Thermal friction and Brownian motion of interstitial defects in irradiated materials

We investigate the origin of long-range correlated jumps observed in molecular-dynamics simulations of Brownian motion of interstitial clusters in irradiated crystalline materials. We show that the presence of long jumps is associated with low thermal friction experienced by a cluster propagating through the crystal lattice in the presence of thermal fluctuations. Using the equation for the friction coefficient, we estimate the length of ballistic trajectories of clusters ejected from high-energy cascades and analyse implications of our findings for understanding the nature of the long-range effect observed in experiments on ion implantation in fcc and bcc metals, and the connection between this effect and the stability of metals under irradiation.