The modelling of electronic effects in molecular dynamics simulations

This thesis describes the development and applications of the continuum-atomistic molecular dynamics (MD) model in the context of radiation damage. By extending the classical MD method to incorporate the electronic excitations represented as an electron fluid and coupled to the ions in the two-temperature (2T) formalism, we have been able to correctly capture the physics governing the atomistic dynamics under huge electronic excitations. The integrated 2T-MD model has been specifically adapted to study three types of non-equilibrium scenarios: laser excitations, swift heavy ion impacts and large-scale high energy collision cascades. Using the 2T-MD model we have estimated the impact of the electron-phonon coupling and the electronic stopping power on the primary radiation damage yield in bcc iron. We have found that the cascade dynamics and the resultant damage from 50-100 keV primary knock-on atom impacts is highly sensitive to the electronic stopping treatment at low projectile velocities, which represents the first rigorous study of this type. By examining the temporal evolution of the structure factor of laser-irradiated gold thin films, we have been able to directly compare the 2T-MD results with Bragg peaks measured by ultrafast electron diffraction and have achieved an excellent agreement between theory and experiment with no fitting parameters. This has enabled us to elucidate the melting dynamics following laser irradiation at a picosecond resolution for the first time and also to validate the two-temperature approach. To simulate semiconductors under electronic excitations, the continuum part of the 2T-MD model, which represents electrons, has been replaced by two continuum equations: one for carrier density and one for their energy, to account for the finite band-gap effects. We have applied such extended method to simulate ion tracks, which result from swift heavy ion impacts. We have achieved a very good agreement with the experimental results on the ion track radii, provided that we are free to adjust the strength of the electron-phonon coupling. We propose future studies in the field of non-equilibrium atomistic modelling. In particular, we discuss ab initio methods and further improvements to hybrid MD to study the effects of the interatomic potential changes in response to high electronic excitations

Modelling the effects of electronic excitations in ionic-covalent materials

High energy radiation events in ionic and covalent materials can lead to highly excited electronic configurations which, over time, relax to the ground state, either radiatively by emitting photons, or non-radiatively. Non-radiative relaxation involves the transfer of energy to the lattice and this can result in lattice heating, defect formation or even phase changes. The effects of the relaxation mechanisms on the atomic configuration are challenging to model accurately by standard methods. The situation is further complicated by interactions between electronic excitations and pre-existing defects, possibly created by other radiation events. In this paper we describe a range of mechanisms by which the electronic energy is transferred to the lattice and the resulting effects on the atomic configuration, along with the different techniques that are used to model these effects

The influence of the electronic specific heat on swift heavy ion irradiation simulations of silicon

The swift heavy ion (SHI) irradiation of materials is often modelled using the two-temperature model. While the model has been successful in describing SHI damage in metals, it fails to account for the presence of a bandgap in semiconductors and insulators. Here we explore the potential to overcome this limitation by explicitly incorporating the influence of the bandgap in the parameterisation of the electronic specific heat for Si. The specific heat as a function of electronic temperature is calculated using finite temperature density functional theory with three different exchange correlation functionals, each with a characteristic bandgap. These electronic temperature dependent specific heats are employed with two-temperature molecular dynamics to model ion track creation in Si. The results obtained using a specific heat derived from density functional theory showed dramatically reduced defect creation compared to models that used the free electron gas specific heat. As a consequence, the track radii are smaller and in much better agreement with experimental observations. We also observe a correlation between the width of the band gap and the track radius, arising due to the variation in the temperature dependence of the electronic specific heat

Hybrid continuum–atomistic modelling of swift heavy ion radiation damage in germanium

The response of germanium to swift heavy ion irradiation is simulated using a hybrid continuum–atomistic approach. The continuum part of the model, which characterises the electronic excitations is an extension of the inelastic thermal spike based on an approximation to the Boltzmann transport equation; while the atomistic part is represented with molecular dynamics. This integrated method can realistically account for the non-equilibrium carrier dynamics in band-gap materials under irradiation, unlike earlier developments based on the two-temperature approach. The model is used to obtain temporal and spatial evolution of carrier density, electronic temperature and lattice temperature for germanium irradiated with carbon cluster ions. Good agreement with experimental data of amorphised latent track radii for different stopping powers is obtained by fitting a constant value for the electron–phonon coupling strength – the only parameter treated as free in the model

Nonthermal solid-to-solid phase transitions in tungsten

The ab initio calculations of phonon dispersions and nonthermal forces along structural deformation paths were used to study nonthermal solid-to-solid phase transitions in photoexcited tungsten. We assumed that electronic excitation can be described by an electronic temperature and demonstrated that nonthermal, i.e., caused purely by electronic excitation, bcc-to-fcc and bcc-to-hcp phase transitions can occur for electronic temperatures between 1.7 and 4.3 eV. These transitions result from soft modes along the Σ line of the Brillouin zone. Structural path calculations at different electronic temperatures indicate that both transitions are likely to take place in nonequilibrium conditions. We further predict that transient fcc and hcp phases of tungsten could be observed for several ps

Determination of the electron-phonon coupling constant in tungsten

We used two methods to determine the effective electron-phonon coupling constant (G 0) in tungsten. Our first principles calculations predict G 0 = 1.65 × 1017 W m−3 K−1. The temporal decay of the femtosecond-resolution optical reflectivity for a (100) surface of bulk W was measured using a pump-probe scheme and analysed using ab initio parameterised two temperature model, which includes both the effects of the electron-phonon coupling and thermal conduction into bulk. This analysis gives G 0 = 1.4(3) × 1017 W m−3 K−1, in good agreement with the theoretical prediction. The described effective method of calculating and measuring G0 in bulk materials can be easily extended to other metals

The nature of high-energy radiation damage in iron.

Understanding and predicting a material's performance in response to high-energy radiation damage, as well as designing future materials to be used in intense radiation environments, requires knowledge of the structure, morphology and amount of radiation-induced structural changes. We report the results of molecular dynamics simulations of high-energy radiation damage in iron in the range 0.2-0.5 MeV. We analyze and quantify the nature of collision cascades both at the global and the local scale. We observe three distinct types of damage production and relaxation, including reversible deformation around the cascade due to elastic expansion, irreversible structural damage due to ballistic displacements and smaller reversible deformation due to the shock wave. We find that the structure of high-energy collision cascades becomes increasingly continuous as opposed to showing sub-cascade branching as reported previously. At the local length scale, we find large defect clusters and novel small vacancy and interstitial clusters. These features form the basis for physical models aimed at understanding the effects of high-energy radiation damage in structural materials

Determination of transient atomic structure of laser-excited materials from time-resolved diffraction data

The time evolution of the Bragg peaks of photo-excited gold nanofilms is measured using transmission ultrafast electron diffraction (UED) with 3.0 MeV electron pulses and the corresponding structure evolution is calculated using two-temperature molecular dynamics (2T-MD). The good agreement obtained between the measured and calculated Bragg peaks, over the full experimental timescale, enables the lattice temperature effects and the structural changes to be disentangled for the first time. The agreement demonstrates that 2T-MD is a reliable method for solving the inverse problem of structure determination of laser irradiated metals in UED measurements

Structural dynamics of laser-irradiated gold nanofilms

We performed relativistic ultrafast electron diffraction (UED) measurements of the structural dynamics of photoexcited gold nanofilms and developed an atomistic model, based on the two-temperature molecular dynamics (2T-MD) method, which allows us to make a direct comparison of the time evolutions of measured and calculated Bragg peaks. The quantitative agreement between the temporal evolutions of the experimental and theoretical Bragg peaks at all fluences suggests that the 2T-MD method provides a faithful atomistic representation of the structural evolution of photoexcited gold films. The results reveal the transition between slow heterogeneous melting at low absorbed photon fluence to rapid homogeneous melting at higher fluence and nonthermally driven melting at very high fluence. At high laser fluence, the time evolution of Bragg peaks calculated using the conventional 2T-MD model disagrees with experiment. We show that using an interatomic potential that directly depends on the electron temperature delivers a much better agreement with UED data. Finally, our ab initio calculations of phonon spectra suggest electronic bond softening, if the nanofilms can expand freely under electronic pressure, and bond hardening, if they are constrained in all three dimensions