1. Department of Physics and Helsinki Institute of Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland
2. Department of Applied Physics, Aalto University, P.O. Box 15100, FI-00076 Espoo, Finland
3. Leibniz-Institut fur Kristallzuchtung, Max-Born-Str. 2, 12489 Berlin, Germany