The Modelling of Radiation Damage in Metals Using Ehrenfest Dynamics

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 identified 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.

Accurate atomistic first-principles calculations of electronic stopping

“…Later stages involve secondary knocked atoms, track formation and melting, as well as long-term irreversible effects on the material that are jointly referred to as radiation damage. [1][2][3][4] Understanding those interactions is highly important for fusion and fission applications, 5 high-energy density physics, 6 medicine, 7 as well as nuclear safety. 8 On average, the stopping power of a material for a given type of projectile is characterized by the projectile's velocity.…”

Ultrafast electron dynamics and orbital-dependent thermalization in photoexcited metals

“…Thus, it is limited in representing a path to thermalization because a thermal state is a mixed state. That is why TDDFT is usually applied to describe phenomena much faster than electron thermalization time [34,35]. Recently it was shown however that TDDFT could indirectly provide a mechanism for the electrons to thermally equilibrate with each other [36].…”

“…Recently it was shown however that TDDFT could indirectly provide a mechanism for the electrons to thermally equilibrate with each other [36]. Moreover the motion of the atoms produce a large number of small electronic transitions affecting the evolution of the electron energy distribution function similar to a thermalization process [34,[37][38][39][40]. For the same reason the external electric potential of a laser pulse contributes to the thermalization of electrons through the photon-electron-ion collisional process and thus TDDFT can reasonably capture the thermalization process during the laser pulse.…”

Resonant charging and stopping power of slow channelling atoms in a crystalline metal

“…By contrast, tight binding (TB) allows us to retain a fully quantum mechanical description of the electrons and their interaction with ions, but in a simpler representation that enables the simulation of orders of magnitude larger systems and for longer periods of time than is the case with DFT. This is the approach to simulating non-adiabatic, quantum, electron dynamics we have developed in our earlier simulations of radiation damage in metals [21][22][23][24][25][26][27][28].…”