Festkörperprobleme 26 Plenary Lectures of the Divisions "Semiconductor Physics" "Dynamics and Statistical Mechanics" "Low Temperature Physics" "Magnetism" "Metal Physics" "Thin Films" "Surface Physics" "Vacuum Technology" of the German Physical Society (DPG) Freudenstadt, April 7…11, 1986 - Grosse, P. (Springer Berlin Heidelberg, 1986)