1. Institute of Nanoengineering and Microsystems, National Tsing-Hua University, Hsinchu 30013, Taiwan
2. Department of Physics, National Tsing-Hua University, Hsinchu 30013, Taiwan
3. School of Physics, Peking University, Beijing 100871, China
4. Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan
5. Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
6. Center for Applied Sciences, Academia Sinica, Nankang, Taipei 11529, Taiwan