using density functional theory
    (2010) Johnston, Karen; Gulans, Andris; Verho, Tuukka; Puska, Martti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Several dissociated and two nondissociated adsorption structures of the phenol molecule on the Si(001)-(2×1) surface are studied using density functional theory with various exchange and correlation functionals. The relaxed structures and adsorption energies are obtained and it is found that the dissociated structures are energetically more favorable than the nondissociated structures. However, the ground state energies alone do not determine which structure is obtained experimentally. To elucidate the situation core level shift spectra for Si 2p and C 1s states are simulated and compared with experimentally measured spectra. Several transition barriers were calculated in order to determine, which adsorption structures are kinetically accessible. Based on these results we conclude that the molecule undergoes the dissociation of two hydrogen atoms on adsorption.
  • Adsorption structures of phenol on the Si(001)-(2×1) surface calculated using density functional theory
    (2010-06-21) Johnston, Karen; Gulans, Andris; Verho, Tuukka; Puska, Martti J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Several dissociated and two nondissociated adsorption structures of the phenol molecule on the Si(001)−(2×1) surface are studied using density functional theory with various exchange and correlation functionals. The relaxed structures and adsorption energies are obtained and it is found that the dissociated structures are energetically more favorable than the nondissociated structures. However, the ground state energies alone do not determine which structure is obtained experimentally. To elucidate the situation core level shift spectra for Si 2p and C 1s states are simulated and compared with experimentally measured spectra. Several transition barriers were calculated in order to determine, which adsorption structures are kinetically accessible. Based on these results we conclude that the molecule undergoes the dissociation of two hydrogen atoms on adsorption.
  • Alkali Postdeposition Treatment-Induced Changes of the Chemical and Electronic Structure of Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers: A First-Principle Perspective
    (2019-01-23) Malitckaya, Maria; Kunze, Thomas; Komsa, Hannu Pekka; Havu, Ville; Handick, Evelyn; Wilks, Regan G.; Bär, Marcus; Puska, Martti J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The effects of alkali postdeposition treatment (PDT) on the valence band structure of Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers are addressed from a first-principles perspective. In detail, experimentally derived hard X-ray photoelectron spectroscopy (HAXPES) data [ Handick, E.; et al. ACS Appl. Mater. Interfaces 2015, 7, 27414-27420 ] of the valence band structure of alkali-free and NaF/KF-PDT CIGSe are directly compared and fit by calculated density of states (DOS) of CuInSe2, its Cu-deficient counterpart CuIn5Se8, and different potentially formed secondary phases, such as KInSe2, InSe, and In2Se3. The DOSs are based on first-principles electronic structure calculations and weighted according to element-, symmetry-, and energy-dependent photoionization cross sections for the comparison to experimental data. The HAXPES spectra were recorded using photon energies ranging from 2 to 8 keV, allowing extraction of information from different sample depths. The analysis of the alkali-free CIGSe valence band structure reveals that it can best be described by a mixture of the DOS of CuInSe2 and CuIn5Se8, resulting in a stoichiometry slightly more Cu-rich than that of CuIn3Se5. The NaF/KF-PDT-induced changes in the HAXPES spectra for different alkali exposures are best reproduced by additional contributions from KInSe2, with some indications that the formation of a pronounced K-In-Se-type surface species might crucially depend on the amount of K available during PDT.
  • All-electron density functional theory and time-dependent density functional theory with high-order finite elements
    (2009) Lehtovaara, Lauri; Havu, Ville; Puska, Martti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We present for static density functional theory and time-dependent density functional theory calculations an all-electron method which employs high-order hierarchical finite-element bases. Our mesh generation scheme, in which structured atomic meshes are merged to an unstructured molecular mesh, allows a highly nonuniform discretization of the space. Thus it is possible to represent the core and valence states using the same discretization scheme, i.e., no pseudopotentials or similar treatments are required. The nonuniform discretization also allows the use of large simulation cells, and therefore avoids any boundary effects.
  • All-electron time-dependent density functional theory with finite elements: Time-propagation approach
    (2011) Lehtovaara, Lauri; Havu, Ville; Puska, Martti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We present an all-electron method for time-dependent density functional theory which employs hierarchical nonuniform finite-element bases and the time-propagation approach. The method is capable of treating linear and nonlinear response of valence and core electrons to an external field. We also introduce (i) a preconditioner for the propagation equation, (ii) a stable way to implement absorbing boundary conditions, and (iii) a new kind of absorbing boundary condition inspired by perfectly matched layers.
  • Analysis of electron-positron momentum spectra of metallic alloys as supported by first-principles calculations
    (2007) Folegati, P.; Makkonen, I.; Ferragut, R.; Puska, Martti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Electron-positron momentum distributions measured by the coincidence Doppler broadening method can be used in the chemical analysis of the annihilation environment, typically a vacancy-impurity complex in a solid. In the present work, we study possibilities for a quantitative analysis, i.e., for distinguishing the average numbers of different atomic species around the defect. First-principles electronic structure calculations self-consistently determining electron and positron densities and ion positions are performed for vacancy-solute complexes in Al−Cu, Al−Mg−Cu, and Al−Mg−Cu−Ag alloys. The ensuing simulated coincidence Doppler broadening spectra are compared with measured ones for defect identification. A linear fitting procedure, which uses the spectra for positrons trapped at vacancies in pure constituent metals as components, has previously been employed to find the relative percentages of different atomic species around the vacancy [A. Somoza et al. Phys. Rev. B 65, 094107 (2002)]. We test the reliability of the procedure by the help of first-principles results for vacancy-solute complexes and vacancies in constituent metals.
  • Atomic relaxations around vacancy clusters in molybdenum and their effects on trapped-positron lifetime
    (1988) Khanna, S. N.; Rao, B. K.; Jena, P.; Esterling, D.; Puska, Martti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The relaxations of first- and second-nearest-neighbor shells of atoms around a monovacancy and around voids corresponding to the removal of 9, 15, 27, and 51 atoms in molybdenum have been calculated by minimizing the total energies with respect to atomic displacements. The energies are obtained by using the tight-binding scheme within the moments and continued-fraction formulation. The sign of the atomic displacements (inward or outward) of both the nearest and next-nearest neighbors varies with the size of the void. In addition, the displacements exhibit oscillations as a function of void size and do not appear to converge to atomic relaxations characteristic on surfaces even for the largest void studied. The relaxation of the first-nearest neighbor has a significant effect on the lifetime of positrons trapped in monovacancies, bringing theory to much better agreement with experiment. This effect, however, diminishes and becomes insignificant as voids grow in size.
  • Atoms embedded in an electron gas: Beyond the local-density approximation
    (1991) Puska, Martti J.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The 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.
  • Atoms embedded in an electron gas: Immersion energies
    (1981) Puska, Martti J.; Nieminen, Risto M.; Manninen, M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Energies 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.
  • Atoms embedded in an electron gas: Phase shifts and cross sections
    (1983) Puska, Martti J.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The 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.
  • Bound and free self-interstitial defects in graphite and bilayer graphene: A computational study
    (2011) Gulans, Andris; Krasheninnikov, Arkady V.; Puska, Martti J.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The role of self-interstitials in the response of layered carbon materials such as graphite, bilayer graphene and multiwalled carbon nanotubes to irradiation has long remained a puzzle. Using density-functional-theory methods with an exchange and correlation functional which takes into account the interlayer van der Waals interaction in these systems without any material-specific empirical parameters, we study the energetics and migration of single- and di-interstitials in graphite and bilayer graphene. We show that two classes of interstitials, “bound” and “free,” can coexist. The latter are mobile at room and lower temperatures, which explains the experimental data and reconciles them with the results of atomistic simulations. Our results shed light on the behavior of graphite and carbon nanotubes under irradiation and have implications for irradiation-mediated processing of bilayer graphene.
  • Broken symmetry in density-functional theory: Analysis and cure
    (2004) Harju, A.; Räsänen, E.; Saarikoski, H.; Puska, Martti J.; Nieminen, Risto M.; Niemelä, K.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We present a detailed analysis of the broken-symmetry mean-field solutions using a four-electron rectangular quantum dot as a model system. Comparisons of the density-functional theory predictions with the exact ones show that the symmetry-breaking results from the single-configuration wave function used in the mean-field approach. As a general cure we present a scheme that systematically incorporates several configurations into the density-functional theory and restores the symmetry. This cure is easily applicable to any density-functional approach.
  • Calculation of positron states and annihilation in solids: A density-gradient-correction scheme
    (1996) Barbiellini, B.; Puska, Martti J.; Korhonen, T.; Harju, A.; Torsti, T.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The generalized gradient correction method for positron-electron correlation effects in solids [B. Barbiellini et al., Phys. Rev. B 51, 7341 (1995)] is applied in several test cases. The positron lifetime, energetics, and momentum distribution of the annihilating electron-positron pairs are considered. The comparison with experiments shows systematic improvement in the predictive power of the theory compared to the local-density approximation results for positron states and annihilation characteristics.
  • Charging mechanism for the bond elongation observed in suspended chains of gold atoms
    (2005) Ayuela, A.; Puska, Martti J.; Nieminen, Risto M.; Alonso, J. A.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Abnormally large bond lengths (3.5–5.0Å) between gold atoms forming small chains suspended between two electrodes have been observed in some experiments. Using the density functional theory we explore the possibility that the elongation could arise from the electrical charging of the chains induced in nonequilibrium by the electron beam of the transmission electron microscope used to image the nanowires in those experiments.
  • Chlorine-impurity-related defects in ZnSe
    (1998) Pöykkö, S.; Puska, Martti J.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Defect complexes formed by chlorine-impurity atoms and native defects in ZnSe are studied by first-principles electronic-structure calculations. The strong tendency for the formation of vacancy-impurity pairs is shown. The chlorine-impurity–zinc-vacancy complex is shown to be the most important source of donor compensation. The results presented are compared with recent experimental results.
  • Clustering and conductance in breakage of sodium nanowires
    (2011) Zugarramurdi, A.; Borisov, A. G.; Zabala, N.; Chulkov, E. V.; Puska, Martti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We study the conductance during the elongation and breakage of Na nanowires described with the ultimate jellium model. A combined approach is used where the nanowire breakage is simulated self-consistently within the density-functional theory, and the wave packet propagation technique is applied for ballistic electron transport. For certain conditions the breakage of the nanowire is preceded by formation of clusters of magic size in the break junction. This affects the conductance G, in particular the shape of the G=3G0 to G=G0 (=2e2/h) step upon elongation. The observed trends can be explained as due to the transient trapping of ballistic electrons inside the cluster, leading to a resonant character of the electron transport through the break junction. The cluster-derived resonances appear as peak structures in the differential conductance which may serve as an experimental signature of clustering.
  • Comment on "Intrinsic n-type Behavior in Transparent Conducting Oxides: A Comparative Hybrid-Functional Study of In2O3, SnO2, and ZnO" Reply
    (2011-02-10) Agoston, Peter; Albe, Karsten; Nieminen, Risto M.; Puska, Martti J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A Reply to the Comment by Stephan Lany and Alex Zunger.
  • Comment on the Positron Surface-State Lifetime
    (1984) Nieminen, Risto M.; Puska, Martti J.; Manninen, M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Received 16 April 1984