Atomistic simulations of swift ion tracks in diamond and graphite

We have used molecular dynamics simulations to study ion tracks in diamond and graphite. Tracks are included using a thermal spike model, i.e. a certain number of atoms within an initial track radius are given an initial excitation energy. The total energy given to the excited atoms and the length of the track determine an “effective” stopping power dE/dx. Electronic excitations in semiconductors and semimetals like diamond and graphite can diffuse far from each other or be quenched before they couple to the lattice. This effect is included by varying the number of atoms that are effectively energized within the track. We use an initial track radius of 3 nm and we find that full amorphization of this region during the first few ps only occurs when the “effective” dE/dx is larger than 6 ± 0.9 keV/nm for graphite and 10.5 ± 1.5 keV/nm for diamond. Since the “effective” dE/dx depends on the electron–phonon coupling, our simulations set bounds on the efficiency of the coupling between the electronic excitations and the lattice in this highly non-equilibrium scenario.