1. Institut für Schichten und Grenzflächen (ISG) and cni - Center of Nanoelectronic Systems for Information Technology, Forschungszentrum Jülich, D-52425 Jülich, Germany
2. Digital Materials Laboratory, Frontier Research System (FRS), RIKEN, 2-1 Hirosawa, Wako-Shi, Saitama, 351-0198, Japan
3. Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1120, USA