Modelling radiation effects in solids with two-temperature molecular dynamics

Abstract

The ability to predict the structural modifications of materials resulting from a broad range of irradiation scenarios would have a positive impact on many fields of science and technology. Established techniques for modelling large atomic systems, such as classical molecular dynamics, are limited by the neglect of the electronic degrees of freedom which restricts their application to irradiation events that primarily interact with atomic nuclei. Ab initio methods, on the other hand, include electronic degrees of freedom, but the requisite computational costs restrict their application to relatively small systems. Recent methodological developments aimed at overcoming some of these limitations are based on methods that couple atomistic models to a continuum model for the electronic energy, where energy is exchanged between the nuclei and electrons via electronic stopping and electron-phonon coupling mechanisms. Such two-temperature molecular dynamics models, as they are known, make it practicable to simulate the effects of electronic excitations on systems with millions, or even hundreds of millions, of atoms. They have been used to study laser irradiation of metallic films, swift heavy ion irradiation of metals and semiconductors, and moderately high ion irradiation of metals. In this review we describe the two-temperature molecular dynamics methodology and the various practical considerations required for its implementation. We provide example applications of the model to multiple irradiation scenarios that accommodate electronic excitations. We also describe the challenges of including the effects of the modification of the interatomic interactions, due to the excitation of electrons, in the simulations and how these challenges can be overcome.

Simulating electronically driven structural changes in silicon with two-temperature molecular dynamics

“…This approach has the advantage that the electronic heat capacity may be expressed as a function of the carrier density, and the optical properties of the material are more straightforwardly accounted for by using the carrier density to attenuate the radiation intensity as it passes through the material. However, as previously noted [30], explicitly including a term for the carrier density is superfluous since the carrier density has a one-to-one relationship with the electronic temperature under (quasi)equilibrium conditions. It follows that any function of the carrier density may instead be written as a function of electronic temperature.…”

“…However, the cost of these techniques make them unsuitable for studying large length or timescale processes such as ablation and spallation. It is within * r.darkins@ucl.ac.uk this domain of large-scale atomistic modeling of radiation damage that two-temperature molecular dynamics (2T-MD) has prevailed as the method of choice [30].…”

Domain decomposition of the two-temperature model in DL_POLY_4

“…Swift heavy ion irradiated systems apply energy deposition that varies radially in two-dimensions from a central point and decays exponentially in time: the electronic temperature grid in these systems typically extends far from the MD cell in these two dimensions with energy-removing boundaries as well as closed or periodic boundaries in the third dimension. A fuller description of the use of 2T-MD (molecular dynamics with TTM) is given in [13].…”

“…A number of simulations have been carried out with modified versions of dl poly 4 that have incorporated the above-mentioned TTM functionalities [13]. These can be divided between radiation damage cascades, laser irradiation and swift heavy ion (SHI) irradiation.…”

Parametric study of the Two-Temperature Model for Molecular Dynamics simulations of collisions cascades in Si and Ge

“…the one developed by Duffy et al in [8]). The only parameter not present in the Duffy et al version of the TTM we use is the activation time of the electron-phonon coupling (t eph ) which acts to delay by a fraction of picosecond the time at which the electronphonon coupling is activated (Zarkadoula et al set it between 0.2 ps and 1 ps in their works [11,10,18]) and whose use is vivedly recommended [11] and well spread in the community [17,10,18,19]. Experiments [20,21] and MD simulations [2] have showed that the response of Si and Ge materials to particle irradiation is very unalike because of their different thermal properties [4].…”