1. Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway
2. Department of Physics, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway
3. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
4. Department of Physics, ETH Zurich, Otto-Stern-Weg 1, 8093 Zurich, Switzerland