12. Artikkelit / Articles

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Browsing 12. Artikkelit / Articles by Department "Department of Applied Physics"

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  • 1/f noise and avalanche scaling in plastic deformation
    (2006) Laurson, L.; Alava, Mikko J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We study the intermittency and noise of dislocation systems undergoing shear deformation. Simulations of a simple two-dimensional discrete dislocation dynamics model indicate that the deformation rate exhibits a power spectrum scaling of the type 1/fα. The noise exponent is far away from a Lorentzian, with α≈1.5. This result is directly related to the way the durations of avalanches of plastic deformation activity scale with their size.
  • Ab initio study of Cu diffusion in alpha-cristobalite
    (2012) Zelený, M.; Hegedüs, J.; Foster, Adam S.; Drabold, D.A.; Elliott, S.R.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We 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 SiO2, 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.
  • 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 study of gamma-Al2O3 surfaces
    (2004) Pinto, Henry P.; Nieminen, Risto M.; Elliott, Simon D.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Starting from the theoretical prediction of the γ−Al2O3 structure using density-functional theory in the generalized gradient approximation, we have studied the (1 1 1), (0 0 1), (1 1 0), and (1 5 0) surfaces. The surface energies and their corresponding structures are computed and compared with predictions for (0 0 0 1) α−Al2O3 and available experimental results for γ-alumina surfaces. (1 1 1) and (0 0 1) surfaces are predicted to be equally stable, but to show quite different structure and reactivity. Whereas a low coverage of highly reactive trigonal Al occurs on (1 1 1), (0 0 1) exhibits a more dense plane of both five-coordinate and tetrahedral Al. Microfaceting of a (1 1 0) surface into (1 1 1)-like planes is also observed. The implications for the structure of ultrathin dielectric films and for the surfaces of disordered transition aluminas are discussed.
  • Ab initio study of oxygen point defects in GaAs, GaN, and AlN
    (1996) Mattila, T.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have studied oxygen point defects with the plane-wave pseudopotential method in GaAs, GaN, and AlN. The calculations demonstrate a qualitatively different behavior of oxygen impurities in these materials. OAs in GaAs acts as a deep center with an off-center displacement and negative-U behavior, in agreement with the experimental data. ON in GaN is found to be a shallow donor with a low formation energy, and is suggested to act as a partial source for the unintentional n-type conductivity commonly observed in GaN. O in AlN is also found to easily substitute for N, which is consistent with the experimentally observed large oxygen concentrations in AlN. However, ON in AlN is shown to be a deep center due to the wide band gap, in contrast with ON in GaN. Our calculations thus predict that isolated oxygen acts as a DX-type center in AlxGa1−xN alloys. Results for other oxygen point defect configurations and for the dominant native defects are also presented. © 1996 The American Physical Society.
  • Ab initio study of point defects in CdF2
    (1997) Mattila, T.; Pöykkö, S.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The plane-wave pseudopotential method is used to study point defects in CdF2. We present comprehensive results for the native defects as well as for dominant impurities. In addition to Fi, VCd and OF were found to be easily formed compensating acceptors. For In and Ga impurities the experimentally observed large Stokes shift could not be established, and the results rule out symmetric atomic relaxation as the mechanism leading to the bistable behavior. The limitations of the present approach utilizing density-functional theory and the local-density approximation in the case of ionic materials are addressed.
  • 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.
  • Accessing nanomechanical resonators via a fast microwave circuit
    (2009) Sillanpää, Mika A.; Sarkar, Jayanta; Sulkko, Jaakko; Muhonen, Juha; Hakonen, Pertti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We demonstrate how to fully electrically detect the vibrations of conductive nanomechanical resonators up to the microwave regime. We use the electrically actuated vibrations to modulate an LC tank circuit, which blocks the stray capacitance and detect the created sideband voltage by a microwave analyzer. We prove the technique up to mechanical frequencies of 200 MHz. Finally, we estimate how one could approach the quantum limit of mechanical systems.
  • Acoustic Emission from Paper Fracture
    (2002) Salminen, L. I.; Tolvanen, A. I.; Alava, Mikko J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report tensile failure experiments on paper sheets. The acoustic emission energy and the waiting times between acoustic events follow power-law distributions. This remains true while the strain rate is varied by more than 2 orders of magnitude. The energy statistics has the exponent β∼1.25±0.10 and the waiting times the exponent τ∼1.0±0.1, in particular, for the energy roughly independent of the strain rate. These results do not compare well with fracture models, for (brittle) disordered media, which as such exhibit criticality. One reason may be residual stresses, neglected in most theories.
  • Adsorption and migration of carbon adatoms on carbon nanotubes: Density-functional ab initio and tight-binding studies
    (2004) Krasheninnikov, A. V.; Nordlund, K.; Lehtinen, P. O.; Foster, Adam S.; Ayuela, A.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We 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.
  • Adsorption of acetic and trifluoroacetic acid on the TiO2(110) surface
    (2004) Foster, Adam S.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We use the first-principles static and dynamic simulations to study the adsorption of acetic (CH3COOH) and trifluoroacetic (CF3COOH)acid on the TiO2(110)surface. The most favorable adsorption for both molecules is a dissociative process, which results in the two oxygens of the carboxylate ion bonding to in-plane titanium atoms in the surface. The remaining proton then bonds to a bridging oxygen site, forming a hydroxyl group. We further show that, by comparing the calculated dipoles of the molecules on the surface, it is possible to understand the difference in contrast over the acetate and trifluoroacetate molecules in the atomically resolved noncontact atomic force microscopy images.
  • 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.
  • AFM tip characterization by Kelvin probe force microscopy
    (2010) Barth, C.; Hynninen, T.; Bieletzki, M.; Henry, C. R.; Foster, Adam S.; Esch, F.; Heiz, U.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Reliable determination of the surface potential with spatial resolution is key for understanding complex interfaces that range from nanostructured surfaces to molecular systems to biological membranes. In this context, Kelvin probe force microscopy (KPFM) has become the atomic force microscope (AFM) method of choice for mapping the local electrostatic surface potential as it changes laterally due to variations in the surface work function or surface charge distribution. For reliable KPFM measurements, the influence of the tip on the measured electrostatic surface potential has to be understood. We show here that the mean Kelvin voltage can be used for a straightforward characterization of the electrostatic signature of neutral, charged and polar tips, the starting point for quantitative measurements and for tip-charge control for AFM manipulation experiments. This is proven on thin MgO(001) islands supported on Ag(001) and is supported by theoretical modeling, which shows that single ions or dipoles at the tip apex dominate the mean Kelvin voltage.
  • Agglomeration of As Antisites in As-Rich Low-Temperature GaAs: Nucleation without a Critical Nucleus Size
    (2005) Staab, T. E. M.; Nieminen, Risto M.; Luysberg, M.; Frauenheim, Th.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    To investigate the early stages of nucleation and growth of As precipitates in GaAs grown at low substrate temperature, we make use of a self-consistent-charge density-functional based tight-binding method. Since a pair of As antisites already shows a significant binding energy which increases when more As antisites are attached, there is no critical nucleus size. Provided that all excess As has precipitated, the clusters may grow in size since the binding energies increase with increasing agglomeration size. These findings close the gap between experimental investigation of point defects and the detection of nanometer-size precipitates in transmission electron microscopy.
  • Aggregation Kinetics of Thermal Double Donors in Silicon
    (2001) Lee, Young Joo; von Boehm, J.; Pesola, M.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A general kinetic model based on accurate density-functional-theoretic total-energy calculations is introduced to describe the aggregation kinetics of oxygen-related thermal double donors (TDD's) in silicon. The calculated kinetics, which incorporates the reactions of associations, dissociations, and isomerizations of all relevant oxygen complexes, is in agreement with experimental annealing studies. The aggregation of TDD's takes place through parallel-consecutive reactions where both mobile oxygen dimers and fast migrating chainlike TDD's capture interstitial oxygen atoms.
  • 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.
  • All-optical reversible switching of local magnetization
    (2007) Shevchenko, Andriy; Korppi, Maria; Lindfors, Klas; Heiliö, Miika; Kaivola, Matti; Il’yashenko, Eugene; Johansen, Tom H.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The authors demonstrate all-optical reversible switching of the magnetization direction in a uniformly magnetized ferrite-garnet film. The magnetization is switched by locally heating the film with a pulsed laser beam. The direction to which the magnetization flips is controlled by two parameters, the beam diameter and the pulse energy, and not by the direction of the external magnetic field. In the experiments, neither the magnitude nor the direction of the external magnetic field is changed. The results of this work illustrate the richness of optical methods to locally control the properties of magnetic materials and suggest all-optical device applications.
  • Alteration of gas phase ion polarizabilities upon hydration in high dielectric liquids
    (2013) Buyukdagli, Sahin; Ala-Nissilä, Tapio
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We investigate the modification of gas phase ion polarizabilities upon solvation in polar solvents and ionic liquids. To this aim, we develop a classical electrostatic theory of charged liquids composed of solvent molecules modeled as finite size dipoles, and embedding polarizable ions that consist of Drude oscillators. In qualitative agreement with ab initio calculations of polar solvents and ionic liquids, the hydration energy of a polarizable ion in both types of dielectric liquid is shown to favor the expansion of its electronic cloud. Namely, the ion carrying no dipole moment in the gas phase acquires a dipole moment in the liquid environment, but its electron cloud also reaches an enhanced rigidity. We find that the overall effect is an increase of the gas phase polarizability upon hydration. In the specific case of ionic liquids, it is shown that this hydration process is driven by a collective solvation mechanism where the dipole moment of a polarizable ion induced by its interaction with surrounding ions self-consistently adds to the polarization of the liquid, thereby amplifying the dielectric permittivity of the medium in a substantial way. We propose this self-consistent hydration as the underlying mechanism behind the high dielectric permittivities of ionic liquids composed of small charges with negligible gas phase dipole moment. Hydration being a correlation effect, the emerging picture indicates that electrostatic correlations cannot be neglected in polarizable liquids.
  • Alternating domains with uniaxial and biaxial magnetic anisotropy in epitaxial Fe films on BaTiO[sub 3]
    (2012) Lahtinen, Tuomas H. E.; Shirahata, Yasuhiro; Yao, Lide; Franke, Kevin J. A.; Venkataiah, Gorige; Taniyama, Tomoyasu; van Dijken, Sebastiaan
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report on domain formation and magnetization reversal in epitaxial Fe films on ferroelectric BaTiO3 substrates with ferroelastica–c stripe domains. The Fe films exhibit biaxial magnetic anisotropy on top of c domains with out-of-plane polarization, whereas the in-plane lattice elongation of a domains induces uniaxial magnetoelasticanisotropy via inverse magnetostriction. The strong modulation of magnetic anisotropy symmetry results in full imprinting of the a–c domain pattern in the Fe films. Exchange and magnetostaticinteractions between neighboring magnetic stripes further influence magnetization reversal and pattern formation within the a and c domains.