1. Solid State Physics Laboratory, ETH Zürich, CH-8093 Zürich, Switzerland.
2. Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan.
3. Institute of Physics, University of Silesia in Katowice, 41-500 Chorzów, Poland.
4. Microsoft Research Cambridge, Cambridge, UK.
5. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
6. Institute of Theoretical Physics, University of Regensburg, D-93040 Regensburg, Germany.