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Now showing items 1 - 16 of 170

  • Dependence of ion channeling on relative atomic number in compounds

    Nordlund, K.   Hobler, G.  

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  • Channeling maps for Si ions in Si: Assessing the binary collision approximation

    Hobler, G.   Nordlund, K.  

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  • Large fraction of crystal directions leads to ion channeling

    Nordlund, K.   Djurabekova, F.   Hobler, G.  

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  • Defect model for the dependence of breakdown rate on external electric fields

    Nordlund, K.   Djurabekova, F.  

    We develop an analytical model for the vacuum electric breakdown rate dependence on an external electric field, observed in test components for the compact linear collider concept. The model is based on a thermodynamic consideration of the effect of an external electric field on the formation enthalpy of defects. Although strictly speaking only valid for electric fields, the model also reproduces very well the breakdown rate of a wide range of radio-frequency breakdown experimental data. We further show that the fitting parameter in the model can be interpreted to be the relaxation volume of dislocation loops in materials. The values obtained for the volume are consistent with dislocation loops with radii of a few tens of nanometers.
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  • Increasing the stiffness of SWCNT bucky paper by heavy ion irradiation

    Salmi, A.   Haeggstrom, E.   Arstila, K.   Nordlund, K.   Keinonen, J.  

    We verify simulation results indicating that carbon ion irradiation increases the stiffness of single wall carbon nanotube bucky paper (SWCNT). 4 MHz longitudinal and 15 MHz shear ultrasonic signals were transmitted by means of dry-coupled acoustic horns through 40plusmn4 mum thick samples and the time-of-flight (TOF) through the samples was measured. From the TOF the velocity of the wave was determined and the out-of-plane elastic and shear moduli were calculated. The nonlinearity parameter B/A for the irradiated and reference sample was measured. The results indicated a significant increase both in the shear modulus (90 %) and in the longitudinal modulus (69 %) for certain irradiation doses as predicted by simulations. In addition, a 36% increase in the acoustic nonlinearity parameter (B/A) was observed. The investigated fluence range was 1middot10 13 to 3middot10 14 cm -2
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  • Optimization of single crystal mirrors for ITER diagnostics

    Litnovsky, A.   Peng, J.   Kreter, A.   Krasikov, Yu.   Rasinski, M.   Nordlund, K.   Granberg, F.   Jussila, J.   Breuer, U.   Linsmeier, Ch.  

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  • Reflection of hydrogen and deuterium atoms from the beryllium, carbon, tungsten surfaces

    Meluzova, D.S.   Babenko, P.Yu.   Shergin, A.P.   Nordlund, K.   Zinoviev, A.N.  

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  • Measurement of Si 311 defect properties using x-ray scattering

    Nordlund, K.   Metzger, T. H.   Malachias, A.   Capello, L.   Calvo, P.   Claverie, A.   Cristiano, F.  

    The 311 defects play a crucial role in the damage healing and dopant redistribution which occurs during the annealing of an ion-beam-doped Si. Using grazing-incidence x-ray scattering we measure the type, length, and width of the 311 defects created with different annealing times. In particular, we show that measurements around (1.3 1.3 0) in reciprocal space can be used to determine all these quantities without the need for pristine reference samples. The results agree well with computer simulation predictions and transmission-electron-microscopy measurements, demonstrating that x-ray methods can be used as a nondestructive, rapid method to characterize the 311 defects. (c) 2005 American Institute of Physics.
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  • Atomic-scale simulations of radiation effects in GaN and carbon nanotubes

    Nordlund, K.   Nord, J.   Krasheninnikov, A. V.   Albe, K.  

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  • Sputtering of hydrocarbons by ion-induced breaking of chemical bonds

    Nordlund, K.   Salonen, E.   Keinonen, J.   Wu, C.H.  

    While the physical sputtering of atoms caused by keV and MeV ions has been studied extensively both by molecular dynamics (MD) simulation and experiments, the mechanisms leading to atom and molecule erosion at energies 1–100 eV are not very well understood. We now describe how low-energy hydrogen ions can cause erosion of carbon atoms and hydrocarbon molecules by entering the region of a carbon–carbon chemical bond and hence breaking it, a process we call `swift chemical sputtering'. In the particular case of hydrogen bombardment of carbon-based materials, we further show that this can lead to erosion yields far exceeding those expected for a physical sputtering process.
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  • Self-Interstitial Atoms at High Temperatures in Dense Metals

    Nordlund, K.   Averback, Robert S.  

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  • Diffuse x-ray streaks from stacking faults in Si analyzed by atomistic simulations

    Nordlund, K.   Beck, U.   Metzger, T. H.   Patel, J. R.  

    Since extrinsic stacking faults can form during postimplantation annealing of Si, understanding their properties is important for reliable control of semiconductor manufacturing processes. We demonstrate how grazing incidence X-ray scattering methods can be used as a nondestructive means for detecting extrinsic stacking faults in Si. Atomistic analysis of diffuse intensity streaks is used to determine the size of the faults, the minimum size at which the streak pattern in the scattering will be visible, and the magnitude of atomic displacements in the center of the stacking fault
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  • Glancing incidence diffuse X-ray scattering studies of implantation damage in Si

    Nordlund, K.   Partyka, P.   Zhong, Y.   Robinson, I.K.   Averback, R.S.   Ehrhart, P.  

    Diffuse X-ray scattering (DXS) at glancing incidence is a potentially powerful means for elucidating damage structures in irradiated solids. Fundamental to the analysis of diffuse X-ray scattering data is a knowledge of the atomic displacement field around defects, which for implantation damage in crystals like Si has been difficult to obtain using analytical solutions of elastic continuum theory. We present a method for predicting the diffuse scattering pattern by calculating the displacement field around a defect using fully atomistic simulations and performing discrete sums for the scattering intensity. We apply the method to analyze experimental DXS results of defects produced by 4.5 keV He and 20 keV Ga irradiations of Si at temperatures of 100–300 K. The results show that the self-interstitial in ion-irradiated Si becomes mobile around 150 K, and that amorphization of silicon by light and medium-heavy projectiles occurs homogeneously through the buildup of interstitial clusters, and not within single cascade events.
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  • Role of Self-Interstitial Atoms on the High Temperature Properties of Metals

    Nordlund, K.   Averback, R. S.  

    Equilibrium concentrations of self-interstitial atoms and divacancies have been determined in Cu by molecular dynamics computer simulations using embedded atom potentials. Near the melting temperature these concentrations are both ~10 -6. Owing to the higher mobility of the interstitial atoms, however, they contribute more to diffusion. In perfect, or pulse-heated crystals, spontaneous Frenkel pair production results in even higher interstitial concentrations
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  • Effect of Atomic Bonding on Defect Production in Collision Cascades

    Nordlund, K.   Averback, R. S.   Diaz de la Rubia, T.  

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  • Collision cascades in metals and semiconductors: defect creation and interface behavior

    Nordlund, K.   Averback, R.S.  

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