Including the Effects of Electronic Excitations and Electron-Phonon Coupling in Cascade Simulations
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0981-JJ04-06
Including the Effects of Electronic Excitations and Electron-Phonon Coupling in Cascade Simulations Dorothy Duffy1,2 and Alexis Rutherford1 1 London Centre for Nanotechnology, UCL, 17-19 Gordon Street, London, WC1H 0AH, United Kingdom 2 Culham Science Centre, EURATOM/UKAEA Fusion association, Culham, OX14 3DB, United Kingdom
ABSTRACT Radiation damage has traditionally been modeled using cascade simulations however such simulations generally neglect the effects of electron-ion interactions, which may be significant in high energy cascades. A model has been developed which includes the effects of electronic stopping and electron-phonon coupling in Molecular Dynamics simulations by means of an inhomogeneous Langevin thermostat. The energy lost by the atoms to electronic excitations is gained by the electronic system and the energy evolution of the electronic system is modeled by the heat diffusion equation. Energy is exchanged between the electronic system and the atoms in the Molecular Dynamics simulation by means of a Langevin thermostat, the temperature of which is the local electronic temperature. The model is applied to a10 keV cascade simulation for Fe. INTRODUCTION Cascade simulations have proved to be a very successful methodology for studying the effects of radiation damage in solids, however such methods have their limitations. One such limitation is that the effect of electronic excitations is generally neglected and, whilst this is reasonable approximation for relatively low energy cascades, such effects increase with increasing cascade energies. For example, a 100 keV atom moving in Fe will lose 17% of its energy to inelastic scattering of electrons. Indeed even for relatively low cascade energies the cooling effect of the electrons may be significant. This was first pointed out by Flynn and Averbach in 1988 [1], who noted that cold electrons moving through a hot cascade region may, depending on the electron-phonon coupling strength, gain energy from the hot atoms and thus contribute to the cooling of the thermal spike. Stoneham [2] noted that, depending on the relative timescales of the processes, the electrons could act either as a heat sink or a heat bath. The effects of electronic stopping have been included in cascade simulations by adding a friction term to the equation of motion for atoms moving above a defined cutoff velocity [3]. The effects of electron-phonon interactions have also included by means of a friction term [4,5] and a Langevin thermostat [6]. In this work we include both effects using the model suggested by Caro and Victoria. We include the electronic temperature explicity in the model. The energy lost by the atoms in the MD simulation, due to electronic stopping or electron-phonon interactions, is gained by the electronic system and the evolution of the electronic temperature is modeled using the heat diffusion equation. Energy is fed back into the atomic simulation by means of a Langevin thermostat, using the local electronic temperature as the thermostat temperature.
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