Schnell, schwer, hoch geladen : Ionen als Werkzeuge für die Festkörperphysik
Ionen eignen sich aufgrund ihrer Eigenschaften hervorragend für die Festkörperphysik: Die Art der Energieabgabe von Ionen in Materie hängt von ihrer Bewegungsenergie ab. Schnelle, energiereiche Ionen werden hauptsächlich durch die Wechselwirkung mit den Elektronen des Materials abgebremst (electronic stopping). Langsame, energiearme Ionen verlieren ihre Energie hauptsächlich durch Stöße mit den Atomkernen des Materials (nuclear stopping).
Ions are a very versatile tool in solid-state physics. Depending on their energy and their charge state, different interactions with solid matter can be studied. In the case of swift heavy ions, the ion/solid interaction is governed by so-called electronic stopping. The energy loss of the projectile is mainly due to electronic excitations that arise in the wake of the ion travelling through the solid. The response of the solid depends on its electronic structure. In the case of a metal, the excitation dissipates before the lattice can be heated. Since the electronic system quickly relaxes into its original state, usually no evidence of the passing ion is found. In the case of an insulator, the coupling of electronic excitations to acoustic and optic modes of the lattice is very strong. As a consequence, the electronic excitation is transferred rather efficiently to the lattice and melting occurs. This very intense, rapid and localized excitation is called a thermal spike. The spike usually leaves the lattice heavily disturbed. The static legacy of this highly dynamic process can be imaged through scanning force microscopy. Each ion produces a nanometer-sized hillock on the surface. In this way, nanoscale modifications of technologically interesting materials become feasible. If the surface is tilted with respect to the beam, chains of hillocks appear. Because the crystal is anisotropic with respect to the electron density, the ion can lose energy only when it passes a layer with a high enough electron density. Another important aspect is the charge state of the ion. Even with slow, highly charged ions; i.e. in the regime of nuclear stopping, an electronic excitation due to the many missing electrons can be injected into the solid. Future experiments will shed light on the corresponding energy dissipation processes.