1. Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Strasse 3, 79104 Freiburg, Germany
2. Centre for Materials Science and Nanotechnology, Department of Physics, University of Oslo, P.O.
Box 1048 Blindern, NO-0316 Oslo, Norway
3. Department of Energy and Process Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway
4. Freiburg Institute for Advanced Studies, Albert-Ludwigs-Universität Freiburg, Albertstrasse 19, 79104 Freiburg, Germany