Robert Zillich - Diplomarbeit
Streuung von 3He Atomen an dünnen 4He Filmen
angefertigt an der Abteilung für Vielteilchensysteme des Instituts für Theoretische Physik
Helium atoms interact via a potential that has a strongly repulsive core and a weakly attractive tail.
The weak attraction plus the light mass of the Helium atoms prevent them from solidifing at zero pressure down to zero temperature. Instead, below a certain temperature, the isotope Helium-4 undergoes a phase transition to superfluidity, which is the most remarkable effect of this liquid (actually, also Helium-3 has a superfluid state, but at a much lower temperature).
The ground state and excitations of a inhomogeneous system of interacting atoms, described by the many-body Schroedinger equation, can be studied using a microscopic theory based on finding the optimal correlations between these atoms.
This formalism is applied to the scattering of a single impurity atom (Helium-3) on a film of several layers of Helium-4 atoms adsorbed to a smooth substrate surface. Due to the coupling to excitations of the film the probability that the impurity atom retains its energy after reflection is smaller than unity. Thus, there is a finite probability for the scattered atom to deposit energy in the film and to either reflect back with decreased energy or to stick on the film. In this work the elastic reflection probabilities are calculated for various energies, substrates, and values of the film thickness; the coupling to ripplons (surface wave modes) and to rotons (specific to the Helium-4 fluid), the phenomenon of "quantum reflection" and its sensitivity to the strength of the substrate potential, and the significance of resonances are investigated.