The modelling of radiation damage in metals using Ehrenfest dynamics
Author(s)
Race, Christopher Peter
Type
Thesis
Abstract
In this thesis we use a time-dependent tight-binding model metal evolving under semiclassical
Ehrenfest dynamics to explore the effects of electron-ion energy exchange on
radiation damage phenomena. By incorporating an explicit model of quantum mechanical
electrons coupled to a set of classical ions, our model correctly reproduces the interaction
of excited ions with cooler electrons and captures phenomena absent in classical
molecular dynamics simulations and in much-used analytical models.
With our simple model we have been able to simulate large numbers of radiation damage
cascades. We have directly explored the electronic excitations stimulated in such
cascades and have found them to be well characterized by an elevated electronic temperature.
We have also analysed the effect of these excitations in weakening the bonding
interactions in our model metal, and the effect of these weakened interactions on the
evolution of replacement collision sequences.
By separating out components of the Hellmann-Feynman forces exerted by the electrons
on the ions, we have identi ed the non-adiabatic force, resulting from the finite response
time of the electrons to ionic motion and responsible for the accumulating electronic
excitations. Based on simplifying physical arguments we have derived a temporallyand
spatially-local expression for this force suitable for incorporation within a classical
MD code at very low computational cost. Data from our simulations show that our
new expression for the non-adiabatic force captures much of the microscopic detail of
the direction and magnitude of the force. We find that it significantly outperforms
commonly used viscous damping models of ion-electron energy transfer.
At higher energies, our simulations of ion channelling reveal a new resonant enhancement
of the electronic charge on the channelling ion and corresponding effects on the stopping
force. We explain these phenomena with reference to the detailed atomic and electronic
structure of our model.
Ehrenfest dynamics to explore the effects of electron-ion energy exchange on
radiation damage phenomena. By incorporating an explicit model of quantum mechanical
electrons coupled to a set of classical ions, our model correctly reproduces the interaction
of excited ions with cooler electrons and captures phenomena absent in classical
molecular dynamics simulations and in much-used analytical models.
With our simple model we have been able to simulate large numbers of radiation damage
cascades. We have directly explored the electronic excitations stimulated in such
cascades and have found them to be well characterized by an elevated electronic temperature.
We have also analysed the effect of these excitations in weakening the bonding
interactions in our model metal, and the effect of these weakened interactions on the
evolution of replacement collision sequences.
By separating out components of the Hellmann-Feynman forces exerted by the electrons
on the ions, we have identi ed the non-adiabatic force, resulting from the finite response
time of the electrons to ionic motion and responsible for the accumulating electronic
excitations. Based on simplifying physical arguments we have derived a temporallyand
spatially-local expression for this force suitable for incorporation within a classical
MD code at very low computational cost. Data from our simulations show that our
new expression for the non-adiabatic force captures much of the microscopic detail of
the direction and magnitude of the force. We find that it significantly outperforms
commonly used viscous damping models of ion-electron energy transfer.
At higher energies, our simulations of ion channelling reveal a new resonant enhancement
of the electronic charge on the channelling ion and corresponding effects on the stopping
force. We explain these phenomena with reference to the detailed atomic and electronic
structure of our model.
Date Issued
2010-06
Date Awarded
2010-07
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Sutton, Adrian
Foulkes, Matthew
Creator
Race, Christopher Peter
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)