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  • 1. Henriksson, Krister O. E.
    et al.
    Lahde, T. A.
    Nyfalt, C. J.
    Riska, C. O.
    Pion decay widths of D mesons2001In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 686, p. 355-378Article in journal (Refereed)
    Abstract [en]

    The pionic decay rates of the excited L = 0, 1 D mesons are calculated with a Hamiltonian model within the Framework of the covariant Blankenbecler-Sugar equation. The interaction between the light quark and charm antiquark is described by a linear scalar confining and a screened one-gluon exchange interaction. The decay widths of the D* mesons obtain a contribution from the exchange current that is associated with the linear scalar confining interaction. If this contribution is taken into account along with the single-quark approximation, the calculated decay rates of the charged D* mesons are readily below the current empirical upper limits if the axial coupling constant of the light constituent quarks is taken to be g(A)(q) = 0.87, but reach the empirical upper limits if g(A)(q) = 1. With the conventional values for g(A)(q), the calculated widths of the D-1 and D-2* mesons fall somewhat below the experimental lower limits, leaving room for other decay modes as well, such as pi pi decay. The unrealistically large contribution from the axial charge operator to the calculated pion decay width of the D-1 meson is suppressed by taking into account the exchange charge effects that are associated with the scalar linear confining and vector one-gluon exchange interactions. The predicted values for the pionic widths uf the hitherto undiscovered L = 1 D-1* and D-0* mesons are found to be smaller than previous estimates.

  • 2.
    Henriksson, Krister O. E.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Nordlund, K.
    Simulations of cementite: An analytical potential for the Fe-C system2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 14Article in journal (Refereed)
    Abstract [en]

    An analytical bond-order interatomic potential has been developed for the iron-carbon system for use in molecular-dynamics and Monte Carlo simulations. The potential has been successfully fitted to cementite and Hagg carbide, which are most important crystalline polytypes among the many known metastable iron carbide phases. Predicted properties of other carbides and the simplest point defects are in good to reasonable agreement with available data from experiments and density-functional theory calculations. The potential correctly describes melting and recrystallization of cementite, making it useful for simulation of steels. We show that they correctly describe the metastability of cementite and can be used to model carbide growth and dissolution.

  • 3.
    Henriksson, Krister O. E.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    Nordlund, K.
    Keinonen, J.
    Annihilation of craters: Molecular dynamic simulations on a silver surface2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 24, p. 15 p.-Article in journal (Refereed)
    Abstract [en]

    The ability of silver cluster ions containing 13 atoms to fill in a preexisting crater with a radius of about 28 angstrom on a silver (001) target has been investigated using molecular dynamics simulations and the molecular-dynamics-Monte Carlo corrected effective medium potential. The largest lateral distance r between crater and ion was about three times the radius of the preexisting crater, namely, 75 angstrom. The results reveal that when r < 20 angstrom and r>60 angstrom the preexisting crater is partially filled in, and for other distances there is a net growth of the crater. The lattice damage created by the cluster ions, the total sputtering yield, the cluster sputtering yield, and simulated transmission electron microscopy images of the irradiated targets are also presented.

  • 4.
    Henriksson, Krister O. E.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    Nordlund, K.
    Keinonen, J.
    Crater annihilation on silver by cluster ion impacts2007In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 255, no 1, p. 259-264Article in journal (Refereed)
    Abstract [en]

    Using the MD/MC-CEM potential we have investigated the impacts of 20 keV Ag-13 cluster ions on (001) silver surfaces having one initial crater. This one was made in the zeroth ion impact. The degree of annihilation of the initial crater was investigated as a function of the lateral distance r(i) between the crater and the ion. The impact points were selected randomly inside a circular area with a radius of 75 angstrom centered on the crater. To reduce the total number of simulations, the circular area was divided into annuli. The initial and final atomic positions in the impact simulations were analyzed and the degree of annihilation of the initial crater was determined. The results indicate that for r <= 60 angstrom there is a net growth of the initial crater, and for distances r is an element of (60,80) angstrom there is a small net filling of the crater.

  • 5. Henriksson, Krister O. E.
    et al.
    Nordlund, K.
    Keinonen, J.
    Fragmentation of clusters sputtered from silver and gold: Molecular dynamics simulations2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 1Article in journal (Refereed)
    Abstract [en]

    Using molecular dynamics simulations and the embedded atom method (EAM) potential we have investigated the sputtered atom clusters produced by 15 keV xenon impacts on silver and 20 keV xenon impacts on gold. Ejected clusters were simulated for long times, up to 0.01-1 mus, in order to investigate the fragmentation of nascent clusters. The size distributions of nascent and final clusters were calculated and fitted to an inverse power law, resulting in exponents close to 2 and 3, depending on the range of the cluster sizes used. These values are in agreement with other simulations and experiments. The results show that clusters are subject to a dramatic breakup, which makes the size of the largest sputtered cluster go down by a factor of 2-4. Despite this, the exponent in the power law does not change very much from the size distribution of nascent to that of final clusters. Considering the uncertainties, the exponent of the final size distribution is 1.0-1.7 times the exponent of the nascent size distribution.

  • 6. Henriksson, Krister O. E.
    et al.
    Nordlund, K.
    Keinonen, J.
    Molecular dynamics simulations of helium cluster formation in tungsten2006In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 244, no 2, p. 377-391Article in journal (Refereed)
    Abstract [en]

    Molecular dynamics simulations of helium implantation into single-crystalline tungsten at 0 and 300 K have been performed. Non-damaging ion energies of 50, 100 and 200 eV were used. Clusters containing up to the order of 100 He atoms were formed. These clusters were nucleated athermally, via the creation of (111) crowdion interstitials and interstitial dislocation loop punching. Ruptures of He clusters were observed, but no associated ejection of W atoms.

  • 7. Henriksson, Krister O. E.
    et al.
    Nordlund, K.
    Krasheninnikov, A.
    Keinonen, J.
    Difference in formation of hydrogen and helium clusters in tungsten2005In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 87, no 16Article in journal (Refereed)
    Abstract [en]

    The experimentally observed large difference in the depths of hydrogen and helium clusters formed in tungsten still lacks a fundamental explanation. Using density functional theory calculations, molecular dynamics simulations, and kinetic Monte Carlo calculations, we show that the fundamental mechanism behind the different clustering depths is significantly different behaviors of interstitial H and He atoms in W: H-H states are unstable for small interatomic distances whereas He-He states are strongly bound.

  • 8. Henriksson, Krister O. E.
    et al.
    Nordlund, K.
    Krasheninnikov, A.
    Keinonen, J.
    The depths of hydrogen and helium bubbles in tungsten: A comparison2006In: Fusion science and technology, ISSN 1536-1055, E-ISSN 1943-7641, Vol. 50, no 1, p. 43-57Article in journal (Refereed)
    Abstract [en]

    The role of self-trapping and defect trapping of hydrogen and helium implanted into tungsten has been investigated using density functional theory (DFT) calculations, molecular dynamics simulations, and kinetic Monte Carlo simulations (KMCSs). The potential energy curves of hydrogen or helium pairs were obtained by molecular dynamics, and the energy of the most essential states was checked with DFT Under assumptions of bubble formation due to trapping by similiar impurity atoms (self-trapping) or defects, KMCSs were carried out using parameters from implantation experiments. The results indicate that self-trapping plays no (or a very small) role in hydrogen bubble formation, whereas helium bubbles form due to strong self-trapping.

  • 9.
    Henriksson, Krister O. E.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    Sandberg, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Wallenius, Janne
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Carbides in stainless steels: Results from ab initio investigations2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 93, no 19Article in journal (Refereed)
    Abstract [en]

    The useful properties of steels are due to a complicated microstructure containing iron and chromium carbides. Only some basic physical properties of these carbides are known with high precision, although the carbides may have a vital impact on the performance and longevity of the steel. To improve on this situation, we have performed extensive density-functional theory calculations of several carbides. The quantitative results are in perfect agreement with the relative empirical stability of the carbides. Also, in contradiction with experimental data, we find that Cr23C6 responsible for the hardness of stainless steels is not the most stable chromium-dominated carbide.

  • 10. Henriksson, Krister O. E.
    et al.
    Vortler, K.
    Dreissigacker, S.
    Nordlund, K.
    Keinonen, J.
    Sticking of atomic hydrogen on the tungsten (001) surface2006In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 600, no 16, p. 3167-3174Article in journal (Refereed)
    Abstract [en]

    The sticking of hydrogen atoms with kinetic energies in the range 0.003-10 eV on a clean (001) tungsten surface has been investigated using molecular dynamics simulations. The atoms are found to stick to the surface at 0 and 300 K, with a sticking coefficient smaller than 0.6 for kinetic energies higher than 3 meV. The adsorption sites for H on the W(001) surface are also presented. The dominant site is in perfect agreement with the experimentally found bridge site.

  • 11. Juslin, N.
    et al.
    Erhart, P.
    Traskelin, P.
    Nord, J.
    Henriksson, Krister O. E.
    Nordlund, K.
    Salonen, E.
    Albe, K.
    Analytical interatomic potential for modeling nonequilibrium processes in the W-C-H system2005In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 98, no 12Article in journal (Refereed)
    Abstract [en]

    A reactive interatomic potential based on an analytical bond-order scheme is developed for the ternary system W-C-H. The model combines Brenner's hydrocarbon potential with parameter sets for W-W, W-C, and W-H interactions and is adjusted to materials properties of reference structures with different local atomic coordinations including tungsten carbide, W-H molecules, as well as H dissolved in bulk W. The potential has been tested in various scenarios, such as surface, defect, and melting properties, none of which were considered in the fitting. The intended area of application is simulations of hydrogen and hydrocarbon interactions with tungsten, which have a crucial role in fusion reactor plasma-wall interactions. Furthermore, this study shows that the angular-dependent bond-order scheme can be extended to second nearest-neighbor interactions, which are relevant in body-centered-cubic metals. Moreover, it provides a possibly general route for modeling metal carbides.

  • 12. Nordlund, K.
    et al.
    Henriksson, Krister O. E.
    Keinonen, J.
    Melting temperature effects on the size of ion-induced craters2001In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 79, no 22, p. 3624-3626Article in journal (Refereed)
    Abstract [en]

    Recent work on the sizes of craters produced by ion impacts of solids has shown that the size of the crater scales with the inverse square of the cohesive energy. This observation is in contrast to the size of craters produced in macroscopic impacts, which scale directly with the inverse of the cohesive energy. It has relied on the assumption that the melting temperature is proportional to the cohesive energy. Using computer simulations, we now show that the size scales in fact with the inverse of the product of the melting temperature and cohesive energy. This provides direct proof that the reason to the different behavior of macroscopic and ion-induced cratering is flow of the liquid produced by the ions.

  • 13.
    Sandberg, Nils
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Henriksson, Krister O. E.
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    Wallenius, Janne
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Carbon impurity dissolution and migration in bcc Fe-Cr: First-principles calculations2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 9Article in journal (Refereed)
    Abstract [en]

    First-principles density-functional theory calculations for C solution enthalpies, H-sol, and diffusion activation enthalpies, H-diff, in body-centered-cubic Fe and Cr are presented. The results for C in Fe compare well with experiments, provided that the effect of magnetic disordering is accounted for. Likewise, in Cr, the calculated Hsol and Hdiff agree well with available experiments. In both materials, the deviation between calculated enthalpies and critically assessed experimental enthalpies are less than 0.05 eV. Further, first-principles calculations for the interaction energies between a solute (e.g., a Cr atom in bcc Fe) and an interstitial C atom are presented. The results are in conflict with those inferred from internal friction (IF) experiments in disordered Fe-Cr-C alloys. A simple model of C relaxation in disordered Fe-Cr is used to compare theoretical and experimental IF curves directly. The results suggest that a more extensive study of the energetic, thermodynamic, and kinetic aspects of C migration in Fe-Cr is needed.

  • 14.
    Wallenius, Janne
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Henriksson, Krister O. E.
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    Sandberg, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, P.
    The role of magnetism in modelling of fuels and structural materials2008In: Transactions of the American Nuclear Society, 2008Conference paper (Refereed)
  • 15.
    Wallenius, Janne
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Sandberg, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Henriksson, Krister
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Atomistic modelling of the Fe-Cr-C system2011In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 415, no 3, p. 316-319Article in journal (Refereed)
    Abstract [en]

    For the purpose of modelling the impact of carbon on radiation damage phenomena in steels, we have performed an extensive set of first principle calculations on the Fe-Cr-C system. The calculated solution and diffusion enthalpies of carbon in iron and in chromium agree well with experimental data, as do the relative formation energies of mono-carbides, cementite. Flagg and M(23)C(6) carbides. Our data further indicate that interstitial carbon is attracted to a solute iron atom in bcc chromium, while the reaction between carbon and a solute chromium atom in bcc iron is repulsive. An empirical potential fitted to data for iron carbides is capable of reproducing melting behaviour of cementite, while the predicted interaction with point defects agrees less well with DFT data than a potential recently published by Hepburn and Ackland.

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