Hyperfast Correlated Dynamics of Radiation Damage and Recovery in Materials.

摘要

The response of solid-state materials to radiation is governed through a host of mechanisms that have time scales ranging from femtoseconds to seconds and years. Metastable liquid-like regions that typically last for several picoseconds and more are commonly observed in ultra-fast experiments and simulations. In this investigation, we make quantitative predictions on correlated dynamical motion of the atoms as the liquid-like state is formed and condensed following an ion or neutron impact. Simulations on three materials -- copper, silicon and argon -- that have very different bond structures reveal an anisotropic and heterogeneous dynamical structure. Of utmost importance are the dynamical correlations during the recovery period, which corresponds to the condensation of the liquid-like state.;Using molecular dynamics simulations and with the appropriate non-equilibrium shock physics formalism, the dynamical metrics of the liquid-like state are evaluated through the density correlator and van Hove self-correlation function, as well as through defect, thermodynamic and hydrodynamic field data, following a confined ion/neutron impact. These correlation functions can also be experimentally accessed or inferred from the state-of-the-art ultrafast pump-probe experimental methods. The hopping mechanism from the van-Hove self-correlation, the fractallike condensation and the fast decay of the density correlator attest to a rapid defect recovery in copper. In contrast, silicon portrays dynamically heterogeneous regions that resist recovery to the underlying lattice structure, and exhibits a non-decaying density correlator that is strikingly analogous to that of a supercooled liquid.;Ion hammering and pump-probe experiments allude to a liquid-liquid phase transition in silicon -- from a high density liquid to a low density liquid -- before silicon is amorphized; the inference, however, is based on indirect interpretations. The simulations presented in this dissertation demonstrate a transitioning to a more complex and rich dynamical structure with a fascinating directional anisotropy that is very different from that in quasi-equilibrium conditions. Thus largescale simulations, as presented in this work, would be of immense value in interpreting the results from experiments performed at ultra/hyperfast timescales.;Lastly, a fundamental understanding of the dynamical attributes of defect recovery is generally unknown; the various defect models currently in use do not account for the dynamical recovery. Insights garnered from our work can however, be advantageously employed in developing realistic models for defect recovery. Although the structural aspects of tolerance of materials to radiation have been elucidated before, the current work throws light on the incipient dynamical interactions that control the structural transformation in a radiation environment.