1. Department of Physics, Umeå University, S-90187 Umeå, Sweden
2. NanoMaterials Group, Department of Applied Physics and Center for New Materials, Aalto University, P.O. Box 15100, 00076 Aalto, Espoo, Finland
3. Ioffe Physical-Technical Institute of the Russian Academy of Sciences, 194021 St. Petersburg, Russia
4. Institute of Physics, St. Petersburg State University, 198504 St. Petersburg, Russia
5. Helmholtz-Zentrum BESSY II, German-Russian Laboratory, D-12489 Berlin, Germany
6. Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden