1. Department of Physics, National Tsing-Hua University, Hsinchu 30013, Taiwan.
2. Department of Physics, The University of Texas at Austin, Austin, Texas 78712, USA.
3. Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan.
4. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
5. Department of Electrophysics, National Chiao-Tung University, Hsinchu 30010, Taiwan.