Abstract
By simulating the passage of heavy ions along open channels in a model crystalline metal using semi-classical Ehrenfest dynamics we directly investigate the nature of non-adiabatic electronic effects. Our time-dependent tight-binding approach incorporates both an explicit quantum mechanical electronic system and an explicit representation of a set of classical ions. The coupled evolution of the ions and electrons allows us to explore phenomena that lie beyond the approximations made in classical molecular dynamics simulations and in theories of electronic stopping. We report a velocity-dependent charge-localization phenomenon not predicted by previous theoretical treatments of channelling. This charge localization can be attributed to the excitation of electrons into defect states highly localized on the channelling ion. These modes of excitation only become active when the frequency at which the channelling ion moves from interstitial point to equivalent interstitial point matches the frequency corresponding to excitations from the Fermi level into the localized states. Examining the stopping force exerted on the channelling ion by the electronic system, we find broad agreement with theories of slow ion stopping (a stopping force proportional to velocity) for a low velocity channelling ion (up to about 0.5 nm fs−1 from our calculations), and a reduction in stopping power attributable to the charge localization effect at higher velocities. By exploiting the simplicity of our electronic structure model we are able to illuminate the physics behind the excitation processes that we observe and present an intuitive picture of electronic stopping from a real-space, chemical perspective.
Highlights
Along high symmetry directions in a crystal lattice, a fast moving ion can travel large distances in a process known as channelling
The conservative ionic potential may be viewed as an approximation to the Born–Oppenheimer potential surface used in quantum molecular dynamics, but neither conventional classical MD nor Born–Oppenheimer quantum MD is able to model the irreversible loss of energy from fast-moving ions to electrons
The most striking feature of figure 3 is the significant increase in the negative charge of the projectile at velocities between 0.5 and 1.5 nm fs−1. This feature is unexpected, but we can understand its origins if we examine the local density of states (LDoS) on the channelling ion
Summary
Max Planck Institute for Iron Research, Dusseldorf, Germany. 4 Present address: EURATOM/UKAEA Fusion Association, Culham Science Centre, Oxfordshire OX14 3DB, UK. 5 Present address: Biophysics Graduate Group, University of California, Berkeley, CA 94720, USA.
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