[article] Perustieteiden korkeakoulu / SCI

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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.
  • Abnormal Auditory Cortical Activation in Dyslexia 100 msec after Speech Onset
    (2002) Helenius, Päivi; Salmelin, Riitta; Richardson, Ulla; Leinonen, Seija; Lyytinen, Heikki
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Reading difficulties are associated with problems in processing and manipulating speech sounds. Dyslexic individuals seem to have, for instance, difficulties in perceiving the length and identity of consonants. Using magnetoencephalography (MEG), we characterized the spatio-temporal pattern of auditory cortical activation in dyslexia evoked by three types of natural bisyllabic pseudowords (/ata/, /atta/, and /a a/), complex nonspeech sound pairs (corresponding to /atta/ and /a a/) and simple 1-kHz tones. The most robust difference between dyslexic and non-reading-impaired adults was seen in the left supratemporal auditory cortex 100 msec after the onset of the vowel /a/. This N100m response was abnormally strong in dyslexic individuals. For the complex nonspeech sounds and tone, the N100m response amplitudes were similar in dyslexic and nonimpaired individuals. The responses evoked by syllable /ta/ of the pseudoword /atta/ also showed modest latency differences between the two subject groups. The responses evoked by the corresponding nonspeech sounds did not differ between the two subject groups. Further, when the initial formant transition, that is, the consonant, was removed from the syllable /ta/, the N100m latency was normal in dyslexic individuals. Thus, it appears that dyslexia is reflected as abnormal activation of the auditory cortex already 100 msec after speech onset, manifested as abnormal response strengths for natural speech and as delays for speech sounds containing rapid frequency transition. These differences between the dyslexic and nonimpaired individuals also imply that the N100m response codes stimulus-specific features likely to be critical for speech perception. Which features of speech (or nonspeech stimuli) are critical in eliciting the abnormally strong N100m response in dyslexic individuals should be resolved in future studies.
  • 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.
  • Activation of Auditory Cortex by Anticipating and Hearing Emotional Sounds: An MEG Study
    (2013) Yokosawa, Koichi; Pamilo, Siina; Hirvenkari, Lotta; Hari, Riitta; Pihko, Elina
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    To study how auditory cortical processing is affected by anticipating and hearing of long emotional sounds, we recorded auditory evoked magnetic fields with a whole-scalp MEG device from 15 healthy adults who were listening to emotional or neutral sounds. Pleasant, unpleasant, or neutral sounds, each lasting for 6 s, were played in a random order, preceded by 100-ms cue tones (0.5, 1, or 2 kHz) 2 s before the onset of the sound. The cue tones, indicating the valence of the upcoming emotional sounds, evoked typical transient N100m responses in the auditory cortex. During the rest of the anticipation period (until the beginning of the emotional sound), auditory cortices of both hemispheres generated slow shifts of the same polarity as N100m. During anticipation, the relative strengths of the auditory-cortex signals depended on the upcoming sound: towards the end of the anticipation period the activity became stronger when the subject was anticipating emotional rather than neutral sounds. During the actual emotional and neutral sounds, sustained fields were predominant in the left hemisphere for all sounds. The measured DC MEG signals during both anticipation and hearing of emotional sounds implied that following the cue that indicates the valence of the upcoming sound, the auditory-cortex activity is modulated by the upcoming sound category during the anticipation period.
  • Adapting to the changing needs of managing innovative projects
    (2016) Rekonen, Satu; Björklund, Tua A.
     Factories | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Purpose – The purpose of this paper is to explore the changes in managerial activities and challengesat different phases of innovative projects. Design/methodology/approach – Six NPD project managers were interviewed in three differentproject phases in a qualitative, longitudinal design. The resulting 18 semi-structured face-to-faceinterviews were content analyzed and categorized according to thematic similarity. Findings – Altogether 19 categories describing managerial concerns in managing innovative projectswere recognized. Task-oriented, rather than people-oriented, approaches were dominant throughoutthe projects, although the reported concerns clearly varied at each phase. The early development phaseemerged as a transition point, where managers had to transform their roles, reported activitiesdecreased, and reported challenges increased. Research limitations/implications – Although based on a small number of participants in a singlesetting, the results highlight the need for longitudinal studies and differentiating between the variousphases of the innovation process, as there was great variance in the concerns of each phase.Furthermore, domain expertise seemed to have a large impact on how the managers reformulated theirrole in transitioning from the front-end to the development phases. Practical implications – The present study emphasizes the need to support managers intransitioning between different innovation phases and to recognize the need to adjust managerial roles.Further, it seemed crucial to establish the practices supporting successful teamwork in the front-endphase before the first phase transition. Originality/value – The study is a rare example of a longitudinal research design examining theimplications and transition between different phases of the innovation process within the sameprojects for project managers.
  • 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.
  • Adult Brain Plasticity Elicited by Anomia Treatment
    (2003) Cornelissen, Katri; Laine, Matti; Tarkiainen, Antti; Järvensivu, Tiina; Martin, Nadine; Salmelin, Riitta
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We describe a study where a specific treatment method for word-finding difficulty (so-called contextual priming technique, which combines massive repetition priming with semantic priming) was applied with three chronic left hemisphere-damaged aphasics. Both before and after treatment, which focused on naming of a series of pictures, naming-related brain activity was measured by magnetoencephalography (MEG). Due to its excellent temporal resolution and good spatial resolution, we were able to track treatment-induced changes in cortical activity. All three subjects showed improved naming of the trained items. In all subjects, a single source area, located in the left inferior parietal lobe, close to the lesioned area, displayed statistically significant training-induced changes. This effect was of long latency as it started 300–600 msec after picture presentation. The change in activation was specific to training, as it could not be accounted for by variation of cortical dynamics associated with increased proportion of correct answers. Our interpretation is that the training effect reflects more effective phonological encoding and storage of the trained items through the engagement of a left hemispheric word-learning system. This is in line with recent functional imaging studies, which have linked left inferior parietal lobe activity to the phonological storage component of the verbal working memory, as well as with theoretical arguments stating that the primary role of the phonological loop is to acquire new words. Finally, the MEG results showed no evidence of increased right hemisphere participation following training, supporting the view that restoration of language-related networks in the damaged left hemisphere is crucial for anomia recovery.
  • 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 that glitters is not BOLD: inconsistencies in functional MRI
    (2014) Renvall, Ville; Nangini, Cathy; Hari, Riitta
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) signal is a widely-accepted marker of brain activity. The acquisition parameters (APs) of fMRI aim at maximizing the signals related to neuronal activity while minimizing unrelated signal fluctuations. Currently, a diverse set of APs is used to acquire BOLD fMRI data. Here we demonstrate that some fMRI responses are alarmingly inconsistent across APs, ranging from positive to negative, or disappearing entirely, under identical stimulus conditions. These discrepancies, resulting from non-BOLD effects masquerading as BOLD signals, have remained largely unnoticed because studies rarely employ more than one set of APs. We identified and characterized non-BOLD responses in several brain areas, including posterior cingulate cortex and precuneus, as well as AP-dependence of both the signal time courses and of seed-based functional networks, noticing that AP manipulation can inform about the origin of the measured signals.