Speaker
Description
Graphene provides the thinnest possible diffraction grating for atom and ion transmission, providing an ideal robust model system for studying the fundamental physics of energy and momentum transfer, and the loss of coherence. Understanding atomic-scale details is only possible with the help of first-principles simulations capable of describing energy loss, which is feasible with the help of time-dependent density functional theory. Our implementation of LCAO Ehrenfest molecular dynamics (ED) method within the projector augmented-wave code GPAW yields satisfactory accuracy at a much-reduced computational cost, which is particularly useful for modeling irradiation processes that require large amounts of vacuum in the simulation cell [1].
After the first demonstration of atom diffraction in transmission through a crystal [2], new measurements have now been performed with an improved graphene sample featuring large monocrystalline domains comparable in size to the beam diameter. In these experiments, the shape of the diffraction pattern is largely determined by the lattice at equilibrium whereas vibrationally-induced distortions are treated perturbatively. By including a model of the target lattice incorporating thermally diffuse scattering via phonon normal modes, we show that the perturbative approach does not hold for helium diffracted at kiloelectronvolt energy through freestanding single-layer graphene. In this case, we enter a new regime of strong coupling, where the projectile strongly interacts with the electron density of several lattice atoms simultaneously, with the weak-coupling regime is retained for atomic hydrogen diffraction [3].
Turning then to ion transmission, the charge of an ion becomes a dynamic quantity when it is in contact with other atoms, molecules, or solids, leading to a complex coupling between stopping processes in matter. We perform experimentally-benchmarked Ehrenfest dynamics simulations of neutral Xe and Xe$^{12+}$ ions transmitted through freestanding single-layer of graphene to reveal the synergy between nuclear and electronic stopping. Importantly, momentum transfer from phonons excludes the high-symmetry bond-center impact parameter from the small acceptance angle of the ion analyzer. Relative to neutral projectiles, ions exert an enhanced nuclear energy transfer, and exhibit non-trivial electronic energy losses. This can result in greater electronic stopping for neutral projectiles compared to ions under certain conditions. Our findings challenge the typical notion that ion stopping force is merely the sum of independent nuclear and electronic stopping components [4].
Funding by the Austrian Science Fund (FWF) via grant P 36264-N is gratefully acknowledged.