1. Institute of Experimental and Applied Physics, University of Regensburg, D-93040 Regensburg, Germany
2. Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan
3. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
4. Institute of Theoretical Physics, University of Regensburg, D-93040 Regensburg, Germany