1. Department of Physics/Centre for Materials Science and Nanotechnology, University of Oslo, Blindern, P.O. Box 1048, Oslo N-0316, Norway
2. Department of Physics, University of Pretoria, Lynnwood Road, Pretoria 0002, South Africa
3. Lawrence Livermore National Laboratory, Livermore, California 94550, USA
4. Leibniz-Institut für Kristallzüchtung, Max-Born-Straße 2, 12489 Berlin, Germany
5. Department of Applied Physics, Aalto University, 00076 Aalto, Finland