1. Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
2. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
3. Graduate Institute of Applied Physics and Department of Physics, National Taiwan University, Taipei 10617, Taiwan
4. Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
5. Institute of Physics, Academia Sinica, Taipei 115, Taiwan