The effect of electronic energy loss on the dynamics of thermal spikes in Cu
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T. Diaz de la Rubia and M.W. Guinan Lawrence Livermore National Laboratory, L-644, Livermore, California 94550 (Received 14 May 1990; accepted 13 November 1990) We present results of a molecular dynamics simulation study of the effect of electron-ion interactions on the dynamics of the thermal spike in Cu. Interatomic forces are described with a modified embedded atom method potential. We show that the electron-ion interaction acts to reduce the lifetime of the thermal spike and therefore the amount of atomic rearrangement that takes place in energetic displacement cascades in Cu. The results point toward the important effect that inelastic energy losses might have on the dynamics of displacement cascades in the subcascade energy regime where the lifetime of the thermal spike is expected to exceed the electron-phonon coupling time.
I. INTRODUCTION
The characterization of the primary state of radiation damage in irradiated materials has been a problem of interest to the physics and materials research community for the last forty years. Component degradation under irradiation in fission and fusion reactors,1 as well as the use of modern ion beam techniques for materials modification,2 have motivated large research efforts aimed at achieving a basic understanding of this problem. Of fundamental importance in the characterization of the primary state of damage is understanding the role played by energetic displacement cascades. Displacement cascades, initiated when an energetic ion or neutron collides with an atom in the solid, deposit a large amount of energy in a localized region of the solid, thereby driving the system to a highly nonequilibrium state containing elevated defect concentrations and, in the case of irradiated alloys, disorder. Due to their inhomogeneous and highly nonequilibrium nature, theoretical modeling of energetic displacement cascades has proven difficult. As a displacement cascade evolves and the characteristic energy of the atoms in the affected region falls below a few eV, the theoretical description of the event must take into account the diffusive (as opposed to ballistic) nature of the process. In the last few years, molecular dynamics computer simulation (MD) has been shown to be a useful tool to describe the dynamics and structure of energetic displacement cascades. For primary knock-on atom (PKA) energies up to several keV,3'4 previous studies have shown that after a brief collisional phase lasting of the order of 10~13 s, a cooling phase ensues in which atom trajectories are diffusive as opposed to ballistic. This phase, commonly termed a thermal spike, lasts several J. Mater. Res., Vol. 6, No. 3, Mar 1991
picoseconds and has been shown to be of critical importance in determining the amount of atomic rearrangement that takes place in energetic displacement cascades. In particular, the crucial role of cascade induced melting on total defect production, point defect clustering, and atomic mixing in Cu and Ni has been clearly demonstrated.4"6 Several problems concerning the proper phys
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